fluid power enginnering:reciprocating compressor

1. MADE BY . SHARMA RAJESH C.
ENROLL.NO. 130140119095

2. TOPIC : RECIPROCATING
COMPRESSOR

3. Introduction to
compressors
 The machine which takes in air or any
other gas at low pressure and
compresses it to high pressure is
called compressor. The compressor is
power consuming machine in which
mechanical work is converted into
pressure energy of fluid. They are also
considered as reversed heat engine.

4. Uses of compressed air
To compress the refrigerant and to
create pressure difference in system.
Operating pneumatic tools.
To operate air motor in mines where fire
risks are more.
To compress air in gas turbine power
plant.
 Spray painting.
For sand blasting.
Operating blast furnaces.

5. Classification of compressors
 The compressors are classified as
follows:
According to design and principle
of operation:
Positive displacement type
compressor: In this type, air/gas is
compressed by positive displacement
of air by piston or rotating elements.
 Examples: Reciprocating compressor
, Rotary compressor, etc.

6. Reciprocatin
g
Rotary

7. Roto dynamic compressors: In this type,
compression is carried out by a rotating
element imparting velocity to the flowing
gas and develops desired pressure, and
compression is achieved by dynamic
action of rotor.
 Examples: Centrifugal compressor

8. Axial flow Mixed flow

9.  According to number of stages:
 Single stage compressor – Low delivery
pressure(≤10 bar)
 Multi stage compressor – High delivery
pressure(≥10 bar)
According to final(delivery) pressure of gas
 Low pressure compressors : final pressure < 10 bar
 Medium pressure compressors : final pressure
between 10 bar to 80 bar
 High pressure compressors : final pressure between
80 bar to 1000 bar
 According to pressure ratio:
 Fan : pressure ratio <1.1
 Blower : pressure ratio >1.1 and <2.3
 Compressors : pressure ratio >2.3

10. According to quantity of air delivered
(capacity):
Low capacity : volume flow rate<10
Medium capacity : volume flow rate between
10 to 300
High capacity : volume flow rate >300
 According to number of cylinders:
Single cylinder

11. Multi cylinder
According to type of fluid to be
compressed
Air compressor
Gas compressor
Vapor compressor

12. According to action of piston:
Single acting compressors: They are the
reciprocating compressor which has piston
working only in one direction. There is one
suction and delivery stroke per revolution of
crank.

13.  Double acting compressors: As from its name it uses
its both sides to compress the air. These types of
compressors have two sets of suction/intake and
delivery/exhaust valve on both sides of the piston .As the
piston moves up and down, both sides of the piston is
utilized in compressing the air. The intake and exhaust
valve operates corresponding to the movement of the
piston or with the stroke of the compressor. The air is
compressed accordingly and delivered continuously as
compared to single-acting air compressor. Here both of
sides are effectively used for compressing the air.

14. According to methods of cooling:
Water cooled air compressors
Air cooled air compressors
According to arrangements of cylinders:
Vertical
Horizontal
V – type

15. RadialW – type
According to types of drives:
Motor driven
I.C. engine driven
Turbine driven

16. ROTO--DYNAMIC
RADIALFLOW
(CENTRIFUGAL)
COMPRESSORS
POSTIVE
DISPLACEMENT
SINGLE STAGE
RECIPROCATING ROTARY
MULTI STAGE
DOUBLE ACTINGSINGLE ACTING
TURBO COMPRESSOR
MIXED
FLOW
EJECTOR
AXIAL
FLOW

17. Reciprocating compressor
 Reciprocating compressors are widely
used for compressing air and it is
satisfactory for all ranges of pressures.
The number of cylinders may be more
than one. In single stage reciprocating
compressor pressure ratio is kept 5 to
8, and the speed varies from 100 to
1500 rpm.

18.  Reciprocating compressors are driven
by electric motors, or I.C. engine. It is
not driven by gas turbines because
turbines have high rotational speed. The
air/gas coming from reciprocating
compressor is not continuous but
intermittent. So the air/gas is stored in
receiver from where continuous flow of
air/gas can be supplied.
Reciprocating compressor generally
seen where there is requirement of high
pressure and low flow. Mostly where the
air is used for hand-tools, cleaning dust,

19. Construction and working
 The single stage, single cylinder, single
acting compressors consists of piston,
cylinder, cylinder head, connecting rod,
crank shaft, flywheel, crank case, water
jacket, suction valve and delivery valve as

20.  The compressor takes inside
successive amount of volume of air
from intake valve and confined it in
closed surface at that time piston
moves downward with the closure of
intake valve. Then there
is compression of air by reducing its
volume .Now the piston moves
upward and compress the air and
then displace the compressed air
through exhaust valve. And then again
intake take place and cycle repeat

21.  These types of compressor also called
positive displacement machines. They are
available in both as lubricated and oil-free.
 The reciprocating compressor is single
acting when the compressing is
accomplished using only one side of piston
and double acting when both the sides of
piston used.

22. Advantages:
Relatively Cheap.
Easy maintenance
Suitable for high pressure
Disadvantages:
Sounds too much. One has to arrange a
room for it or put it into isolating box.
High outlet temperature of compressed air.
High oil content in air piping.

23. Single stage reciprocating compressor :
 Let us consider piston to be at top
dead centre (TDC) and move towards
bottom dead centre (BDC). Air gets into
cylinder during this stroke and is
subsequently compressed in next
stroke with both inlet valve and exit
valve closed. After piston reaching
BDC it reverses its motion and
compresses the air inducted in
previous stroke. Compression is
continued till the pressure of air inside
becomes sufficient to cause deflection

24.  At the moment when exit valve plate gets lifted the
exhaust of compressed air takes place. This
piston again reaches TDC from where downward
piston movement is again accompanied by
suction. This is how reciprocating compressor
keeps on working as flow device. Reciprocating
compressor described above has suction,
compression and discharge as three prominent
processes getting completed in two strokes of
piston or one revolution of crank shaft.

25. Work done:
 Compression of air in compressor may be
carried out following number of
thermodynamic processes such as
isothermal compression, polytropic
compressor or adiabatic compressor. Fig.
shows the thermodynamic cycle involved
in compressor. Theoretical cycle is shown
neglecting clearance volume but in actual
cycle clearance volume cannot be
negligible.
 Clearance volume is necessary in order to
prevent collision of piston with cylinder
head, accommodating valve mechanism

26.  The compressor is work consuming machine,
hence work done is negative.
 Let,
 pressure of air at inlet of compressor
 pressure of air at outlet of compressor
 volume of air before compression
 volume of air after compression
 absolute temperature of air before
compression
 absolute temperature of air after compression

27. Isentropic Compression
,
1
2
1 1
1
1
1
p
W p V
p


 
  
   
   




polytropic compression
1
2
1 1
1
1
1
n
n
pn
W p V
n p

 
  
   
   
 

28. Isothermal compression
2
1
1 ln
p
W RT
p
 
  
 
If compression process is isothermal, pV = C,
temperature remains constant, the change of internal
energy during compression process is zero. Thus as per
law of conservation of energy the entire work of
compression is related to the cooling medium
(surroundings). It is clear from for p-V, the area 1-2”-3-
4 is less than area 1-2-3-4 and area 1-2’-3-4. It means
that in isothermal compression no energy is wasted in
heating the air or increasing the internal energy. It has
been found that work required for compression is
minimum when the process is isothermal.

29. Methods of cooling:
 Faster heat dissipation from inside of
compressor to outside by use of fins over
cylinder. Fins facilitate quick heat transfer
from air being compressed to atmosphere
so that temperature rises during
compression can be minimized.
 Water jacket may be provided around
compressor cylinder so that heat can be
picked by cooling water circulating through
water jacket. Cooling water circulation
around compressor regulates rise in
temperature to great extent.

30.  The water may also be injected at the end of
compression process in order to cool the air
being compressed. This water injection near
the end of compression process requires
special arrangement in compressor and also
the air gets mixed with water and needs to be
separated out before being used.
 In case of multistage compression in different
compressors operating serially, the air leaving
one compressor may be cooled up to ambient
state or somewhat high temperature before
being injected into subsequent compressor.
This cooling of fluid being compressed
between two consecutive compressors is
called intercooling and is frequently used in
case of multistage compressors.

31. Single stage reciprocating
compressor with clearance:
 With clearance volume the cycle is
represented on Fig. The work done for
compression of air polytropically can be
given by the area enclosed in cycle 1-2-3-4.
Clearance volume in compressors varies
from 1.5% to 35% depending upon type of
compressor. In the cylinder of reciprocating
compressor (V1-V4) shall be the actual
volume of air delivered per cycle. Vd = V1 –
V4. This (V1 – V4) is actually the volume of
air in hated in the cycle and delivered

33. Work done for polytropic :
 
1
2
1 1 4
1
1
1
n
n
pn
W p V V
n p

 
  
    
   
 
3 4
1 3 3
1 1v
V V
V V V

 
   
  
Volumetric efficiency:

34. The volumetric efficiency decreases with
increase pressure ratio . At r = 1 ∴ p2 = p1 and
efficiency = 100%. It means that no compression
takes place. For given value of C and the delivery
pressure is limited due to decrease in in single
stage reciprocating compressor. Hence it is
necessary to go for multistage compressor to get
desired pressure with satisfactory value of
.However higher pressure ratio up to 8 in a single
stage refrigerating compressor is used as
compared to air compressor because n in
refrigeration system is lower than that of air.

35. The efficiency decreases with
increase in the clearance ratio C.
At C = 0, clearance volume= 0,
efficiency= 100%, no clearance is
provided.
When the value of C and r are
constant, efficiency increases with
n.
1
2
1
1
n
v
p
C C
p

 
      

 

36. Free Air Delivery (FAD):
 The free air delivery is the actual volume
of air delivered by the compressor when it
reduced to ambient pressure and
temperature conditions. It is expressed in
and represents the capacity of compressor.
Following factors reduce the FAD less than
swept volume and volumetric eff.
 The presence of clearance volume.
 The throttling of the air when it passes
through the inlet and outlet valves.
 Heating of incoming air and
 Leakage.

37. Actual (indicator) diagram for
single stage reciprocating
compressor

38. Mean effective pressure
The area of indicated diagram represents the
work done per cycle. The average height of the
indicator diagram in proper unit of pressure is
called the indicated mean effective pressure ( )mp
Area of indicator diagram X spring constant
Length of indicator diagram
mp 
workdone / cycle
swept volume / cycle
mp 

1
2
1
1
1
1
n
n
m v
pn
p p
n p


 
  
  
   
 


39. Indicated power
The power consumed inside the cylinder, as mea
sured from the indicator diagram is termed as the
indicated power.
Wo
In
rk
dica
done
ted powe
/ min
r(IP) = kW
60X1000

But, work done = mL.A.N(xp ×i), Nm
m .L.A.N
60X10
p
IP = (x ×i), i
0
W
0
n k 
x = no. of cylinders per stage
i = 1 for single acting
i = 2 for double acting

40. Compressor efficiencies
•Mechanical efficiency( ): It is ratio of
Indicated power (IP) to the Brake power (BP) of
compressor.
m
The power required to drive the compressor is
called the brake power or shaft power of the
compressor. 2
. ,
60000
NT
B P kW

 
. .
.
m
I P
B P
 
It varies from .85 to .96.

41. iso
iso
isothermal work input
actual work input

 

 
•Isothermal efficiency( ) : It is the ratio of
isothermal work input (minimum work input) to
actual work input(polytropic work input).
 
 
 
 
  
  
  
 

2
1 1
1
n-1
n
2
1 1
1
p
p V ln
p
pn
p V - 1
n - 1 p
It is 100% when compression process is
isothermal (best because compressor needs
minimum work).

42. •Adiabatic efficiency( ): It is the ratio of
actual work input to adiabatic work input.
adη
ad
Actual work input
Adiabatic work input

 

 
 
  
  
  
 
 
  
  
  
 
n-1
n
2
1 1
1
γ-1
γ
2
1 1
1
pn
p V - 1
n - 1 p
=
pγ
p V - 1
γ - 1 p

43. Multi-stage reciprocating air
compressor
There are several disadvantages to compress the air at
high pressure in single stage reciprocating compressor.
To eliminate the limitations of single stage, the air is
compressed by more than one stage. As we know, if
pressure ratio is increased in single stage reciprocating
compressor the volumetric efficiency decrease. The
temperature of the air after compression is so high as to
cause mechanical problems and the amount of heat is
actually the energy loss. so compressor has to handle
more volume of air at high temperature.

44. The increase in temperature needs heavy
working parts. With this reasons, in multi-
stage compression the overall
compression ratio is divided into two or
more stages to have lower compression
ratios. An intercooler is used between two
stages so that the compressed air from the
first stage cools down before it enters
second stage with low temperature.

45. Advantages of multi-stage
compression
The work done in compressing the air is
reduced, thus power can be saved.
Prevents mechanical problems as the air
temperature is controlled.
The suction and delivery valves remain in
cleaner condition as the temperature and
vaporization of lubricating oil is less
The machine is smaller and better balanced
Effects from moisture can be handled better, by
draining at each stage
Compression approaches near isothermal

46. Disadvantages:
A multi-stage compressor is more expensive in
initial cost than same capacity single stage
reciprocating compressor. This is due to multi-
stage compressor needs more than one cylinder,
intercoolers with water supply system.

47. Two stage- reciprocating air
compressor

49. Condition for minimum work or
maximum efficiency
The work required per cycle for the above two
stage reciprocating compressor without
clearance and perfect intercooling is given by:
 
    
    
    
 
n-1 n-1
n n
32
1 1
1 2
ppn
W = p V
p p
+
n -1
- 2
The work input will be minimum when
2
0
dW
dp

3 2
2 1
, pressure ratio per stage is equal
p p
p p
 
2 3 1p p p 

50. Minimum work required for two
stage compressor
 
  
  
  
 
n-
3
1
2n
min 1 1
1
-
p2n
W = p V
n - 1 p
1
If there are N numbers of stages, then condition o
minimum work required is
3 N+12 4
1 2 3 N
p pp p
= = = _ _ _ _ = = z say
p p p p
 
1
N 1
N+1
N
1
2
1
pressure ratio through compressor= =
pp
z =
p p
 
 
 
1
1
1 1
1
1
1
n
Nn
N
min
pNn
W p V
n p


 
  
    
 


 


51. Following conditions are necessary
for compressing air with minimum
work input in a multi-stage
compressor with intercooler:
•Compression ratio in each stage is equal.
•Air is cooled to initial temperature after
each stage of compression.
•Work required in all stages is equal.

52. Actual p-V (indicator) diagram
for two stage compressor

53. References:
Fluid Power Engineering by J.P.Hadiya,
Books India Publications.
Images and information from Google.

54. THANK YOU