STUDY OF COMPRESSOR

An air compressor admits atmospheric air, compresses it and delivers the high

pressure air to a storage vessel from which it is conveyed by a pipeline to wherever the

supply vessel from which it is conveyed by a pipe line to wherever the supply of compressed

air is required. A compressor is driven by a prime mover.

Even though compressors have innumerable applications it is commonly used in the

following purposes:

Operating tools in factories

Excavating

Spray painting

Refrigerators etc.

Compressors are classified as positive displacement type and rotodynamic type.

Positive displacement compressor is again classified as reciprocating type and rotary type,

while rotodynamic type is classified as centrifugal type and axial type compressors. In our

lab, we have reciprocating compressor test rig and so the study is related to reciprocating

compressor.

WORKING OF RECIPROCATING AIR COMPRESSOR

A reciprocating compressor consists of a cylinder, piston, inlet and outlet valves. The

arrangement of compressor is shown in the figure below:

Reciprocating air compressor

During the downward motion of the piston, the pressure inside the cylinder falls

below the atmospheric pressure and the inlet valve is opened due to the pressure difference.

The atmospheric air is admitted into the cylinder until the piston reaches bottom dead centre

position. As the piston starts moving upwards, the inlet valve closes and the pressure starts

increasing continuously until the pressure inside the cylinder is above the pressure of the

delivery side which is connected to the receiver. Then the delivery valve opens and the high

pressure air is delivered to the receiver during the remaining upward motion of the piston. At

the end of the delivery stroke, a small volume of high pressure air is entrapped in the

clearance space. The high pressure air left in the clearance space expands as the piston starts

moving downwards in the next cycle and its pressure falls until it is just below the

atmospheric pressure. Then the inlet valve opens and the air from outside is taken in and the

cycle is repeated. The suction, compression and delivery of the air take place within two

strokes of the piston or one revolution of the crank.

AIR COMPRESSOR TERMINOLOGY

Definitions of the following terms are necessary in the study of the operation and

theory of reciprocating compressors.

1) Single acting compressor

In single acting reciprocating compressor, the suction, compression and delivery of the air

take place on one side of the piston only. Such compressor would have one delivery strokes

per revolution of the crankshaft. Also it should have a piston rod, cross head and connecting

rod arrangement.

2) Double acting compressor

In double acting reciprocating compressor, the suction, compression and delivery of the

air take place on both sides of the piston. Such compressor would have two delivery strokes

per revolution of the crankshaft. Also it should have a piston, cross head and connecting rod

arrangement.

3) Single stage compressor

In single stage compressor, the compressor of the air from the initial pressure to the final

pressure is carried out in one cylinder only.

4) Multi stage compressor

When the compression of air from the initial pressure to the final pressure is carried out in

more than one cylinder, then the compressor is known as multi stage compressor.

5) Ratio of compression or pressure ratio

It is the ratio of absolute discharge pressure to absolute inlet pressure.

6) Free air delivered (F.A.D)

The free air delivered is the actual volume of air delivered at the atmospheric pressure

and temperature, and expressed in cubic meter per minute.

7) Displacement of the compressor

The swept volume of the piston in the first cylinder is known as displacement of the

compressor.

8) Actual capacity of the compressor

The actual free air delivered by the cylinder per cycle or per minute is known as actual

capacity of the compressor.

9) Volumetric efficiency

The ratio of actual free air delivered by the compressor per stroke to the displacement of

the compressor is known as the volumetric efficiency.

10) Isothermal efficiency

The work required to run the compressor becomes minimum if the compressor follows

isothermal compression process. Isothermal efficiency of the compressor is the ratio of

isothermal work to the actual work.

The following practical methods are used to achieve nearly isothermal compression for

high speed compressors.

1. Spray injection- injecting water into compressor cylinder at the end of compression

stroke with objective of cooling

2. Water jacketing- water is circulated through the annular space around the cylinder

3. Inter cooling- Inter cooling is used in addition to the water jacketing, by dividing the

compression process into two or more stages. The air compressed in first stage is

cooled to its original temperature in a heat exchanger known as inter cooler before it

is taken to the second stage.

4. External fins- Effective cooling can be achieved for small capacity air compressor by

providing fins on the external surface of the cylinder.

MULTI STAGE COMPRESSION

The volumetric efficiency of a reciprocating compressor is a function of clearance

ratio, pressure ratio and index of expansion. Therefore the maximum pressure ratio attainable

with a single stage reciprocating compressor is limited by the clearance volume. But there is

practical limitation for the reduction of the clearance volume. Therefore to get high pressure

air, it becomes necessary to use multi stage compression. In multi stage compression, cooling

of the air after it leaves each stage is possible. In the two stage compressor with intercooler,

the air is first taken into the low pressure (LP) cylinder. After compression 2 some desired

intermediate pressure, the air from the LP cylinder is passed through the intercooler. The

temperature of air leaving the intercooler will be reduced to a great extent. It is possible to

cool the air in the intercooler up to initial temperature, by properly regulating the supply of

cooling water in the intercooler. Finally, the air is compressed in the high pressure (H.P)

cylinder and is discharged to the cylinder. Both HP and LP cylinders are mounted on the

same shaft, and are driven by the same prime mover.

Advantages of multi stage compressors with intercooler are:

Work done per kg of air is reduced

Higher volumetric efficiency

Effective lubrication due to low temperature

Loss of air due leakage is less

Two stage compressor with intercooler

EXPT NO: NAME:

DATE: ROLL NO:

EXPERIMENT ON TWO STAGE AIR COMPRESSOR

AIM

To conduct experiment on two stage air compressor and to find out

(a) Volumetric efficiency

(b) Isothermal efficiency and to plot the following graphs:-

1. Volumetric efficiency vs delivery pressure

2. Isothermal efficiency vs delivery pressure

APPARATUS

Manometer, Digital rpm indicator, stops watch

SPECIFICATION

Diameter of piston, D= 0.07 m

Stroke length, L= 0.085 m

Energy meter constant, Ec= 1600 rev/ kwh

Orifice diameter = 15mm= 0.015 m

PRINCIPLE

1. Volumetric efficiency

ƞvol = (actual volume of air intake/ theoretical volume) × 100

2. Isothermal efficiency

Ƞiso = (Isothermal power/ Input power to compressor) × 100

PROCEDURE

1. The water present if any in the receiver is drained out using the drainage cock.

2. The outlet valve of the receiver is kept open to facilitate starting and then the

motor is switched on.

3. When the compressor reaches its normal speed the outlet valve of the receiver is

closed and the compressor is allowed to build the required pressure.

4. When the pressure reaches the desired valve, the outlet valve is adjusted so that

the delivery pressure remains constant at that pressure. At this point manometer

reading, speed of the motor and energy meter readings is noted down.

5. The experiment is repeated for different values of pressures and the above set of

reading are noted down.

After completing the experiment, switch of the motor and release the air from the

receiver.

TABULAR COLUMN

SAMPLE CALCULATION FOR SET NO:__________

1) Inlet pressure, P1=Atmospheric pressure=1.01325x105N/m

2

2) Total pressure, P2 =Gauge pressure (Pg) + Atmospheric pressure N/m2

(Where, Gauge pressure = Pressure gauge reading in Kgf/cm2)

Pg = Kgf/cm

2 = N/m

2

P2= N/m2

3) Manometric Head, Ha= h × (ρw– ρ air) / ρair m of air

Where h = manometer difference in ‘m’ of water= m

ρair= Density of air at RTP (RTP-Room Temperature and Pressure) in Kg/m3

ρw = Density of water =1000kg/m3

Density of air at NTP= 1.293 kg/m3

Density of air at RTP =ρair =

= kg/m

3

t = ambient temperature = 0C

Manometric Head, Ha=___________ m of air

4) Actual volume of air intake per second at RTP, Va in m3/s

Va = Cd A =

= m3/s

Where, Cd = Coefficient of discharge of the orifice meter=0.62

A = area of orifice in m2= πd

2/4 = m

2

d=diameter of orifice in m = 0.015 m

g=Acceleration due to gravity in m/s2 =9.81m/s

2

5) Swept volume, Vs in m3/s

Vs = (πd2 x L x Nc)/ (4 x 60)

= = m3/s

Where, D =Diameter of piston in m = 0.07m

L = Stroke length in m = 0.085 m

Nc = Speed of the Compressor in rpm (925 rpm constant at all load condition)

= rpm

6) Volumetric efficiency (%)

ηvol = (Va / Vs) x 100 %

= = %

7) Input to the compressor (KW)

Input power = (3600 x n)/ (Ec x t) kW

= = kW

t = time required for` n` pulses in sec

n=

t= sec

Ec = Energy meter constant=

8) Isothermal power (KW)

Piso = P1Va loge (P2/ P1) /1000 =

= KW

P1 = Atmospheric pressure=1.01325x105 N/m

2

Va = Actual volume intake in m

3/s

P2= N/m2

9) Isothermal Efficiency (%)

ηiso= (Isothermal power / input to the compressor) × 100 %

= = %

RESULT

1. Maximum volumetric efficiency =---------------

2. Maximum Isothermal efficiency =---------------

INFERENCE