2009

J.ILANGUMARAN

SL&HOD i/c

3/6/2009

R&A/C MACHINES

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SYLLABUS 2471 REFRIGERATION AND AIR CONDITIONING MACHINES

5 Hours / Week Total Hours : 70

UNIT – I a. COMPRESSORS - Reciprocation compressor – Constructional details and working of single acting

single stage reciprocating compressor – Classification – Rotary compressor – Roller type compressor - vane type compressor – Centrifugal compressor – Comparison of centrifugal with reciprocating compressor

b. CONDENSERS - Working of condensers – factors affecting condensing capacity – Types of cooling medium – Quantity of cooling medium - amount of condensing surface- velocity of cooling medium - Air cooled condensers – Natural convection air cooled condenser – forced convection air cooled condenser – Chassis mounted - remote air cooled condenser - Water cooled condenser – waste water system - Re-circulated water system – Types of water cooled condensers – Tube in tube - Shell and coil, Shell and tube condensers - Evaporative condenser

UNIT II a. EVAPORATORS - Types of evaporators – Bare type coil evaporator - finned evaporator – Plate

evaporator – Shell and tube evaporator – Shell and coil evaporator – Tube in tube flooded evaporator – Dry expansion evaporator – Natural convection evaporator – No problem – distributors.

b. EXPANSION DEVICES - Types of expansion devices – capillary tube - Hand operated expansion valve - automatic expansion valve – Thermostatic expansion valve – Thermostatic expansion valve with internal equalizer - external equalizer – low side float valve – High side float valve.

UNIT III a. COOLING TOWERS AND SPRAY PONDS - Types of cooling towers – Natural draft cooling towers

– Atmospheric natural draft spray type – Atmospheric natural draft splash – deck type cooling towers – Mechanical draft cooling towers – Forced draft, induced draft – Advantages and disadvantages of mechanical draft and natural draft cooling tower.

b. HUMIDIFIERS AND DEHUMIDIFIERS - Method of humidification – injecting the steam – steam injection type humidifier – Atomizing the water - Atomization types – Impact type – Hydraulic separation type – Mechanical separation type – Heated air type – Air washer humidifier - Dehumidification – Reducing the air temperature below DPT or by refrigeration spray type humidifier..

UNIT IV a. AIR CLEANING & AIR FILTERS - Advantages of removal of impurities – Effect of dust on health –

method of air cleaning – Air filters – Dry filters – Viscous filters – Wet filters – Electric filters – Centrifugal dust collectors.

b. FANS AND BLOWERS - Introduction – Forced draft – ID fan – fan types - centrifugal and axial flow propeller fan - tube axial fan - van axial fans – fans in series - fan is parallel.

UNIT V a. REFRIGERANT PIPING - Piping materials – joints and fittings – low pressure vapour line sizing

and layouts – high pressure vapour line sizing and layouts – liquid line sizing and layouts – refrigerant piping for multiple compressor and multiple evaporator systems – pressure losses in piping

b. ACCESSORIES AND CONTROLS – Compressor service valves – use of gauge manifold – vibration eliminators – pressure controls – temperature controls – oil separator – oil pressure control switches – liquid receivers – liquid receiver service valves – purge valves – pressure relief valves – filters and driers – sight glasses – solenoid valves – suction accumulators – suction pressure regulators – relays and overload protecting devices Reference Books:

1. Principles of refrigeration By Roy J Dossat, Wiley Publictions 2. A text book of Refrigeration and Air-conditioning By Arora and Domkundwar 3. A text book of Refrigeration and Air-conditioning By R.S.Khurmi and J.K. Gupta

Pub: Eurasia Publishing house (P) Ltd., Dist: S. Chand & Co Ltd., 152, Anna Salai, Chennai – 2 Ph. 28460026

7361, Ram nagar, Delhi. Ph. 23672080 4. ASHRAE handbook Equipment volume

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CHAPTER 1.COMPRESSOR

1. Define the following terms relating to a Reciprocating Refrigerant - Compressor.

a. Bore : Bore is the inner diameter of the cylinder of a compressor in which the piston reciprocates during operation.

b. Stroke : Stroke is the movement of the piston from TDC to BDC OR BDC to TDC during operation.

c. Top Dead Centre : (TDC) It is the upper most position that a piston reaches during operation.

d. Bottom Dead Centre : (BDC)

It is the lower most position that a piston reaches during operation.

e. Clearance: There is always a gap left in between the Top dead centre and valve plate. It is to provide cushioning and to prevent piston slap on the valve plate.

f. Positive displacement: Positive displacement machines ensure positive admission and delivery, preventing the undesired reversal of flow within the machine as by the use of valve in reciprocating compressor.

g. Piston displacement It is defined as the volume swept by the piston inside the cylinder in unit time. Unit is m3/Sec.

Let

D be the Bore of a cylinder in meter, L be the stroke in meter, N be the speed of the crank in rpm, n be the no. of cylinders

Piston displacement (Pd) = π D2 X L X N X n m3/sec.

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Where π D2 = Area of the cylinder.

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h. Actual Volume flow rate:

It is nothing but the amount of vapour refrigerant flowing through the suction line per unit time. Its unit is m3/sec.

Vact = Vs - (Vc’-Vc) where Vs = strike volume of compressor.

Vc’ = volume of gas after re-expansion. Vc = clearance volume.

i. Mass flow rate:

It is the weight of refrigerant flowing per unit time. The ratio of the volume flow rate to the specific volume will give the mass flow rate.

Mass flow rate = Volume flow rate kg/sec.

Specific volume

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j. Refrigerating capacity:

It is the cooling capacity of the compressor per unit time. The refrigerating capacity is defined as the product of Refrigerating effect and the mass flow rate of refrigerant. The unit is KJ/sec or K. Watt.

Refrigerating Capacity = Ref. Effect X Mass flow rate

Where,

Ref. Effect is the Amount of heat removed in evaporator per kg of refrigerant used.

k. Volumetric Efficiency:

It is ratio between the suction volume or actual volume and the piston swept volume or theoretical volume.

Vact Volumetric Efficiency =----------

Vthe

l. Compression Ratio:

It is the ratio of absolute discharge pressure to the absolute suction pressure. It is always more than one.

2. Explain the function of a compressor in a vapour compression Refrigeration system.

Function of a compressor in vapour compression system:

The low pressure, low temperature vapour refrigerant is drawn into the compressor from the evaporator through the inlet valve during the operation. During discharge stroke of the piston, the vapour refrigerant is compressed to a high pressure and high temperature than the suction line. This high pressure and temperature vapour refrigerant is discharged into condenser through the delivery valve.

3. Classify all the compressors that are used in vapour compression system. Classification of Compressors:

1. According to the method of compression A. Reciprocating compressors B. Rotary compressors and C. Centrifugal compressors.

2. According to the no. of working stroke A. single acting compressors

B. Double acting compressors.

3. According to the method of drive employed A. Direct drive compressors. . B. Indirect drive compressors.

4. According to the no. Of stages. A. Single stage OR single cylinder compressors and B. Multi-stage OR Multi-cylinder compressors.

5. According to the Location of Prime Mover and construction.

A. Semi-Hermetic Compressors. B. Hermetic Compressors.

6. According to the method cooling

A. Air cooled compressors. B. Water cooled compressors.

7. According to Compression Ratio

A. Low Compression ration B. Medium Compression ratio. C. High Compression ratio.

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8. According to Installation:

A. Portable Compressors B. Semi fixed Compressors C. Fixed Compressors.

9. According to the power consumption A. Low power consumption B. Medium power consumption C. High power consumption.

10. According to the Pressure limit

A. Low pressure compressor B. Medium pressure compressor C. High pressure compressor

11. According to Principle of compression a. Positive displacement compressors

(i) Reciprocating compressors (ii) Rotary compressors

a. Vane type compressors b. Blade type compressors c. Screw type compressors b. Roto-dynamic displacement compressors

Centrifugal compressors.

12. According to the speed of compressor A. Low speed compressor B. Medium speed compressor C. High speed compressor.

13. According to the capacity :

A. Low capacity compressors up to 5 TR B. Medium capacity compressors 5 to 15 TR C. High capacity compressors 15 TR

14. According to manufacturer : A. Voltas B. Blue star C. Kelvinator D. Kirloskar 15. According to the availability in market : A. Easily available. B. Rarely available. 16. According to cost : A. Low cost compressors B. Medium cost compressors C. High cost compressors.

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3. Explain the construction and working principle of a Reciprocating compressor with neat sketch. Also represent in P-V diagram.

The reciprocating motion compresses the vapour refrigerant i.e. back and forth movement of the piston. The reciprocating compressors are available in various sizes and capacities.

There are two types of reciprocating compressor. The single acting reciprocating compressors usually

have their cylinders arranged vertically and the double acting compressors have their cylinders arranged horizontally. Reciprocating compressor has 3 main parts. They are cylinder, piston and valves.

Cylinder : Compressor cylinders are usually constructed in close-grained cast Iron, which is easily machined

and not subjected to warping. For small compressor the cylinder and crank case housing are case in one piece. The cylinder and crank case housing are usually cast separately, flanged and bolted together.

Piston : Piston employed in refrigeration compressors is of two types. 1) Automotive 2) Double-trunk. The Pistons used depend on methods of suction gas intake and on the location of the suction valve. Double trunk is used in medium and large capacity compressors. These pistons are provided with oil grooves to facilitate lubrication of the cylinder wall. Pistons are manufactured from close-grained cast Iron. However a number of aluminium pistons are used. Double trunk pistons are equipped with one to Three compression rings at the top and one or two oil rings at the bottom. Automotive type pistons which have 50 mm dia is usually equipped with 2 compression rings and one oil ring, its also some time located at the bottom of the piston. Aluminium pistons are equipped with at least one compression ring.

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Suction and Discharge Valves : The suction and discharge valves are mounted on the valve plate. To minimise the wire drawing losses the valve should be designed to provide the longest possible opening with the least possible effort. To minimise back leakage of the vapour through valves, the valves should be designed to close quickly and lightly and it is designed of lightweight for a low lift. The valves are grouped as (i) Puppet (ii) Ring plate (iii) Reed. To facilitate rapid closing of valve, most discharge valves and some suction valves are spring loaded. Crank and Connecting rod : The one end of the connecting rod is fitted to the piston and the other end is fitted with crank and mounted to the driving shaft eccentrically. The crank and connecting rod is enclosed by a casing called crank case , provided with an oil return path from the top of valve plate and filled with lubricating oil. When the vapour is sent into the cylinder through the suction valve, the piston will be at the bottom most position. Then the piston is moved towards top dead centre by the rotation of crank wheel. Because of the decrease in volume, the pressure increases. Due to the increase in pressure the vapour is sent out through the discharge valve. The cycle is thus repeated and the vapour is compressed and sent out continuously. During the suction stroke, the vapour flows from suction line to cylinder due to pressure drop inside the cylinder. During the compression stroke the vapour flows from cylinder to delivery line due to increase in pressure inside the cylinder by decrease in volume of cylinder by piston. The fig (2) shows the pressure velocity diagram of a vapour compression system. From A to B pressure decreases and the volume is increased. This volume is called as the volume of re-expansion. From B to C the intake volume is increased at constant pressure. This volume is called as sucked vapour volume. From C to D the process is called compression. During compression, the pressure is increased but the volume is decreased from D to A and this is called as discharging. During discharge the volume is decreased at constant pressure.

5. What is a lubricant? To reduce friction, power loss and the wear and tear of the moving parts, a foreign material known as ‘Lubricant’ is applied between the rubbing surfaces. The lubricant keeps the matting surfaces apart by forming a thin film in between them. Lubricants may be solid or semisolid or liquid. The liquid lubricant generally used is mineral oil. Grease is used as a semisolid lubricant in some places.

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6. What are the desirable properties of lubricants? Explain.

Viscosity: It is defined as the internal resistance of lubricating oil for flowing. The viscosity of the oil depends on the adhesive and cohesive forces. The viscosity of lubricating oil is usually measured in “Saybolt Seconds Universal”. The viscosity value should be sufficient to form a thin layer between moving parts. Miscibility: It is the ability of the lubricating oil to mix with the refrigerant used in the system. All the lubricating oils should be miscible with the popular refrigerants. Pour point: It is the lowest temperature at which the lubricant can be poured. Pour point is important when selecting the lubricant for low temperature systems. Pour point should be as low as possible. Cloud point: It is the temperature nearer to the freezing point at which the oil becomes cloudy. If the cloud point is too high wax will precipitate from the oil in the refrigerant controls. So that the cloud point should be as low as possible. Floc point: It is the lowest temperature at which the precipitation starts. It must be as low as possible for a good lubricant. Flash point: It is the temperature at which a splash is formed when a flame is introduced in the lubricant’s surface. The lubricant will catch the fire on the surface and will not burn continuously. This value must be as high as possible. Fire point: It is the temperature at which the lubricant will catch fire and burn continuously. This property should also be high for good lubricants. Dielectric strength: It is the resistance for the flow of Electric current. It should be as high as possible. Chemical stability: It is the ability to keep up its properties under different pressure and temperature conditions. It should also be high as possible. Freezing point: It is the temperature at which the liquid solidifies. It must be as low as possible. Boiling point: It is the temperature at which the liquid boils. It must be very high. Emulsification: It is the ability of the lubricating oil to accept the dispersed water particles inside it. The lubricating oil should not form emulsion when brought in contact with water Non-reacting property: It is the property of the lubricating oil not to react with material of the components used in the system. Availability: It should be freely available in the market. Cost: The cost must be reasonable

7. Define the process of lubrication. Lubrication is a process of applying a foreign substance to the moving surfaces so as to reduce the friction, wear and tear, power loss etc. On application, the lubricant should form a stable thin film in-between the moving parts and keeps the rubbing surfaces apart.

8. Describe the methods of lubrication. The most commonly used lubrication methods are (i) Splashed lubrication (ii) Forced feed lubrication

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(i) Splashed lubrication: In this method the lubricating oil is filled inside the crank case up to the centre and it is splashed by a dipper or by the crank wheel when the compressor is in operation. In this method there is a possibility of splashing excess oil to the moving parts and there is also a possibility of supplying insufficient oil to the moving parts. This type is mostly used in low capacity compressors. A modified type of flash lubrication sometimes called as flooded lubrication employs “slinger rings”, discs, screws or similar devices to raise the oil to a level above the crankshaft. This method is practically suitable for small and high-speed compressors. (ii) Forced feed lubrication: In this method the lubricating oil is stored in a chamber and it is supplied to all the moving parts by a circulation pump. This method has many advantages than the previous method. It is mainly used in high capacity compressors. After making lubrication the oil is drained back to the sump, which is located in the crankcase. Since the circulation of oil is made by using a pump, some amount of power from the compressor shaft is utilised for its operation. Oil strainers are provided at the inlet of the oil pump to prevent the entry of foreign material in to the pump. In some other models externally mounted mechanical forced feed lubricators will lubricate the cylinders.

9. State the necessity of lubricating a refrigerant compressor. 1. To reduce the frictional resistance between piston and cylinder and other moving parts. 2. To reduce the wear and tear of the piston, crank, cylinder and other moving parts. 3. To provide cooling effect to the cylinder. 4. To reduce the noise and vibration

10. What is a Refrigerant? It is a working substance used in refrigeration system to absorb the heat in evaporator by change of state from liquid to vapour and to release the heat in a condenser as the substance returns from vapour state to liquid.

11. How will you classify the Reciprocating compressors? 1. According to the principle of operation a. Single acting compressor b. Double acting compressor 2. According to number of stages a. Single stage compressor b. Multistage compressor 3. According to pressure limit a. Low pressure compressor b. High pressure compressor c. Medium pressure compressor 4.. According to method of drive employed a. Direct drive compressors b. Indirect drive compressors 5. According to the location of the prime mover a. Hermetic type compressor b. Semi hermetic compressor 6. According to the compression ratio a. Low compression ratio b. Medium compression ratio c. High compression ratio 7. According to installation a. Portable compressors b. Semi fixed compressors c. Fixed compressors 8. According to the method of cooling a. Air cooled compressors b. Water cooled compressors 9. According to capacity a. Low capacity compressors b. Medium capacity compressors c. High capacity compressors 10. The other general points like cost, manufacturer, availability in the market etc. can also be

considered for classification.

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12. What is a Hermetically sealed compressor? When the compressor and motor operate on the same shaft and enclosed in a common casing, they are known as hermetically sealed compressors. This type of compressor eliminates the use of crankshaft seal, which is necessary in ordinary compressors in order to prevent the leakage of refrigerant. On either rotary or reciprocating principles these compressors may operate and may be mounted with the shaft in either in the vertical or horizontal pistons. The hermetic units are widely used in small capacity refrigerating systems such as domestic refrigerators, home freezers, and window air-conditioners.

13. What are Semi-sealed and permanently sealed compressors?. Semi sealed compressor: The Semi-sealed compressor can be dismantled easily. Its housing is closed tightly with the help of bolts and nuts. The service and maintenance of both the motor and compressor can be done easily. Permanently sealed compressor: It is the compressor in which the housing is welded at the joints and can’t be separated easily. If there is any disturbance inside the compressor, the casing is to be cut and it should be re-welded after the correction is over.

14. Compare the sealed and open type compressors. Sealed compressor Open type compressor 1.The leakage of refrigerant during operation is completely avoided. 2.It is less noisy 3.Power is transmitted from motor to compressor directly 4. Being more compact requires less space 5. It has less vibration

The leakage of refrigerant can’t be avoided during operation since there is always a very small amount of leak in the shaft seal It is more noisy Power is transmitted by means of a transmission system using belt or chain or gears or pulleys etc. Requires large space ( Separately for motor, compressor and the transmission system) It has more vibration (depends on the type of mounting)

6. It is used up to 5 TR capacity 7. Construction is complicated 8. During service the system is disturbed 9. Mostly used in domestic appliances 10. Cost is high

It is used for all capacities. Construction is simple Servicing the compressor need not disturb the system Used in all kind of systems Cost is low when compared to sealed compressors

15. Explain the factors affecting the volumetric efficiency and performance of a reciprocating compressor in detail. The following factors are affecting the volumetric efficiency and performance of the reciprocating compressors. a. Clearance: Certain amount of vapour will always be present in the clearance space at the end of the compression stroke. The pressure of this vapour will be equal to the discharge vapour pressure. During the suction stroke this clearance vapour expands first then the vapour present in the suction line will enter into the cylinder. Some volume of the cylinder is occupied by the re-expansion of the clearance vapour and the remaining volume is only filled with the suction vapour. So the presence of the clearance affects the volume of the refrigerant handled by the compressor and hence the volumetric efficiency is affected. To get maximum volumetric efficiency the clearance volume of a compressor should be as low as possible. b. Cylinder heating: It is the heating of suction vapour by the heat produced by the friction between the piston and the cylinder. The wall of the cylinder will be at high temperature and this will heat the suction vapour. The heated suction vapour will occupy more volume inside the cylinder than its original requirement. This also affects the amount of vapour brought inside the cylinder during suction stroke and hence the volumetric efficiency is decreased. The heating of cylinder increases with the compression ratio. c. Wire drawing: It is defined as the restriction of the area for a flowing fluid, causing a loss in pressure by friction without the loss of heat or performance of work in throttling. When the refrigerant flows through the

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suction port wire drawing occur. Hence at reduced pressure the suction vapour will occupy more volume inside the cylinder. So the volumetric efficiency of the compressor decreases with wire drawing effect. d. Piston & valve leakage: Any reverse leakage of gas through either in suction or in discharge or in piston will decrease the volume of vapour pumped by the compressor. The pressure in the cylinder is lowered at the beginning of suction stroke. Small amount of high-pressure vapour from the discharge line will leak back into the cylinder. Similarly at the start of the compression stroke some vapour will flow back in to the suction line if there is leakage. The vapour will also leak into the crankcase if the piston is leaking. Due to these leakages the volumetric efficiency is affected. e. Evaporator temperature and pressure (Suction temperature and pressure) The refrigerating capacity decreases as the suction pressure decreases. If the vaporising pressure is low and the density of suction vapour will also be low. Therefore the mass of refrigerant circulated through the compressor per unit time decreases with the decrease in suction temperature for a given piston displacement and hence the volumetric efficiency is affected. Similarly, If the evaporator temperature is increased, the suction pressure increases. At this condition the amount of gas occupied in the cylinder is more and hence volumetric efficiency will increase. f. Condenser or Discharge Temperature & Pressure. Due to increase in the temperature of the refrigerant vapour, the pressure increases and the power consumption increases and the volumetric efficiency decreases. Similarly if the pressure of the vapour in the condenser is low, the temperature is also low. Now the power consumption is low and the volumetric efficiency increases. g. Compression Ratio. The ratio between absolute discharge pressure and absolute suction pressure is called compression ratio. Absolute Discharge Pressure Compression Ratio = ------------------------------------- Absolute suction pressure Absolute Pressure = Gauge Pressure + Atmospheric Pressure Increasing the discharge pressure or lowering the suction pressure can increase either the Compression Ratio. Volumetric Efficiency varies inversely with the compression ratio. If the compression ratio is high, the volumetric efficiency is low and the power consumption is high. If the compression ratio is low the volumetric efficiency is high and the power consumption is low.

16. Define Wet and Dry compressions. Wet compression: It is the compression process in which a small amount of liquid refrigerant droplets are compressed along with the vapour refrigerant. Dry compression: It is the compression process in which there are no liquid refrigerant droplets. It is the compression of only dry vapour refrigerant.

17. Explain the performance of reciprocating compressor under wet compression. Case (i) :- Droplets are evaporated completely during suction stroke The volumetric efficiency is increased and also the power consumption is increased. Case (ii):- Droplets are evaporated and compressed only during the compression stroke. It will make a pressure turbulence. The compressor noise and vibration will be high. But the volumetric efficiency and power consumption will increase. Case(iii):- The liquid droplets will remain in the clearance space after the completion of compression stroke. Now the liquid droplets are evaporated during the re-expansion period of the clearance vapour. Therefore the volumetric efficiency decreases. Case(iv):-The refrigerant enters into the cylinder with large amount of liquid drops. The parts of the compressor will be damaged. Since a liquid can’t be compressed.

18. What is compressor capacity control and why it is necessary? The compressor capacity control is nothing but the change of the capacity of the compressor according to the load on the evaporator and condenser. Otherwise it will be difficult to maintain the required temperature in the evaporator. The capacity of the compressor is increased or decreased and whatever necessary can be done with an efficient control system.

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We can summarise the necessities as follows: 1. It is necessary to change the capacity of the compressor with respect to the load on the evaporator otherwise it will be difficult to maintain the required temperature in evaporator. 2. To increase the condenser pressure. 3. To reduce the refrigerant pressure with the help of an inlet valve control, by which the throttling of the gas will be done to reduce the pressure. 4. To vary the speed of compressor as the discharge pressure is the function of compressor speed.

19. Explain the various methods of capacity control used in Refrigerant compressors. 1.Speed Control: It is the simple and satisfactory method of controlling capacity of a compressor. The capacity of a compressor is proportional to the speed of the driving motor. When the suction pressure of the vapour is high, the speed of motor and compressor capacity must be increased. Due to this reason, electric motors with multiple speed ranges are used. By using this motor, we save power consumption and increase the efficiency. 2. Suction valve lift control: A capacity control with the Multi cylinder compressor may be used by forcing the suction valve to remain open in one or more cylinders and making inactive according to our requirement during the compression. 3. Hot gas by pass system: In this method a part of hot compressed gas is bypassed back to the suction line through a pressure-reducing valve. The valve admits the hot gas in suction line as the evaporator pressure tends to drop. This is maintaining the constant suction pressure. This method is not efficient as the suction temperature tends to increase thus resulting in the loading of the compressor instead of unloading however this method is simple and can be used in conjunction with other methods. Refer fig 3.

4. On-Off Control: This is adopted for parallel connected multiple compressor systems. By switching off few compressors, which are excess to our required capacity, we can save the power consumption. In smaller unitary equipment like refrigerators, air-conditioners, water coolers etc., simple ON-OFF system with a thermostat is used. This control is particularly used where there is a sudden large demand followed by periods of small or no demand.

Switching off is preferable to run at partial load because of low part-load efficiency of squirrel-gauge induction motors that are employed in refrigerant compressors. 5. Three way valve control: The three-way valve can be used for capacity control of compressors. During normal operation of the valve will be kept as shown in fig (4) under controlled position the valve is kept as shown in fig (5). The discharge refrigerant vapour is sent back into the suction line for re-cycling and the compressor will be operating without supplying the refrigerant into the condenser.

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20. Why a Thrust Bearing is used in Reciprocating Compressor? By using more than one number of cylinders, random vibrations and thrusts will be overcome in reciprocating compressors. But there will always be a thrust opposite to the action of the piston in single cylinder compressors. So, in order to face the opposite thrust, in single-cylinder systems we use a special bearing called thrust bearing. If we use an ordinary bearing, the bearing will be damaged.

21. Define the principle of Roto-dynamic. It is the working principle of the centrifugal compressor. It is the method of increasing the angular momentum of the moving particle i.e. vapour refrigerant, to increase the pressure.

21. Describe the construction and working principle of a blade type rotary compressor with a neat sketch. Construction : Refer fig (6) The rolling piston type employs a cylindrical steel roller. It rotates on an eccentric shaft. The later is being mounted concentrically in a cylinder. Because of the eccentric shaft, the cylindrical roller is eccentric with the cylinder and touches the cylinder wall at the point of minimum clearance. As the shaft turns the roller, it rolls around the cylinder wall in a direction of the shaft rotation and always maintaining contact with the cylinder wall, in relation to the camshaft. The inside surface of the cylinder roller moves in counter clockwise direction which is opposite to the direction of the rotation of the shaft. A spring-loaded blade mounted in a slot of the

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cylinder wall bears firmly against the roller at all times. The blade moves in and out of the cylinder slot to follow the roller as the roller moves around the cylinder walls. Cylinder heads or end plates are used to close the cylinder at each end and to serve as supports for shaft. Suction & discharge ports are located in the cylinder wall near the blade slot, but on opposite sides. Working: The flow of vapour through both suction and delivery parts are continuous except for the instant that the roller covers one or the other of the ports. The suction & discharge vapours are separated in the cylinder at a point of contact between the roller and the cylinder wall. In contact of cylinder with the roller changes continuously as the roller travels around the cylinder. The whole cylinder assembly is enclosed in housing and placed in the bath of oil. The high-pressure vapour is discharged into the space, which is present above the oil level, inside the housing. Then it passes into the discharge line. All rubbing parts or surfaces in the compressor including the end plates are highly polished and closely fitted. Although no suction valves are used, feed check or flapper valve is installed in the discharge line for restricting the vapour to return back into the cylinders when the compressor is operating. Oil films form seal between the high and low press areas. When the compressor stops the oil seal is broken and the high & low pressure equalises in the compressor. The cheek valve must be placed in the suction line to prevent the high-pressure discharge gas from backing up through compressor and suction line into the evaporator. When the compressor cycles off, the theoretical displacement is, H (A2-B2) VP = ---------------- 4 Where H = The height of the cylinder A = The diameter of the cylinder B = The diameter of rolling piston.

23. Describe the construction and working of vane type rotary compressors with neat sketch.

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Construction : Refer fig (7) The vane type rotary compressors employ a series of rotating vanes (i.e.) blades, which are installed in equal intervals around the periphery of a slotted rotor. The rotor shaft is mounted eccentrically in a steel cylinder. So that the rotor nearly touches the wall on the side, the two being separated only by oil film at this point. Directly opposite to this point, the clearance between the rotor and the cylinder wall will be large. Heads or end plates are installed on the ends of the cylinder to seal and to hold the rotor shaft. Large compressors are usually pressure lubricated with rotary gear pump driven directly from the compressor crankshaft. In some cases, the oil is chilled and introduced into the rotary cylinder at key points to provide lubrication and cooling for the blades and oil seal or shaft seal for all running surfaces. Working: The suction vapour drawn into the cylinder through suction ports in the cylinder wall is entrapped between the adjacent rotating vanes. The vapour is compressed by the reduction in volume, which results as the vanes rotate from the point of maximum rotor clearance to the point of minimum rotor clearance. The discharge ports are located so as to allow discharge of the compressed vapour to the desired point during the compression process and that point being the design point. The rotating vane type of rotary compressor also requires the use of a check valve in the suction or discharge line to prevent the discharge gas from leaking back through the compressor and suction line to the evaporator when the compressor cycles. Although the capacity of a rotary compressor varies directly with speed, capacity control is most frequently accomplished by relieving low-pressure refrigerant gas. At the lowest degree of compression with bypassing the gas at the black pocket to the suction line only partial compression of the total gas flow is happened.

24. Describe the construction and working of a screw type rotary compressor with neat sketch. Construction: Refer fig (8) It essentially consists of two helical grooved rotors as shown in fig, enclosed in a cylindrical housing. The male rotor consists of 4 lobes and female rotor consists of 6 flutes for meshing against male rotor. The 4 lobes of male rotors can drive a 6 flutes female wheel at two third of its speed. Working: Gas entering the machine at one end fills the available flutes and the inter lobe space nearest to the suction opening. When re-meshing starts, the volume decreases and pressure increases and the vapour charge moves in helical path and gets compressed. The teeth (lobe) will act like a stationary piston. The discharge port is uncovered and located in the cylindrical casing at the opposite end to the intake. Advantages: 1. No Balancing requirement and there are no surgical problems. 2. High compressor efficiency. 3. It is most suitable for handling high-pressure refrigerant. 4. It can handle large suction vapour volumes. Disadvantages: 1. Complicated oil system & its cooling arrangement.

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25. Classify all the centrifugal compressors. Classification of centrifugal compressors : 1. According to the construction of blade. a. Partially opened type. b. Fully opened type. 2. According to the type of blades. a. Backward curved blade impeller b. Forward curved blade impeller. c. Straight blade impeller. 3. Type of casings. a. Volute type casing. b. Vortex type casing. c. Turbine type casing. 4. According to the refrigerant used a. R22, R134a using compressors b. Ammonia using compressors 5. According to power consumption a. Low power consumption b. Medium power consumption c. High power consumption 6. According to noise produced a. Low noise producing b. Medium noise producing c. High noise producing 7. According to the capacity a. Low capacity compressors b. Medium capacity compressors c. High capacity compressors 8. According to the size a. Small size b. Medium size c. Big size 9. According to the cost a. Low cost compressors b. Medium cost compressors c. High cost compressors 10. According to the availability in the market a. Easily available b. Rarely available 11. According to the Manufacturers a. Kirloskar b. Voltas c. Blue star Etc. 12. According to the number of stages a. Single stage compressor b. Multi stage compressor

26. Explain the construction and working of a centrifugal compressor with neat sketch. Refer Fig (9) Construction : A centrifugal compressor consists essentially of a series of impeller wheels, mounted on a steel shaft and enclosed in a cast iron casing. Compressors employed with two, three or four wheels are common. More wheels may be used when the required increase in pressure is sufficiently large. To resist corrosion and erosion, the impeller blades are made either by stainless steel or by high carbon steel with a lead coating. Centrifugal compressors have large discharge and moderate compression ratios. So that it can be used for large capacity systems. Centrifugal compressors are available in sizes ranging from approximately 125 kW to 35000 kW. These compressors have high rotating speeds between 3000 and 18000 rpm.

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Working: The working principle of centrifugal compressors are similar to those of centrifugal pump or fan low pressure., low velocity vapour from the suction line is drawn into the inlet cavity or ‘eye’ of the impeller wheel along the axis of the rotor shaft. On entering the impeller wheel, the vapour is forced radial outward between the impeller blades by action of the centrifugal force developed by rotating wheel and is discharged from the blade tips into the compressor housing at high velocity and at increased temperature and pressure. The high pressure, high velocity vapour discharged from periphery of the wheel is collected in specially designed passages in the casing which reduce the velocity of the vapour and direct the vapour to the inlet of the condenser. Common refrigerants used in centrifugal compressors are R11, R12, R13, R22, R500 and Ammonia.

27. Compare reciprocating and rotary compressor.

COMPARISON OF RECIPROCATING AND ROTARY COMPRESSOR

28. Compare Reciprocating compressor with centrifugal compressor.

S. No.

Reciprocating Compressor

Centrifugal Compressor

1.

2. 3. 4. 5. 6. 7.

Complicated in construction because of more no. of parts. Suitable for less discharge. Maintenance cost is more. Higher speeds are not possible. Delivery will be pulsating. Life is short. Vibration is more.

Simple in construction because of less no. of parts. Suitable for large discharge. Maintenance cost is less. Higher speeds are possible. Delivery will be continuous. Life is comparatively large. Vibration is less

S. No.

Reciprocating Compressor Rotary Compressor

1. 2. 3. 4. 5. 6. 7.

8. 9. 10.

Discharge is discontinuous or pulsating. Noise and vibrations are more. Power consumption is more. Efficiency is low. Compression ratio is high. Construction is complicated Used in small plants like Refrigerator, window air-conditioner i.e. small volume Refrigerant circulation. Life is short. Suction and discharge valves are essential. High speed is not possible as centrifugal compressor.

Discharge is continuous Less noise and vibration Less power consumption Efficiency is high. Compression ratio is low. Simple in construction Used in large volume Refrigerant-circulating system i.e. large plants. Life is long. Those valves are not necessary. High speeds are possible.

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29. Why does a reciprocating compressor consume high power when compared with other types? In reciprocating compressor, we get the output or compressed vapour in the compression stroke only. But we cannot get any output in the suction stroke. By the rotation of crank wheel, we get the output by only half of the rotation but we spend the power for full rotation. So, we spend more power. In other types like centrifugal, vane, blade type compressor, compressed vapour is sent out continuously. Therefore reciprocating compressors consume more power than other type of compressors for a given amount of Refrigerant circulation.

30. How will you select a compressor for a Refrigeration and A/C system? By considering, 1. The Refrigeration capacity a. High Mass flow rate. b. Low Mass flow rate. c. Medium mass flow rate. 2. The Refrigerant used. a. R134a compressor b. R22 compressor c. NH3 compressor and etc. 3. The compression ratio. a. High compression ratio. b. Medium compression ratio. c. Low compression ratio. 4. The type of compressor a. Reciprocating compressor. b. Rotary compressor. c. Centrifugal compressor. 5. The power consumption. a. High power consumption. b. Medium power consumption. c. Low power consumption. 6. The type of drive. a. Direct drive. b. Indirect drive. 7. The size. a. Big size. b. Medium size. c. Small size. 8. The material used for manufacture a. Closely rained cast iron. b. High carbon content steel. c. Alloy metals. 9. The cost a. Low cost. b. Medium cost. c. High cost.

10. The manufacturer. a. Kirloskar. b. Blue star. c. Voltas & Kelvinator etc. 11. The space available and installation. a. Portable compressor. b. Semi fixed compressor. c. Fixed compressor.

8.

9. 10. 11.

Compression is done by the motion of piston. It requires more lubrication. Power consumption is high. It works on the principle of positive displacement.

Compression is done by motion of impeller. Requires less lubrication. Power consumption is low. It works on the principle of Roto-dynamic displacement.

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CHAPTER – 2. CONDENSERS AND COOLING TOWERS.

1. Define the following terms. a). Condenser Capacity:

It is defined as the amount of heat energy that can be removed by the condenser.

Actual Capacity = m (h1 -h2) Kj/sec. m - Mass flow rate of refrigerant, Kg/sec. h1 - Enthalpy of vapour refrigerant, at the inlet of the condenser. KJ/Kg. h2 - Enthalpy of liquid refrigerant, at the outlet of the condenser. KJ/Kg. Theoretical Capacity = AU (T1 ~ T2) Kj/Sec. A - Area of heat transfer , m2 U - Overall heat transfer coefficient of the condenser tubes, Kj / m2 Sec K T1 - Temperature of Refrigerant, K. T2 - Temperature of cooling medium, K.

b). Condenser Load:

It is the amount of heat energy that must be removed by the condenser. This is equal to the heat absorbed in the evaporator and the heat added in the compressor by the work of compression.

c). Condenser Efficiency:

It is defined as the ratio between the actual capacity and the theoretical capacity of the condenser.

Actual Capacity

Condenser Efficiency = -------------------------------------- Theoretical Capacity d). Tower Range:

It is the temperature difference between the water leaving the tower and the water entering the tower.

e). Tower Approach:

It is the difference between the Dry bulb temperature of water leaving the tower and Wet bulb temperature of entering air.

Tower Approach = Wto - WBAT

f). Tower Efficiency:

It is defined as the ratio between the tower range and the temperature difference between water inlet (Wti) and Wet bulb temperature of air at inlet (WBAT).

Tower range

Tower Efficiency = --------------------- X 100 Wti - WBAT g). Fouling Rate:

It is defined as the rate of deposition of dust particles over the surface of heat transfer. It is also the rate of deposition of scale inside the heat transfer tube.

h). Hard water and Soft water:

Hard water:- The water containing salts, dust and impurities is called hard water. It does not give lather with soap easily and immediately. Soft water:- The water which are not containing any hardness producing salts, dust and impurities is called as soft water. It gives good lather with soap immediately. i). Evaporator Capacity:

It is defined as the amount of heat absorbed by the evaporator per unit time.

Theoretical Capacity = AU (T1 ~ T2) Kj/sec. A - Area of surface contact. m2 U - Overall heat transfer coefficient of the evaporator pipes and fins Kj/m2 sec K T1 - Refrigerant temperature. T2 - Temperature of surrounding.

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j). Evaporator Load:

The amount of heat that must be absorbed by evaporator per unit time is known as evaporator load. The capacity of the refrigeration of air-conditioning system is the load of evaporator.

k). Evaporator Efficiency:

It is the ratio between the actual capacity of Evaporator and the theoretical capacity.

Actual Capacity (Qact)

Evaporator efficiency = ------------------------------------------ X 100 % Theoretical Capacity (Qtheo)

Qact in Refrigerant side

Qact = m ( h1 - h2 ) Kj/Sec. m - mass flow rate of refrigerant circulated. Kg/ sec. h1 - Enthalpy of refrigerant entering. Kj/Kg h2 - Enthalpy of refrigerant leaving. Kj/Kg

Qact in Airside, (taking air as a cooled medium)

Qact = m cp. δt Kj/Sec. m - mass flow rate of air. Kg/sec CP - Specific heat capacity of air at constant pressure. Kj/Kg K

δt - Temperature difference between inlet and outlet air. K

2. Classify all the condensers that are used in vapour compression system. Classification: 1. According to the cooling medium used.

a). Air, cooled condenser. b). Water-cooled condenser. c). Evaporative condenser.

2. According to the capacity of condenser. a). Low capacity - 0 - 5 Tons. b). Medium capacity - 5 - 15 Tons. c). High capacity - Above 15 Tons.

3. According to the type of construction. a). Natural draft air-cooled condenser.

(i) Bare tube. (ii) Wire meshed tube. (iii) Plate and tube. (iv) Plate and Plate.

b). Forced draft Air-cooled condenser. (i) Finned tube.

c). Water-cooled condenser. (i) Double pipe. (ii) Shell and tube. (iii) Shell and coil Type.

4. According to the fouling rate. a). High Fouling rate. b). Medium Fouling rate. c). Low Fouling rate.

5. According to the circulation of cooling medium. a). In air-cooled condenser.

(i) Natural circulation. (ii)Mechanical circulation.

1.Forced circulation. 2.Induced circulation.

6. According to the use of water in water-cooled condenser. a). Waste water system. b). Re-circulated system.

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7. According to the type of mounting.

a). Chassis mounted b). Remote type.

8. According to the cost. a). Low cost. b). Medium cost. c). High cost.

9. According to the Availability. a). Cheaply Available. b). Rarely Available.

10. According to the manufacturer. a). Kelvinator. b). Kirloskar. c). Blue star etc.

3. Compare Air-cooled condensers with water-cooled condensers.

S. No.

Air-cooled condenser Water-cooled condenser.

1.

2.

3.

4.

5.

6.

7.

8.

Efficiency is less. Used at any place, we want. Used for low capacity. Size of this when used for high capacity is big. Operating cost for low capacity is less. There is no cooling tower, pumping and piping arrangement. There is no scale formation. Noisy when we do not use proper fan.

Efficiency is high. Only used in the places where water is cheaply available. Used for High and Medium capacity system. Size of this when used for low capacity is big. Operating cost for High capacity is less. There is a cooling tower, pumping and piping arrangements. There is scale formation on the inner wall of the pipes. It is not noisy.

9.

10.

11.

12.

13.

14.

15

Easily cleaned. Fouling Rate is less. Simple in construction. Installation and maintenance cost is less. Servicing is not essential. Fans, Blowers are used. Poor Heat carrying capacity

Cleaning is not easy as air cooled condenser. Fouling Rate is high. Complicated in construction. Installation and maintenance cost is high. It is frequently serviced. Fans, Blowers are not used. High heat carrying capacity

4. Describe chassis mounted and remote type air-cooled condensers. A chassis mounted air-cooled condenser is one that is mounted on a common chassis with the compressor and compressor driving motor so that it becomes an integral part of a packaged air-cooled condensing unit.

A remote type air-cooled condenser is located separately and mounted usually some distance away from compressor.

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5. Describe the construction and working of various types of condensers.

a. Bare tube Refer Fig (2.1)

Only tubes are arranged without any plates and wires. These are used as natural draft air-cooled condensers and they have poor heat transfer rate.

b. Wire meshed Tube: Refer Fig (2.2)

This arrangement increases the heat transfer area. No. of wire meshes are attached with tubes. This type is better than bare tube.

c. Plate and Tube: Refer Fig (2.3)

The tube, which carries the refrigerant, is supported on a plate, by screwing or riveting. This will help to increase the heat transfer area. This is better than previous two types.

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d. Plate and plate: Refer Fig (2.4)

Two plates are attached by roll bonding. In between the two plates, refrigerant is flowing through gaps and guided ways, which are provided as shown in fig. Roll bonding is a process of joining two metal plates by pressing them after heating to red hot state. This method is better than the previous types, having direct contact of the plates with the refrigerant.

e. FinnedTube: Refer Fig (2.5) High conductivity thin sheets (fins) are arranged compactly as shown in fig. This type is used for forced air circulation. This method is better than all other previous methods. It has high capacity. This is used in large plants.

f. Double pipe condenser: Refer Fig (2.6)

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Double pipe method is shown in fig. It consists of concentric cylindrical tubes one inside the other. The water is taken in the inner tube from one side while refrigerant vapour is made to flow from opposite direction in the annular space between the two pipes. Due to counter flow, maximum amount of cooling can be achieved. The counter flow arrangement gives higher Log Mean Temperature Difference (LMTD) and therefore gives high rate of heat transfer. This is easy in construction.

g. Shell and Coil: Refer Fig (2.7)

This type of condensers can be used for the units up to 50 Tons. The arrangement is shown in fig. It consists of a welded steel shell and one or more coils inside it. The refrigerant vapour enters at the top of the shell and gets condensed as it comes in contact with low temperature coil surface. The liquid refrigerant is collected at the bottom. This condenser is simple in construction and main advantage is the high overall heat transfer coefficient. But the coil cannot be cleaned and serviced easily.

h. Shell and tube condenser: Refer Fig (2.8)

These are similar in construction to flooded type chillers. This is universally used for all high capacity plants from 2 to 1000 tons. This condenser consists of No. Of copper plates arranged parallel to each other. Cylindrical steel tubes are used in system with ammonia as refrigerant. Water is circulated through the tubes. Refrigerant vapour enters at the top and collected at the bottom as liquid. The bottom portion of the condenser serves as liquid receiver. So some amount of vapour refrigerant is also present at the top of the liquid surface.

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The headers, which are provided with both the ends, are removable. So the tubes can be perfectly cleaned by removing the header.

6. Describe the wastewater and re-circulated systems.

i. Waste water system: Refer Fig (2.9) In the waste water system, the water from the city main is used to cool the hot vapour flown through condenser. Then the used water is drained out to the sewer. These condensers are used only in the places where the water is freely available.

ii. Re-circulated System: Refer Fig (2.10) In this system, the warm water used in the condenser is sent to the cooling tower. There the warm water is sprayed and the latent heat of water is removed and cooled. The evaporation of the surface layer of the drop will be taking place when the water is sprayed in the presence of air. Then the cold water is again pumped to the condenser to use it as a cooling medium. The process is repeated. This system is used where water is available in limited volume.

7. Discuss the economical water flow rate and water velocity through a water-cooled condenser.

ECONOMICAL WATER FLOW RATE & VELOCITY: The mass flow rate and velocity of water are the most important considerations for the rate of heat transfer. When the amount of water circulated is less than the requirement, the capacity will decrease. When the amount of water circulated is more than the requirement the capacity will be more but unnecessary spending of work input to circulate the water by pump will happen. So there must be a critical value of water circulation. If not, the system efficiency will decrease, the operating cost will increase and the system’s performance will decrease.

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The heat transfer increases as the velocity of water increases. The head pressure losses due to friction also increase due to increase in water velocity.

The rated velocity of water in a water cooled condenser is calculated using the following formula

V = (0.465) * (A/B * C1/C2) 0.83 m/sec

Where, A - Cost in Rs per unit heat transfer (h) gain B - Cost in Rs per unit head (hf) loss C1 - Power cost per unit.in Rs C2 - Water cost in Rs per litre of per Kg. By reducing the velocity, the layer adjacent to the tube gets vaporises. Therefore volume flow rate will decrease. By increasing the velocity, the heat transfer will increase. Therefore the designed condition of velocity of water should be between 2.4 m/sec and 2.6 m/sec.

8. Derive the relation for finding the economical water flow rate through a water-cooled condenser. Let us consider, C1 - Cost of unit power (RS/kw) C2 - Cost of water per kg (RS/kw) C - Total cost = Power cost + Water cost p - Increase in power per degree temperature rise in condenser (kw / K) ∆T -Temperature rise in the condenser (K) The actual increase in power = p ∆T

p ∆T

Theoretical increase in power = ----------- η η = Efficiency of motor and compressor Actual increase in power η = ---------------------------------------------- Theoretical increase in power p ∆T x C1 The total theoretical cost of power = ------------ ……….(1) η Let us also consider, Q - Rate of heat transfer in the condenser ( Kj / sec ) m - Mass flow rate of water through the condenser (Kj/sec) CP - Specific heat capacity of water at constant pressure (Kj /kg K) ∆T - Temperature rise in condenser (K) Now Q = m CP ∆T Kj/sec Q ∴m = --------- -------(2) CP ∆T

The cost of water to the above mass flow rate = m C2

Q From (2) = ------------ x C2 -----------(3) CP ∆T

We know that The Total cost = Power cost + Water cost

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p ∆T Q From (1) & (2) Total Cost = ---------- C1 + --------- C2 η CP ∆T

Differentiating with respect to ∆T p C1 Q C2 -1 0 = ---------- + -------- * ---- η Cp ∆T

2

p C1 Q C2

0 = ---------- - -------- η Cp ∆T

2

Q C2 1 pC1

------ X ----- = ------- Cp ∆T

2 η η Q C2

∆T2 = --------------

p C1 Cp _ _ 1/2 | η Q C2 | ∆T

= | --------------- | | p C1 Cp | |_ _ | Combining equation (2) & (4) _ _ 1/2 We get Q | p C1 Cp | m = ----- | ---------- | Cp | η Q C2 | |_ _| _ _ 1/2 The Economical mass flow rate | Q p C1 | m = | ------- | Kg/sec | η Cp C2 | |_ _|

9. Discuss the effects of non-condensable gases present in the condenser using a p-h diagram. Refer fig (2.11)

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Non-condensable gases like air, chlorine, oil, vapour that are present in the condenser will raise the condensing pressure. The higher condensing pressure 1. Increases the work of compression. 2. Reduces the Refrigerating effect. 3. Increases the power consumption.

4. Totally the COP is reduced. From p-h diagram 1-2-3-4 is the ordinary cycle of refrigerant and 1\ - 2\ - 3\ -4\ is the cycle with non-condensable gases. ∆p is the increase in discharge pressure due to accumulation of non condensable gases. ∆h1′ is the decrease in refrigerating effect. ∆h2′ is the increase in work of compression. Non condensable gases are produced by, 1. Decomposition of Lubricant oil. 2. The breakdown of the Refrigerant under high temperature conditions. 3. Due to the chemical reaction of Refrigerant (or) lubricating oil (or) of both on the metal.

10. Describe the evaporative condenser with neat sketch. Refer Fig (2.12) These condensers have combined features of both air and water cooled condensers. The construction is looking like assembling air and water cooled condensers in one structure. In this type water is sprayed over the coils, which carry the refrigerant. Air is also sucked by a fan, which causes the air to flow surrounding the coil. First the refrigerant rejects its heat to water which in turn rejects its heat to air. That is the refrigerant is cooled by water and the water is cooled by the air. This evaporative condenser is used in large capacity systems and packaged air-conditioners. The main advantage is low volume of water flow requirement. But this type requires more space, additional and complicated construction.

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11. Explain the selection of condenser. The condenser are selected by considering the 1. The condenser capacity. a). High capacity. b). Low capacity. c). Medium capacity. 2. The cooling medium used. a). Air-cooled condenser. b). Water-cooled condenser. c). Evaporative condenser. 3. The water circulation. a). Waste water system. b). Re-circulated system. 4. The Refrigerant used. a). R12 condenser. b). R22 condenser. c). NH3 condenser. 5. The construction and availability of space. I a). Shell and coil. b). Shell and tube. c). Double pipe. II a). Bare tube. b). Plate and tube. c). Plate and plate. d). Wire meshed tube. e). Finned tube. III a). Remote type. b). Chassis mounted. 6. The size. a). Small size. b). Big size. c). Medium size. 7. The cost. a). Low cost. b). Medium cost. c). High cost. 8. The availability a). Cheaply available. b). Rarely available. 9. The comfort & Installation. a). Portable. b). Fixed. c). Semi-fixed. 10. The Fouling Rate. a). High Fouling rate. b). Medium Fouling rate. c). Low fouling rate. 11. The manufacturer. a). Blue star. b). Kelvinator. c). Voltas etc.

12. What is the function of a condenser in vapour compression Refrigeration system? Function of the condenser. 1. The condenser removes the heat from the vapour Refrigerant. (The heat received from the evaporator and the heat added by the compressor to the refrigerant). 2. The condenser converts the vapour refrigerant into liquid refrigerant. Simply, it is used as a cooling equipment or heat exchanger.

13. How the process of cooling is done in a cooling tower and spray pond? Refer Fig (2.13) Cooling tower is a device that is used to cool the warm water, coming out of the condenser for re-circulation.

29

When the water received from the condenser is sprayed in a cooling tower, the water droplets fall downwards. At the same time, the air will be flowing from bottom to top of the cooling tower. When the droplets come in contact with the air which is flown opposite to the direction of droplets, the outer layer of the droplets get evaporated. The evaporated water absorbs some amount of heat from the remaining quantity of droplet. Thus the remaining droplets get cooling effect. The evaporation of outer layer of water droplets depends on

1. Temperature of the water sprayed. 2. Temperature of the air flown. 3. The humidity (R.H), of air. 4. The velocity of air flown. 5. The quantity of air flown. 6. The size of the water droplet Etc. In the case of a spray pond, the warm water received from the condenser is sprayed in open atmosphere. The air present in the atmosphere cools the water sprayed, by the evaporation of outer layer of droplets.

15. Classify all the cooling towers used in our field. Classification of cooling towers. 1. Natural draft cooling tower. 2. Mechanical draft cooling tower. a). Induced draft. b). Forced draft. 3. Spray pond.

16. Describe the following cooling towers with neat sketch. 1). Natural draft cooling tower : refer fig (2.14). The arrangement of the tower is shown in fig. The tower should be located in open space or on the roof of the buildings where the movement of air would be free. Here the hot water from condenser is sprayed at the top of the tower. The free air is flown through the water droplets from bottom to top of tower by natural circulation. The water droplets lose their heat i.e. Latent heat. The cold water is collected at the bottom and circulated through the condenser. These are used for load under 200 ton capacity.

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2). Forced draft cooling towers: refer fig (2.15). The construction of the forced draft cooling tower is shown in fig. The warm water received from the condenser is sprayed at the top of the tower. The fan or blower from the bottom forces air up. Therefore, water droplets lose their latent heat to the air by the evaporation of the surface layer. Therefore, the cold water is collected at the bottom and sent again to the condenser.

3). Induced draft cooling tower: refer fig (2.16). The construction of the induced draft cooling tower is shown in figure. The warm water received from the condenser is sprayed at the top. The blower is located at the top. It sucks the air, which enters at the bottom of the tower. The water droplets lose their heat to the air by evaporating their surface layers. The cold water is collected at the bottom and sent again to the condenser. Advantages of induced draught cooling tower: 1) It provides even air distribution over the coil. 2) It eliminates the re-circulation of discharged hot air. 3) The pressure inside the tower will always remain lower than the atmosphere. This will improve the evaporation rate than the forced draft.

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4). Deck type cooling tower: refer fig (2.17) The construction of this tower is shown in figure. It is just similar to atmospheric spray tower except the water-distributing troughs which help to break the water into small droplets. The objective of the deck is to provide additional evaporation area. It gives 20 to 30 % high efficiency than the atmospheric spray tower for the same size and for the same quantity of water flow.

17. Describe the spray pond with neat sketch. Refer Fig (2.18)

The simple construction of a cooling spray pond is shown in figure. The warm water received from the

condenser is sprayed in open atmosphere. The spray ponds are always provided or surrounded with wooden walls to prevent the winds from carrying the water particles.

It requires more floor area. Water treatments may be necessary for control of Algae and hardness. It has very low heat transfer rate. Loss of water including evaporation is more. These are the main disadvantages using spray pond.

18. Discuss the problem scale formation in water-cooled condenser. In water-cooled condensers, particularly in re-circulated systems, the salts and dust particles present in the water deposit over the inner surface of the tubes and container. Therefore, it forms a layer of hard mass called scale. The scale resists the flow of water and heat transfer. The efficiency of condenser is affected. The condenser pressure and heat is increased. Work of compression is increased and the refrigerating effect is decreased. Totally the C.O.P is affected.

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To remove the scale the following methods are used 1) Metal brushing 2) Acid picking In metal brushing method, a metal brush is inserted into the condenser tubes back and forth until the scales are removed. The abrasive action of the metal wires will remove the scales. The operator should be very careful in using the brush because the copper tubes are mild and soft than the scales and the abrasion process should not make any harm to the tubes. In acid pickling method, the cooling tower is water is added with sulphuric acid or hydrochloric acid in low concentrations. The water is circulated through the condenser tubes as usual. The acidic water will pickle the scales from the tubes. The acidic water is normally circulated for a period of 6 hrs or more until all the scales are removed.

19. Explain the methods of softening water. 1) Boiling: By simply boiling the hard water we can remove some salts which cause temporary hardness. These salts are removed as precipitates in the bottom of the boiler. Boiling Mg (HCO3)2 --------------------> MgCO3 + CO2 + H2O Boiling Ca (HCO3)2 --------------------> CaCO3 + CO2 + H2O 2) lime soda process (cold & hot): In the lime soda process by adding lime Ca (OH)2 and soda we can remove some salts as below : By adding lime: Ca (HCO3)2 + Ca(OH)2 ---------------------> 2 CaCO3↓ + 2H2O Mg (HCO3)2 + Ca(OH)2 --------------------- > MgCO3↓ + CaCO3 ↓ + 2H2O By adding washing soda: Ca (HCO3)2 + Na2CO3 ---------------------> CaCO3↓ + 2NaHCO3↓

CaCl2 + Na2CO3---------------------> CaCO3↓ + 2NaCl MgCl2 + Na2CO3---------------------> MgCO3 ↓ + 2NaCl CaSO4 + Na2CO3---------------------> CaCO3↓ + Na2SO4↓ MgSO4 + Na2CO3---------------------> MgCO3↓ + Na2SO4↓ Hot lime soda process is done in the water at 42 ° C for 8 hours. Cold lime soda process is done for twenty-four hours at atmospheric temperature and this is a slow process. 3. Ion exchange process: Refer fig (2.19) In this process the ions of the hardness producing salts are removed by passing through two various ion exchangers. The first is an anion exchanger and the later is a cation exchanger. The anion exchanger is filled with acid resin and the cation exchanger is filled with basic resin. When water is passed through the anion exchanger the positively charged ions of the salts are picked up by acid

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resin. When the water is passed through the cation exchanger the negatively charged ions are removed. Finally we get de-ionised water, which is free from hardness producing salts. In this process the acid and basic resins lose their power after some time. They are regenerated by passing sulphuric acid (H2SO4) through the ion exchange chamber.

20. How will you remove the dust particles and impurities that are present in water? The water filters remove the dust particles and impurities that are present in the water. There are two types of water filters, they are 1. Sand filter for low content of dust. 2. Sedimentation for high content of dust. In the sand filter the water containing dust particles is passed through the sand filled with gravel. The dust particles are deposited over the gravel when they flow through them. Thus the dust particles are removed. In the sedimentation process the water is allowed to stay in a big tank for one or two days undisturbed. The dust particles present in the water are slowly accumulated at the bottom of the tank by the action of gravity. The water from the top which is free from dust can be removed by pumping. This is a slow process.

21. Describe the methods of removal of scales in condenser tubes. A. Rodding or brushing: Scale formation in the condenser tubes are hard mass. Ordinary brushes do not easily remove them. So in order to remove the scales we can use special brushes made of metal wires. This brush removes the scale formation when we apply it on the scaled surfaces. This is called as “Rodding” or Brushing.

B. Acid pickling: Acid pickling is a process of removing the scales from the inner walls of condenser tubes. The hydrochloric acid is flown through the condenser tubes, which are affected by scale formation. The acid picks up the scales from the tube and thus the scales are removed. This process is known as Acid picking.

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CHAPTER 3 : EVAPORATORS

1. What is the function of the evaporator of a vapour compression cycle? Function of evaporator: The evaporator of a vapour compression cycle is a heat exchanger, which absorbs the heat from the surrounding by the evaporation of liquid refrigerant flowing through it. 2. Describe the flooded and dry expansion type evaporators with heat sketch. Refer figure 3.1 The flooded evaporators consist of an accumulator at the inlet, which serves as a liquid reservoir, from which the liquid refrigerant is circulated by gravity to the coil. The liquid level in the accumulator is maintained by a low side or by high side float control. The vapour, which is generated by the boiling action of the refrigerant in the coils is separated from the liquid in the upper port of accumulator and drawn directly into the suction line along with the flash gas. This results when pressure of the refrigerant is reduced from condensing pressure to the evaporator pressure. Advantages:

1) These evaporators give high heat transfer. 2) Several evaporators are used in conjunction with one accumulator without using more than one throttle valve, if all of them are used for same temperature.

Dry expansion type evaporators: Refer figure 3.2 In the liquid chillers, the chilled liquid is fed to the coils, which are used for cooling air. But if the coils of the evaporator with refrigerant passing through them are used directly to cool air by natural or forced convection, the evaporator is called as dry or direct expansion evaporator.

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Liquid refrigerant is fed into the dry expansion evaporator through an expansion device. The length of evaporator coil is constructed according to rate of heat transfer or rate of heat absorption of the liquid refrigerant. The entire liquid refrigerant must be evaporated before it reaches the evaporator outlet. Normally, the refrigerant expansion device is so adjusted as to feed the liquid refrigerant into the evaporator at the same rate (i.e.) the rate at which the liquid is fed into evaporator depending upon the rate of heat transfer or vaporisation. The amount of the liquid flow through the evaporator will vary with the load on the evaporator (i.e.) if the load on the evaporator is considerably low, the amount of liquid refrigerant flow through it is also low. The efficiency of the evaporator is more when the load on the evaporator is more.

3. State the applications of the following types of evaporators. A. Bare tube: 1. Used in the systems where the ammonia is used as refrigerant for handling the large amount. 2. Spiral bare tubes are used for liquid chilling. 3. Large ceiling-hung bare tube coils are employed in natural convection air circulation and are used for frozen storage rooms and storage coolers. 4. They are used also either ‘dry ‘ or as spray coils in conjunction with centrifugal blowers to provide high velocity chilled air for blast cooling or freezing. B. Wire meshed tube: This type of evaporator is used in refrigeration plants. This will increase the rate of heat absorption of the evaporator. C. Plate and tube: 1. They are generally used in home freezers, beverage coolers, ice cream plants and locker plates. D. Plate and plate: This type is used in domestic refrigerators, frozen food industry. E. Finned tube: This type of evaporator is used in the systems where the forced convection air circulation is adopted. Finned coils are best suited to air cooling applications where the temperature of the coil is maintained above 0 oC (i.e.) in the window air conditioners. F. Shell and tube: Used as chillers, in general dry expansion shell and tube evaporators are used in the large capacity systems requiring capacities ranging from 7 kW to approximately 1000 kW. Flooded expansion shell and tube evaporators are used in systems requiring capacities ranging from approximately 35 kW through several thousand kW. G. Shell and coil : They are used for small applications having high but frequent peak loads. It is used for chilling the drinking water. It is recommended to use in the applications of chilling liquid above 3 ° C. It is used for the chilling of beer and other beverages in ‘draw bars’. It is also used to cool secondary refrigerant. H. Double pipe: These evaporators are used for wine cooling and in petroleum industry for chilling of oil. [Note: the line sketches for all the above models will be more or less similar to the models that are already discussed for the various types of condensers] I. Baudelot - cooler: This is generally used for milk chilling and wine cooling. It is also used for chilling the water for carbonation in bottling plants.

4. What are natural convection and forced convection evaporators? 1. Natural convection evaporator: These evaporators are used where low velocity of air and minimum dehydration of the products is desired. The air circulation depends upon the temp, difference between evaporator and the space to be cooled. High circulation rates will be produced with higher temp difference. The air circulation depends upon the shape, size and location of evaporator. Baffles are used in small cooler capacity, which provides better air distribution. In general they are used in household refrigerator, water cooler and small freezers. I) Bare pipe: These evaporators are usually made either of copper pipes in small evaporators or of steel pipes in large capacity evaporators. These are used in liquid chilling, ammonia plant, storage coolers and frozen storage rooms.

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II) Plate and tube : In this type the evaporator coil is fixed with a plate. These are used in home freezers, beverage coolers, ice cream plants etc. III ) Plate and plate : In this type two plates are fastened by roll bonding and provided with guide ways for the flow of the refrigerant through it. These are used in domestic refrigerator, frozen food industries. 2. Forced connection evaporator: The forced convection evaporator is more efficient than natural convection evaporator is because it requires less cooling surface. High evaporative pressures can be used to save considerable power input to the compressor. These evaporators are provided with fins to increase rate of heat transfer. These are normally used in most of the air-conditioners.

5. What are liquid chillers? Liquid chillers are the devices that are used for chilling liquids such as water, milk, wine, beer, petroleum oil etc. There are five types of liquid chillers, they are 1. Double pipe cooler 2. Baudelot cooler 3. Shell and coil cooler 4. Shell and tube cooler 5. Tank type cooler

6. Describe the Baudelot cooler with neat sketch. Refer Figure (3.3) Baudelot cooler : The baudelot cooler is shown in figure. It effectively consists series of horizontal pipes which are located one under the other and are connected together to form a refrigerant circuit or circuits. There is a collector tray at the bottom of the horizontal pipes to collect the chilled liquid ie) milk. There is a header on the top of the baudelot cooler to spray the droplets on the surface of the horizontal pipes.

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The hot milk is filled in the spray header. The hot milk is sprayed over the tubes, which have the flow of liquid refrigerant. After absorbing the heat from the liquid to be chilled the refrigerant flowing through the tubes will evaporate. The chilled liquid is collected at the bottom in the tray.

7. What are direct and indirect systems? Explain them with suitable examples. Direct systems refer fig ( 3.4 ) When the heat is taken from the refrigerated or conditioned space by direct expansion of the liquid refrigerant inside the coil, which is situated in the refrigerated or conditioned space, then the evaporator is known as direct expansion evaporator and the system is called as direct system. For example let us assume an air conditioning system adopted with direct system. The liquid refrigerant from the condenser enters into the evaporator that is placed in the unit. The air is passed over the coil and thus the air gets cooling effect. The cold air is used for the A/C system. This is a direct system. Indirect system: Refer Fig ( 3.5a & b ) In this system a secondary refrigerant which is pumped through a secondary coil is used to absorb the heat from a conditioned or refrigerated space. We have to note that the secondary refrigerant will not boil inside the secondary coil. It absorbs the sensible heat only. A primary refrigerant coil is used to cool the secondary refrigerant with a vapour compression refrigeration system before circulating the secondary refrigerant into the secondary coil. Since a secondary refrigerant is used to absorb the heat this system, this is known as indirect system.

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For example, let us consider an air-handling unit with indirect system. At first the liquid refrigerant from the condenser enters into the evaporator coil which is located within a tank having two paths. At one path the hot liquid enters in and through another the chilled liquid passes away. The evaporator present in this tank is known as primary evaporator and the refrigerant used in this evaporator is primary refrigerant. The chilled liquid is then pumped to another cooling coil situated in an air-handling unit.

This cooling coil is called as secondary cooling coil since the secondary refrigerant (i.e.) chilled liquid is flowing through it. The air from the fan is passed over the secondary cooling coil and gets cooled. Thus it is used in an A/C plant. The hot liquid from secondary cooling coil is returned back to the tank which is located with the primary evaporator and thus the process is repeated. This system is known as indirect system. 8. Discuss about the evaporator capacity control. Evaporator capacity control:

Evaporator capacity control is the method of controlling the capacity of the evaporator according to our requirement of cooling. Capacity of evaporator is nothing but the absorption of heat by evaporation of liquid refrigerant.

To control the capacity of the evaporator, we should control the circulation of liquid refrigerant by the expansion devices according to the evaporator load. We can also control the flow of liquid refrigerant in the multi-stage (Parallel connected) compression system by switching off few compressors that are excess than the requirement.

In air conditioning plant dampers are used to regulate the amount of air motion by the fans. By varying the blower speed we can also control the capacity.

9. Write notes on system balancing. System balancing: When the amount of heat absorbed in the evaporator is equal to the amount of heat removed in the condenser, then the system is said to be balanced. When the condenser load is equal to the evaporator load then the system is said to be balanced. The system balancing can be achieved by designing correct capacity equipment and by using suitable controls and valves. The controls and valves that are used in a system are the thermostatic expansion valve, pressure cut-out, suction and discharge regulators etc. In the design of refrigerant system, one of the most important considerations is that of establishing the proper relationship or balance between the vaporising and condensing section of the system. The rate at which the vapour is removed from the evaporator and condensed by the condensing unit should always be equal to the rate at which the vapour is produced in the evaporator by boiling of refrigerant. Since all components in a refrigerating system are connected together in series, the refrigerant flow rate through all the components should be the same. Therefore the capacity of all the components must necessarily be the same. When the system components are selected to have capacities at the system design conditions, then the point of system equilibrium or balance will occur at the system design conditions. On the other hand when the components that are selected do not have equal capacities at the same design conditions, then the system balance will be established at operating conditions other than the system design conditions and the system will not perform satisfactorily.

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CHAPTER 4: EXPANSION DEVICES 1. What are the functions of an expansion device in a vapour compression refrigeration system? Functions of the expansion valves in a vapour compression refrigeration system 1. Expansion devices reduce the high pressure of the liquid into low pressure before fed into the evaporator. 2. They maintain the designed pressure difference between the high and low-pressure side. So the liquid vaporises in the designed pressure inside the evaporator. 3. They regulate the flow of refrigerant according to the load on the evaporator.

2. Name various types of expansion devices. 1. Capillary tube. 2. Hand operated expansion valve. 3. Automatic expansion valve. 4. Thermostatic expansion valve. i) With internal equaliser ii) With external equaliser iii) With pilot control solenoid valve 5. Low side float valve. 6. High side float valve. 7. Electronic expansion valve

3. Describe the following expansion devices with neat sketch. A. Hand expansion valve: Refer Fig (4.1) It is the simplest type. But it requires an operator to regulate the flow of refrigerant to evaporator. The conical shaped needle valve extends down into the valve port and restricts the flow area through the port. When closed, the valve rests on its conical seat. The use of this valve is limited to systems, operating under nearly constant loads for long periods of time such as in ice making plants and cold storage. It is not suitable for installations where the load varies and compressor runs intermittently to maintain a constant pressure, since it is operated by hand only. In this case the flow direction of refrigerant is changed to the normal (i.e.) 90. So a considerable amount of pressure drop occurs.

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B. Automatic expansion valve: Refer Fig (4.2) Construction & working: It works in response to the pressure changes in the evaporator due to an increase or decrease in load. The arrangement of the valve is shown in figure. This consists of a needle valve, a valve seat, a diaphragm and a spring. The screw can adjust the tension of the spring. This valve maintains a constant pressure in the evaporator. Flooding of more or less of the evaporator coil is depending upon the changes in the evaporator load. Two opposing forces control the opening of the valve in the seat. They are i) The tension in the spring at the top of the diaphragm. ii) The pressure in the evaporator acting at the bottom of the diaphragm. When the compressor is running the valve will maintain an evaporator pressure in equilibrium with spring pressure and designed evaporator pressure, then the valve operates automatically to maintain a constant evaporator pressure. When the evaporator pressure falls down, the diaphragm moves downward to open the valve. This allows more liquid to enter into the evaporator and increasing the pressure till the designed evaporator pressure rises to move the diaphragm upward to reduce or close the opening of the valve till the designed pressure in the evaporator is reached. The only disadvantage is poor efficiency of operations (response) compared with other expansion devices. C. Thermostatic expansion valve: Refer Fig (4.3) The thermostatic valve controls the flow of refrigerant through the evaporator in such a way that the quality of the vapour leaving the evaporator will be always in superheated condition. The arrangement of the system is shown in figure. It consists of a needle valve, a seat, a metallic diaphragm, a spring and an adjusting screw. In addition to this a sensing bulb or feeler is mounted on the suction line near the outlet of the evaporator. The bulb is partially filled with the same liquid refrigerant as used in the system. The opening & closing of the valve depends on the given forces that are acting on the diaphragm. i) The spring pressure acting on the bottom of the diaphragm. ii) The sensing bulb pressure acting on the top of the diaphragm. Under normal operating conditions, the spring pressure acting at the bottom of the diaphragm balances the sensing bulb pressure acting on the top of the diaphragm. We can also say the operation of the valve is controlled by the difference between two temperatures (saturation temp & feeler bulb temp).

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If the load on the evaporator increases, it causes the liquid refrigerant to boil faster in the evaporator. The temperature of the feeler bulb and pressure increases. The feeler bulb pressure is transmitted to the diaphragm through a capillary tube. The diaphragm moves downwards and opens the valve to admit more liquid to the evaporator till the pressure equilibrium on the diaphragm is reached. If the load on the evaporator decreases, less volume of liquid evaporates in the evaporator coil. The excess liquid will be cooled in the feeler bulb. Therefore the pressure of bulb decreases and the diaphragm moves upward. This reduces the opening of the valve till the evaporator pressure, maintains equilibrium with feeler bulb pressure. Mostly these valves are set for 5O C super heat. i) It has got high efficiency. ii) Its ability to provide effective use of all evaporators surface under all load conditions. D. Externally equalized thermostatic expansion valve : In order to overcome the effect due to pressure drop in evaporator, the valve with external equaliser as shown in fig (4.4) is used.

In this externally equalised thermostatic expansion valve, the pressure at the bottom of the diaphragm is maintained equal to the evaporator outlet pressure with the help of an external equaliser. Thus the ill effect of the evaporator pressure drop is overcome.

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A small diameter equaliser tube connects the diaphragm with the evaporator outlet as shown in fig. The externally equalised valve operates at the designed super heat regardless of the evaporator pressure drop. E. Internally equalized thermostatic expansion valve: ref. Fig (4.5): The ordinary thermostatic expansion valve with internal equaliser is known as internally equalised thermostatic expansion valve. The standard thermostatic valves are internally equalised. Simply taking away the partition wall makes it or a hole is drilled in the partition wall will make direct contact with the liquid at the inlet of the evaporator. If the pressure drop in the evaporator is high, the pressure at the outlet of the evaporator or at the sensing bulb will be less by the amount equal to the pressure drop. The internally equalised thermostatic valve will operate with excessive super heat. Thus the flow of the refrigerant to the evaporator is reduced and hence the net refrigerating effect reduces.

F. Low side float valve: refer fig (4.6) : Low side float valve maintains constant level of liquid refrigerant by supplying the quantity of liquid required taking the load in the evaporator. Its function depends upon the liquid level in the low pressure side of the system. The float is hollow and made of light metal. It is attached to a pointed arm lever and can make up and down with the rise (or) fall in the liquid level in the float valve housing.

By the sucking action of the compressor the rate of evaporation increases with the increase of load, causing the liquid level to fall below the normal level in the float housing. Therefore the float valve comes down with the liquid level and opens the needle valve to enter more liquid refrigerant. As soon as sufficient liquid has entered, the float valve is taken to the proper height and the needle is seated to prevent further entry of liquid, when liquid level rises due to less evaporation. This is used in commercial (or) industrial applications.

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G. High side float valve : refer fig (4.7) The float valve supplies the liquid refrigerant to the evaporator only when sufficient liquid level is reached in float chamber. This float valve keeps the liquid level steady towards the pressure. The construction is similar to low side float valve. It is located in the high side of the system and indirectly controls the liquid level. In the float chamber the float is connected to the needle with the pivoted arm. The liquid flows from the condenser and liquid level increases in float chamber. Thus the float valve is opened to allow the entry of liquid in evaporator and starts evaporating. As soon as the “heat load” changes, there is a change in the evaporation rate with respect to the rate of change in condensation. Liquid returning rate also changes accordingly. There is no difficulty in float control with the presence of oil since float is directly attached to needle. We have to add one more valve known as intermediate valve after the float chamber. They are nothing but pressure reducing valves.

4. Differentiate internal equalising and external equalising. S.No Internal Equalising External Equalising

1.

2.

3.

4.

5.

6.

In the Internal equalising the partition wall is removed or the wall is drilled for the action of the liquid refrigerant on the bottom of the diaphragm. The spring pressure and the liquid refrigerant pressure act against the sensing bulb vapour pressure. This valve operates with the excessive super heat results in sensing bulb. The pressure drop in the evaporator is directly sensed by the diaphragm and the valve is regulated. There is no connection in between the evaporator outlet and valve. Thus it is operated by the inlet pressure changes. It reduces pressure drop in evaporator.

In the external equalising the evaporator vapour pressure is acting through the external equaliser pipe at the bottom of the diaphragm. The sensing bulb vapour pressure is equalised by the sum of the pressure of the evaporator outlet vapour and the spring. It operates at the desired super heat regardless of the evaporator pressure drop. The pressure drops in the evaporator decrease rate of evaporation and this evaporator outlet pressure (drop) regulates the valve. There is no connection in between the evaporator inlet and valve. Thus it is operated according to the evaporator outlet pressure changes. It does not reduce the pressure drop in the evaporator.

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5. What is a sensing bulb? The sensing bulb is a bulb in which the liquid refrigerant or any low boiling pint liquid is partially filled. It is located at the evaporator outlet to sense the evaporator outlet temperature. In general the sensing bulb is a device which senses the evaporator temperature and allows liquid to evaporate or condense according to that temperature.

6. Describe the installation of externally equalised thermostatic expansion valve along the pilot control solenoid valve. Refer Fig (4.8) The fig shows the installation of externally equalised thermostatic expansion valve along with a pilot control solenoid valve. Solenoid valve is given power connection along with the power connection to the compressor. When the system is started the solenoid valve is given power connection. Now the line 1 which is connected with the liquid line is closed. The lines 2 and 3 are opened. Since 3 is connected with the vapour line, the vapour pressure acts through equaliser connection against the diaphragm in the expansion valve. The line 2 is connected with the equaliser line, and allows the vapour to the equaliser. The line 3 is closed the lines 1 and 2 are opened to give liquid pressure against the diaphragm to close the valve stem when the power supply to the compressor is dropped. Once again, when the power is switched on, lines 3 and 2 are opened first and the liquid present in the line 2 drains through line 3. Then the line 1 is closed and the lines 3 and 2 are opened to perform the normal cycle.

7. Compare hand expansion valve and automatic expansion valve.

S.No. Hand Expansion Valve Automatic Expansion Valve 1.

2.

3.

4.

5.

6.

The valve is operated by hand (Manually). It does not regulate the flow of refrigerant to the evaporator with the load. By adjusting the adjusting screw, we can directly control the flow of refrigerant to the evaporator. Construction and working is simple, so it’s cost will be less. This valve maintains constant refrigerant flow to the evaporator. The opening or closing of valve is dependent on rotation of the valve stem by screwing with the hand.

This valve operates automatically according to the pressure changes in evaporator. It regulates the flow of refrigerant to the evaporator with the load. By adjusting the screw, we can adjust the spring pressure acting against to the evaporator pressure. Construction and working are complicated, so its cost is high. This valve maintains constant pressure in the evaporator. The opening or closing of valve depends on the spring pressure and the evaporator pressure.

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8. Differentiate automatic expansion valve and the thermostatic expansion valve. S.no. Automatic expansion valve Thermostatic expansion valve

1. The spring pressure is acting on the diaphragm.

The sensing bulb vapour pressure is acting on the diaphragm.

2. The pressures of the spring and evaporator operate this valve.

The spring pressure and the evaporator outlet temperature operate this valve.

3. Liquid refrigerant directly acts on the diaphragm.

A partition is provided to avoid the contact of liquid to the diaphragm.

4. It has no connection with the outlet of the evaporator.

It has an evaporator outlet connection in the case of external equaliser.

5. The spring pressure acts on the diaphragm is equalised by the evaporator pressure.

The spring pressure acts in the bottom of the diaphragm is equalised by sensing bulb vapour pressure.

6. Cost is low. Cost is high. 7. The opening and closing of the valve depends

upon the pressure in the spring and the pressure of the evaporator.

The opening and closing of valve depends with the pressures of the spring and the sensing bulb vapour.

8. It is not operated with any other control. It is operated along with a pilot control solenoid valve in many installations.

9. Poor efficiency and response compared with thermostatic expansion valve.

High efficiency and good operation compared with any other valve.

9. Compare low side float valve with high side float valve. S.no Low side float valve High side float valve 1. The float valve is attached with the inlet of the

high-pressure liquid refrigerant. The float valve is attached with the outlet of the low-pressure liquid refrigerant.

2. Depending upon the evaporation in the flooded evaporator the float valve will open or close.

Depending upon the liquid level in the evaporator the valve will open or close.

3. Since the pressure is maintained constant there will not be any other valves in between the float chamber and evaporator.

Since the pressure is not maintained constant an intermediate valve is fitted in between float chamber and evaporator.

4. As soon as sufficient liquid is evaporated, to bring down the float ball to the proper height the needle is seated and prevents further entry of liquid.

As soon as sufficient fluid is entered to bring up the float ball to the proper height, the needle is seated and prevents further release of liquid.

5. There will a pressure drop in evaporator when the float is at closed condition.

There is an increase in pressure when the valve is at open condition.

10. What is a capillary tube? Capillary tube is a smaller diameter tube in which the capillarity occurs. When a capillary tube is dipped or inserted in a liquid, the level of the liquid in the tube will fall or rise than the level of the liquid outside the tube in the container. This phenomenon is called as capillarity. 11. How does a capillary tube function as an expansion device. Refer Fig (4.9) The capillary tube is shown in figure. It is normally used with the small capacity hermetic sealed compressors such as domestic refrigerator, water cooler, freezers etc. It is made of copper having small bore about 0.5 mm to 2.5 mm and having a length of 0.5 to 5 meters. It is installed between the condenser outlet and the evaporator inlet. A fine mesh screen is provided at the inlet of the tube in order to protect the tube from blocks.

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When the liquid which is coming from the condenser enters the tube, the frictional resistance is offered by the small diameter and the pressure of the liquid drops. Since frictional resistance is directly proportional to the length and inversely proportional to the diameter, the tube made to have long length and small diameter will produce high-pressure drop. Then the low-pressure liquid is throttled at the outlet of the capillary or at the inlet of the evaporator. During the throttling, there must be some pressure loss. Thus the capillary tube acts as an expansion device.

12. What are the advantages and limitations of a capillary tube to use it as an expansion device? Advantages: 1. Simple in construction. 2. Very low cost. 3. There is no need to maintain. 4. System using capillary tube does not require a receiver. 5. When the compressor stops, the refrigerant continuously flows from high-pressure side to low pressure side through the capillary until the pressure is equalised. This requires less starting torque to start the compressor. So a low starting torque motor can be used. Disadvantages: 1. The refrigerant must be free from moisture and dust. Otherwise it will block in the tube and will stop the flow of refrigerant. 2. It cannot be used with high fluctuating load conditions. 3. It cannot be used in large capacity plants and with open type reciprocating compressor. 4. It cannot regulate the flow of refrigerant. 5. Formation of oil block, moisture block and dust blocks are common. 6. The refrigerant must be critically charged according to the capacity.

13. Capillary tube is not used in open type system. Explain why? Capillary tube is not used in open type systems because 1) It cannot be used under high fluctuating load conditions. 2) It cannot regulate the flow of refrigerant according to the evaporator requirement as in the case of other expansion devices. 3) We should use accumulator, drier, and strainer as extra devices. This will disturb critical charging and increase the initial cost. 4) An open type compressor may lose sufficient quantity of refrigerant by seepage around the shaft seal. It makes the system in-operative within short time, since critical charging is disturbed.

14. What is critical charging? Critical charging is nothing but charging the system with exact amount of refrigerant which is required for that particular system. This critical charging is very important in capillary tube using systems.

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15. Why critical charging necessary in systems that are using capillary tube? Critical charging is necessary in systems using capillary tube. If the system is not critically charged, the excess amount of refrigerant will be stored in the condenser and liquid line during the working period. This excess liquid will be drained into the evaporator during off-cycle period since the capillary tube is always open and equalises the pressure. It is highly recommended to eliminate the liquid line in systems which are using capillary tube.

16. What are the multi-outlet expansion valves and why they are necessary? a).When the evaporator has more than one refrigerant circuit, the refrigerant which is coming from the expansion valve is delivered into number of evaporator circuits through a refrigerant distributor. In some cases, the distributor is an integral part of the expansion valve itself. Since expansion and distribution of the refrigerant occur simultaneously within the valve, it is known as multi outlet expansion valves. b). By using multi outlet expansion valve, the amount of discharge will increase. c). By using multi outlet expansion valve the rate of heat absorption will increase. d). Multi outlet valves are necessary in very large capacity systems to increase the capacity of evaporator by increasing the rate of evaporation of the refrigerant.

17. “Distributors” - What are they? Distributors are nothing but the devices, which are used to divide and distribute the liquid refrigerant into equal amount and supply it to all the tubes of the multi-pass circuits. It is also used to mix the vapour which is formed by flashing in the expansion valve, with the liquid homogenously. 18. Describe the following distributors. 1. Manifold: Refer Fig (4.10) The Refrigerant is filled in a Header and distributed to several lines called outlets. This type is used when number of outlet are maximum. In this model there will be no mixing of vapour and liquid. It is advisable to keep the manifold horizontally. 2. Pressure drop Type: Refer Fig (4.11) This type is shown in fig. By using pressure drop type we can reduce the pressure in the expansion valve just above the required range. Since, there is a pressure drop in this distributor also. This pressure drop is used to minimise the evaporation of the liquid on the way to evaporator from expansion valve.

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3.Venturi type: Refer Fig (4.12) The construction is similar to a venturi-tube as shown in figure. This distributor is fixed very close to the evaporator. In this type, the path of flow of liquid is reduced uniformly as shown. This model mixes the vapour and liquid evenly in all tubes. 4. Centrifugal type: Refer Fig (4.13) The flow through this type is very similar to the flow of liquid in a centrifugal pump. Hence, the vapour is uniformly mixed with the liquid. Here the disc has got multiple outlets as shown in figure. The liquid experiences a swirl inside the disc and delivered through the slots, which are provided in the periphery of the disc.

19. Discuss the necessity of distributor in a system. Distributors are used to divide or distribute the liquid to the multi circuits of the evaporator. Hence increases the rate of heat absorption. It is also used to mix the vapour with the liquid and distribute to the all the outlets uniformly.

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20. Compare the merits and demerits of all four distributors. S.No MERITS DEMERITS 1. MANIFOLD DISTRIBUTOR

1. Construction is simple. It should not be installed in vertical position

2. A baffle is often fitted in the header in order to minimise the tendency of overfeeding into the evaporator circuits.

When compared with other distributors the pressure loss that occurs in this type is less.

3. To reduce the velocity of the refrigerant an elbow is installed between the expansion valve and the header inlet.

This type of distributor depends upon the level of mounting and low entrance velocities to ensure even distribution of the refrigerant circuits.

4. It helps to prevent unequal distribution of the refrigerant to evaporator circuits.

If it is vertically used, the flow of liquid refrigerant to the upper lines is affected and thereby there will not be even distribution.

5. Any number of refrigerant flow lines is possible. 6. Low cost. 7. It helps to increase the rate of heat transfer by

passing large quantity of refrigerant at same time. 2. VENTURY TYPE :

1. This type utilises the principle of a venturi tube with large percentage of pressure recovery.

It provides a minimum overall pressure loss.

2. It is compact when compared with manifold type. Pressure loss is confined only to wall friction. 3. It depends on contour flow for equal distribution

of the refrigerant. 4. There is a gradual reduction in velocity. 5. There will be a little turbulence on the flow. 6. It may be mounted in any position. 3. PRESSURE DROP TYPE :

1. In this type the pressure drop is more when compared with other.

Construction is complicated when compared with the manifold type.

2. There is a nozzle to reduce the pressure and hence the velocity of the refrigerant increase.

3. A conical button is mounted to provide even distribution of the refrigerant.

4. The orifice determines the capacity of the distributor.

5. The pressure drop prevents separation of flash gas from the liquid.

4. CENTRIFUGAL TYPE :

1. Centrifugal type develops a swirling motion in the refrigerant and maintains a homogenous mixture of liquid with flash gas. It permits the refrigerant with high velocity.

1. Construction is complicated when compared with other types.

2. It provides even distribution of mixture of refrigerant and the flash gas to each of the evaporator tubes.

It is costlier than manifold type.

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21. Discuss the location of the expansion valve in a system. Location of expansion valve: For best performance, the thermostatic expansion valve should be installed as close to the evaporator as possible. When it is necessary to locate a hand valve at the outlet of the expansion valve, the hand valve should have a full sized port. Since there will be enough liquid inside the liquid charged expansion valve to ensure the control of the valve and that will remain with the bulb under all conditions. A liquid charged thermostatic expansion valve could be installed in any position (i.e.) power head up, down or sideways either inside or outside of the refrigerated space. Gas charged valves must be installed so that the valve body is always being warmer than the sensing bulb, preferably with the power head up. With the exception of the manifold type distributor, when a refrigerant distributor is used, the valve should be installed as close the distributor as possible.

2. Discuss the location of sensing bulb and equaliser port in the case of an externally equalised thermostatic expansion valve. Location of sensing bulb: i) When an external remote bulb is used (mounted on the outside, rather than the inside of the refrigerant piping) the bulb should be clamped firmly (with metal clamps) to a horizontal section of the suction line near the evaporator outlet, preferable inside the refrigerated space. ii) Since the remote bulb must respond to the temperature of the refrigerant vapour in the suction line, the entire length of the bulb should be in good thermal contact with the thermal line. When an iron pipe or steel pipe suction line is used, the suction line should be cleaned thoroughly at the point of bulb location and painted with aluminium paint.

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iii) The sensing bulb shall be located that it is not unduly influenced by temperature other than the suction line temp, particularly during compressor off cycle period. However when it is necessary to locate the bulb outside the confined space, both the bulb and suction line must be well insulated from surroundings. iv) Consider the bulb is located on the suction up to the point where the line leaves the refrigerated space. The “heat conducted” along the suction line from the outside may cause the bulb pressure to increase to the extent that the valve will open and permit liquid to fill the evaporator during off cycle. v) In the air-handling unit, when pressure cut0outs are employed, the bulb may be located either outside or inside the duct but always out of the direct air streams. vi) On the brine tanks bulb should always be located below the liquid level at the coldest point. LOCATION OF EXTERNAL EQUILIZER:

i) In general the external equaliser connection is made on the suction line, 150 to 200 mm beyond the expansion valve bulb on the compressor side.

ii) ii) The external equaliser may be connected either to one of the feeder tubes or to one of the evaporator return bends at approximately the midpoint of the evaporator.

iii) When the external equaliser is connected to a horizontal line, it should be installed on top of the line in order to avoid the drainage of oil or liquid into the equaliser tube.

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23. What is cross charging? Expansion valves whose bulbs are charged with fluids other than the system refrigerant are called “cross charged valves “, because the pressure temperature curve of the fluid crosses the pressure temperature curve of the refrigerant. So cross charging is a method of filling the sensing bulb with a liquid different from the refrigerant used in the system.

24. Explain the construction working of the electronic expansion valve with a neat sketch? Electronic expansion valve: These are operated by Microprocessors to maintain a specified a superheat at the lead compressor

entering gas using a thermistor. The high-pressure liquid refrigerant enters valve through bottom. A series of calibrated slots are located inside of orifice assembly. As refrigerant passes through orifice, pressure drops and refrigerant changes to a condition with liquid and vapour. To control refrigerant flow for different conditions, sleeve moves up and down over orifice, thereby changing the orifice size.

A linear stepper motor moves the sleeve. It moves in increments and is controlled directly by a

processor module. As stepper motor rotates, motion is transferred into a linear movement by lead screw. Through stepper motor and lead screws, discrete steps of motion are obtained. This expansion valve is also used to limit cooler suction temperature. This makes it possible for chillers to start at higher cooler water temperatures without overloading the compressor.

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CHAPTER 5:

REFRIGERANT PIPING AND ACCESSORIES 1. What is the purpose of providing piping and accessories? Piping systems are provided for carrying water or refrigerant from one place to another. By providing proper piping arrangement we can transfer the refrigerant or water without any leakage. Accessories are the devices or extra fittings that are installed along with the basic or essential components of a system to improve the efficiency and performance of the system. They will also improve the function and operation of the equipment in the system.

2. Name the materials that are commonly used for refrigerant, water and brine piping. The materials that are most commonly used for refrigerant piping are i) Black steel ii) Wrought iron iii) Copper iv) Brass and v) Rarely stainless steel. Brass and copper may not be used with ammonia because it reacts with the non-ferrous materials in the presence of moisture. Piping materials that are commonly used for water are cast iron, wrought iron, galvanised iron, PVC etc.

3. What are pipe joints, fittings and supports? The joints that are used in refrigerant piping are screwed joint, flanged joint, flared joint, welded joint, brazed joint and soldered joint. The pipe fittings used are ‘U’ bend, elbow, 90° bend, union, ‘T’ fitting, reducer ‘T’, diffuser ‘T’, concentric diffuser, eccentric diffuser etc. Suitable ceiling hangers or wall brackets should support all piping. i) Supports should not be more than 3m apart. ii) A support should be placed not more than 2 ft (0.6m) away from other, when there is a change in direction. iii) All valves in horizontal piping should be installed with the valve stems in a horizontal position whenever possible. iv) All valves should be supported independently.

4. Discuss the general points that must be considered in providing refrigerant piping. The refrigerant piping should be so designed and installed as to i) Assure adequate supply of refrigerants to all components. ii) Assure positive and continuous return of oil to the crankcase. iii) Avoid excessive refrigerant pressure losses, which unnecessarily reduce the capacity and efficiency of the system. iv) Prevent liquid refrigerant from entering the compressor during the running period or off cycle period. v) Avoid trapping of oil into the evaporator from suction line, which may subsequently return to the compressor in the form of a large slug with possible damage to the compressor. vi) Refrigerant piping should be as short as possible, which is economical and efficient.

5. Discuss the piping arrangement in the suction line under various possible locations of evaporator and compressor. The size of the suction piping is more critical than that of other refrigerant lines under sizing of the suction piping will cause an excessive refrigerant pressure drop. On the other hand, over sizing of the suction piping will result in decrease in the velocity of the refrigerant, which will not be sufficient for the return of the lubricating oil. Case (i): The evaporator is installed below the level of the compressor. In this case a riser must be provided in the suction line, and the riser must be trapped.

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Case (ii): The evaporator is located above the compressor. In this case if the system is not operated on a pump down cycle the suction line should be trapped immediately beyond the expansion valve bulb. So the liquid refrigerant cannot drain by gravity from the evaporator to the compressor, during off cycle period. If the system is operated on a pump down cycle, the trap may be omitted since all the liquid is pumped from the evaporator before the compressor cycles off. Some of the piping arrangements of suction line with evaporators and compressors are shown in the diagrams

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6. Discuss the design of refrigerant piping in discharge line under various possible arrangements of compressor and condenser. The design of discharge piping is also similar to the suction piping. The pressure drop in discharge line tends to decrease the compressor discharge pressure and reduce the capacity and efficiency of the system. So the discharge piping is sized to provide minimum refrigerant pressure drop. All horizontal discharge piping should be pitched downward in the direction of refrigerant flow. So that the oil pumped over from the compressor into the discharge line will drain toward the condenser and not back to the compressor head. Let us consider the compressor is mounted below the level of the condenser. In this case a riser is necessary. The discharge line riser must be designed, so that the vapour velocity in the riser under minimum load conditions will sufficiently entrain the oil and carry it up in the riser. When the compressor is not operating, the oil adhering to the inside surface of the discharge riser tends to drain by gravity to the bottom of the riser. If the riser is more than 3 m long the amount of oil draining from the riser may be quite large therefore the discharge line from the compressor is looped to the floor to form a trap. Additional traps, one of each 7.5 m of vertical rise should be installed in the discharge riser, when vertical rise exceeds 7.5 m. The traps may be omitted if an oil separator is used. The various possible arrangements are shown in Figures.

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7. Discuss the design of liquid line with receiver in between condenser and evaporator. Since the refrigerant is in the liquid state, any oil entering the liquid line is easily carried along by the refrigerant to the evaporator. So there is no problem of oil return in liquid lines. For this reason the design of liquid line is less critical than suction and discharge lines. The problem encountered being the flashing of liquid before it reaches the refrigerant control. To avoid flashing, the liquid must be maintained above the saturation pressure corresponding to the temperature of the liquid. Condenser to receiver piping: The condenser to receiver piping must be so designed and sized as to allow free draining of the liquid from condenser at all times. If the pressure in the receiver is permitted to rise above that in the condenser, vapour binding of the receiver will occur. The through flow type receivers may be having bottom inlet or top inlet. When a top inlet through flow type receiver is used, equalisation of the receiver pressure to the condenser can be accomplished directly through the piping. The velocity of liquid in the piping must not exceed 0.5m/s. All horizontal piping should be pitched toward the receiver. When a stop valve is placed in the line it should be located to a minimum distance of 200mm below the liquid outlet of the condenser..

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With the surge type receiver the liquid refrigerant enters and leaves the receiver through the same opening. In addition to all other considerations stated in the case of through flow type receiver, an equalising line is used in this type in between the receiver and the condenser. In this case the refrigerant velocity may be up to 0.75 m/sec. The minimum vertical distance between the outlet of the condenser and the maximum liquid level in the receiver should never be less than 300mm. This is to prevent the back up of liquid in the condenser.

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8. Discuss water and brine piping. Water or brine is universally used as a cooling medium in all secondary evaporator-cooling systems. We should also provide water piping in systems, which use water-cooled condensers. The water piping is required to carry the water from source to the condenser, condenser to cooling tower etc. The pressure drop that allowed in the water pipeline is 8mm of Hg per metre length of pipe. The velocity is limited to 2m/sec. The design of the brine pipe is exactly same as water pipeline except brine. Friction multipliers are used to calculate the pressure loss in pipeline to take into account the higher density of the brine. The general considerations are as follows: i) Excessive pressure loss should be avoided with the help of standard joints and fittings and neglecting unnecessary bends. ii) The length of the pipe is as short as possible. This will minimise the cost and the efficiency will increase. iii) The cooling tower is always mounted above the level of a centrifugal circulating pump to provide self-priming. Commonly G.I pipes are used for water and brine piping.

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9. What is a liquid indicator? Explain the use of liquid indicators. A liquid indicator is also called as sight glass. It is normally installed in the liquid line of a refrigeration system & provides a means of determining wisely whether or not the system has a sufficient charge of refrigerant. If the system is short of refrigerant, the vapour bubbles appearing in the liquid stream will be easily visible in sight glass. The sight glass is installed as close to the liquid receiver as possible. Typical sight glass is shown in Figure

10. Describe the construction, working and use of the following accessories. 1. Impingement oil separator 2. Chiller type oil separator 3. Through-flow type receiver 4. Surge type receiver 5. Refrigerant dehydrators 6. Strainers 7. Pressure relief valves 8. Compressor service valves 9. Manual valves 10. Heat exchangers 11. Receiver tank valves or liquid receiver service valves 12. Suction accumulators 1. Impingement type oil separator: The impingement oil separator consists of a series of screens or baffles through which the oil laden refrigerant vapour must pass. On entering the separator, the velocity of the refrigerant vapour is considerably reduced because of the larger area of the separator. The oil particles are caused to impinge on the surface of the screens, when the refrigerant flows through the separator. The oil then drains by gravity from the screens to the bottom of the separator. From there it is returned to the crank case of the compressor through float and pressure reducing valves. The construction is shown in the Figure.

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2. Chiller-type oil separator: It is also called as “oil chiller”. The construction is similar to a water-cooled condenser. Water is circulated through the tubes and the vapour passes through the shell. The oil is separated from the vapour by precipitation on the cold water tubes. The oil may be manually drained from the sump or automatically drained to the compressor through a float valve. The water flow through the separator must be carefully controlled so that the refrigerant vapour is not cooled below its condensing temperature. In some installations the chiller type is installed in the liquid line. To eliminate the possibility of liquid refrigerant draining from the oil separator into the compressor crankcase during the off cycle period, the “oil drain line” from the separator should be connected to the suction inlet of the compressor rather than to the crankcase. 3. Through-flow type receiver: In a through-flow type receiver the liquid refrigerant is first sent into the receiver and then it is taken out from the receiver to the evaporator. The inlet in the receiver may be at the top or at the bottom. The construction is shown in Figure

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4. Surge type receiver: In a surge type receiver the liquid enters and leaves the receiver through the same opening, usually made at the bottom. During operation excess refrigerant that is not used by the evaporator is surged into the receiver. When the amount of refrigerant evaporates in the cooling coil is more than the amount cooled in the condenser, the excess requirement is now supplied from the receiver. In normal practice a surge type receiver is installed with an equaliser line between the receiver and the condenser. This ensures the surging of liquid into the receiver under all conditions of the condenser, since the pressures in both condenser and receiver are balanced through the equaliser. A typical arrangement is shown in the Figure. 5. Refrigerant dehydrators: Refrigerant dehydrators are also called as driers and recommended for all refrigerating systems employing halocarbon refrigerants. In small systems the drier is usually installed in the liquid line directly. In larger systems a by-pass arrangement is employed. The construction of the drier is shown in Figure. In that a hygroscopic material is packed inside a hollow cylindrical body. The hygroscopic materials used are phosphorous pentoxide, silica gel etc. It is called as drier cartridge. Depending on their construction they are called as i) Throw away type. ii) Re-chargeable type. In throw away type the drier cartridge made of a hygroscope material can’t be replaced. After prolonged use the drier must be thrown away and a new drier shall be fitted in the system. But rechargeable driers provide the facility of refilling the hygroscopic material into the body of the drier.

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6. Strainers: Strainers are filters and are used to arrest dust particles or impurities if any present in the refrigerant. Strainers should be installed immediately in front of the expansion device. Most of the refrigerant compressors are equipped with strainers in their suction inlet chamber. The construction of a strainer is similar to a drier except that a filtering pad is placed inside the cylindrical body. In the market, filter cum driers are also available. These filters cum driers perform both the functions of a strainer and drier. 7. Pressure relief valves: Pressure relief valves are safety valves designed to relieve the pressure in the system to atmosphere. Most refrigerating systems have at least one pressure relief valve mounted on the receiver tank or water-cooled condenser. A typical pressure relief valve is shown in Figure. The two pressures operate all pressure relief valves are i) Spring pressure ii) System pressure A fusible plug is sometimes substituted for the pressure relief valve. It is a metal alloy designed to melt at some predetermined fixed temperature.

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8. Compressor service valves: Compressor service valves are usually designed to bolt directly to the compressor housing. They have ‘front seat’, ‘backseat’ and “cracked” type position adjusting arrangements. The front seat controls the flow between the refrigerant lines and the compressor. The back seat controls the gauge port of the valve. If the valve is kept in between front and back seat positions, it is called as intermediate or cracked. With the intermediate position, both the refrigerant line and the gauge port are open to the compressor. The construction is shown in Figures. 9. Manual valves: The manual valves may be of the globe, angle or gate type. These are primarily used in water and brine lines. Gate valves have a very low-pressure drop, but do not permit throttling. So they can be used only where full flow or no flow conditions are desired. The globe and angle valves are suitable for throttling. Out of the above two, the angle valve offers least resistance for flow and hence recommended in practical use. Depending on the construction, the manual valves are either packed or pack-less type.

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Packed valves are covered with cap seals in their valve stem to eliminate the leakage, when the valve is not in use. Pack-less valves are leaking, when they are not in use. The manual valves are shown in the Figure . 10. Heat Exchangers: Heat exchangers are used in a system to transfer the super heat of liquid refrigerant to the vapour refrigerant. Hence the C.O.P. of the system is increased. According to the type of flow heat exchangers are classified as follows: i) Parallel flow ii) Counter flow iii) Cross flow We can also call the condensers and evaporators as Heat exchangers, since heat is transferred from one to other. So by considering the construction, the heat exchangers may also be of the types i) Shell and tube ii) Shell and coil iii) Double pipe etc.

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11. Receiver tank valves or liquid receiver service valve: Receiver tank valves are usually of the packed type. When designed for installation on the top of the receiver, the valves must be provided with dip tubes, so that the liquid refrigerant can be drawn from the bottom of the receiver. Some valves also have tapings to accommodate a relief valve or fusible plug. 12. Suction accumulators

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11. State the location of the following accessories in a system. a) DRIER or DEHYDRATOR: This is installed in the liquid line just before the expansion valve either directly in the line or in a bye pass line. b) OIL SEPARATOR: Oil separator is installed in the discharge line of the compressor. Normally it is installed close to the compressor. c) PRESSURE RELIEF VALVE: This is at the top of a receiver tank or a water-cooled condenser. d) COMPRESSOR SERVICE VALVE: This is connected and assembled with the inlet and outlet ports of a refrigerant compressor. e) RECEIVER: Receiver is installed in the liquid line of the system, just after the condenser. Normally it is installed below the level of the condenser. f) HAND SHUT OF VALVE: These valves are used in many places in the refrigerant line, normally below and after all accessories. g) HEAT EXCHANGER: This is installed in the system, connecting the low-pressure vapour line and liquid line.

12. Discuss the necessity of oil separators in a system. The oil separators should be used in a system where the oil return is likely to be inadequate or difficult to accomplish. When the amount of oil in circulation is excess this will cause an undue loss in the efficiency of the heat transfer surfaces. Specifically oil separators are recommended for i) All systems employing non-miscible refrigerants. ii) Low temperature systems iii) All systems employing non-oil returning evaporators. iv) In a system where the capacity control and long suction or long discharge risers which will cause serious piping design problems. *******************

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CHAPTER 6 - FANS AND BLOWERS 1. What are fans and blowers? Fans and blowers are Roto-dynamic machines, which propel air or gas continuously. When the quantity of air handled is large then it is also known as blower. The fan is normally used for small air handling capacity. In general fans are axial flow and blowers are radial or mixed flow. Fan produces high velocity of air motion with low static and total pressures and the blowers are capable of developing high static pressures.

2. Classify all the fans and blowers that are used in Ref. & A/C systems. i) ACCORDING TO FLOW OF AIR: a) Axial flow fans b) Radial flow fans c) Mixed flow fans ii) ACCORDING TO THE DIRECTION OF ROTATION OF IMPELLER: a) Clockwise blast b) Anticlockwise blast iii) ACCORDING TO CAPACITY: a) Low capacity b) Medium capacity c) High capacity iv) ACCORDING TO TYPE OF DRIVE: a) Direct drive b) Indirect drive v) ACCORDING TO THE CONSTRUCTION OF BLADE: a) Forward curved blades b) Backward curved blades c) Straight blades vi) ACCORDING TO THE SPEED OF FAN MOTOR: a) Single speed b) Variable speed vii) ACCORDING TO THE TYPE OF IMPELLER: a) Open type impeller b) Semi- open type impeller c) Closed type impeller viii) ACCORDING TO THE MOTOR USED : a) Single phase motor b) Three phase motor ix) ACCORDING TO THE MANUFACTURER: a) Kirloskar b) Kruger c) Nadi

3. Mention the places where fans and blowers are used in Ref. & A/C systems. i) Cooling towers v) Desert coolers or Air washers ii) Forced draft air cooled condensers vi) Exhausts iii) Air handling unit of an A/C system vii) Unitary air conditioners, etc. iv) Ducts

4. Describe the following with neat sketch. 1. Propeller fan: fig (a) A Propeller type of axial flow fan consists of a propeller or disc type wheel, which operates within a mounting ring as shown in fig. The design of the ring surrounding the wheel is important because it presents the air discharged from being drawn backward into the wheel around its periphery. The propeller fans are used only when the resistance to air motion is small. They are useful for the ventilation of attic spaces, lavatories and bathrooms, removal of cooking odours from kitchens and many other applications where little or no ductwork is involved.

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2. Tube axial fan: A tube axial fan consists of a propeller wheel housed in a simple cylinder as shown in fig. The wheel may be driven either from an electric motor within the cylinder directly connected to its shaft or may be driven through a belt arrangement from a motor mounted outside the housing. These fans are easily installed in round ducts. They are more efficient than propeller fans. The air discharge from tube axial fan follows a spiral path as it leaves the cylindrical housing. 3. Vane axial fan : A vane axial fan combines a tube axial fan wheel mounted in a cylinder with a set of air guide vanes. This fan eliminates spiral flow of the discharge air and reduces the turbulence of flow. The efficiency of operation and the pressure characteristics are better than those of tube axial fan. The straight-line flow of air leaving the fan assures quiet operation

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4. Centrifugal blower: Centrifugal fans are widely used for duct air-conditioning system because they can efficiently more large or small quantities of air over a greater range of operating pressures. All centrifugal fans have an impeller or wheel mounted in a scroll type or housing as shown in fig. The impeller is turned either by the direct drive or more frequently by an electric motor employing pulleys and belt. The centrifugal force created by the rotating impeller moves the air outward along the blade channels. The scroll into a single large air stream combines the outward moving air streams. This air stream leaves the fan through the discharge outlet. The fan impeller may have the following three types of blades. a). Forward curved blades. b). Backward curved blades. c). Straight blades or Radial blades. a). Forward curved blades: Large number of centrifugal fans installed in air conditioning system has impellers with forward curved blades as shown in fig. Since the blades are very shallow in depth, the diameter of the housing, air inlet opening more nearly approaches to that of the impeller. The ample inlet opening together with streamlines hub of the wheel, promotes a smooth flow of air into the rotating blades. This increases the efficiency of the fan and reduces the noise. The forward curved blades are more capable of overcoming the attached duct system resistance when their operation is at low speeds. b). Backward curved blades: The centrifugal fan impeller may have backward curved blades as shown in fig. The backward curved blades must be operated at a much higher speed of rotation than the forward curved blades if the same static pressure is to be produced in each case. In some cases the higher speed may be an advantage because of a possible direct connection to the driving motor. The fan impeller backward curved blades operate at high efficiency and have no over loading power characteristic. They also offer the advantage of wide ranges of capacity at constant speed with small changes in the power requirements. The number of impeller blades varies in centrifugal fans. The backward curved blade impeller has usually 10 to 16 blades. While forward curved blade impeller has 24 to 64 blades.

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BACKWARD BLADE c). Straight blade impeller: The centrifugal fan impeller may have radial blades, shown in fig. The blades run straight out from a central hub with high structural strength. These fans provide very high pressure at very high speed.

STRAIGHT BLADE GENERALLY AVAILABLE MODELS OF FANS AND BLOWERS

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5. Explain the characteristic and performance of the following using graphs. a). Axial flow fans: Fan and system characteristics: The ductwork’s element such as elbows, Tee’s, registers, dampers etc., offer resistance to the air flow and cause loss in pressure. The change in pressure loss is called system characteristics. Any air-conditioning or ventilating system that has a ductwork, dampers, coils etc has definite system characteristics. The loss of the total pressure in a system at a given volume flow must be equal to the total pressure developed by the fan at the same volume. The point of intersection of system characteristic curve and the fan performance curve satisfies this condition. This point is the particular fans “operating point” in particular system. The system characteristics are independent of fan used. A fan performance curve is a graph of fan’s volume rate plotted against pressure horse power or efficiency of the fan. The performance curve of axial fan is shown in the fig. The axial flow fans are said to have non-overloading power characteristics, which means that the driving motor cannot be overloaded if the fan and motor are properly selected. b). Centrifugal blower with forward blade impeller: The centrifugal fans with forward curved blades require an ever-increasing amount of power as the air volume is increased. However this type of centrifugal fan provides greater static pressure for a given blade-tip velocity than the other types and it is commonly used in air-conditioning systems in spite of this disadvantage.

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c). Centrifugal blower with backward blade impeller: We see that centrifugal fans with backward curve blades require maximum horsepower. The operating condition, which requires the maximum horsepower, is close to the combination of volume and static pressure under which the fan operates most efficiently. The fans of this type are said to have non-overloading power characteristic, which means that the driving motor cannot be overloaded if the fan and motor are properly selected. d). Centrifugal blower with straight blade impeller: We see that centrifugal fans with straight blade require maximum horsepower. The total efficiency widely increases to some extent and correspondingly decreases after some time with percentage volume delivery. The static efficiency increases up to 30 % of volume delivery. The static and total pressure falls with the further increase in % volume delivery.

6. Describe various types of inlets and outlets connected to a blower. FAN INLETS: Fan inlets are formed passages to permit entrance of the air to the fan impeller with minimum entry losses and turbulence. i) The streamlined orifice reduces the entry losses to a minimum and of guiding the fluid into the fan blades most rationally, reducing shock and eddy losses. ii) Losses that arise from improper inlet connections and ductwork cannot be evaluated as equivalent to the loss of an elbow or other variable gas passage.

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iii) Directional passage do not exert their inherent pressure losses but also create unbalanced, restricted or whirling flow into the fan inlet and seriously reducing the basic pressure and efficiency. iv) Generally the inlet should be designed to permit the gas flow, applicable to the action with the fan inlet open to atmosphere or with a section of straight inlet duct. FAN OUTLETS: The outlet of a centrifugal fan is the termination of the scroll shaped housing, that is the point of connection of fan to the external system. When a straight outlet duct is connected, the change from spiral flow in the housing is partially completed at the fan outlet, continued and finally completed at the fan outlet. When the fan outlet discharges directly to atmosphere or to a large plenum chamber the complete velocity pressure is dissipated and wasted. Gradual reduction in velocity by proportional expanding outlet connections reduces the final velocity pressure and converts appreciable portion of velocity into useful static pressure. The arrangement of the outlet connection can contribute to the elimination of losses. Avoidance of abrupt or excessive expansion, restrictive sections and rapid change in direction is good practice. 7. Describe various types of blast from a centrifugal blower. Considering the direction of rotation of the impeller the blast is divided into the types as i)clockwise blast and ii) anticlockwise blast. The type of blast is also described by considering the angle in which the air is delivered from the outlet. The possible directions of delivering the air from the outlet of a blower are shown in Figure

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8. State and explain fan similarity laws. FAN SIMILARITY LAWS: FIRST LAW: It states that the quantity of air delivered from a fan or blower is directly proportional to the speed (N) of the blade or impeller and it is also proportional to the cube at the diameter (D3) of the blade or impeller Q ∝ N Q ∝ D 3 Q ------- = Constant N D 3

For another similar fan, Q 1 ------- = Constant N1 D1

3

For any number of fans, Q Q1 Q2 Q3 ------- = ---------- = --------- = ---------- = Constant N D 3 N1 D1

3 N2D23 N3D3

3

SECOND LAW: It states that the total pressure developed by a fan or blower is directly proportional to the square of speed of the impeller or blade, to the square of the diameter (d2) of the blade or impeller and also proportional to the density of air (ρ). P T ∝ N2 P T ∝ D2 P T ∝ ρ ∴ P T --------- = Constant N2 D2 ρ For another similar fan, P T1 --------- = Constant N1

2 D12 ρ

For any number of similar fans, P T P T1 P T2 --------- = --------- = ----------- = Constant N2 D2 ρ N1

2 D12 ρ N2

2 D22 ρ

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The above law holds good for static pressure (Ps) also. ∴ P S --------- = Constant N2 D2 ρ For another similar fan, P S1 --------- = Constant N1

2 D12 ρ

For any number of similar fans, P S P S1 P S2 --------- = --------- = ----------- = Constant N2 D2 ρ N1

2 D12 ρ N2

2 D22 ρ

THIRD LAW: It starts that the power (P) of a fan or blower is directly proportional to the cube of the speed of the impeller or blade to the fifth power of the diameter of impeller or blade and also to the density of air. (ρ) ∴ P α N 3 P α D 5

P α ρ P ∴ --------- = Constant N3 D5 ρ For another similar fan, P1 ∴ ------------- = Constant N1

3 D15 ρ

For any number of similar fans, P P1 P 2 --------- = --------- = ----------- = Constant N3 D5 ρ N1

3 D15 ρ N2

3 D25 ρ

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9. What are the fan classes and standards? FAN CLASSES: The fan classes are as follows: Class I fans: 3.75 inches of water, Total pressure as a maximum value. Class II fans: 6.75 inches of water, Total pressure as a maximum value. Class III fans: 9.75 inches of water, Total pressure as a maximum value. Class IV fans: Above 9.75 inches of water, total pressure values. FAN STANDARDS: Fan standards are established for 25 sizes by setting up maximum wheel diameter and maximum outlet areas correspond to these sizes. The series is based on geometric progression advocated by the American Standards Association. The standards are named from A to Y. For any index (or ) standard name the wheel diameter and other dimensions shall not exceed than the standards. 10. Show in line sketch that how will you measure static and velocity pressures of air flow in ducts?

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11. How will you find the quantity of air flow in a duct. To calculate the quantity (discharge) Quantity (discharge) Q = Velocity * area Q = v * a Where a - is the cross sectional area of the duct in m2 v - is the mean air velocity in m/sec.

12. How are the capacity of fans and blowers controlled? Capacity control in fans and blowers is done as follows: 1. By using dampers or guide vanes or blades we can adjust the inlet opening. 2. By changing the diameter of the blade or impeller we can vary the discharge. 3. By adjusting the speed of the motor. (We can use a variable speed motor or variable speed arrangements.) 4. By changing the type of blade and impeller. 5. By changing the blade angles.

13. What are the sources of fan noise. Fan noise depends on the following factors: i) Speed of fan or blower. ii) Type of fan used. iii) Blade angles. iv) Type of drive. v) Type of mounting. vi) Type of inlets and outlets connections. vii) Type of delivery. viii) Application. ix) Type of lubrication.

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14. How will you select fan or blower for a system? i) ACCORDING TO THE CAPACITY (i.e.) Total pressure, static pressure & volume flow. a) High capacity fan b) Medium capacity fan c) Low capacity fan ii) TYPE OF APPLICATION: a) Used in condenser b) Used in evaporator c) Used in ducts to increase the velocity iii) ACCORDING TO THE TYPE OF IMPELLER (or) BLADE: a) Forward blade b) Backward blade c) Straight blade d) Closed type impeller e) Semi open type impeller f) Open type impeller iv) ACCORDING TO NUMBER OF IMPELLER BLADES (or) VANES: Having 2, 4, 6, 14, etc. v) ACCORDING TO THE TYPE OF CASING:

a) Volute casing b) Vortex casing c) Diffuser or turbine casing

vi) ACCORDING TO THE POWER CONSUMPTION: i) Low power consumption ii) High power consumption iii) Medium power consumption

vii) ACCORDING TO THE MATERIAL USED: i) Cast iron , steel ii) Non ferrous iii) Wrought iron etc.

viii) ACCORDING TO SIZE: i) Big size ii) Medium size iii) Small size

ix) ACCORDING TO THE COST: i) High cost ii) Medium cost iii) Low cost

x) ACCORDING TO NOISE PRODUCTION: i) Noisy ii) Noiseless

xi) ACCORDING TO THE MANUFACTURER: i)Kruger ii)Nadi

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CHAPTER 7 - AIR CLEANERS AND AIR FILTERS 1. What are air cleaners and air filters? These are devices, which are used to remove the dust, bacteria and odour from the air. 2. Explain the use and necessity of air filters? The air filters are used to i) Safeguard the occupants from the health hazards that are suspended in air. ii) Improve the quality of a product in a manufacturing industry. iii) Ensure efficient operation of machines and equipment. iv) Avoid the nuisance caused by the presence of dust and unwanted odour. v) Protect machines and equipment from corrosion. vi) Reduce the required refrigeration capacity of the air conditioning system. The above are achieved by removing the impurities from the outside air. We know the air taken from the atmosphere carries dust, bacteria and odour. These are harmful to health. To safe guard the health of the occupants from the above harmful sources, it is necessary to remove all possible ingredients from the air before taken into the air conditioning system.

3. Describe various methods of air cleaning. The method of air cleaning depends on the nature of dust. Most of the ducts can be removed using air filters. The following are the methods of air cleaning. a) Air filtration b) Air sterilisation c) Air ionisation d) Odour suppression A) AIR FILTERATION: Mostly all types of ducts are removed by filtration. The use of a particular air filter depends upon the nature of the dust and it’s concentration. Air filtration is done in many ways. The following are some types of air filters. i) Natural filters ii) Electronic filters iii) Mechanical filters B) AIR STERILIZATION: It is the process of removing bacteria and germs from air. The germs and bacteria are killed when the air is sterilised with ultra violet light. Sterilising lamps can be mounted directly in air conditioning spaces with a shield to prevent eye injury. Bacterial mist is also used to kill the bacteria. This must consist of tiny droplets of propylene glycol or tri-ethylene glycol, and are dispersed in air with concentration of one gram to 50 to 200 million cubic centimetres of air. The application of air sterilisation is not economical because if the people work in germ free offices and live in sterile homes, can’t escape from bacteria as they move in crowded vehicles, stores and restaurants, where the possibilities of infection are more. C) AIR IONIZATION: The ions present in the air have an effect on the health of people and the working efficiency. The ion con tent of the air naturally increases with the temperature of air. At present air ionisation has no much practical significance in comfort air conditioning. D) ODOUR SUPPRESSION: Odour removal or suppression from air is absolutely necessary for comfort air conditioning. The odour creates dullness and reduces working efficiency. Some odour bearing materials and vapours carried with the air are removed by absorbing materials. The activated carbon placed in perforated container is used for removing odour. Whenever the activated carbon becomes ineffective, it can be made effective by heating it to a temperature of 550oC . The use of Ozone for the treatment of odours is made successful for many years. The unwanted odours are suppressed by the masking effect of ozone. The ozone has pleasing effect when it is added 0.1 parts per million parts of air. Greater concentration of ozone is rather harmful and the quantities of the order of 15 parts per million are exceedingly irritating to respiration membranes and the eyes. This method of suppressing odour with ozone is not universally accepted.

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4. Describe the following filters. a) Dry filters b) Viscous filters c) Wet filters d) Electronic filters and e) Centrifugal dust collector. a) DRY FILTERS: These are subdivided into two as i) Clearable or reusable filters. ii) Throw away filters. Dry filters are usually made of cloth, coarse papers, wool or cellulose felt. The dust in the air will be trapped or screened when it passes through the filtering medium. The velocity of air allowed through these filter ranges between 2m to 15 m/min. The filtering materials are often arranged in bag forms to provide necessary surface without excessive surface requirements. Shaking or rapping action of the filters during operation can do the cleaning of these filters. Dry filters are capable of collecting 99 % or more dusts as small as 0.5 micron. Throwaway filters are made of glass wool, plastic fibres, steel wool, animal hairs or vegetable fibres. These filters are available in the market in different sizes and forms. The pads of above mentioned materials are set into permanent steel frames. If a resin powder is added with dry wool, then electrostatic charge develops which is sufficient to attract and retain small diameter aerosols. Addition of resin powder increases the filtering efficiency. The dry type air filters remove tiny dust particles of 0.3 to 10 micron diameter very satisfactorily. It can’t be used where the dust concentration in the air is higher than 2.5 grams per 1000m 3 . The dry type filters are suitable for removing dust in all ordinary applications in metropolitan and industrial communities but are not suitable for filtering the air drawn through a hand over or sand blasting or a grinding operation in an industrial plant. The dry type filters are not capable of removing smoke from air. b. VISCOUS FILTERS: These filters are made in the form of pads and bats using glass wool, steel wool, plastic fibres or copper mesh. The pads are impregnated with “viscosine”, which is an oily substance. Some viscous filters can be washed in gasoline and can be used again. The “viscosine” used must have the following properties. i) Constant viscosity over a wide temperature range. ii) Germicidal action to prevent the growth of bacteria. iii) Should not evaporate more than 1 % of its weight during the lifetime of the filter. It is in the form of a continuous roll of material coated with the oil and is driven by motor across the air stream. The filtering screen is continuously running over rolls at the top and bottom while the bottom roll is in a sump filled with viscous liquid, which acts to wash the screen and to recharge with the fresh viscous liquid. The filter curtain is rotated by in the speed of 10 cm per hour. The advantage of this type over others is the reduction in maintenance cost. It has the clearing efficiency of 97 % with 0.7cm of water heat loss at 150 m/min air velocity through the filter. c. WET FILTERS: Water spray type air washer is used as air filter. Such type of air filters is called as wet filters. In this type the dust particles are wetted by water spray and then owing to the additional weight of the water, the particles fall to the bottom. The effectiveness depends on the ability to become wet by water. It is almost impossible to wet a greasy particle like “pollents”. These types of filters are used for absorption of soluble gases in industries. Sometimes chemicals are added in water to absorb some specific gases. The wet filters are always used with a filter at their outlets. It is nothing but, providing a dry filter along with a wet filter. d) ELECTRONIC FILTERS: The principle of the electronic filter is shown in Figure 7.4. The air is passed between a pair of oppositely charged conductors and it becomes ionised, as the voltage applied between the conductors is sufficiently large (8000 V to 15000 V). As the air is passed through this ionised chamber both negative and positive ions are formed and the later being larger in quantity (20% negatively charged and 80% positively charged). The air carrying the ions and coming out of the ionising chamber is further passed through the collecting unit.

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The unit consists of a set of vertical metal plates spaced 15 to 20 mm apart. Alternate plates are positively charged and earthen to attract the negatively and positively charged dust particles respectively. As the alternate plates are grounded, high intensity electrostatic field exerts a force on charged particles and drives them towards the grounded plates. To remove the dust accumulation, the collector plates are cleaned periodically by washing them with hot water spray. The advantages of the air cleaner are i) Low initial cost ii) low working cost iii) low maintenance cost iv) ease of operation v) smaller installation space vi) effectiveness on very small dust particles and vii) less power requirement. The disadvantages are i) The necessity of protecting the entire apparatus by placing a wire mesh from the high potentials and the closeness of the charged plates. ii) The necessity of a pre filter for reducing the load. iii) The decrease in efficiency corresponding with the increase in velocity of air and iv) Formation of ozone during ionisation. e) CENTRIFUGAL DUST COLLECTOR: These are used mostly in industrial air conditioning. In this cleaner, a high velocity air stream is directed into a conical chamber. This produces a whirling air current within the chamber and throws the heavier dust particles to the sides and fall out of the air stream. The dust particles are collected at the bottom of the cleaner and the clean air taken out from the top of the cleaner. The advantage of this air filter is the rugged, which does not require attention in service. The disadvantages are the effectiveness in removing only larger dust particles and the high power requirement. 5. Explain the use of activated Carbon (Charcoal) for removing gases and odours. The activated carbon, equipped in a filter is used in re-circulation air systems to clean the air for reuse and to remove the gaseous and vaporous contaminants. The activated carbon removes organic contaminants by the principle of adsorption. It is ineffective to remove gases such as H2 and N2. The activated carbon can remove particles in one micron size and smaller. (The particles visible to the naked eye are about 175 microns.) This sub micron material is accumulated and held throughout the carbon media on the surface of the carbon molecules rather than within them as in absorption. Different carbon materials capture different size particles. Activated carbon made from hard wood captures larger gaseous particles than that made from coconut. A pre filter is always used with an activated carbon filter to remove large dust particles, which would clog-up on the pores of the carbon and reduce the efficiency. Activated carbon filters remove odours from the inside air. The efficiency of a carbon filter is 99.9 % on all the particles in the range of 1 to 5 microns.

6. What is odour? Indicate the sources of odour. Odour is defined as the sense of smell. The sources of odour are a) Dusts, smoke, vapour and any unwanted gases that are present in air. b) Bacteria c) Decayed products d) Dead bodies of living being e)Chemicals

7. Explain the method of measuring odour. Odour is measured by two methods. a) Sensory measurement b) Analytical measurement

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a) SENSORY MEASUREMENT: It is one way of measuring odour by human sense. The following properties are taken into account to measure odour on sensory measurement. i) ACCEPTABILITY: Human body accepts some kind of odour with pleasure and rejects some as bad, unpleasantly by nature, so that the order is classified by this property. ii) QUALITY: Each and every odour has some unique quality. For example the odour generated from a mango is different from the odour generated from a jasmine. So by identifying their quality the odour can be classified. iii) INTENSITY: The odour of different products differs in their intensities also. For example the odour a lemon is mild and the odour of ammonia is dense. By considering the intensities the odour is categorised. With the above properties of the odour we can classify the odour and measure it. The unit for measuring odour is called as “ Odour Units “. b ) ANALYTICAL METHOD : In this method the constituents of the air is analysed and separated using the method of gas Chromatography. After identifying the particles or constituents the odour from the air can be classified and measured. 8. Describe the methods of removing odour. Most odours are either eliminated or reduced in concentration by the following methods. a) Process modification b) Dilution method c) Sensory modification method d) Adsorption e) Combustion or incineration. a) PROCESS MODIFICATION: A slight change in the process minimise odour generation. Modification in the manufacturing process could involve lowering the process temperatures. This is commonly done in foundry ovens, mineral wool plants, varnish cookers and paint baking ovens. The temperature in a fertiliser plant using brewery waste is adjusted to assure that there is no vaporisation. b) DILUTION METHOD: If the odour is not dangerous it can be diluted until the concentration is below the objectionable level. This is a very common approach in industries. In the production of polyurethane the terylene- disocynate is diluted to a non-detectable concentration. c) SENSORY MODIFICATION METHOD: There are two sensory modification methods masking does not alter the composition of the original odour. In sensory modification, perfume or deodorant may be added directly to the process. Perfumes, colognes and deodorants are “odour masking elements”, which release a pleasant odour to overcome an unpleasant smell. Use of ozone for odour removal is also a masking method. The concentration of ozone required to destroy odour is normally toxic. The toxic qualities of ozone causes drowsiness and headache and reduce their ability to perceive odours. Ozone treatment has been employed in processing of fish and other foods, paint, varnish, plastics, petrol, paper and fertiliser plants. Odour neutralisation i s the method used to eliminate or diminish the intensity of the original odour. Selection of neutralisation agents depends mainly on the experience of the person in the field. d) ADSORPTION: Charcoal (Activated carbon) is used mostly in locations where noxious gases are plentiful. Charcoal is an absorbent of odours in the same that Silica- Gel is an adsorbent for moisture. Charcoal is able to absorb condensable vapour and gases, which come in contact with its surface and hold them until released by reactivation. It has strong affinity for organic gases and hydrocarbons. The small charcoal containers are capable of handling 1 cu-m of air with a pressure drop of 0.5 cm of water. e) COMBUSTION (or) INCINERATION: This method is used in removing odours from industrial exhausts. The gases containing objectionable odours are passed over a catalyst, which is maintained at 300 oC. The odour is removed by catalytic combustion. Direct combustion at 650 oC is also used for destroying odours from gases. The combustion of air pollutants that are organic and gaseous in nature converts into harmless products. The carbon dioxide is odourless and water vapour present in the exhaust is simply super heated system. The cost of catalytic oxidation is less when compared to all other methods. Catalytic combustion systems are now popular in emission control applications, food processing, chemical and metal finishing industries.

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9. Describe the factors that are to be considered in selecting an air cleaner. The factors to be considered in evaluating the suitability of air filter for a particular application include: i) The degree of air cleanliness required. ii) Amount of air handled. iii) Type and amount of particular matter in the air to be filtered. iv) The method of disposal of the collected dust. v) Better protection of valuable objects, equipment or plants. vi) Improved operation of plant and equipment leading to increased reliability and product quality. vii) Reduced housekeeping expenses. viii) Better and healthier working conditions. Each type of filter has its own merits and demerits. The proper selection of air filter requires a detailed measure of experience and judgement as well as a detailed knowledge of the particular circumstances. For example, a) Viscous impingement filters are suitable as pre-filters and unsuitable for applications where fine dust is a problem. b) The automatic moving curtain filters are most suitable for handling large airflow. c) The electronic filters are suitable for complete filtration of coarse and fine particles and unsuitable for collection of lint and d) Finally the dry type air filters are more versatile regarding possible installations.

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CHAPTER 8

HUMIDIFIERS, DEHUMIDIFIERS AND AIRWASHERS 1. What are humidifier and dehumidifier? Humidifier is the equipment that is used to add moisture to the air. The process of adding moisture to air is known as humidification. Dehumidifier is the equipment that is used to remove moisture from air. The process of removal of moisture from air is known as Dehumidification.

2. Describe various methods of humidificaton. Any one of the four methods can do the humidification. a) Steam injection b) Atomising the water c) Evaporating the water d) by air washing a) STEAM INJECTION: In this method steam is produced in a steam generator and injected into the air just above atmospheric pressure. The steam condenses to a very fine mist as it is dispersed, and evaporates instantly to the gaseous state, raising the R.H. b) ATOMIZING THE WATER: In this method the water is converted into very fine droplets (i.e.) Atomised and directly mixed with air. When the fine droplets are mixed with air they immediately evaporate to the gaseous state, raising the R.H. c) EVAPORATING THE WATER: In this method the water is boiled inside the path of airflow and the evaporated water vapour is mixed with air, raising the R.H. Heating the water with the help of a heater does the evaporation. d) BY AIRWASHING: By simply spraying the water in the path of airflow, the surface layers of the drops are evaporated by absorbing heat from the remaining parts of the drops. This evaporated water vapour is mixed with air, raising the R.H.

3. What are the general considerations for the humidification process? The selection of water quality for humidification is very important as the water introduced into air to raise the R.H affects humidification equipment operation. All humidification systems except steam humidifier consist of an arrangement for exposing water surface either in the form of small droplets or as wet surfaces. The heat required to change the water into vapour is taken from air, which is to be humidified, and cooling of air takes place. These are the general points that must be considered for doing humidification process.

4. Describe the following humidifiers with a neat sketch. a) Steam injection type humidifier f) Pan and coil type humidifier b) Atomisation type humidifier g) Heated water type humidifier c) Impact type humidifier i) Spray type air-washer humidifier d) Hydraulic separation humidifier j) Capillary or cell type humidifier e) Mechanical separation humidifier STEAM INJECTION TYPE HUMIDIFIER: The steam flows from the available source through a strainer and pressure reducer into the outer steam jacket of the distribution manifold. The steam heats the manifold and prevents dirt from reaching the working parts and the pressure reducer regulates the steam at the required pressure for the humidifier. If a central plant steam source is not available, a steam generator must be installed to handle humidification requirements. It may also be necessary to provide each distribution manifold with it’s own electric steam generating capacity Most steam humidifiers are installed in ducted systems to ensure proper mixing. Steam releases it’s latent heat of vaporisation as it condenses.

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Some advantages of steam humidifiers are i) Low initial cost (when the steam is already available) ii) The ability of tolerating greater variety of attitudes. But they require a minimum distance of 1m straight duct to allow the steam to be completely absorbed into air. iii) Low weight and less space requirement. iv) Easily supported without any reinforcing. v) Noise of operation is negligible. vi) Does not carry any harmful impurities. Some disadvantages of the steam humidifiers are i) Can’t be used for humidifying for cold storage applications, since air is heated in steam humidification. ii) The odour from a steam humidification process may be unpleasant to the occupants. b) ATOMIZATION TYPE HUMIDIFIERS: In this type compressed air is used to draw water by aspiration from a supply tank. This water is blown into fine mist into the duct carrying the air to the conditioned space. The compressed air passing through a narrow section of the pipe at a high velocity creates suction, which lifts the water from reservoir and mixes in the air. The mixture of air and mist is passed into the main duct. Then the air from the main duct is sent to the conditioned space. The humidifier does not add heat to room. The heat required for evaporation of water mist is taken from the air causing a decrease in DBT of air. These are available in capacities ranging from 5 to 50 kg/hr. This humidifier is noisy due to the high velocity of air. c) IMPACT TYPE HUMIDIFIER: In this type of humidifier a fine set of water is directed against a hard target at high velocity. Due to this impact the water is converted into a fine spray. The air, which is forced through the chamber, picks up the water by evaporation. The principle of this type is atomising the water. Eliminators are used to avoid the loss of water particles, which are carried along with the air stream. The spray formed in this type is not so fine as formed in the previous type. This humidifier is quiet in operation and the percentage of humidification can be improved by using hot coil surface as targets. d) MECHANICAL SEPARATION HUMIDIFIER : In this type the spray of water is caused due to the centrifugal action given to the water by the disks. The water rises from the reservoir to the disks due to the rotation of the tube. The fan blows the air through the formed spray. So the water is evaporated and carried with air. This type is preferable for small air supply. The main drawback is the requirement of high speed to lift the water up to spraying disks. e) HYDRAULIC SEPARATION HUMIDIFIER: In this type a stream of water is caused to rotate by helical slots in nozzle. Because of this action the stream will break up into a fine spray as a result of it’s own motion. The rate of evaporation depends on the velocity of jet and the air temperature. This type is used in industrial air-conditioning. f) PAN AND COIL TYPE HUMIDIFIER: This type works in the principle of evaporating the water. The heating coil placed in the pan warms the water and the evaporation takes place in the surface. The water vapour coming out from the surface is absorbed into the air . The rate of evaporation depends on the water surface area exposed to air., rate of air flow and mixing motion. A pan and coil type humidifier using steam as a heating medium. This humidifier is simple in operation and maintenance. Close control of R.H is not possible in this case. g) HEATED WATER TYPE HUMIDIFIER: When air is forced through an evaporative medium, which is wasted by water, picks up pure water vapour and gets humidified. The water is brought to the evaporative medium at high temperature (warm conditions). This type is known as heated water type humidifier. h) HEATED AIR TYPE HUMIDIFIER: In the heated air evaporative unit, a rotating evaporative media moves through a reservoir of cold water and the fan and the heating coil are energised. Incoming air flows around the heater to raise the temperature and the heated air then goes through the wetted media and picks up the pure water vapour. The evaporative type humidifiers are used in capacities from 5 to 20 kg/hr.

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i) SPRAY TYPE AIR WASHER HUMIDIFIER: The Principle of this type is Air washing. This is most effective and commonly used type among all. A

small pump supplies the water to the spraying nozzles under high pressure. The non-evaporated water is collected and re-circulated again and again. A float valve in the supply line controls the level of water in the tank. The eliminator plates provided after the sprays remove the water droplets from air. A heater installed before the spray serves to heat both air and spray water. Many different types of humidifiers are in use.

The advantages of this type are

1. By regulating the quantity of water and water temperature the air can be made to leave the humidifier at any desired R.H.

2. As air comes in intimate contact with water the soluble and objectionable gases will be dissolved in water.

3. It is easy in operation and its maintenance cost is considerably low. j) CAPILLARY OR CELL TYPE HUMIDIFIER: In this type capillary cells are used as surfaces instead of spray nozzles. This humidifier provides intimate contact between air and water so it gives high humidifying efficiency of the order of 98 % . This humidifier cleans the air better than the conventional type Glass Fibres are used on the downstream end of the humidifier to act as eliminator. These humidifiers are rarely used because of the cost and size of the equipment.

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5.Discuss the necessity of removal of moisture. In addition to comfort air conditioning Dehumidification (removal of moisture) is necessary for the storage, manufacture and packing of variety of products. 1. Rust and oxidation type corrosion will not occur below 40% R.H. 2. The storage room humidity must be kept low to prevent formation of mold. Hygroscope materials are stored in humidity controlled areas to prevent them from taking moisture. 3. Handling powdery hygroscope products such as flour, Cocoa is practically impossible if moisture content of the air is not regulated.

4. Many packing operations require dry air. If small amount of water vapour is present during the packaging of potato chips, instant coffee etc they will become rancid and stale after a very short period.

5. In electronic industry the moisture causes multitude problems.

6. Describe various methods of Dehumidification. There are three common methods to accomplish dehumidification.

1. By reducing the temperature air below its DPT (Dew point temperature) This is accomplished by passing the air over a cooling coil whose surface temperature is maintained below D.P.T. of air.

2. By absorption of moisture of air. This is accomplished by passing the air through absorption bed. In this method the moisture air does not enter in to chemical combination with the medium through which it is passed.

3. By adsorption of moisture from air: This is accomplished by passing air through a chemical. The moisture in the air enters into chemical combination with drying agent.

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7. Describe the following Dehumidifiers. 1. Refrigeration Dehumidifier (By reducing the air temperature below DPT ) 2. Spray type Dehumidifier 3. Absorption Dehumidifier 4. Adsorption Dehumidifier 5. Gear Heat pump Dehumidifier.

1. REFRIGERATION DEHUMIDIFIER: The cooling coils used for dehumidification are fin and tube type. Either water or brine is used as a cooling medium, depending on the D.P.T. of air. Direct expansion evaporators can also be used. Whenever the D.P.T. of air is below 0 C there is a possibility of forming snow on the surface of the coil. This also reduces the area of airflow, which causes higher-pressure losses in the system.

2. SPRAY TYPE DEHUMIDIFIER: A refrigeration machine is used to cool the water supplied to the spray. No makeup water is necessary in this de-humidifier because the amount of water in the system increases as the moisture condenses from air. An overview pipe is required to drain the added water. The spray type de-humidifier is extensively used in large installations where the duct system carries the air from a control unit to the various rooms being air-conditioned. This system is flexible in operation. Adjusting the flow of refrigerant in the secondary cooling coil can positively control the required temperature of spray as per D.B.T. of air. The sprays also have a cleaning action upon the air.

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3. ABSORPTION DE-HUMIDIFIERS : An absorbent changes chemically or physically when combined with water. For example material like H2SO4 absorbs water and change chemically. In practical use liquid absorbent de- humidifiers are used. It is a very recent development in A/C field. The liquid absorbents, which commonly used are Lithium Bromide and Lithium Chloride solutions. The stronger the concentration and lower the temperature of the solution, more the water vapour that will be absorbed from the air. Lithium Chloride also removes bacteria from air, making it particularly applicable in food and pharmaceutical industries. All incoming air must be filtered before passing it to the liquid absorbers. LITHIUM BROMIDE ABSORPTION SYSTEM: A fan usually on the dry airside draws most air in to the unit. The solution is sprayed a coil. As the air gives up its moisture to the solution, it is simultaneously also cooled by the coolant. Controlling the refrigerant or brine flow can control the temperature of the air coming out of the generator. A small scavenger air stream is used in the regenerator to pick up water vapour from the heated solution and this air exhausted from the plant. A liquid de-humidifier is automatically controlled by a system that maintains the solution level. This unit can be used for continuous de-humidification. CALCIUM CHLORIDE ABSORPTION SYSTEM: The fundamental principle is same as bromide system but the application is slightly different. The Calcium Chloride solution is sprayed or to the cooling coils concurrently with the airflow. The water vapour from the air is absorbed by calcium chloride spray. In addition to this large percentage of sensible heat is released by the air. This heat is removed by passing the air over the cooling coil. This system normally uses a cooling tower. The only disadvantage of this system is the high initial cost.

4. ADSOR PTION DE-HUMIDIFIER : Adsorbing type material takes up the water vapour from the air and holds it without any chemical reaction. Silica gel is a good adsorbing type for water vapour in the air. It is capable of adsorbing and holding about 40 % of it’s own weight of water to the great surface area available through it’s porous structure. The moisture adsorbed by this can be removed by reactivation process. When Silica gel is used in de-humidifying unit two beds are provided one of which is used while the other is dried out. The arrangement of de-humidifying the air using one bed and reactivation of the other bed is shown in diagrams 8.14 and 8.15. The reactivation of the Silica gel is simple. The adsorption process rises the temperature of air leaving the unit to a maximum of about 75 C and lowers D.P.T. The capacity of this de-humidifier ranges from 1 to 3000 m3/min. 5. GEAR HEAT PUMP DE-

HUMIDIFIER : The air is first cooled and then reheated. There is then a possibility of transferring heat from point A to point B in the cycle. The evaporator of the heat pump effectively provides the temperature difference driving the heat transfer across the air to air heat flow. Extra cooling and therefore de-humidifying is provided, compared with evaporator alone. The geared de-humidification process about three times the capacity of conventional of other de-humidifiers.

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8. What is an air washer? Air washer is equipment in which the water is sprayed in the path of air flow. A small pump supplies the water to spray nozzles under high pressure. The sprayed water is collected and re-circulated again and again.

9. Describe the following air-conditioning processes that can be carried out in an air washer.

a) Cooling and humidification b) Cooling and de-humidification c) Heating and humidification. In an air washer, when water is sprayed in the path of airflow several air-conditioning processes can be performed by suitably controlling the following properties.

a) Temperature of air b) Temperature of water c) Quantity of air circulated d) Amount of water sprayed e) R.H. of air

a) COOLING AND HUMIDIFICATION :

The cooling and humidification process is done by keeping the temperature of water equal or less than the temperature of air and above the D.P.T. of air. Under this condition adding the water vapour into the air will do the humidification process and simultaneously the temperature is reduced.

b) COOLING AND DE-HUMIDIFICATION : The cooling and de-humidification process is performed in an air washer by maintaining the temperature of water or brine, sprayed in the path of airflow, less than the D.P.T. of air. The cooling of the water or brine is done using a refrigeration system.

c) HEATING AND HUMIDIFICATION : This is a very simple process in which the temperature of water is maintained above the temperature of air. So the humidification and heating processes are done simultaneously. Here the temperature of water is carefully maintained above the temperature of air using a separate heater.

10. What is adiabatic air washer?

If there is no transfer of energy from water to air or air to water during the process of an air washer then it is called as adiabatic air washer. In an adiabatic air washer the enthalpy of the air remains same both at the inlet and outlet of the air washer.

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CHAPTER 9

CONTROLS AND PROTECTING DEVICES 1. What is the purpose of providing the controls and protecting devices in Ref. & A/C

field? a) To protect the equipment and system from abnormal operating conditions. b) To run the system with maximum efficiency. c) To increase the life of the system. d) To study the performance of the system

2. List all the controls and protecting devices that are used in Ref. & A/C system. a) Solenoid b) Water regulating valve c) Suction pressure regulator d) Pressure cut outs e) Thermostats f) Oil pressure failure switch g) Overload protector h) Pressure relief valves i) Relays

3. State the function, use and the place of installing the following devices. A. SOLENOID VALVE : A solenoid is a coil of wires which when carrying electric current has the characteristics of an

electromagnet. When this coil is attached to the system stem of a valve, a plunger is pulled into the coil providing the desired action of valve. Hence it is called solenoid valve.

Solenoid valve is usually, connected in the liquid line before the expansion device to stop the flow of liquid refrigerant during off cycle period.

B. SUCTION LINE PRESSURE REGULATOR : Suction line pressure regulator is connected to suction side of the compressor and is adjusted

to stop the flow if suction line pressure drops below a desired level. It ensures normal operating pressure range of the compressor. It maintains a constant pressure in the suction line. It is mounted in the suction line. It is mostly used in large capacity systems.

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C. WATER REGULATING VALVE: In the water-regulating valve, the sensing bulb is attached in the water outlet of the condenser. The valve is mounted in the water inlet of the condenser according to the fluctuation in load that is sensed by the sensing bulb with response to the temperature changes in outlet water.

It is used to maintain economical water flow rate of the condenser. It is generally used in large capacities.

D. PRESSURE CUT OUTS: Pressure cut outs are the devices which are used to disconnect the supply to the compressor when the pre-set or safe value of pressure is exceeded. If it senses the suction pressure and correspondingly controls the compressor, it is a low pressure cut out. If it senses the discharge pressure and correspondingly controls the compressor it is a high pressure cut out. If it senses both the suction and discharge pressures it controls the compressor then it is known as dual pressure cut out.

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E. THERMOSTATS :

Thermostats are the controls that are used to increase the efficiency of the system by cut off the electric supply to the compressor after attaining the required temperature in the conditioned space. It is used for economic operation of the compressor and to increase the efficiency of the system. It is installed in the input electric supply line to the compressor and its sensing bulb is kept in the conditioned space. But the bimetal thermostat is kept in conditioned space and it regulates the input electric supply to the compressor.

F. OIL PRESSURE FAILURE SWITCH : Oil pressure failure switch is a switching device to stop the compressor motor. It gives dependable protection against major breakdowns on pressure lubricated refrigeration compressors by guarding against low lubricating oil pressures. It is used to protect the system from the ill effects of wear and tear, friction and breakdown of the system. It is installed and connected with compressor crankcase, outlet of the gear pump and along with input electric supply to the compressor motor by some mechanism.

G. RELAYS : Relays are the equipment used to disconnect the input supply to the starting coil of the motor after the motor reaches the rated speed. In open type compressor motor the centrifugal switches are used. They are connected in the rotating shaft of the motor. In hermetically sealed compressor the electric relays are installed in the input electric supply line to the compressor.

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H. OVERLOAD PROTECTOR : Overload protector disconnects the supply to the compressor while it consumes high current. It is used to protect the compressor from ill effects such as burnout of motor coil, damage of motor etc. It is installed in the input electric supply of the compressor.

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4. Describe the working of the following controls. A. DIRECT ACTING SOLENOID VALVE :

In the direct acting solenoid valve, the valve stem attached to the coil armature controls the main valve port directly. When the circuit is completed to the coil the energised pulls the plunger upward therefore the valve gets open. This allows the fluid to pass through the valve until such time as the circuit is broken to the coil. The plunger will then drop and the valve will rest on the seat, shutting off the flow of gas. Valves that are equipped with manual operation have a knob on the bottom of the valve body. In the direct acting solenoid valve the plunger will operate the valve to close or open the flow path directly.

B. PILOT OPERATED SOLENOID VALVE : In large capacities the solenoid valve used are pilot operated. In this type the coil armature controls only the pilot port rather than the main valve port. In this type, the plunger is just modified as shown in the Figure. The operations are as same as direct acting solenoid valve. When the circuit is completed the energised coil pulls the plunger upward and the valve gets opened. This allows the fluid to pass through the valve until the circuit is broken. The plunger will then drop and the valve will rest on the seat, shutting off the flow of fluid. C. LOW PRESSURE CUTOUT: This type of pressure cut-outs are connected with the suction line of the compressor. The suction vapour is introduced into the bellow. Due to the low pressure the bellow will be contracted. Therefore the contact is opened and the supply to the motor will be cut. Thus the low pressure cut out safeguards the compressor from abnormal low pressure operating conditions.

D. HIGH PRESSURE CUTOUT : High pressure cut out is shown in the figure. The high pressure cut out is connected to the discharge line of the compressor. Thus the discharge vapour is introduced into the bellow of the cut out. At abnormal operating conditions, due to high pressure, the bellow gets expansion. The contacts are opened by the expansion of the bellow. Therefore the electric supply to the compressor will be cut.

E. DUAL PRESSURE CUT OUT :

The dual pressure cut out is shown in the figure. It is installed between the suction and discharge line. Here both low pressure and high-pressure cut outs are combined together. When there is an abnormal condition in either suction line side or discharge line side. It will disconnect the supply to the compressor. If a diaphragm replaces the bellows, this type of pressure cut out is known as diaphragm type pressure cut out. Principle and other operations are same as bellow type.

F. SUCTION PRESSURE REGULATOR : It is mounted at the outlet of the evaporator. It regulates the pressure of the refrigerant to the compressor at constant rate. In large capacities the fluctuation in load will be more. At the beginning of the system there will be maximum load. At that condition large amount of vapour will be delivered from evaporator. Therefore the suction line pressure will increase. At minimum loads, low quantity of vapour will be delivered from evaporator. So the suction pressure will decrease. At maximum load, the compressor consumes high power. To prevent the fluctuations we use a suction pressure regulator. The construction of the regulator is as shown in figure. The regulator has a spring at a constant tension. It controls the valve. By means of the vapour pressure, the valve closes or opens. At the rate pressure the valve will open and release the vapour till the pressure returns below the desired level.

G. CONDENSOR WATER REGULATING VALVE :

A water-regulating valve automatically controls the water flow rate through a water-cooled condenser on a waste water system. The valve is installed on the water line at the inlet of the condenser. When the compressor is in operation the valve acts to modulate the flow of water through the condenser in response to the changes in the condensing pressure. An increase in the condensing pressure tends to collapse the bellows further and open the valve wider against the tension of the range spring, thereby increasing the water flow rate through the condenser. Likewise as the condensing pressure decreases the valve moves toward the close position. So that the flow rate through the condenser is reduced accordingly. When the compressor cycles off the water, valve remains open and water continues to flow through the condenser until the pressure in the condenser is predetermined minimum, at which time the valve closes off completely and shuts off the water flow.

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When the compressor cycles on again the water valve remains closed until the pressure in the condenser builds up to the opening pressure at which time the valve opens and permits water to flow through the condenser. The opening pressure of the valve is approximately 50 K Pa above the shut off pressure.

H. BIMETAL THERMOSTAT : The bimetal strip is made up of two dissimilar metals (usually invar and brass or invar and steel) bonded into a flat strip. Invar is an alloy, which has a very low coefficient of expansion. The change in length of the invar per degree of temperature change will always be less than that of the brass or steel. Increasing the temperature of the bimetal element causes the bimetal to warp in the direction of the invar is left side, where as decrease in the temperature of the bimetal causes the bimetal to warp in the direction of the brass or steel. This change in the configuration of the bimetal with the changes in temperature can be utilised directly or indirectly to open and close electrical contacts or to actuate other compensating mechanisms.

I. DIAPHRAGM TYPE THERMOSTAT : In this diaphragm type thermostat the sensing bulb which is filled with a gas, a liquid or a saturated mixture of the two is connected by a tube with a diaphragm. Increasing the temperature of the bulb or tube increases the pressure of the confined fluid, which acts through the diaphragm, and the system of levers to close electrical contacts or to actuate other compensating mechanisms. Decreasing the temperature of the tube will have the opposite effect. J. BELLOW TYPE THERMOSTAT: In this bellow type thermostat a tube with a bellow connects the sensing tube or bulb, which is filled with a gas, a liquid or a saturated mixture of the two. Increasing the temperature of the bulb or tube increases the pressure of the confined fluid that acts through the bellow and the system of levers to close electrical contacts or to actuate other compensating mechanisms. Decreasing the temperature of the bulb will have the opposite effect. K. HOT WIRE RELAY: The hot wire relay depends on the heating effect of the high starting current to cause the thermal expansion of a special alloy wire, which in turn acts to open the starting contacts and remove the starting winding from the circuit as shown in figure. The hot wire relay contains two set contacts “S” and “R” which are in series with the starting and running windings respectively. Both sets of contacts are closed at the instant of starting. Hence both the windings are connected to the line. The high starting current heats the wire and causes it to expand sufficiently to pull contact “S” open and remove the starting winding from the circuit. After the starting winding is out of the circuit the normal current through the running coil will generate enough heat to maintain “S” contacts in the open position but not enough to cause additional expansion of wire and open contacts “R”. However if for any reason the motor draws a sustained over current, the wire will expand further and pull open contact “R” removing the running winding from the circuit contact. “R” is actually a overload contact which acts as overload protection for the motor. The mechanical arrangement of the two sets of contacts is such that the contacts “R” cannot open without opening contact “S”. Since the action of hot wire relay depends on the amount of current flow through the alloy wire these relays must be sized to fit the current characteristics of the motor. They are best applied to the split phase type motor. L. CURRENT COIL RELAY: The current coil relay is used primarily with capacitor start motors. It is a magnetic type relay and is actuated by the change in the current flow in the running winding during the starting and running periods. The coil of the relay, which is made of large wire, is connected in series with the running winding. The relay contacts, which are normally, open one connected in series with starting winding. When the motor is energised the high locked rotor current passing through the running winding and through the relay coil produces a relatively strong magnet around the coil and causes the relay armature to pull in and close the starting contacts energising the starting winding. With the starting winding energised the motor begins to rotate and a counter e.m.f. is induced in the stator winding which opposes the line voltage and reduces the current through the windings and relay coil. As the current flows through the relay coil, the coil field becomes too weak to hold the armature, where upon the armature fall out of the coil field by starting contacts. The motor then runs on the running winding alone. M. POTENTIAL RELAY: Potential or voltage coil relays are employed with capacitor start and run motors. The potential relay differs from the current coil relay in that the coil is wound with many turns of small wire and is connected in parallel with the starting winding rather than in series with the starting capacitor and are closed when the motor is not running. When the motor is energised both the starting and running windings are in the circuit.

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As the motor starts and comes up to speed the voltage in the starting winding increases to a value considerably above that of the line voltage as a result of the action of capacitor in series with this winding. N. OIL PRESSURE FAILURE SWITCH: The function of the oil pressure failure switch is to cycle the compressor off. When the useful oil pressure developed by the oil pump falls below a predetermined minimum or in that event that the oil pressure fails to build up to a minimum safe level within a predetermined time interval after the compressor is started. There are two pressure bellows opposed to each other. One is connected to the crank case and reflects crankcase pressure whereas the other is connected to the discharge of the oil pump and reflects total oil pressure. The pressure differential between the two bellows is equal to useful oil pressure and is utilised to actuate the differential switch of the oil pressure failure control. A time delay incorporated to the oil pressure failure control allows the compressor to operate 90 to 120 seconds with the oil pressure below the safe level. This permits the compressor to start with zero oil pressure and also prevents unnecessary shut down of the compressor in the event of that the oil pressure momentarily falls below the minimum limit. The compressor can be restarted when the oil pressure failure control is reset manually. The resistor in series with relay heater limits the current flow through the heater and makes the oil pressure failure control adaptable to both 115 V and 230 V control circuits. Since the oil pump operates only when the compressor starting the total oil pressure will be exactly equal to the crankcase pressure during the compressor off cycle, so that both the timing relay heater and holding coil are energised when the compressor is started. Continued operation of the relay heater will cause the bimetal of the timing contacts. This breaks the holding coil circuit and stops the compressor. If the useful oil pressure falls below the cut-out point of the switch while the compressor is operating, the differential pressure switch closes and energises the relay heater. The continued operation of the heater will open the timing switch and stop the compressor. O. OVERLOAD PROTECTOR: It is important to recognise that line fuses and circuit breakers are designed to protect the circuit only and do not provide over current protection for the motor. Therefore unless the motor is equipped with a built in thermal overload protection a separate protection must be provided in the circuit of each motor. To satisfy the need for over current protection many magnetic motor starters are equipped with overload relays. The overload protector consists of essentially two parts: i) a heater element installed in the motor cycle and ii) a set of contacts installed in the holding coil circuit. In that even the motor is subjected to sustain over current. The temperature of the heater element increases above normal and the excess heat given off by the heater causes warping of a bimetal element which opens the overload contacts in the holding coil circuit. This de-energises the motor from the power source.

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5. What is the basic principle of a control system? The control system is considered as the brain of A/C plant. Since the successful and economical working of the plant depends upon the efficiency of the control system. Control systems are used to control the temperature, humidity and pressure. This is accomplished by the controls of the dampers, regulating the quantity of steam (i.e.) hot H2O supplied to the heater and the quantity of chilled H2O to the cooling coil or spray chamber.

The required conditions in the A/C space and successful operations of the equipment used in A/C systems are dependent on the control system.

The control system feels or senses variation from desired setting and then transmits signals to operate valves, dampers, motor and other controlling devices, which provide necessary corrective action.

6. Describe the various components of a control system using line sketch.

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The basic elements of a control system are shown in the figure. The common elements are controlling elements, actuating element, limit control system and equipment to be controlled. The basic components are as follows: i) CONTROLLER :

It consists of three elements Sensing element:

This element is used to sense the controlled property such as humidity, temperature, purity, noise, vibration, quantity and velocity.

Controlling element: The sensed property is transferred to the controlling element such as diaphragm, bellow and amplifier. Pre-set value: It is the value of our requirement in the sensed property.

The controller compares the sensed property with pre-set value and according to that, it transfers a signal to actuator by a transmission system. ii) ACTUATOR :

The actuator makes necessary action according to the signal received. The actuator may be bellow, diaphragm, servomotor, relay, contacts or any mechanism.

iii) CONTROLLED DEVICE : It is the device, which is to be controlled. It may be a cooling coil or heating coil or humidifier or de-

humidifier, air cleaner or damper or a valve.

iv) CONTROLLED MEDIUM : The medium, which is to be controlled, is controlled medium. It may be air or water or stream or brine

or electric current.

v) CONTROLLED PROPERTY : The controlled property may be humidity or temperature or purity or noise or vibration or quantity or

velocity. The controlled property is nothing but the sensed property.

vi) TRANSMISSION SYSTEM : The mechanism, which is used to transmit the signal from controller to actuator, is called as

transmission system.

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7. Describe various sensing elements.

i) BIMETAL STRIPS : Combining two different metals having different coefficients of thermal expansion makes Bimetal

strips.

The bimetal, which commonly used, are invar and brass. Former has low coefficient of thermal expansion while later has high coefficient of thermal expansion. According to the variation in temperature it produces a signal in deflection.

ii) SEALED BELLOW TYPE THERMOSTAT: The low boiling point liquid present in the sensing bulb evaporates according to the temperature in the

outlet of the A/C space and the vapour expands or contracts the bellow and opens or closes the contacts.

Instead of bellow we can also use a diaphragm.

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iii) ELECTRIC RESISTANCE TYPE : The change of electrical resistance of a material with temperature is used to operate the actuating

element, which controls the equipment in the air conditioning system which is responsible for the temperature rise or fall. This property can be used to control the temperature within moderate and satisfactory limits of fluctuations. Arrangement is shown in figure.

iv) THERMO COUPLE : A set of two wires of dissimilar metals held together at one end and free to other end is known as

thermocouple. If the held junction is subjected to higher or lower temperature than the free end, the emf is developed in the circuit as a result of temperature difference. This property is used to operate the regulatory element as magnetic coil or motor.

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v) ELECTRONIC SENSING ELEMENT : This is a quite concept in a design of temperature regulator. These controllers use an electronic

amplifier, which is a device for increasing the magnitude of weak signal coming from temperature sensing element. The increased signal is used through suitable electric apparatus to operate valves or dampers. By proper amplification of weak signal small change in temperature can be controlled. This arrangement of amplification tends to cause shorter cycling, less overshooting and gives more satisfactory results. With the addition of feedback potentiometer an electronic thermostat can be used for proportional operation of valves of dampers.

8. Describe various humidity sensing elements. i) HAIR TYPE HUMIDOSTAT :

Human hair or animal membrane is used as water vapour absorbing material from air. The quantity of water vapour is dependent upon the R.H of the air. The human hair is suitably connected to actuate indicating or recording instrument. This type of humidostat is useful up to 60 oC and 15 to 95 % R.H. ranges.

ii) ELECTROLYTIC WATER ANALYZER : In this type of humidostat a measured quantity of air is passed through a cell of hygroscope material

such as silica gel, phosphorous pentoxide or calcium chloride. Moisture is absorbed quantitatively and continuously decomposed by electrolysis between platinum electrodes.

According to the Faraday’s law the decomposition of current is directly proportional to the rate of decomposition of water and of moisture content in sample at known flow rate. This type is generally used for low level of moisture below 1000 ppm.

iii) WATER VAPOUR RECORDER : In this type of humidostat air stream is divided into two equal parts, one of which is thoroughly dried.

The dried and moist air streams are passed through two absorbers. The temperature difference between absorbers is detected by thermopile and measured by peak to peak voltmeter.

This is generally used for low level moisture streams.

iv) AUTOMATIC DEW POINT RECORDER :

The places where low amount of moisture content is present must be determined and especially where an absolute humidity measurement, independent of temperature is desired, hygrometers and wet and dry bulb thermometers are not so well suited. The general electric dew point measuring instrument uses a mirror surface, which is cooked, to a temperature at which a layer of dew first forms. The temperature at which this occurs is recorded and corresponds to the dew point temperature. Instrument consists of four components.

A two stage refrigeration system to maintain temperature –70 C.

A gas chamber with dual photoelectric system.

An electronic power unit and amplifier.

A temperature recorder calibrated in dew point.

The refrigeration system can maintain the temperature of –70 C or less in the space behind the mirror. While the cooling rate remains constant, it is possible to raise the temperature in the space behind the mirror by means of a heater as shown in figure. The heater mirror assembly has a low heat capacity and hence a low thermal inertia, therefore it gives fast response.

A gas flow rate of 2 – 4 cm3 per minute is continuously passed through the gas chamber to provide

for the formation of dew on the mirror, which is detected by the photo-tube viewing system. Two ports at an angle to the mirror surface allow a beam of light from the small light source to enter and leave the chamber.

The entering beam reflects from the mirror surface and leaves the chamber to be measured by a photo-tube. The presence of dew on the mirror reduces the intensity of the reflected light by an amount proportional to the size of the dew spot.

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If the dew spot tends to grow, the reflected light intensity drops and the electronic circuit supplies more power to the heater preventing this growth by raising the temperature of the mirror. If the dew spot tends to diminish in size the reflected light is increased and less power is supplied to the heater. A thermo-couple is mounted directly on the mirror and a temperature recorder reads the dew point temperature.

v) DUN- MORE CELL :

In this type of humidostat, a double winding on insulator is coated with hygroscope salt (usually lithium chloride) which becomes more conductive as its equilibrium moisture content increases. Ambient humidity determines conductivity of coating and governs current flow between wires. Compensatory is required if it is used for varying temperature.

9. Describe the construction and working of the following transmission system. TWO POSITION CONTROL PNEUMATIC SYSTEM : The two- position is for normal condition in an air-conditioned space. Whenever the temperature or humidity increases above or falls below the predetermined value the sensing element operates the flapper and closes the nozzle, the pressure of the air supplied to the actuator increases to actuate. TWO POSITION ELECTRICAL CONTROL SYSTEM: The simple and most commonly used electrical controller is a two-position thermostat with bimetallic element operating a solenoid coil actuator. The arrangement of the system is shown in the figure. Whenever the temperature or humidity increases above or falls below the predetermined value, the sensing element makes the contact and completes the electrical circuit through solenoid coil, which energises and operates the actuator arm for controlling the equipment. PROPORTIONAL PNEUMATIC CONTROL SYSTEM: The proportional control of the damper or valve is achieved by the insertion of pilot valve in the circuit diagram. The pilot valve in the circuit is operated by means of bellows under control of low-pressure air permit rapid changes in air pressure going to actuator. When the flapper closes the nozzle, the diaphragm expands and moves the valve port to the left. The supply airline is closed and relatively leak port is opened more giving a rapid drop in control pressure. With the nozzle opening the leak port is closed and full supply is given to the actuator. The above mentioned arrangement gives narrow operation range which can be modified with insertion of feedback bellows to provide full range proportional control. The arrangement with feedback bellow is shown in figure. As the output pressure changes with the change of flapper position, the feedback bellow expands or contracts to move the flapper in the opposite direction.

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PROPORTIONAL ELECTRICAL CONTROL SYSTEM:

The input supply is given through a resistance on the resistor a pointer is placed. That pointer is hinged at one end and the pointer is attached with the sensing element. According to the sensed property the point will be actuated. Thus according to that actuation the resistance is varied by this system.

10. Describe the following control systems using neat diagrams.

A) PREHEATING AND HUDIFICATION CONTROL SYSTEM

The arrangement of the system is shown in the fig. The latent heat load or sensible heat load changes in the room. The temperature or humidity change in the room will be corrected by correcting the steam flow in the heating coil or spray quantity in the humidifier, which are controlled by the thermostat and humidostat as shown in the fig. Actuators operate the valves after getting the signals from the thermostat and humidostat.

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B) COOLING, DE-HUMIDIFICATION AND REHEAT CONTROL SYSTEM

The system arrangements are shown in the fig. The sensible heat load or the latent heat load changes in the room. Then the temperature or humidity change in the room will be corrected by correcting the quantity of chilled water flow through the cooling coil and the de-humidifying coil and also correcting the quantity of steam flow through the heating coil. The flow of both fluids is re-circulated by thermostat and humidostat, which are placed in the re-circulated air duct as shown in the fig. The actuators A operate the valves after getting the signals from humidostat and thermostat. A separate thermostat as well as humidostat as shown in the fig controls the temperature and humidity of air, coming out of the cooling coil and dehumidifying coil.

C) ALL YEAR ROUND AIR-CONDITIONING CONTROL SYSTEM The system arrangement is shown in the fig. The amount of outdoor and re-circulated air is controlled by the thermostats C. These are designed in such a way that when the outdoor air temperature is either above or below a certain selected value, the amount of outdoor air is changed by the damper motor until a minimum amount as set by the air selector is reached.

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D) FACE AND BY PASS CONTROL SYSTEM The arrangement of the system is shown in the fig. Say the outside conditions are fixed and room load conditions are changed. As the sensible heat load in the room is changed (decreased) the temperature of the air leaving the room (temperature of re-circulated air) will increase. Then the motor starts through the sensing element, and increases the bye pass by increasing the opening of the dampers and decreases the flow quantity through the face dampers. It is also possible to arrange the system in such a manner when the dampers close completely the heating medium is shut off by the operation of the motorised valve. This system can be effectively used for wide range of conditions.

E) ZONE CONTROL SYSTEM. Zoning is the division of a building in to separately controlled sections by providing partitions. Zone control system is used in a building, which possesses a number of exterior aspects of some considerable area, each of which is normally affected differently according its exposure to atmospheric changes.

Changes in the direction of sun, wind and rain result in varying temperatures of rooms. The heat input to these rooms should be different as per the conditions. This is done by sectionalising the “circulating mains”, as shown in the fig. This is to enable each group of rooms to be served by a separate circuit. Its respective thermostat controls a regulating valve, which is introduced in each section. A separate thermostat is used to control the heat given by the boiler according to the steam requirement form the boiler.

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11. Define the following term. Also give suitable examples. A) Controlled Device:

It is the device, which is to be controlled by the controlling system according to the requirement of the Air conditioning system. The controlled device may be the following.

1. Cooling coil. 2. Heating coil. 3. Humidifier. 4. Dehumidifier. 5. Air cleaner. 6. Damper. 7. Valve etc.

B) Controlled Variable : The controlled variable is the medium, which is to be regulated by the controlled device according to the sensed property. The controlled variables may be the following

1. Air 2. Water 3. Steam 4. Brine 5. Electric Current etc.

C). Controlled Property: The controlled property is nothing but the property of controlled medium, which is sensed by the element it may be

1. Humidity. 2. Temperature. 3. Purity. 4. Noise. 5. Vibration. 6. Quantity. 7. Velocity.

D). Sensing Element:

It is nothing but the integral part of the controller, which is used to sense the property of the out coming variable from all space. It may

1. Humidity sensing element. 2. Temperature sensing element. 3. Purity sensing element. 4. Noise sensing element. 5. Vibration sensing element. 6. Velocity sensing element.

E). Controlling Element:

The sensed property is transferred controlling element by comparing the present value. The controlling element is a device, which is transferring the signal absorbed from sensing element to the actuator. The controlling element may be

1. Bellow. 2. Diaphragm. 3. Amplifier.

F). Controller:

Controller is the main part of the controlling system, which compares the sensed property with pre-set value, and according to that it produces a signal by the controlling element. It consists of sensing element, Pre-set value and controlling element.

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G). Pre-set value :

This is our required value of controlled property. It is compared with the sensed property by the controller. The pre-set value may be

1. Temperature. 2. Humidity. 3. Purity. 4. Noise. 5. Quantity. 6. Velocity etc

H). Transmission System:

This is the signal carrier of a control system from controller to the Actuator.

It may be a

1. Two - position pneumatic control Transmission system. 2. Two- position electrical control Transmission system. 3. Proportional control pneumatic Transmission system. 4. Proportional control electrical Transmission system.

I). Actuator:

Actuator is the actuating element, which is actuating according to the signal produced by the controlling element. Actuator may be

1. Bellow. 2. Diaphragm. 3. Servomotor. 4. Relay. 5. Contacts and any other mechanism.

12. List the general considerations for selecting the control system.

1. The controlled device. 2. Controlled medium. 3. Controlled property. 4. Variation in the load of the controlled device. 5. Variations in the load of the surroundings 6. Requirement of the system or pre-set value to meet the comfort to occupants.

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ACR PPT SLIDES R&A/C MACHINES

J.ILANGUMARAN

3/6/2009

j.ilangumaran