Compressor

1. Ch. 9 Air Compressor
Prepared by.:- Nilesh Rana
Asst. Prof. , MED.
SNPIT&RC, Umrakh

2. WHAT IS COMPRESSOR?
 Compressors are mechanical devices that compresses
gases. It is widely used in industries and has various
applications
 An air compressor is a device that converts power (using an
electric motor, diesel or gasoline engine, etc.) into potential
energy stored in pressurized air (i.e., compressed air)
 an air compressor forces more and more air into a storage
tank, increasing the pressure.
• Def.:- A machine which takes in air at low pressure and
compress it to high pressure with the help of some suitable
arrangement i.e. a reciprocating piston and cylinder
arrangement or a rotary arrangement, is called an air
compressor.

3. How They Are Different From Pumps?
Major difference is that compressors handles
the gases and pumps handles the liquids.
As gases are compressible, the compressor
also reduces the volume of gas.
Liquids are relatively incompressible.

4. WHY WE NEED?
Air conditioners, (car, home)
Home and industrial refrigeration
Hydraulic compressors for industrial machines
Air compressors for industrial manufacturing

5. Uses of Compressed air
The compressed air has number of application in
different industries. They are listed as under:
• For filling the air in tube of vehicles
• In automobile service station to clean vehicles.
• For spray painting in paint industries.
• In vehicle to operate air brakes.
• For cleaning workshop machines.
• For operation of pneumatic tools i.e. rock drills,
vibrators etc.

6. Classification of compressors
• 1. According to method of compression
– a. Reciprocating compressor: This type of compressor
compresses air by reciprocating action of piston inside
a cylinder. It is suitable for producing high pressure.
– b. Rotary Compressor: In a rotary compressor, air or
gas is compressed due to the rotation of impeller or
blades inside a casing similar to a rotary pump.
– c. Centrifugal compressor: A machine in which
compression of air to desired pressure is carried out
by a rotating impeller as well as centrifugal action of
air.

7. • 2. According to method delivery pressure
– a. Low pressure - up to 1.1 bar
– b. Medium pressure - 1.1. to 8 bar
– c. High pressure – 8 to 10 bar
– d. Very high pressure - above 10 bar
• 3. According to principle of operation
– a. Positive displacement: In this type, pressure of air is
increased by reducing the volume of it.
• Here air is compressed by positive displacement of air with piston
or with rotating element. Examples: Reciprocating compressor
– b. Roto dynamic or steady flow compressor: In this type,
compression of air is carried out by a rotating element
imparting velocity to the flowing air and developed desired
pressure.
• Here compression is achieved by dynamic action of rotor.
Examples: Centrifugal, Axial flow, etc

8. • 4. According to the number of stages
– a. Single stage compressor - pressure up to 5 bar
– b. Multistage compressor - pressure above 5 bar
• 5. According to method the number of cylinder
– a. Single cylinder
– b. Multi cylinder
• 6. According to method the pressure limit
– a. Fans - pressure ratio 1 to 1.1
– b. Blowers - pressure ratio 1.1 to 2.5
– c. Compressor - pressure ratio above 2.5
• 8. According to fluid to be compressed
– a) Air compressor
– b) Gas compressor
– c) Vapour compressor

9. Classification of compressors

10. Reciprocating air compressor
• Construction
– It consists of the cylinder in which a piston
reciprocates.
– The piston is driven by crank through connecting
rod.
– The crank is mounted in a crankcase.
– There are two valves, i.e. intake valve and delivery
valve
– they operate automatically by the difference of
pressure across the valve.

11. Operation of a compressor
• Case-(1) Operation without clearance
• It is assumed that in an ideal compressor there is no
clearance volume at the end of the stroke.
• In this type of compressor, when piston is moving away
from TDC, pressure inside cylinder will decrease and
volume will increase.
• Hence pressure difference across the valve is created
• The spring operated inlet valve will be opened
automatically for intake of air.
• Therefore the atmospheric air enters into the cylinder
at constant pressure P1 with increase in volume. This
process is shown by (4-1) on p- V diagram.

12. Reciprocating air compressor

13. Operation of a compressor (Cont.)
• The piston moves towards the TDC from BDC during the second
stroke.
• The air is compressed with increase of pressure.
• This is shown by the curve (1-2).
• When the pressure of compressed air becomes equal to the
pressure of receiver in which the air is delivered, the spring
operated delivery valve opens automatically and the air is forced
into the receiver at constant pressure P2 from the cylinder.
• This process is shown by horizontal line 2-3.
• Again piston moves away from TDC
• Thus the cycle is completed and the same cycle will be repeated
• Area of the diagram (1-2-3-4) represents the work required to
compress air from pressure P1 to P2 for Polytropic compression.

14. Point 4- Suction Valve opens
Point 1- Suction valve close
Point 2- Delivery valve opens
Point 3- Delivery valve close
Vs – Swept volume= (V1- V4)
4-1:- Suction process- Air of volume V1 enters into the compressor at pressure P1 and temperature T1
1-2: According to Law PVn=C , Constant air is compressed from pressure P1 to P2.
Volume decreased from V1 to V2.
Temperature rises from T1 to T2
1-2’ :-Isothermal Compression process
1-2” :- Adiabatic Compression Process
2-3 :- Delivery process- Compressed air from volume V2 and pressure P2 , temperature T2 delivered from
the compressor.

15. • The area under P-V diagram represents work done in the
cycle.
• The work done is least when the compression is isothermal
(1-2’) and Highest during adiabatic compression.(1-2”)
• In practice, Isothermal compression is not possible because
to achieve it compressor need to run at very slow speed.
• In adiabatic compression, to reduce work done and to
approach isothermal compression, compressor is cooled by
air cooling or water cooling.
• In actual practice the compression neither isothermal nor
adiabatic but it follows PVn=Constant…

16. Work done per cycle
• The following assumptions are made in deriving the power required
to drive the compressor.
1. There is no pressure drop through suction and delivery valves.
2. Complete compression process takes place in one cylinder.
3. There is no clearance volume in the compressor cylinder.
4. Pressure in the suction line remains constant. Similarly,
pressure in the delivery line remains constant.
5. The working fluid behaves as a perfect gas.
6. There is no frictional losses.

17. • The cycle can be analyzed for the three different
case of compression. Work required can be
obtained from the p - V diagram.
• Let,
• p1 = Pressure of the air (kN/m2), before
compression
• V 1 = Volume of the air (m3), before compression
• T1 =Temperature of the air (K), before compression
• p2, V2 and T2 be the corresponding values after
compression.
• m - Mass of air induced or delivered by the cycle
(kg).
• N - Speed in RPM.

18. Work done during Polytropic
Compression

19. Work done during Adiabatic Compression
In this n is replaced by γ

20. Work done during Isothermal process

21. Operation of a compressor
• Case- (2) Operation with clearance
• In actual compressor always there is clearance
volume at the end of stroke.
• The small clearance is required because of,
• (1) Preventing striking of piston at cylinder head,
• (2) Thermal expansion due to high temperature
at the end of compression,
• (3) Maintaining machine tolerance.

22. Operation of a compressor (Cont.)
• Clearance and clearance volume:
• When the piston reaches top dead centre (TDC) in the cylinder, there is a dead space
between piston top and the cylinder head. This space is known as clearance space and
the volume occupied by this space is known as clearance volume, Vc.
• Working of Compressor with clearance volume:-
• The clearance volume is expressed as percentage of piston displacement. Its value
ranges from 5% - 10% of swept volume or stroke volume (Vs).
• At the end of delivery of high pressure air (at point 3), a small amount of high pressure
air at p2 remains in the clearance space.
• This high pressure air which remains at the clearance space when the piston is at TDC is
known as remnant air.
• It is expanded polytropically till atmospheric pressure (p4=p1) is reached.
• The inlet valve is opened and the fresh air is sucked into the cylinder.
• The suction of air takes place for the rest of stroke (upto point 1).
• The volume of air sucked is known as effective suction volume (V1 - V4).
• At point 1, the air is compressed polytropically till the delivery pressure (p2) is reached.
• Then the delivery valve is opened and high pressure air is discharged into the receiver.
• The delivery of air continues till the piston reaches its top dead centre, then the cycle is
repeated.

23. Vc = Clearance volume (m3)
Vs = Swept( stroke) volume (m3)
= V1- V3
Vas = Effective swept volume (m3)
= V1 –V4
4-1 : Suction Process (Constant pressure process)
1-2 : Polytropic Compression process follow PVn=C
2-3 : Discharge process (Constant pressure process)
3-4 : Polytropic expansion process PVn=C.
Effect of clearance volume:
1. Suction volume (volume of air sucked)
is reduced.
2. Mass of air is reduced.
3. If clearance volume increases, heavy
compression is required.
4. Heavy compression increases
mechanical losses

24. • Work done per cycle with clearance volume:
Work done/Cycle = Net area 1-2-3-4-1
= (W. D. for area under (5-4-1-2-3-6-5) – W.D. for
area under (5-4-3-6-5))
= [ n/n-1* (P2V2 – P1V1)] - [ n/n-1* (P3V3 – P4V4)]
From P-V Diagram P1 = P4 and P2= P3
W= [ n/n-1* (P2V2 – P1V1)] - [ n/n-1* (P2V3 – P1V4)]
= [ n/n-1* P1V1 *{(P2V2 / P1V1)-1}] - [ n/n-1* P1V4 *{(P2V3 /P1V4)-1}]
For Polytropic process
P2V3
n = P1V4
n
V3/V4 = (P1/P2)1/n
Also V2/V1 = (P1/P2)1/n
= (P2/P1)-1/n
So P2V2 / P1V1 = (P2/P1)* (P2/P1)-1/n
= (P2/P1)1+(-1/n) = (P2/P1)(n-1)/n

25. P2/P1 = Pressure ratio
Also P1*Vas = mad RT1
Mad = mass of air delivered w.r.t. Vas (Kg/Cycle)
W= [(n/n-1)*madRT1* {(P2/P1}n-1/n – 1}] Kj/Cycle.

26. Important terms used in Reciprocating
Air compressor
• Indicated Power:-
– I.P. = W*N/60000 KW
• W= work required (J/Cycle)
• N = Compressor speed (RPM)
– For double acting compressor:-
• I.P. = 2*W*N/ 60,000 KW

27. Volumetric Efficiency

28. Formula - Volumetric Efficiency

29. Formula - Volumetric Efficiency