Compressors are mechanical devices used for increasing the pressure of a gas. Compressors used for producing high pressure air are called air compressors. Air is drawn from the atmosphere by suction process, which is then compressed to the required pressure and delivered to the receiver

1. Aldel Education Trust’s
ST. JOHN COLLEGE OF ENGINEERING AND MANAGEMENT, PALGHAR
(ST. JOHN POLYTECHNIC)
DEPARTMENT OF MECHANICAL ENGINEERING
SUB: POWER ENGINEERING AND REFRIGERATION (PER)
22562
TOPIC:AIR COMPRESSORS
PREPARED BY:-
Prof. Pranit Mehata
Lecturer, SJCEM
7972064172

2. INTRODCUTION
Compressors are mechanical devices used
for increasing the pressure of a gas.
A machine which takes in air or gas during
suction stroke at low pressure and then
compresses it to high pressure in a piston
cylinder arrangement is known as
Compressor.
External work must be supplied to the
compressor to achieve required
compression. Fig. Energy flow from prime mover to compressor

3. CLASSIFICATION OF COMPRESSORS

4. USES OF COMPRESSED AIR
To drive a compressed air engine
For producing an air blast for a workshop
For spraying the fuel (atomizing) into a boiler furnace
For operating pneumatic drills and tools
For operating pneumatic brakes for locomotives and rolling stock
Pneumatic conveying and for pumping of water by compressed air
For working compressed air engines especially in mines
In gas turbine power plants and air-conditioning plants
The several uses of compressed air are as follows:

5. COMPRESSOR TERMINOLOGY
Single-acting compressor: If the air admission from the atmosphere is on only one side of
the cylinder.
Double-acting compressor: When, the air from the atmosphere is drawn on both sides of
the piston.
Single-stage compressor: If the total compression is done fully in one cylinder.
Multi-stage compressor: If the compression is carried out in more than one cylinder, and
every cylinder carries out a part of the compression.

6. COMPRESSOR TERMINOLOGY
Free Air Delivered (FAD): The actual volume of air delivered by the compressor when reduced
to normal temperature and pressure condition. Capacity of compressor is generally given in
terms of free air delivery. Its SI Unit is 𝑚3
𝐶𝑦𝑐𝑙𝑒 .
Displacement or Swept Volume of the Compressor: The volume displaced by piston
movement between two dead centers is called displacement or swept volume. For a single acting
compressor, swept volume, 𝑉
𝑠 =
𝜋
4
× 𝐷2
Compressor capacity:-It is the volume of air delivered by the compressor. It is express in
𝑚3
min

7. COMPRESSOR TERMINOLOGY
Intake Pressure: The absolute pressure of air entering the compressor at inlet.
Discharge Pressure: The absolute pressure of air leaving the compressor at outlet.
Compression ratio or Pressure ratio: The ratio of discharge pressure to the intake
pressure.
Piston Speed: The distance travelled by piston in one minute. It is given as 2𝐿𝑁. It
is measured in 𝑚 𝑚𝑖𝑛.

8. RECIPROCATING COMPRESSOR
If compression is done in a
conventional cylinder with a closely
fitted piston making reciprocating
motion, then the compressor is called a
reciprocating compressor

9. SINGLE STAGE RECIPROCATING
COMPRESSOR
Fig. Single Stage Reciprocating Compressor

10. WORK REQUIRED FOR SINGLE ACTING
SINGLE STAGE RECIPROCATING
COMPRESSOR
❖ There are two possibilities of work done:
1. Work done without clearance volume
2. Work done with clearance volume
❖ Amount of work required depends on
nature of compression
❖ Three compression process
1. Polytropic compression (𝑃𝑉𝑛 = 𝐶)
2. Isothermal compression (PV = 𝐶)
3. Isentropic Compression (𝑃𝑉𝛾
= 𝐶)

11. WORK REQUIRED DURING
ISOTHERMAL PROCESS
Work done on air during Isothermal compression is given by,
𝑊 = 𝐴𝑟𝑒𝑎 4 − 1 − 2 − 3
𝑊
= 𝐴𝑟𝑒𝑎 (𝑂 − 3 − 2 − 𝐴) + 𝐴𝑟𝑒𝑎 (𝐴 − 2 − 1 − 𝐷) − 𝐴𝑟𝑒𝑎 (𝑂 − 4
− 1 − 𝐷)
𝑊 = 𝑃2𝑉2 + 𝑃2𝑉2 log𝑒
𝑉1
𝑉2
− 𝑃1𝑉1
For Isothermal process, 𝑃1𝑉1= 𝑃2𝑉2
𝑊 = 𝑃1𝑉1 log𝑒
𝑃2
𝑃1
𝑊 = 𝑚𝑅𝑇1 log𝑒
𝑃2
𝑃1

12. WORK REQUIRED DURING
POLYTROPIC PROCESS
Work done on air during Polytropic compression is given by,
𝑊 = 𝐴𝑟𝑒𝑎 4 − 1 − 2 − 3
𝑊
= 𝐴𝑟𝑒𝑎 (𝑂 − 3 − 2 − 𝐵) + 𝐴𝑟𝑒𝑎 (𝐵 − 2 − 1 − 𝐷) − 𝐴𝑟𝑒𝑎 (𝑂 − 4
− 1 − 𝐷)
𝑊 = 𝑃2𝑉2 +
𝑃2𝑉2 −𝑃1 𝑉1
𝑛 − 1
− 𝑃1𝑉1
𝑊 =
𝑛
𝑛 − 1
× 𝑚𝑅 × (𝑇2 − 𝑇1)
For Polytropic process, 𝑃1𝑉1
𝑛
= 𝑃2𝑉2
𝑛
𝑊 =
𝑛
𝑛 − 1
× 𝑚𝑅𝑇1 × [
𝑃2
𝑃1
𝑛−1
𝑛
− 1]

13. WORK REQUIRED DURING
ADIABATIC (ISENTROPIC) PROCESS
Work done on air during Isentropic compression is given by,
𝑊 = 𝐴𝑟𝑒𝑎 4 − 1 − 2 − 3
𝑊
= 𝐴𝑟𝑒𝑎 (𝑂 − 3 − 2 − 𝐶) + 𝐴𝑟𝑒𝑎 (𝐶 − 2 − 1 − 𝐷) − 𝐴𝑟𝑒𝑎 (𝑂 − 4
− 1 − 𝐷)
𝑊 = 𝑃2𝑉2 +
𝑃2𝑉2 −𝑃1 𝑉1
𝛾 − 1
− 𝑃1𝑉1
𝑊 =
𝛾
𝛾 − 1
× 𝑚𝑅 × (𝑇2 − 𝑇1)
For Polytropic process, 𝑃1𝑉1
𝛾
= 𝑃2𝑉2
𝛾
𝑊 =
𝛾
𝛾 − 1
× 𝑚𝑅𝑇1 × [
𝑃2
𝑃1
𝛾−1
𝛾
− 1]

14. WORK REQUIRED FOR SINGLE ACTING
SINGLE STAGE RECIPROCATING
COMPRESSOR WITH CLEARANCE VOLUME
 Clearance volume is the volume left in the cylinder when
piston reaches TDC
Let
𝑉
𝑐 = Clearance volume
𝑉
𝑠 = Swept volume = 𝑉1 − 𝑉
𝑐
𝑉
𝑎 = Actual volume = 𝑉1 − 𝑉4
Let the compression and expansion processes follow
the same law, 𝑃𝑉𝑛
= 𝐶
Work done per cycle,
W = area (1–2–3–4–1)
Fig. p-V diagram with
clearance

15. WORK REQUIRED FOR SINGLE ACTING
SINGLE STAGE RECIPROCATING
COMPRESSOR WITH CLEARANCE VOLUME
Work done per cycle,
𝑊 = 𝑎𝑟𝑒𝑎 (1– 2– 3– 4– 1)
= 𝑎𝑟𝑒𝑎 1– 2– 5– 6– 1 − 𝑎𝑟𝑒𝑎 3– 4– 6– 5– 3
∴ W =
n
n − 1
P1V1
P2
P1
n−1
n
− 1 − P4V4
P3
P4
n−1
n
− 1
Now, 𝑃3 = 𝑃2, 𝑃4 = 𝑃1
∴ 𝑊 =
n
n−1
P1(V1−𝑉4)
P2
P1
n−1
n
− 1 𝑊 =
n
n−1
P1Va
P2
P1
n−1
n
− 1

16. POWER REQUIRED TO DRIVE SINGLE STAGE
RECIPROCATIG AIR COMPRESSOR
Power required in driving the compressor is given as
𝑃 =
𝑊𝑁
60 × 103
𝑘𝑊
N=N for Single Acting
N=2N for Double Acting

17. MULTISTAGE COMPRESSION
 Multistage compression is carried out in more than one cylinder.
Disadvantages of Single Stage Compression
Handling of very high pressure range in one cylinder resulting in
leakage past the piston,
Ineffective cooling of the gas,
Necessitating robust construction of the cylinder to withstand the
high delivery pressure.
The volumetric efficiency of a single-stage compressor with fixed
clearance decreases with an
Increase in pressure ratio and thus reduces the capacity.

18. MULTISTAGE COMPRESSION
The air can be cooled perfectly at pressures intermediate between the
suction and delivery pressures resulting in less power required as
compared to a single-stage compressor for the same pressure limits
and quantity of free air delivered.
The mechanical balance of the machine is better due to phasing of
the operations.
The pressure range and hence the temperature range in each stage
can be kept within desirable limits.
Work required for single stage compression increases due to increase
in pressure ratio, which can be reduced with multistage compression.
NECESSITY OF MULTISTAGE COMPRESSION

19. TWO STAGE RECIPROCATING AIR
COMPRESSOR WITH INTERCOOLING
Two stage compression with intercooling Schematic
arrangement
p-V diagram

20. TYPES OF INTERCOOLING
Perfect
Intercooling Imperfect
Intercooling

21. PERFECT INTERCOOLING
When the temperature of the air leaving the
intercooler (i.e. T3) is equal to the original
atmospheric air temperature (i.e. T1) then
the intercooling is known as complete or
perfect intercooling.
In this case, the point 3 lies on the
isothermal curve as shown in below
figures:

22. IMPERFECT INTERCOOLING
When the temperature of the air leaves the
intercooler (i.e. T3) is more than the
original atmospheric air temperature (i.e.
T1), then the intercooling is known as
incomplete or imperfect intercooling.
In this case, the point 3 lies on the right
side of the isothermal curve as shown in
below figure:

23. WORK REQUIRED FOR TWO STAGE
RC WITH INTERCOOLER
𝑊 =
𝑛
𝑛 − 1
× {𝑃1𝑉1[(
𝑃2
𝑃1
)
𝑛−1
𝑛 − 1] + 𝑃1𝑉1[(
𝑃3
𝑃2
)
𝑛−1
𝑛 − 1}
𝑊 =
𝑛
𝑛 − 1
× 𝑃1𝑉1[(
𝑃2
𝑃1
)
𝑛−1
𝑛 + (
𝑃3
𝑃2
)
𝑛−1
𝑛 − 2]
When Intercooling is
imperfect
When intercooling is
perfect

24. CONDITION FOR MAXIUM EFFICIENCY OF
TWO STAGE RECIPROCATING COMPRESSOR
Efficiency is maximum when work required for compression is minimum.
Same
pressure
ratio

25. CONDITION FOR MAXIUM EFFICIENCY OF
TWO STAGE RECIPROCATING COMPRESSOR
𝑃2 = 𝑃3 ∙ 𝑃1
𝑃2
𝑃1
=
𝑃3
𝑃2
𝑊 = 2 ×
𝑛 − 1
𝑛
× 𝑃1𝑉1[(
𝑃2
𝑃1
)
𝑛−1
𝑛 − 1]
Condition for Maximum Efficiency
Same pressure ratio in each stage
Work required for each stage is same

26. VOLUMETRIC EFFICIENCY
The volumetric efficiency of a
reciprocating compressor is defined as
the ratio of the actual free air delivered to
the swept volume of the compressor. The
free air delivered is (V1 − V4)whereas the
sweptvolume is (V1 − VC).
ηV =
Va
Vs
=
V1 − V4
V1 − VC
Very high speed
Leakage through piston seals
Too large a clearance volume
Obstruction at inlet valves
Overheating of air by contact with cylinder
walls
 Inertia effect of air
Factors Affecting Volumetric
Efficiency

27. ISOTHERMAL EFFICIENCY
 It is defined as the ratio of isothermal work to that of actual indicated work.
𝜂𝑖𝑠𝑜 =
𝑚𝑅𝑇1 log𝑒
𝑃2
𝑃1
𝑛
𝑛 − 1
× 𝑚𝑅𝑇1 × [(
𝑃2
𝑃1
)
𝑛−1
𝑛 − 1]
𝑃𝑖𝑠𝑜 =
𝑃1𝑉1 log𝑒
𝑃2
𝑃1
× 𝑁
60 × 103
𝜂𝑖𝑠𝑜 =
𝐼𝑠𝑜𝑡ℎ𝑒𝑟𝑚𝑎𝑙 𝑃𝑜𝑤𝑒𝑟
𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑒𝑑 𝑃𝑜𝑤𝑒𝑟
Cooling cylinder by spraying water during
compression stroke
Circulation of water surrounding to cylinder by
providing jackets
Installing inter cooler between two cylinders
Providing greater fins on cylinder
By selecting suitable material for cylinder
Methods to improve Isothermal
Efficiency

28. MECHANICAL EFFICIENCY
It is defined as the ratio of indicated power to the shaft power.
𝜂𝑚𝑒𝑐ℎ =
𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑒𝑑 𝑃𝑜𝑤𝑒𝑟
𝑆ℎ𝑎𝑓𝑡 𝑃𝑜𝑤𝑒𝑟

29. ROTARY AIR COMPRESSORS
In a rotary compressor, the compression of air is achieved due to the rotating blades fitted
in a rotor.
It requires less starting torque as compared to reciprocating compressors because of direct
coupling with the prime over.
Usually, rotary compressors operate at high speed and supplies higher quantity of air than
reciprocating compressors

30. TYPES OF ROTARY COMPRESSORS
ROTARY COMPRESSORS
LOBE TYPE
CENTRIFUGAL
SCREW
COMPRESSOR
VANE TYPE

31. LOBE TYPE
 The roots blower is used to supply air
from 0.15 to 1500 𝑚3 𝑚𝑖𝑛with pressure
ratio up to 3.6 per blower.
 Rotational speeds up to 12,500 rpm are
used
 It can be directly coupled to a steam
turbine or a gas turbine shaft without
any intermediate gearing

32. VANE TYPE
The vane blowers require less power
than the root blowers for the same
capacity and pressure rise.
They are used to deliver up to 150
𝑚3 𝑚𝑖𝑛 of air at pressure ratio up to
8.5.
The speed is limited to 3000 rpm.

33. SCREW COMPRESSOR
 A screw compressor is a type of rotary
compressor which compresses air due to
screw action.
 The main advantage of using this
compressor is that it can supply
compresses air continuously with
minimum fluctuation in delivery pressure.
 It is usually applied for low pressure
applications up to 8 bars

34. AXIAL FLOW COMPRESSOR
 In an axial flow compressor, the air flows
throughout the compressor parallel to its axis.
 The axial compressor is generally driven by an
internal combustion engine or a turbine.
 The work input to the rotor shaft is transferred by
the rotor moving blades to the air, thus accelerating
it.
 The space between the moving blades form
diffuser passages so that the velocity of air relative
to the blades is decreased as the air passes through
them and consequently, the pressure rises.

35. DIFFERENCE BETWEEN
Parameters Reciprocating Compressor Rotary Compressor
Suitability Suitable for low discharge at high
pressure
Suitable for high discharge at low pressure
Working
principle
Compression is done by reciprocating
motion of piston inside the cylinder
Compression is done by rotary motion of
blades
Air Supply Intermittent or discontinuous Continuous
Size For same discharge the size of air
compressor is large
For same discharge the size of air
compressor is small
Delivery
pressure
Upto 1000 bar Upto 10 bar
Pressure
ratio
High Small

36. DIFFERENCE BETWEEN
Parameters Reciprocating Compressor Rotary Compressor
Work
required for
compression
High Less
Maintenance
cost
More Less
Lubrication Complicated Simple
No. of moving
parts
More Less
Wear and
tear
More Less
Speed Low High

37. DIFFERENCE BETWEEN
Parameters Reciprocating Compressor Rotary Compressor
Cleanliness of
air
Less clean will be delivered More clean air will be delivered
Mechanical
efficiency
Low High
Flow rate of
air
3000 𝑚3
𝑚𝑖𝑛 300 𝑚3
𝑚𝑖𝑛
Application Pneumatic tools, spray painting, coal
mines, auto garages etc.
Oil refinery, refrigeration and air
conditioning plants, turbo jet engine etc.

38. THANK YOU

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