Air compressor unit no 3 of PER subject of MSBTE

1. Unit No:- 03
AIR COMPRESSORS
 COURSE OUTCOME:-
CO562.3 : Maintain reciprocating air compressors
Prof. D. C. Jadhav
Marks : 14

2. Syllabus:-
 3.1 Reciprocating compressors: Applications, working of single stage and two stage
compressors with PV diagrams, Intercooling.
 3.2 Testing of reciprocating air compressors: Pressure ratio, Compressor capacity, FAD,
Volumetric efficiency, Isothermal efficiency, Numerical, Methods of energy saving.
 3.3 Rotary compressors : Screw, Centrifugal, Lobe type, Vane type compressors and axial
flow compressors, Comparison of rotary and reciprocating compressors.

3. Introduction:-
 Air compressor is a power absorbing machine which provides high pressure air. It takes the air
from the atmosphere, compresses it to a high pressure and high pressure air will be stored in a
storage vessel (reservoir) from where it can be taken out for use.
 There are many uses of high pressure air in industry. The main uses of compressed air are:
1. For inflating automobile tyres.
2. To clean workshop machines, generators etc.
3. To operate air operated drills, hammers (Pneumatic tools).
4. To inject fuel in the diesel engine cylinder.
5. In spray painting.
6. To operate air brakes in automobiles
7. In automobile service station to clean vehicle.

4. Classification of Air Compressor:-
Compressor
Reciprocating Rotary
Single Stage Multi stage Positive
Displacement
Non-Positive
Displacement
Single
Acting
Double
Acting
Single
Acting
Double
Acting
Screw
Type
Vane
Type
Roots
blower
Lysholm
Comp.
Centrifugal Axial Flow

5. Single Stage Reciprocating air compressor :-
-The principal parts of reciprocating compressor are
same as that of engine.
-Fig. shows single stage reciprocating air compressor
with PV diagram. Crank shaft is coupled to the prime
mover or electric motor. Inlet and delivery valves are
automatic in their operation. They are opened and closed
by pressure difference on both sides of valves.
-In working, there are two strokes, suction stroke and
delivery stroke.
-During suction stroke piston moves downward. Due to
which pressure in cylinder falls bellow atmospheric
pressure and intake valve opens and air is taken in the
cylinder.
-In delivery stroke, piston moves upwards with
compression of air in cylinder. Both the inlet and
delivery valves is closed and compression proceeds; at
the end of compression stroke the pressure of air
increases above the receiver pressure and this pressure
delivers to the receiver.
-The receiver is a vessel, which acts as a storage tank.
Crank shaft
Connecting rod
Fig. Single stage compressor with P-V
Diagram

6.  The figure shows the P-V diagram for single stage reciprocating air
compressor without clearance.
 During the suction stroke the air is drawn into the cylinder along line
4-1 at constant pressure P1 which is slightly below the atmosphere.
 At point 1, the piston completes the suction stroke and starts its
compression stroke. At this time, all the valves are closed; the air
inside the cylinder is compressed along the curve 1-2.
 At point 2, the pressure P2 is reached which is slightly higher than the
receiver pressure.At this point discharge valve opens delivery of
compressed air takes place along line 2-3 at constant pressure P2.
 The piston has now reached at top of cylinder and again starts its
suction stroke & the pressure in the cylinder will be lowered again P1
& the cycle of operations will be repeated.The net work done
required is represented by area 1-2-3-4.
P-V Diagram of single stage
compressor

7. Work required for single stage single acting reciprocating compressor:-
Work required for compression :
W = A(1-2-3-4) = A(0-a-2-3) + A(a-b-1-2) – A(0-b-1-4)
W = P2V2 +
P2V2 − P1V1
𝑛−1
+ P1V1
W =
nP2V2 − P2V2+P2V2− P1V1−n P1V1+P1V1
𝑛−1
=
nP2V2−n P1V1
𝑛−1
=
n
𝑛−1
[P2V2 - P1V1]
=
n
𝑛−1
P1V1[
P2V2
P1V1
– 1]
Consider process 1-2 : PVn = C i.e P1V1
n = P2V2
n
W =
𝑛
𝑛−1
P1V1
P2
P1
P2
P1
−
1
𝑛
− 1
∴ W =
𝑛
𝑛−1
P1V1
P2
P1
𝑛−1
𝑛
− 1
But P1V1 = mRT1 ∴ work required (W) =
𝑛
𝑛−1
mRT1
P2
P1
𝑛−1
𝑛
− 1
∴ I.P = work required x
𝑁
60
∴ I.P = W x
𝑁
60
∴ I.P =
𝑛
𝑛−1
mRT1
P2
P1
𝑛−1
𝑛
− 1 x
𝑁
60
Pressure
(P)
Volume
(V)
V 1
V 2
P 1
P 2
3 2
14
0 ba
Suction
Compression
Delivery

8. Two Stage Reciprocating air compressor :-
In a single-stage compression, the pressure ratio is less. But sometimes we require a higher pressure ratio.
So, in order to get a large pressure ratio, we can employ a large pressure in a single cylinder or compress the
air in two or more cylinders in series.
Inter cooler
Cold water InHot water
out
Delivery
valve
Delivery
valve
Intake
valve
Intake valve
L. P.
Compressor
H. P
Compressor
Cold
air
Hot
air Discharge air at
delivery
pressure
Working:- Air is admitted to L.P.(Low-pressure cylinder) and
is compressed to some intermediate pressure between the
intake and deliver pressure. Air at high temperature and
pressure is brought to an intercooler( intercooler is a device
used for cooling air between two stages or it is a heat
exchanger), where the air is cooled down nearly to the
temperature of intake air to L.P. cylinder by regulating the
supply of cooling water in the intercooler.
The temperature of the air leaving the intercooler depends
upon the cooling efficiency of the intercooler. This high
pressure cold air is again compressed to required delivery
pressure in H. P (High pressure cylinder) compressor.
Complete or Perfect Intercooling:-
When the temperature of the air leaving the intercooler is
equal to the original atmospheric air temperature, then this
is known as Complete or Perfect Intercooling.
Incomplete or Imperfect Intercooling:- When the temperature of the air leaving the intercooling is more than the original
atmosphere air temperature, then this is known as incomplete or Imperfect Intercooling.
The work saving in perfect intercooling is more than work saving in incomplete intercooling.

9. Work required for two stage single acting reciprocating compressor:-
o The figure shows the P-V diagram for two stage reciprocating air
compressor with perfect intercooling
o Air is sucked at atmospheric pressure in L.P cylinder at P1 during suction
stroke. Then it is compressed along 1-2’
o From condition 2’ it is delivered to an intercooler where heat from the air
is rejected by cooling water at constant pressure P2.
o If air is cooled to intake temperature of L.P. it is called as perfect cooling
this cooled air is admitted into H.P. cylinder.
o In H.P. stage it is compressed along curve 2-3 to pressure P3 and then
delivered to receiver at constant pressure P3.
o The shaded area 2-2’-3’-3 is saving in work with two stage compressor
with perfect intercooling
Let, P1 = intake pressure, V1= Volume at point 1, P2 = pressure at inlet to H.P
V2 = Volume at point 2, P3 = Delivery pressure.
Pressure
(P)
Volume
(V)
P1
P2
P3
3 3’
2
2’
1
Intercooler
pressure
Suction
Delivery
Then total work for compression and delivery of air is,
W = Work done in L.P + Work in H.P
W =
𝑛
𝑛−1
P1V1
P2
P1
𝑛−1
𝑛
− 1 +
𝑛
𝑛−1
P2V2
P3
P2
𝑛−1
𝑛
− 1
∴ W =
𝑛
𝑛−1
P1V1
P2
P1
𝑛−1
𝑛
+
P3
P2
𝑛−1
𝑛
− 2

10.  Advantages of Multistage Compression
1. Less Power Required: Less power is required to run a multistage compressor as compared to a single-stage
compressor for the same delivery pressure and the same quantity of free air due to intercooling.
2. Increased volumetric efficiency
3. Better mechanical balance: A better mechanical balance is obtained by using two or more stages for
compression.
4. Better lubrication: Better lubrication is possible due to lower working temperature and pressure.
5. Reduced size of cylinder
6. Reduced leakage loss
7. Reduced cost of compressor
 Disadvantages of Multistage Compression
1. It requires more no of cylinders.
2. Arrangement of intercooler is required to be done to reduce work.
3. It requires more floor space.
4. System becomes complicated and costly.

11. Compressor terminology:-
Intake Pressure :-
It is the pressure at which air is taken in cylinder of compressor.
Discharge pressure:-
It is the pressure at which air is delivered by the compressor.
Compression ratio( pressure ratio):-
Compression ratio is the ratio of delivery pressure to suction pressure.
Free air delivered (FAD):-
It is the volume of air delivered under the condition of temperature and pressure existing at compressor
intake.
Swept volume:-
It is actual volume of air taken in during suction stroke. It is expressed in m3
Efficiency of compressor:-
Volumetric efficiency:-
It is the ratio of volume of free air delivered per stroke to the volume of air swept by piston during the stroke.
∴ Volumetric efficiency =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑓𝑟𝑒𝑒 𝑎𝑖𝑟 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑒𝑑 𝑝𝑒𝑟 𝑠𝑡𝑟𝑜𝑘𝑒
𝑠𝑤𝑒𝑝𝑡 𝑣𝑜𝑙𝑢𝑚𝑒 𝑝𝑒𝑟 𝑠𝑡𝑟𝑜𝑘𝑒
Factors which reduce volumetric efficiency are:
1. As clearance volume increases, volumetric efficiency decreases.
2. Leakage at inlet valves.
3. Piston ring leakage
4. As pressure ratio increases, volumetric efficiency decreases
5. High speed of rotation of crank.

12. Isothermal Efficiency:-
It is the ratio of isothermal power to the indicated power in kW.
∴ 𝐼𝑠𝑜𝑡ℎ𝑒𝑟𝑚𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝐼𝑠𝑜𝑡ℎ𝑒𝑟𝑚𝑎𝑙 𝑃𝑜𝑤𝑒𝑟
𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑒𝑑 𝑃𝑜𝑤𝑒𝑟
Mechanical Efficiency:-
The ratio of Indicated Power to the Brake power of compressor is called as mechanical efficiency.
∴ Mechanical 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑒𝑑 𝑃𝑜𝑤𝑒𝑟
𝐵𝑟𝑎𝑘𝑒 𝑃𝑜𝑤𝑒𝑟
Methods of energy saving in air compressor:-
To reduce work required to compression following methods are adopted,
i) Spraying cold water into cylinder during compression
ii) Providing cooling jackets
iii) Multistaging of compressor

13. Rotary Compressor:- Rotary
Positive
Displacement
Non-Positive
Displacement
Screw
Type
Vane
Type
Roots
blower
Lysholm
Comp.
Centrifugal Axial Flow
In a rotary compressor the air is entrapped between two sets of engaging surface and
the pressure of air is increased by squeezing action of air

14. Roots blower or Lobe type rotary compressor:-
Suction
side
Driver Lobe
Casing
Driven Lobe
Delivery side
Construction:-
They consist of a pair of involute
profiled lobes/rotors rotating inside an
oval shaped casing, closed at ends by
side plates. One lobe is the driving lobe,
which is driven by the external power
while the driven lobe is driven by a
pair of equal ratio gears. Both the lobes
thus, rotate at same speed but in
opposite direction.
Working:-
During rotation, volume of air at atmospheric pressure is trapped between the rotors and the casing.
This air is positively displaced with change in volume until the space is open to high pressure region
and this high pressure air is delivered to the receiver. This can be shown in fig.

15. Rotary screw compressors:-
 Construction:-
Rotary compressors uses two Asymmetrical rotors that
are also called helical screws to compress the air. The
rotors have a very special shape and they turn in
opposite directions with very little clearance between
them. The rotors are covered by cooling jackets. Two
shafts on the rotors are placed that transfer their
motion with the help of timing gears that are
attached at the starting point of the shafts
 Working principle-
Air sucked in at one end and gets trapped between the
rotors and get pushed to other side of the rotors .The
air is pushed by the rotors that are rotating in opposite
direction and compression is done when it gets
trapped in clearance between the two rotors. Then it
pushed towards pressure side.

16. Vane Type Rotary Compressor:-
Construction:-
This is an another type of rotary compressor. There is a
fixed casing in Vane type compressor in which a rotary
rotor disc is placed which has slots that are used for
holding the sliding plates.
Working:-
Whenever rotor rotates the disc also rotates thus
allowing the sliding plates to slide as the inner surface of
casing is eccentric. Whenever the plates moves away
from the center a huge amount of air get trapped inside it
and with the rotation the sliding plates converge due to
its shape and the trapped air get compressed. This results
in compression of air.

17. Centrifugal compressor:-
Construction and working:-
Centrifugal compressor consists of a rotating
member known as impeller wheels mounted on steel
shaft and enclosed in cast iron casing. The impeller
wheel consists of two discs, a hub disc and cover disc
with number of blades mounted radially between
them. An impeller has rotary vanes, which provides
closed radial passages for flow of air. Centrifugal
compressors also known as dynamic compressors. A
centrifugal compressor imparts kinetic energy into
the air stream by increasing the velocity of the air
using a rotating element and then converts this
kinetic energy into potential energy in the form of
pressure. Air is drawn into the center of a rotating
impeller with radial blades and is pushed toward the
center by centrifugal force. This radial movement of
air results in a pressure rise and the generation of
kinetic energy. Before the air is led into the center of
the impeller, the kinetic energy is also converted into
pressure by passing through a diffuser and volute.

18. Comparison between reciprocating and rotary compressor:-
Reciprocating Compressor Rotary Compressor
1 Compression of air takes place with the help of piston
and cylinder arrangement with reciprocating motion of
piston
Compression of air takes place due to
rotary motion of blades
2 Delivery of air is intermittent Delivery of air is continuous
3 Delivery pressure is high i.e. pressure ratio is high Delivery pressure is low i.e. pressure
ratio is low
4 Flow rate of air is low Flow rate of air is high
5 Speed of compressor is low because of unbalanced
forces
Speed of compressor is high because of
perfect balancing.
6 Reciprocating air compressor has more number of
moving parts, so it needs proper lubrication and more
maintenance.
Rotary air compressor has less number
of moving parts therefore less
maintenance is required.
7 Size of compressor is large for given discharge. Size of compressor is small for the given
discharge.
8 Air delivered is less clean, as it comes in contact with
lubricating oil.
Air delivered is more clean, as it does
not comes in contact with lubricating