Air Compressors

IV Semester ME 404 Applied Thermodynamics Module II

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DEPT. MECH. ENGG, CE ADOOR

Air Compressors

In many industrial purposes high pressure gases or air are required. A

machine providing gas at high pressure is called a compressor and work

must be done upon the gas by an external agency. An air compressor takes

in air at atmospheric pressure, compresses it at the cost of the work

supplied, and delivers the high pressure air to a storage vessel called

receiver from which it may be conveyed by the pipe line to a place where the

supply of compressed air is required.

Classification of Air Compressors

The compressors are broadly classified as

Positive displacement type

Roto-dynamic type

In positive displacement type compressors, compression is achieved by

trapping air into an enclosed space and then reducing its volume. Since a

fixed amount of air is trapped each time, its pressure rises as its volume is

reduced. When the pressure rises to the required level, then it is expelled

from the enclosed space and a fresh charge of low-pressure air is drawn in

and the cycle continues. Since the flow of air to the compressor is not

steady, the positive displacement type compressor is a pulsating flow device.

However, since the operating speeds are normally very high the flow appears

to be almost steady on macroscopic time scale. Since the flow is pulsating

on a microscopic time scale, positive displacement type compressors are

prone to high wear, vibration and noise level. Depending upon the

construction, positive displacement type compressors can be classified into:

i. Reciprocating type

ii. Rotary type with sliding vanes (rolling piston type or multiple vane

type)

iii. Rotary screw type (single screw or twin-screw type)

In roto-dynamic compressors, the pressure rise of air is achieved by

imparting kinetic energy to a steadily flowing stream of air by a rotating

mechanical element and then converting into pressure as the air flows

through a diverging passage. Unlike positive displacement type, the roto-

dynamic type compressors are steady flow devices, hence are subjected to

less wear and vibration. Depending upon the construction, roto-dynamic

type compressors can be classified into:

i. Radial flow type, or

ii. Axial flow type

Centrifugal compressors (also known as turbo-compressors) are radial flow

type, roto-dynamic compressors.


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Reciprocating compressors

Reciprocating air compressors compresses air in a cylinder by reciprocating

motion of piston and supplies high pressure air with intermittent discharge.

Rotary compressors have compressed air at low pressure but with

continuous and high discharge. Reciprocating air compressors may be

single acting or double acting.

Single Stage Air Compressor

Figure given below shows the schematic of a single stage reciprocating air

compressor.

Reciprocating compressors consist of a piston moving back and forth in a cylinder, with suction and discharge valves to achieve suction and compression of air. Its construction and working are somewhat similar to a two-stroke engine, as suction and compression of air are completed in one revolution of the crank. Reciprocating air compressors are positive displacement machines, meaning that they increase the pressure of the air by reducing its volume. This means they are taking in successive volumes of air which is confined within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by a piston

within a cylinder as the compressing and displacing element. Crank is coupled to the prime mover or electric motor. Suction and delivery valves are automatic in their operation. They are open and closed by pressure difference on both the sides of valves and a spring is provided to closed valve in case pressure is equal on both the sides. In working there are two strokes, suction stroke and delivery stroke. During suction stroke piston moves downward due to which pressure in cylinder falls down below atmospheric and intake valve opens with atmospheric air is taken in during


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whole the stroke. In delivery stroke piston moves inward with compression of air in the cylinder. Both the inlet and delivery valve are closed and compression proceeds; as the end of compression stroke the pressure increases above the receiver pressure. The high pressure air overcomes the spring force on delivery valve; valve opens and air is discharged to the receiver. The receiver is a vessel act as a storage tank. Ideal Indicator Diagram (with clearance neglected) Working of air compressor is represented on the indicator diagram (p-V diagram). The indicator is drawn for compressor neglecting the clearance.

The process d – a is suction stroke during which air is drawn into cylinder at constant pressure p1. At the end of suction stroke both suction and delivery valve are closed and air is then compressed by inward movement of piston. Process is represented by curve a-b; the process follows increase in pressure and reduction in volume. At point b, the pressure in the cylinder is slightly greater than the pressure in the receiver. At this point delivery valve

gets opened and air is discharged at constant pressure along line b-c. Again the suction valve opens and the cycle of operation is repeated. The net work required for compression and delivery of air per cycle is represented by the area a – b – c – d. The condition for minimum work The amount of work done on the air depends upon the nature of compression curve; means whether it is isentropic or isothermal


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compression. However in actual practice compression will be between isentropic and isothermal. Refer the p – V diagram below. Compression a-b2 is isentropic, a-b is polytropic and a-b1 is isothermal.

Isothermal, polytropic, and isentropic compression processes on a T-s

diagram


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Ideal indicator diagram for a reciprocating compressor with clearance

Actual indicator diagram for a reciprocating compressor

The sequence of operation in the cylinder is as follows : Compression Starting at maximum cylinder volume, point a, slightly below the inlet pressure p1, as the volume decreases the pressure rises until it reaches p2 at b; the discharge valve does not open until the pressure in the cylinder exceeds p2 by enough to overcome the valve spring force. Discharge Between b and c gas flows out a pressure higher than p2 by the amount of the pressure loss through the valves; at C, the point of minimum volume, the discharge valve is closed by its spring. Expansion From c to d, as the volume increases, the gas remaining in the clearance volume expands and its pressure falls; the suction valve does not open until the pressure falls sufficiently below p1 to overcome the spring force. Intake


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Between d and a gas flows into the cylinder at a pressure lower than p1 by the amount of pressure loss through the valve. The total area of the diagram represents the actual work of the compressor on the gas. The cross-hatched areas of the diagram above p2 and below p1 represent work done solely because of pressure drop through the valves and port passages. This work is called the valve loss. WORK OF COMPRESSION IN STEADY FLOW The steady flow energy equation per unit mass for a compression process, assuming that changes in potential and kinetic energy are negligible may be written (refer Figure below)


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In the p-v plot of Figure shown above, the work of compression for each type of process is represented by the area between the path of that process and the axis of pressure. It is evident that the work of reversible isothermal compression is less than the work of reversible adiabatic compression; the work of reversible polytropic compression is intermediate between the others if n lies between g and unity. This is the case in reality as the polytropic case will involve some cooling but not enough to obtain isothermal compression. In a real compressor the work will be greater than the work of the reversible compression process because of friction. In such cases the path of compression may be represented by pVn = constant. WORK OF COMPRESSION – RECIPROCATING COMPRESSORS The total area of the indicator diagram represents the actual work of the compressor on the gas. The idealized machine to which an actual machine is compared has an indicator diagram like Figure given below, in which there are no pressure loss effects, and the processes a – b and c – d are reversible polytropic processes.

Assuming no state change in the intake d – a and discharge b – c processes, and assuming equal values of the exponent n in the compression a – b and expansion processes c – d, the ideal work of compression can be found by taking the integral of pdv around the diagram. If mf is the mass of fluid taken in and discharged per machine cycle, then the total work interaction per cycle is


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Thus, we see that the work per kg of fluid flow is the same as obtained from the steady flow analysis. It is therefore unnecessary to make any further analysis of the work of the idealized reciprocating compressor since all desired results have already been obtained by the steady flow analysis. EFFICIENCY OF A COMPRESSOR I. Indicated Power The power consumed inside the cylinder, as measured from the indicator diagram is termed as the indicated power. If pm is the mean effective pressure, then the indicated power of the compressor is obtained from, , Watts

2. Brake Power The actual power input to the compressor (brake power or shaft power) is more than the indicated power due to the irreversibilitites and friction. Brake Power = Indicated power + Friction Power 3. Mechanical Efficiency The mechanical efficiency of the compressor is the ratio of the indicated power to the brake power.


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4. Isothermal efficiency (ηt) Actually due to high speed running of compressor the compression process may be assumed to be near adiabatic or polytropic process following law of compression as pVn = C with value of ‘n’ varying between 1.25 and 1.35 for air. It is thus obvious that actual compression process should be compared with isothermal compression process. A mathematical parameter called isothermal efficiency is defined for quantifying the degree of deviation of actual compression process from ideal compression process. Isothermal efficiency is defined by the ratio of isothermal work and actual indicated

work in reciprocating compressor.

5. Isentropic Efficiency ( ηs) It is the ratio of isentropic work of compression to the actual indicated work of compression

6. Polytropic Efficiency (ηp) It is the ratio of reversible polytropic work of compression to the actual indicated work.

7. Volumetric efficiency For a given compressor inlet condition, the mass flow rate of air depends on the volumetric efficiency of the compressor. The volumetric efficiency, ηV is defined as the ratio of actual volumetric flow rate of air to the maximum possible volumetric flow rate, which is equal to the compressor displacement rate, i.e.,


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Actual volume flow rate = mass flow rate x specific volume at compressor inlet VOLUMETRIC EFFICIENCY AND CLEARANCE

In order to prevent the piston from striking the cylinder head, a clearance volume is provided. The ratio of clearance volume to swept volume is called clearance ratio (C)

The flow capacity of positive displacement compressors is expressed in terms of volumetric efficiency ηvol. The volumetric efficiency is the ratio of actual volume of air intake to the swept volume.

Swept volume or piston displacement is, Therefore,

For the polytropic expansion along c-d


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Thus the volumetric efficiency depends on the clearance ratio and the index of expansion. In actual case, the actual air that is sucked in is get heated up as it comes in contact with the hot valves and the hot cylinder walls. So the condition of air at inlet of the compressor is quite different from the ambient conditions. Thus the volumetric efficiency with reference to ambient conditions is given as, Rotary Compressor Rotary compressors have rotors in place of pistons and give a continuous, pulsation free discharge air. They are directly coupled to the prime mover and require lower starting torque as compared to reciprocating machine. They operate at high speed and generally provide higher throughput than reciprocating compressors. Also they require smaller foundations, vibrate less, and have a lower number of parts - which means less failure rate. Rotary compressors may work on the principle of positive displacement and dynamic action both. Rotary compressors having positive displacement may be of following types: (i) Roots blower (ii) Screw type or helical type compressor (iii) Vane type compressor Rotary compressors employing dynamic action may be of centrifugal type or axial type depending upon the direction of flow. These centrifugal type or axial compressors may also be termed as non-positive displacement type steady flow compressors.


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(i) Roots blower: Roots blower is a positive displacement type rotary compressor. It has two rotors having two or three lobes having epicycloid and hypocycloid or involute profiles such that they remain in proper contact. Figure below shows two lobe rotors in a roots blower. To prevent wear and tear two rotors have clearance in between. Out of two rotors one is driven by prime mover while other one is driven by first rotor. When two rotors rotate then their typical geometry divides the region inside casing into two regions i.e. high pressure region and low pressure region.

Air at atmospheric pressure enters the casing and is trapped between rotor A and the casing. When the rotor rotate then air trapped in volume space V is displaced towards high pressure region due to rotation of rotor. Exit end is connected to receiver in which air is gradually transferred and the pressure inside receiver increases due to cumulative effect of air being transferred from atmospheric pressure region to receiver region. In one revolution this positive displacement of air trapped between rotor and casing from inlet end to receiver end shall occur four times in case of two lobe rotor as shown. While in case of three lobes rotor this transfer shall occur six times. Every time when V volume of air is displaced without being compressed to the receiver side high pressure region, then the high pressure air rushes back from receiver and mixes irreversibly with this air until the pressure gets equalized. Thus, gradually air pressure builds up and says this pressure becomes p2. For inlet air pressure being p1, the work done per cycle,


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Air Compressors

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