3. COMPRESSOR – A device which takes a definite quantity of fluid ( usually gas, and
most often air ) and deliver it at a required pressure.
Air Compressor – 1) Takes in atmospheric air,
2) Compresses it, and
3) Delivers it to a storage vessel ( i.e. Reservoir ).
Compression requires Work to be done on the gas,
Compressor must be driven by some sort of Prime Mover ( i.e. Engine )
Air Compressors
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4. Reciprocating Rotary
Single – acting
Double - Acting
No. of Sides of Piston
in operation
No. of Stages
for Compression
Centrifugal
Single – stage
Multi - stage
Classification
Air Compressors
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5. Reciprocating Compressor - Working
2. Principle of Operation
Fig. shows single-acting piston actions in
the cylinder of a reciprocating compressor.
The piston is driven by a crank shaft via a
connecting rod.
At the top of the cylinder are a suction
valve and a discharge valve.
A reciprocating compressor usually has
two, three, four, or six cylinders in it.
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7. Reciprocating Compressor – Equation for Work
Volume
Pressure
P1
P2
V1V2
3 2 2”2’
4 1 (Polytropic)
(Adiabatic)
(Isothermal)
CVP n
CVP
CVP
Operations : 4 – 1 : Volume V1 of air aspirated into Compressor, at P1 and T1.
1 – 2 : Air compressed according to PVn = Const. from P1 to P2.
→ Temp increase from T1 to T2.
2 – 3 : Compressed air at P2 and V2 with temperature T2 is delivered.
8. Reciprocating Compressor – Equation for Work
During Compression, due to the excess temperature above surrounding, the air will
exchange the heat to the surrounding.
Compression Index, n is always less than γ, the adiabatic index.
As Compressor is a work consuming device, every effort is desired to reduce the work.
Work done = Area under P-V curve
1 – 2” : Adiabatic Compression = Max. Work.
1 – 2 : Polytropic Compression
1 – 2’ : Isothermal Compression = Min. Work.
9. Reciprocating Compressor – Equation for Work
Thus, comparison between the Isothermal Work and the Actual Work is important.
Isothermal Efficiency, ηiso =
Isothermal Work
Actual Work
Thus, more the Isothermal Efficiency, more the actual compression approaches to the
Isothermal Compression.
P1
P2
V1V2
3 2 2”2’
4 1(Polytropic)
(Adiabatic)
(Isothermal)
CVP n
CVP
CVP
Actual Work = Wact = Area 4-1-2-3-4
Wact = Area (4-1) – Area (1-2) – Area (2-3)
1
1
1
2211
2211
1122
2211
22
1122
11
n
VPVP
VPVP
n
VPVP
VPVP
VP
n
VPVP
VP
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12. Reciprocating Compressor – Equation for Work
P1
P2
V1V4
6 2
5 1
CVP n
3
4
V3
Effective Swept Volume, V1-V4
Swept Volume, V1-V3=Vs
Total Volume, V1
Clearance Volume,
V3=Vc
Clearance Volume :
Volume that remains inside the cylinder
after the piston reaches the end of its
inward stroke.
Thus, Effective Stroke Volume = V1 – V4
Actual Work = Wact = Area 1-2-3-4
Wact = Area (5-1-2-6) – Area (5-4-3-6)
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13. Reciprocating Compressor – Equation for Work
n
m
n
m
act
P
P
VP
n
n
P
P
VP
n
n
W
1
1
2
41
1
1
2
11
1
1
1
1
n
act
P
P
P
P
VVP
n
n
W
/1
2
1
1
2
411 1
1
n
m
n
m
act
P
P
VP
n
n
P
P
VP
n
n
W
1
4
3
44
1
1
2
11 1
1
1
1
P1
P2
V1V4
6 2
5 1
CVP n
3
4
V3
Effective Swept Volume, V1-V4
Swept Volume, V1-V3=Vs
Total Volume, V1
Clearance Volume,
V3=Vc
But, P4 = P1 and P3 = P2
14. Reciprocating Compressor – Volumetric Efficiency
Volumetric Efficiency :
Ratio of free air delivered to the displacement of the compressor.
Ratio of Effective Swept Volume to Swept Volume.
Volumetric Efficiency =
Effective Swept Volume
Swept Volume
V1 – V4
V1 – V3
=
Vc
Vs
= = γ
Clearance Volume
Swept Volume
Clearance Ratio =
Presence of Clearance Volume
Volumetric Efficiency less than 1. ( 60 – 85 % )
P1
P2
V1V4
6 2
5 1
CVP n
3
4
V3
Effective Swept Volume, V1-V4
Swept Volume, V1-V3=Vs
Total Volume, V1
Clearance Volume,
V3=Vc
( 4 – 10 % )
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15. Reciprocating Compressor – Volumetric Efficiency
↑ Pr. Ratio ↑ Effect of Clearance Volume
….Clearance air expansion through greater volume before intake
Cylinder bore and stroke is fixed.
Effective Swept Volume (V1 – V4) ↓ with ↑ Pr. Ratio
↓ Volumetric Efficiency
3
4
31
3
31
3
3
3
31
4
31
3
31
4
31
3
31
4331
31
41
1
1
1
V
V
VV
V
VV
V
V
V
VV
V
VV
V
VV
V
VV
V
VV
VVVV
VV
VV
vol
P1
P2
V1V4
6 2
5 1
3
4
V3
Effective Swept Volume,
V1-V4
Swept Volume, V1-
V3=Vs
Total Volume, V1
Clearance Volume,
V3=Vc
16. Reciprocating Compressor – Volumetric Efficiency
11
11
11
11
/1
4
3
/1
4
3
31
3
4
3
31
3
4
3
31
3
n
vol
n
vol
vol
vol
P
P
P
P
VV
V
V
V
VV
V
V
V
VV
V
P1
P2
V1V4
6 2
5 1
3
4
V3
Effective Swept Volume,
V1-V4
Swept Volume, V1-V3=Vs
Total Volume, V1
Clearance Volume,
V3=Vc
17. Reciprocating Compressor – Multistage
High Pressure required by Single – Stage :
1. Requires heavy working parts.
2. Has to accommodate high pressure ratios.
3. Increased balancing problems.
4. High Torque fluctuations.
5. Requires heavy Flywheel installations.
This demands for MULTI – STAGING…!!
18. Reciprocating Compressor – Multistage
Series arrangement of cylinders, in which the compressed air from earlier cylinder (i.e.
discharge) becomes the intake air for the next cylinder (i.e. inlet).
Intercooler :
Compressed air is cooled
between cylinders.
L.P. = Low Pressure
I.P. = Intermediate
Pressure
H.P. = High Pressure
L.P.
Cylinder
I.P.
Cylinder
H.P.
Cylinder
Intercooler
Intercooler
Air Intake
Air Delivery
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19. Reciprocating Compressor – Multistage
Intake Pr.
P1 or Ps
Delivery Pr.
P3 or Pd
3
2
9 5
4
1
CVP n
8
7
6
Intermediate Pr.
P2 CVP
Without Intercooling
Perfect Intercooling
L.P.
H.P.
Volume
Overall Pr. Range : P1 – P3
Single – stage cycle : 8-1-5-6
Without Intercooling :
L.P. : 8-1-4-7
H.P. : 7-4-5-6
With Intercooling :
L.P. : 8-1-4-7
H.P. : 7-2-3-6
Perfect Intercooling : After initial compression in L.P. cylinder, air is cooled in the
Intercooler to its original temperature, before entering H.P. cylinder
i.e. T2 = T1 OR
Points 1 and 2 are on SAME Isothermal line.
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20. Reciprocating Compressor – Multistage
Ideal Conditions for Multi – Stage Compressors :
A. Single – Stage Compressor :
CVP
3
2
9 5
4
1
CVP n
8
7
6
L.P.
H.P.
Single – stage cycle : 8-1-5-6
1
1
5
11 1
1
n
n
P
P
VP
n
n
W
Delivery Temperature,
n
n
P
P
TT
1
1
5
15
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21. Reciprocating Compressor – Multistage
CVP
3
2
9 5
4
1
CVP n
8
7
6
L.P.
H.P.
B. Two – Stage Compressor (Without Intercooling) :
Without Intercooling :
L.P. : 8-1-4-7
H.P. : 7-4-5-6
n
n
n
n
P
P
VP
n
n
P
P
VP
n
n
W
1
4
5
44
1
1
4
11
1
1
1
1
This is SAME as that of Work done in Single – Stage.
Delivery Temperature also remains SAME.
Without Intercooling
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22. Reciprocating Compressor – Multistage
CVP
3
2
9 5
4
1
CVP n
8
7
6
L.P.
H.P.
C. Two – Stage Compressor (With Perfect Intercooling) :
With Intercooling :
L.P. : 8-1-4-7-8
H.P. : 7-2-3-6-7
n
n
n
n
P
P
VP
n
n
P
P
VP
n
n
W
1
2
3
22
1
1
4
11
1
1
1
1
Delivery Temperature,
12
1
2
3
1
1
2
3
23 , TTas
P
P
T
P
P
TT
n
n
n
n
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23. Reciprocating Compressor – Multistage
CVP
3
2
9 5
4
1
CVP n
8
7
6
L.P.
H.P.
C. Two – Stage Compressor (With Perfect Intercooling) :
With Intercooling :
L.P. : 8-1-4-7-8
H.P. : 7-2-3-6-7
n
n
n
n
P
P
P
P
VP
n
n
W
1
2
3
1
1
2
11 2
1
Now, T2 = T1
P2V2 = P1V1
Also P4 = P2
Shaded Area 2-4-5-3-2 : Work Saving due to Intercooler…!!
24. Reciprocating Compressor – Multistage
Condition for Min. Work :
CVP
3
2
9 5
4
1
CVP n
8
7
6
L.P.
H.P.
Intermediate Pr. P2 → P1 : Area 2-4-5-3-2 → 0
Intermediate Pr. P2 → P3 : Area 2-4-5-3-2 → 0
There is an Optimum P2 for which Area 2-4-5-3-2 is
maximum,
i.e. Work is minimum…!!
n
n
n
n
P
P
P
P
VP
n
n
W
1
2
3
1
1
2
11 2
1
0
2
1
2
3
1
1
2
2
dP
P
P
P
P
d
dP
dW
n
n
n
n
For min. Work,
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25. Reciprocating Compressor – Multistage
Condition for Min. Work :
0
111 1
1
2
1
3
1
1
21
1
n
n
n
n
n
n
n
n
P
n
n
PP
n
n
P
0
2
1
2
3
1
1
2
2
dP
P
P
P
P
d
dP
dW
n
n
n
n
n
n
n
n
n
PP
P
P 1
3112
2
/1
2
31
2
2 PPP
2
3
1
2
312
P
P
P
P
ORPPP
CVP
3
2
9 5
4
1
CVP n
8
7
6
L.P.
H.P.
26. Reciprocating Compressor – Multistage
P2 obtained with this condition (Pr. Ratio per stage is equal) is the Ideal Intermediate Pr.
Which, with Perfect Intercooling, gives Minimum Work, Wmin.
n
n
P
PP
VP
n
n
W
1
1
2/1
31
11 1
1
2
n
n
P
P
VP
n
n
W
1
1
2
11 1
1
2
n
n
P
P
VP
n
n
W
2
1
1
3
11 1
1
2
Equal Work per cylinder…!!
27. Reciprocating Compressor – Efficiency
How to Increase Isothermal Efficiency ?
A. Spray Injection : Assimilation of water into the compressor cylinder towards the
compression stroke.
Object is to cool the air for next operation.
Demerits : 1. Requires special gear for injection.
2. Injected water interferes with the cylinder lubrication.
3. Damage to cylinder walls and valves.
ater must be separated before delivery of air.
B. Water Jacketing : Circulating water around the cylinder to help for cooling the
air during compression.
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28. Reciprocating Compressor – Efficiency
How to Increase Isothermal Efficiency ?
C. Inter – Cooling : For high speed and high Pr. Ratio compressors.
Compressed air from earlier stage is cooled to its original
temperature before passing it to the next stage.
D. External Fins : For small capacity compressors, fins on external surfaces are useful.
E. Cylinder Proportions : Short stroke and large bore provides much greater surface
for cooling.
Cylinder head surface is far more effective than barrel surface.
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29. Reciprocating Compressor – Efficiency
Clearance Volume : Consists of two spaces.
1. Space between cylinder end & the piston to allow for wear.
2. Space for reception of valves.
High – class H.P. compressors : Clearance Vol. = 3 % of Swept Vol.
: Lead (Pb) fuse wire used to measure the gap between
cylinder end and piston.
Low – grade L.P. compressors : Clearance Vol. = 6 % of Swept Vol.
: Flattened ball of putty used to measure the gap
between cylinder end and piston.
Effect of Clearance Vol. :
Vol. taken in per stroke < Swept Vol. ↑ Size of compressor
↑ Power to drive compressor.
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30. P1
P2
V1V4
6 2
5 1
3
4
V3
Effective Swept Volume,
V1-V4
Swept Volume, V1-V4=Vs
Total Volume, V1
Clearance Volume,
V3=Vc
Reciprocating Compressor – Work Done
n
n
n
n
P
P
VP
n
n
P
P
VP
n
n
W
1
4
3
44
1
1
2
11 1
1
1
1
Assumption : Compression and Expansion follow same Law.
Work / cycle = Area 1-2-3-4-1
P3 = P2 and P4 = P1
n
n
a
n
n
P
P
VP
n
n
P
P
VVP
n
n
W
1
1
2
1
1
1
2
411
1
1
1)(
1
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31. P1
P2
V1V4
6 2
5 1
3
4
V3
Effective Swept Volume,
V1-V4
Swept Volume, V1-V4=Vs
Total Volume, V1
Clearance Volume,
V3=Vc
Reciprocating Compressor – Work Done
n
n
P
P
TRm
n
n
W
1
1
2
11 1
1
m1 is the actual mass of air delivered.
Work done / kg of air delivered :
n
n
P
P
TR
n
n
W
1
1
2
1 1
1
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32. Rotary compressors
These compressors use rotors in place of pistons, giving a pulsating free
discharge air. These rotors are power driven. They have the following
advantages over reciprocating compressors:
oThey require a lower starting torque
oThey give a continuous, pulsation free discharge air
oThey generally provide higher output
oThey require smaller foundations, vibrate less, and have lesser
parts, which means less failure rate
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