Fundamentals of Reffregeration

1-Fundamentals of Reffregeration1-1ApplicationPreservation of foods/slightly above Freezing tempAir conditioningBeverage cooling & Humidity controlIndustrial process-Nitrogen plant, BT200Research 1-2First Law of thermodynamicheat and mechanical energy are mutually converted work is convertible to heat

11-3Sensible HeatHeat which causes a change in temperatureIf heat is added and temperature rises as heat is added1-4Latent heatHeat added to or removed from a substance during a change of state, but no change in temperature Latent heat of fusion Latent heat of condensation Latent heat of vaporization Latent heat of sublimation

1-5Application of Latent heatAs ice melts, its temperature remains constant. Nevertheless, it absorbs a considerable amount of heat in changing from ice to water. When a substance passes from a liquid to a vapour, its ability to absorb heat is very high. The temperature at which a substance changes its state depends on pressure. The higher the pressure, the higher the temperature needed to bring about the change. Conversely, if pressure lowered, the temperature is lowered.A liquid under low pressure will boil at a lower temp. If the vapour resulting from this boiling is then compressed, it will condense back in to a liquid at a higher Temp. Each substance has a different latent heat value.

In the evaporator, the pressure is greatly reduced. The refrigerant boils, absorbing heat from the evaporator. This produce low temperature and cools the evaporator. The compressor pumps this vaporized refrigerant out of the evaporator and compresses it in to the condenser. Here the heat that was absorbed is released to the surrounding. Having lost this heat of vaporization, the refrigerant becomes a liquid again. The cycle is then repeated.

2-Refrigeration systems and classification2-1PrinciplesAs a liquid changes to a vapour, it absorbs large quantity of heat the converse is true changing from vapour to liquid.The boiling point of a liquid can be changed by changing the pressure exerted on the liquid. In other words, the boiling point of a liquid can be raised by increasing its pressure and it can be lowered by reducing its pressure.2-2ClassificationEvaporation (Mechanical)Absorption (gas)Thermo-electric /peltier effect2-3According to useHigh-temperature system(25-45C) Air conditioningMedium-temperature system(25-0C) Food storageLow-temperature system(0 and below)

3-Components of refrigeration system3-1CompressorHeart of vapour-compression system Reduce evaporator pressure Increase condenser pressure(causing rise in temp) Refrigerant circulation

Compressor typesPositive displacementReciprocating/ piston typeRotaryScrollScrewdynamicTurbo/centrifugal fieldFurther classificationOpen/shaft seal to protect Ref. leakageHermetic/weldedSemi- hermetic13

3-2The electric windingDirect/Indirect contact with refrigerant vapour and oilCooled by the Refrigerant, oil or external meansIf the electric motor burns occurred(corrosive products), all the system should be cleaned before new motor installed.Oil in the refrigeration system is not desirable.Different designs are available for oil free compression for different refrigerants. According to solubility of oil in the refrigerant and density

3-3EvaporatorsBare tube evaporator/simplestPlate- surface evaporator/easy to manufactureFinned evaporator/increased surface area efficiently

3-4CondensersMounted at the back of cabinet/should not be obstructedAir cooled/natural or forcedWater cooledEvaporative3-5AccumulatorsSafety device to prevent liquid refrigerant from flowing in to the suction line and the compressor.If it flows in to the compressor it will cause knocking and damage the compressor3-6Liquid receivesStorage tank to reserve liquid Refrigerant.Insures enough liquid refrigerant in liquid line free of gas.Mostly used in systems with the low side float type or the expansion valve type.3-7Electric motors for refrigeration systemsFor most fractional horse power appliance, split phase induction motor is used.Two stator winding is used, one for starting and one for running.The starting windings are disconnected when approximately 75% of its running speed.No rotor winding/sqirrel cage windingCapacitor start induction motorCapacitor in starting winding/in seriesHigh starting torque

3-8Motor starting device and protection 3-8-1 Starting Limit current & produce torque Starting relays(outside of the hermetic compressor) 1-Magnetic -current(amperage) relays -potential(voltage) relays 2-PTC positive temperature coefficient 3-Thermal Relay 4-solid state(electronic) Relay24PTCStart winding gets current through PTCWhen the device is cold it has low resistance and current can flow.The PTC heats up very quickly and its resistance gets very high. The starting winding now disconnected.

3-8-2Protection

Bimetal switch(Located at important place in the Unit) Excessive current draw Excessive over-heating(compressor) -poor air circulation -poor refrigerant circulation -low level refrigerant -friction

3-9Refrigeration control -operation control-temperature & humidity -Refrigerant control- rate of flow in evaporator 3-9-1Operation control

1-pressure control 2-Temperature control(thermostat) 3-Thermostat -sensing bulb(remote bulb) -Bimetals -solid state(PTC or NTC)

1-Remote bulb control

The bulb filled with a liquid that is easily evaporated connected by capillary tub with the bellow.The screw adjustment-spring pressure. -cut-in and cut-out temp. -differential temp service tec.

2-Pressure control

As the temperature at the evaporator (low side) warms, the pressure increase and hence the below pressure. (Switch is closed) As the temperature at the evaporator (low side) low, the pressure decrease and hence the below pressure. (Switch is open)

3-9-2Refrigerant control(throttling)

Rapid expansion of refrigerantThe Function is to control rate of flow from liquid line to evaporator and to maintain pressure difference.Types of throttling valve - hand expansion valve -capillary tube -Thermostatic expansion valve(TEV) -Low pressure side float -High pressure side float -Thermal-electric expansion valve -Automatic expansion valve

1-Capillary tubesSmall diameter tube connected directly b/n condenser and evaporator.Principle restriction in fluid flow line pressure goes down as the refrigerant moves through the tube until the liquid starts to evaporate. sudden pressure and temperature drop at last quarter.AdvantageNo moving part-no wear or stick/ simple designPressure balance when the unit stopsMinimum starting torque

2-The hand expansion valve

Screw to control size of orifice Used only for large refrigeration systemThe operator should look after the plant the whole time.

3-Thermostatic expansion valve (TEV)

PrincipleConstant super heat temperature at the suction line end of evaporator.Pressure balance b/n bulb pressure(Diaphragm) -Evaporator pressure -spring pressure

4-Refrigerant and Dryers4-1IntroductionPrimary refrigerant-phase changeSecondary refrigerant-No phase change

Qualities - Boils at low temperature-except water - Non flammable -Non explosive -Not be poisonous -Flow easily -Able to conduct well

Requirement Non Toxic, Non poisonous Non Explosive Non corrosive Non Flammable Leaks easily detected, located Operate under low pressure/low boiling point stable gas moving part in the fluid be easily lubricatedPressure-Temperature curve Refrigerant in state of equilibrium Reading the pressure at any particular Temperature Example R12 at 100F(38C) is 117psi(909kpa) The temperature and pressure value for the condensing are always higher than the equilibrium.

Grouping and classification of refrigerant national refrigertion safety code(NRSC) Group one/ safest Group two/ Toxic and somewhat flammable Group three/ Flammable

Group one Refrigerants Allowable quantities up to 20lb hospital and residntial AC systems up to 50lb public assembly(indirect)/residential in precaution R-11 R-12 R-22 R-500/73.8% R12 and 26.2% R-152a R502/48.8% R-22 and 51.2% R-115 R503/40.1% R-23 and 59.9% R-13 R744/CO R-22 colourless, odourless liquid at-29C(bp) & atmospheric pressure non toxic, non irritating, non corosive, non flammable inert, thermally stable to above 427CR-22 synthetic, stable, non toxic, noncorrosive, non irritating, non flammable(-41C bp) application is in fast freezing unit(-20 to -40C)R-11 synthetic, stable, non flammable, non toxic used as flushing agent for cleaning when overhauling system after system has had motor burnout or great deal of moisture Group two toxic, irritating to breath, somewhat flammable R-717 Ammonia R-40 Methyl chloride R-764 Sulphur DioxideAmmonia commonly used in industrial systems colourless gas(bp=-33C) at atmospheric pressure low bp makes it possible to have refrigeration without using pressure in evaporator. caution in operating amonia valve/couse instant loss of consciousness and damage the eye(wwear mask) R-717 attacks copper and bronze, does not corrode steel or iron lighter than oil/ separation is easy excess oil easily removed from evaporator by opening the valve bottom Group three may form combustible mixture when mixed with air R-600 Butane R-170 Ethane R-290 propane

Water as a refrigerant the latent heat of water at 100C is 2260kj/kg(effective) b/p=100CRefrigerant applications Low temperature -126C R-14 -99 C R-503 -82 C R-13 Medium Temperature-46 C R-502 -30 C R-12 High Temperature 4 C R-114 48 C R-113 28 C R-123 24 C R-11 Type of compressor centrifugal Rotary ReciprocatingRefrigeration oil - circulate with the refrigerant/mixed -Lubricate and cool compressor moving parts -Direct contact with motor winding -must withstand extreme temperatures/ high, low -harmless to refrigerant and equipment -fluidity of oil-refrigerant mixture determined by Refrigerant used temperature properties of the oil solubility of refrigerant solubility of oilproperties of Good refrigerant oil Low wax content Good thermal stability/hard carbon deposit Good chemical stability Low pour point/ability to remain fluid at lowest temperature. low viscosityIn addition manufacturers add chemicals to inhibit the formation of sludge or foaming/ oil removed from system Should be clear/no discolouration/ if this happens, place new dryer.Dirty oil may be acidic and will burn the handsContainers must be kept tightly sealed4-2Halocarbon compounds(chlorine, fluorine,bromine)R-11 Trichluoro monofluro methane CClFR-12 Dichloro difluoro methane CClFR-13 Monochloro trifluoro methane CClFR-22 Monochloro difluoro methane CHClFR-40 Methyl chloride CHClR-113 Trichloro trifluro ethane CClFCClFR-114 Dichloro tetra fluro ethane CClFCClF

4-3Inorganic refrigerants

R-717 Ammonia NHR-718 Water HOR-729 Air R-740 Carbon dioxide COR-764 Sulphur dioxide SO

Hydro chloro fluorocarbons HClFCHydro fluoro carbons HFC 4-4Environmental Impact

-Ozone depletion potential(ODP)-Global warming potential(GWP)-Cancer

4-5Moiler diagram and thermodynamic properties

Pressure- enthalpyTemperature- enthalpy

Sub-cooled liquid saturated liquid line Mixture saturated vapour line Superheated vapourCooling loadRefrigerating rate4-6Coefficient of performance(COP) The measure of efficiency of refrigeration cycle Heat absorbed from space Heat supplied to compressor

C.O.P.= Refrigerating effect Heat of compression4-7Refrigerant and moistureMoisture Acid corrosion sludge Blockage

Driers Liquid line Driers Suction line Driers

Liquid line driers parts - course filter - Felt pad -De hydrating material(desiccant) Types of desiccants Absorption/chemically react/for short time use Adsorption/chemically stick-silicon jell -activated alumina

5-Refrigeration Materials and hand tools1-Piping an tubing2-welded and brazed tubing seamless copper tubing/except ammonia/good for Freon - aluminum tubing/Evaporator in domestic refrig. - stainless tubing/ Condenser3-Tubing cutters- Hand Held tubing cutter - Hack saw and sawing fixture4-Tubing Reamers5-Pinch-off tools6-Tubing benders

7-Pressure Gauge8-Valve Manifolds9-Charging Hose10-Pocket thermometer11-Welding Unit12-Charging Cylinders13-Fitting Brush14- Flaring

6-Servicing of Refrigeration