This document provides information about reciprocating compressors. It defines reciprocating compressors as using pistons driven by a crankshaft to compress gas in a cyclic manner. The key components of reciprocating compressors are identified as the compression cylinder, piston, intake and outlet valves, crankshaft, and connecting rod. The thermodynamic cycle and p-V diagram of reciprocating compressors are discussed. The document also covers indicated power, mechanical efficiency, conditions for minimum work, clearance volume, volumetric efficiency, and multistage compressors.

1. RECIPROCATING COMPRESSOR
Presented by
Prof. PRABHA CHAND
Department of Mechanical Engineering
National Institute of Technology, Jamshedpur

2. 2
Contents
1. W
orkingCycleandp-vDiagram
2. Indicated Powerandwork
3. Mechanical Efficiency
4. Conditions for minimumwork
5. Isothermal Efficiency
6. Clearance V
olume
7. V
olumetric Efficiency
8. MultistageCompressor

3. 3
What iscompressors?
 Compressor is a mechanical device that increases the
pressureof gasbyreducingitsvolume.
 It usesmechanicalworktotakeanamountoffluidanddeliveritata
requiredpressure.
 Anefficientcompressorincreasespressurewithminimumwork
 The amountof fluid is limited by the volume of the compressor
cylinder whichisfixed.

4. 4
Applications
 Inagasturbineoraircraft/jetengine
 Inrefrigerationandair-conditioningsystem
 Incar
 Inworkshops
 Forpneumaticsystem
 Formedical purposes
 Etc…

5. 5
Typeof compressors

6. Reciprocatingcompressors
Reciprocatingcompressorsalsoknownaspistoncompressorbecauseituse
pistondrivenbyacrankshaft andoperatesinacyclicmanner
.
Theycanbedirect-orbelt-drivenbyelectricmotorsorinternalcombustion
engines,canbeeitherstationaryorportableandcanbesingleormulti-
staged.
Reciprocatingcompressorsaretypicallyusedwherehighcompression ratios
arerequiredperstagewithouthighflowrates,andtheprocessfluidis
relativelydry
.
Smallreciprocatingcompressorsrangefrom5to30hparecommonly
seeninautomotiveapplications.
Largerreciprocatingcompressorswellover1,000hp(750kW)are commonly
foundinlargeindustrialandpetroleumapplications.Discharge pressurescan
rangefromlowpressuretoveryhighpressure(>180MPa).Incertain
applications,multi-stagedouble-actingcompressorsaresaidto bethemost
efficient compressorsavailable,andaretypicallylarger,and morecostlythan
comparablerotaryunits.
6

7. 7
1. Air intake,
2.Compressor pump,
3.Outlet,
4. Drive belt,
5. Motor,
6. Control switch,
7. Relief valve,
8. Pressure gauge,
9. Manifold,
10. Regulator,
11. Supply line,
12. Air tank/receiver,
13. Water drain,

8. Basiccomponentsof reciprocatingcompressor
Compressioncylinder– provide
confinement for theprocessgas
during compression.
Piston – moves through the cylinder in a
reciprocatingactiontocompressthegas.
Intakevalve– tolettheairtodrawninto
thecylinder.
Outlet valve – to let air out of the
cylinder. Crank shaft – connect to the
motorto providetherotarymotion.
Connecting rod – connect the crank and
thepiston– providethereciprocatingups
anddownmovement.
8

9. Workingprincipleofreciprocatingcompressor
9
Refrigerant vapor to air
Whatis:
- topdeadcenter,TDC
- bottomdeadcenter,BDC
- bore,d
- stroke,L
- sweptvolume,Vs
- inducedvolume, Vin

10. 10
Thermodynamicscycleandp-vdiagram
4-1 Induction/intakeprocess
Pistontravel fromTDCtoBDC.
Inductionvalveopens
Airisinducedintothecylinder
Volumeandmassincreases ↑
Pressureandtemperature is
constant duringthisprocessat P1, T1
1 -2 Compressionprocess
Piston travel fromTDCtoBDC
Inlet valvecloses
Pistoncompressesair, Vol ↓
Pressurerises until reachP2 at (2)
T
emperaturealsoincreases
2 -3 Deliveryprocess
Deliveryvalveopensat pt
(2) Highpressureair is
delivered
Note: Compressionprocessis
reversiblepolytropicand
followsthelawpVn =C
Thethermodynamicscycleofreciprocatingcompressorisalwaysshowgraphicallythrough
thep-vdiagram.

11. 1
1
WorkandIndicatedpower
p2Vb  p1Va 
n 1
n
n 1
W
1 a

p2Vb  p1va  (n 1) p2vb  (n 1) p1va
 p2Vb  p1Va
n 1

p2Vb  p V 
Thework doneonair for onecycle is
theareainthegraph(areaabcd)
in  area abcd
cycle
 area abef  area bc0e  area ad0f

2 1
Win
Since p1Va  mRT1 and p2Vb  mRT2
So, work input per cycleis given by
n 1
 mRT T 
mRT2  mRT1 
n 1
n
n
cycle
 where m N  m
and N  rpm
Indicated Power
1

 T 
IP 
n
m R T
n 1 2
0 f
e
d
P2
c b
a
V
p
P1

12. Summary

13. 13
Example 3.1
Singleactingcompressor withoutclearance
Asinglestagereciprocatingcompressoroperatesbyinducing1 m3/minof
airat1.013barand15ºCanddelivers it at 7 bar. Assumethecompression
processbeingpolytropicand the polytropicindexis1.35.
Calculate:
(a) massof airdeliveredperminute, and
(b) indicatedpower.

14. 14
Mechanical Efficiency, ηm
 Because there are movingmechanical parts in the compressor, it is likelythat
losseswill occurduetofriction.
 Therefore power required to drive the compressor is actually more higherthan
theindicatedpower
.
 Sothemechanical efficiencyof thecompressor isgivenby:
ηm
Power system
[Power required]
Compressor
[Indicated power]
>

15. 15
Example 3.2
Mechanical Efficiency
If the compressor of previous example is to be driven at 300
rev/min and is a single-acting, single-cylinder machine, calculate
the cylinder bore required, assumingastroke to bore ratio of 1·5/1.
Calculate the power of the motor required to drive the compressor
if the mechanical efficiency of the compressor is 85% and that of
the motor transmissionis90%.

16. 16
 Weknowthat theworkdoneis equalto theareaunderthegraph.
 Smaller areameanslessworkandthebetter thecompressor.
 For reciprocating compressors, the pressure ratio is fixed, so the
height of p-vdiagramisfixed.
 Thevolumeof cylinder is alsofixed sotheline 4-1is fixed.
 Therefore thearearepresentingwork depends ontheindexfor n.
 For n=1,
pV=constant (isothermal)
 For n=,
pV =constant (isentropic)
 So,theprocesscanbepolytropic, isothermal orisentropic
Conditionsfor MinimumWork

17. 17
o pV
o pV
o pVn
=constant (isothermal)
=constant (isentropic)
=constant (polytropic)
o Fromhereit canbeseenthat the
isothermal processisthebest
becauseit requiresminimumwork.
o Soitisbestthatthegastemperatureis
constantthroughout the compression
cycle.
Conditionsfor MinimumWork

18. Conditionsfor MinimumWork
**Isothermalefficiencyindicatesisothermalworkcomparedtotheindicatedwork.

19. Air is compressedin asingle-stage reciprocating compressor from 1.013
barand15°Cto7bar.Calculatetheindicatedpower requiredforafree
airdeliveryof0.3m3/min,whenthecompressionprocessisasfollows:
(a) isentropic
(b) isothermal
(c) polytropic,withn=1.25
Whatisthedeliverytemperatureateachcase?
Ifthecompressorissingle-actingandhasastroketoboreratioof1.2/1
andistorunat1000rev/min,calculatethesizeofboreandstroke
required.
19
Question3.3
Best conditionfor minimumwork

20. 20
Exercise
Isothermal efficiency
Asingle-stagereciprocatingcompressorinduced1.23kg/minof air at a
pressureof1.023barandtemperature23°Canddeliversit at8.5bar.If
polytropicindexis1.3, determine
(a) Indicatedpower,
(b) Isothermal power, and
(c) Isothermal efficiency

21.  In actual compressors, piston does
not reach the top of wall of the
cylinder head.
 Instead, it reaches maximum stroke
(TDC) at a certain distance from the
cylinder head.
cylinder where piston does
 The remaining volume of the
not
travel through
is call the clearance volume VC.
 The volume where the piston does
travel through is called the swept
volume, VS.
 Purpose of Vc : to give freedom for
working parts and space for valve
operations
ClearanceVolume, Vc

22. (ii) with clearancevolume
(i) without clearancevolume
Cycledifferentbetweenwithandwithoutclearance
volume

23. 23

24.  In actual compressor, the piston cannot
expel all the gases during delivery at
point 3.
 So, there are some compressed gas
trapped and left in the cylinder.
cylinder head, this compressed
expands according to pVn=C until
 When piston moves away from the
gas
the
pressure falls to p1 (reaches point 4).
 At point 4 the inlet valve opens and gas is
drawn in.
 The volume drawn in from point 4 to 1 is
called as induced volume, Vin is smaller
than the swept volume because of the
expansion process of the compressed
gas.
C
V =Clearancevolume
S
V =Swept volume
Thep-Vdiagramfor compressorwith, Vc

25.  Because of the expansion of gas
remaining in the VC, induced volume
is reduced from swept volume VS to
(V1–V4) which is the effective volume
 Mass of air per unit time
m
1  m
2 and m
3  m
4
 Mass delivered per unit time = mass
induced per unit time
m m
2  m
3  m
1  m
4
TheEffect of ClearanceVolume, Vc

26. m
1  m
4  m mass induced per unit time
 

min

 

min

m m  N  kg 







1
1 
 
p
 p2 
2  1 
 1   1 
  
p
n 1
m
1  m
4 RT2  T1 
Indicated powerW 
For compressor with N cycles per unit time (N rpm)
Therefore
with
1 4
m m  m  N  kg 
1
2
n 1
or
n
n1
n
 mRT1
n 1
n
W mRT  T 
V
V1
and      ,
T p
 T2   p2   p2 n
n
n
n1 1
IndicatedPowerfor CompressorwithClearanceVolume

27.  The Free Air Delivery (FAD) is the volume of air drawn into a compressor
fromtheatmosphere.
 Itisastandardizedmeasureofthecapacityofanaircompressor.
 Normally, the pressure inside the compressor during the induction processis
slightlylowerthantheatmosphericpressureoutsideofthecompressor.
 Fore.g:
FreeAir Delivery(FAD)

28. FreeAir Delivery(FAD)

29.  Volumetric efficiency is another definition to measure the
performance of a compressor.
 It can be defined as the ratio of the actual delivered gas volume to
the swept volume of the cylinder or,
 Where,
 If the volumetric efficiency is 100%, it
means the compressor has no clearance
volume.
VolumetricEfficiency, ηv

30.  The volumetric efficiency also can be written as:
 or
pressure ratio and clearance ratio increase.
 This efficiency is made worse if leaks occur past the valves or piston.



 From the definition, the volumetric efficiency decreases as




  1
1
s  1 
v
P
V
Vc  P2 n
 1
VolumetricEfficiency, ηv

31.  A single-acting compressor completes one compression cycle with one
revolutionofthecrank
 Adouble-acting compressor completes two compression cycles with onerevolutionofthe
crank
 Sothemassinduceperrevolutionistwicethanasingleactingwhere
 
 

min


min
 a d
m 2  N  m kg  or m 2  N  m  m  kg 
Delivery Delivery
Induction
Induction
DoubleActingCompressors

32. Gas is compressed in a reciprocating compressor from 1 bar to 6
bar. The FAD is 0.013 m3/s. The clearance ratio is 0.05. The
expansion processofthecyclefollowsthelawofpV1.2=C.Thecrank
speedis360 rev/min. Calculate,
(a) theswept volume, and
(b) thevolumetricefficiency
Question3.4
FAD

33. FAD
Asingle-stage, single-acting air compressor running at 1000 rev/min delivers air at 25bar
.For this
purpose the induction and freeairconditions can be taken as1.013barand 15°C,and theF
ADas
0.25 m3/min. Theclearance volume is 3%of the swept volume and the stroke/bore ratio is 1.2/1.
Calculate:
(a) theboreand stroke
(b) thevolumetricefficiency
(c) theindicatedpower
(d) theisothermalefficiency
T
aketheindexofcompressionand re-expansionas1.3
(Answers: 73.2mm;87.8mm;67.7%
; 2kW;67.7)
Question3.5

34. Question3.6
FAD
Thecompressorof Question3.5hasactual induction conditions of
1 barand 40°C,andthedeliverypressureis25bar.Takingthe
boreas calculatedinQuestion3.5, calculatetheFADreferred
to1.013bar and 15°C,and theindicatedpower
required. Calculatealsothevolumetric efficiencyand
compareit withthat of Question3.5.
(Answers: 0.226m3/min; 1.98kW; 61.4%)

35. Whendeliverypressureisincreasedtoa
highervalue, several weaknesseswill
occur:
i. inducedvolumewill beless
ii. increaseindeliverytemperature
iii. decreaseinvolumetricefficiency(Vin
become less were else no change in
Vs)
Toovercomethosematter,multi-staging
compressorisintroduced.
MultistageCompressors

36. Pi,Ti Pi,T1 P2,T2
P1,T1
Coolant in Coolant out
Intercooler
LP Compressor HP Compressor
 It consist of more than one compressor where the air passes through an
intercooler before entering the next compressor.
 The size of the next compressor is smaller to compromise Vs.
 In the intercooler, heat is transferred to the surrounding and temperature
will decreased. It will be brought back to its inlet temperature (before
induction process).
 It is assumed that all compressors will have the same polytropic index.
MultistageCompressors

37. e-f :
a-b : PVn=Ccompression
b-e : Qfromairtosurrounding
TemperaturedropsfromTbtoTe.
IdeallyTe=Ta
PVn=Ccompression
Advantages:
i. Slight increaseintemperature ii.
iii. Increaseinvolumetricefficiency
Savinginwork (shadedarea)
NOTES:
 Sincenomassisallowtoescapeduringitstravel, mLPC =mHPC
 If pressureratio andtheratio of Vc/Vsis the same,volumetric efficiency for both
compressoristhesame.
a
b
e
f
g
c h
d
Vc
p
V
Vs
P2
Pi
P1
LP CPMPRESSOR
HP
CPMPRESSOR
MultistageCompressors

38. TheIdeal IntermediatePressure

39. IndicatedPowerfor z-StagesCompressor

40. A single acting air compressor runs at 700 rpm and compresses air in two-
stages to 80 bar from an induction pressure of 1 bar with an ideal
intermediate pressureandcompleteintercooling. Thefree air delivery is 0.06
m3/s at the pressure 1.013 bar and 20oC with the clearance volume is 3%of
the swept volume in each cylinder. The index of the compression and re-
expansion is 1.3 in both cylinders and the temperature at the end of the
induction stroke in each cylinder is 30oC. The mechanical efficiency of the
compressor is85%. Calculate:
i. theindicatedpower required,
ii. thesavinginpower over single-stagecompressionbetweenthesame
pressure,
iii. theswept volumeforeachstage, and
iv. therequiredpoweroutputofthedrivemotor.
Exercise3.7 MultistageandIntercooling

41. In a single acting, two-stage reciprocating air compressor, 4.5 kg/min of air is
compressed from 1.013 bar and 15oC surrounding conditions through a
pressure ratio of 9 to 1. Bothstages have the same pressureratio, and the law
of compressionandexpansionin both stages is PV1.3=C. The clearance volume
ofbothstagesare5%oftheirrespectivesweptvolumesand itrunsat 300rpm.If
intercoolingiscompleteandacoolingwatercoolstheairto32oC calculate:
i. indicatedpower,
ii. volumetricefficiency,
iii. swept volumefor thehighpressurecompressor, and
iv. themassflowrateofthecoolingwaterifwaterenterstheintercooler
at20oCandexitat46oC.
v. Alsoshowsthepowersavingonp-vdiagramandgetthevaluein
percentage.
Exercise3.8 MultistageandIntercooling