compressor notes.pdf

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1. Module 5
CHAPTER
8
Compressors
Syllabus
Reciprocating
Air Compressor, Single stage compressor
-
computation
of work done,
isothermal
efficiency,
effect of clearance volume,
volumetric efficiency,
free air
delivery,
Theoretical and actual indicator diagram.
Multistage
compressors-Constructional
details of multistagecompressors,
Need of
multistage,
Computation
of work done, Volumetric efficiency,
Condition for maximum
efficiency,
Inter cooling
and after cooling (numerical),
Theoretical and actual indicator
diagram
for multi stage compressors.
Rotary
Air Compressors Classification,
Difference between compressors
and
blowers, Working
and constructional details of roots blower,
screw type
and vane type
compressors.
Compressor and their Classification
8.1 Definition of a Compressor
The compressors
are work absorbing
devices. They
can be defined as follows
Compressor:
A compressoris a device which
is used to increase the pressure ofa fluid
on the
expense of work energy supplied.Usually,
the compressors
are driven by motors, I.C. engines
or
gas turbines.
Air compressor:A compressor
with working fluid
as air is called an air compressor.
8.2 Classificationof Compressors
8.2
Compressorscan be classifiedin the followingways:
Based on design and principle of operations : Based on the principle of operations .
compressors can be classified as:
Positive displacement Compressors,
and
i) Non-positive or Steady flow compressors
)
ositive displacementcompressors:
These compressors
are further divided into:
(a)
Reciprocatingcompressors
(b) Rotary compressors

2. Themodynamics(MU)
8-2
Compreees
nt.
Due to
positive
(a)
displacement of air by the piston in the cylinder, the air is compressed and.
vessel called receiver.]
These are capable
of producing
Volume flow raes or With Tow mass flow ratC3
The reciprocating compressors
are considered as
ppen steady flow
systems Tti
the rate of heat and work transfers are at uniform rate..
Reciprocating compressors may by single acting compressor or
douhla
A reciprocating compressor
uses the piston cylinder arrangement. De
delivered to s large pressureratios
plies that
ouble actingMcompressor.
A single acting reciprocating compressor has one delivery stroke per revolution
of t two
delivery strokes ter
crankshaft, while the double acting reciprocating compressor has two delivery strol
revolution of the crankshai
Inpositive displacement rotary compressor the positive displacement of air or fluid :.
low.
to a rotating part causing compressionof fluid. These compressors rotate at hiph
therefore they can handld large volume or mass flow rate )but the pressure ratios are
etc.
Examples of positive displacementrotary compressors
are root
blower, vane blower eto
are
Non-positiveor steady flow compressors
:
Non-positive displacement
compressor
(b)
also called as
steady flow compressorse.g.lcentritugal compressors and axial
COnpresSOTS. Lnese are rotary compressors.
In these compressors,the fluid is not containedin a confined
space (i.e. by so boundarieslike
piston cylinder arrangement
of a
reciprocatingcompressor),but t moves at steady rate through the machine..
In
rotary compressorsthe dynamichead
imparted
to a fluid of solid boundar
(e.g. impeller of
centrifugal compressor ) causes the
pressure rise of the fluid.
Based on number of stages: Compressors based on number of stages are
classified as:
) Single stage compressor
Multistagecompressori)
The Single stage compressors are either air cooled (small size) or water cooled
compressors(big sizes). Normally
a single stage compressor is
employed when the
pressure ratio is limited upto 5.
Multistagecompressorsare used to achieve higher pressure ratios exceedingthe (ü)
compressionratio more than 5. Generally, the reciprocatingcompressorscan deliver the
following maximum pressures
Keciprocatmgcompressoraximum delivery pressures
Upto 5 bar Single stage compressor
5 bar to 35 bar Two stage compressor
Three stage compressor 35 bar to 80 bar
Four stage compressor
More than 80 bar
Based on
pressure limits
Depending upon the dischargepressures,the
compressors
are also classified as
Low
pressure compressors: Delivery pressure upto 1.1 bar
Medium
pressure compressors:Delivery pressure upto
7 bar
ii) High pressure
compressors: Delivery pressure more than 7 bar.
()

3. Thermodynamics(MU)
Cornpressors
Base on capacity of compressors
)
(i)
Low capacity compressors: Volume flow ratio
upto 10 m/min or less
Medium capacity compressors: Volume flow rates 10 m to 300 m per minute.
High capacity compressors: Volume flow rates above 300 m'/min.
(Gi)
8.2.1 Classification between Fan, Blower and
Compressors
per American Society
of
Mechanical
Engineers (ASME), the fans, blowers and compressors
As pe
are classifiec fied according
to
pressureratiosachieved.
These are as follows
Fans: Pressure ratio upto 1.1
Blower: Pressure ratio from 1.1 to 2.5 A
()
Pressure ratio above 2.5 Compressors:
(i)
8.2.2 Air Pumps
Compressors used for creating vacuum are called air pumps or exhausters.
Applications of Compressors
8.3 Applications / Practical uses of Compressed Air
MU May 12
University Question
Enumerate the various uses of air compressor
(May 12)
The compressed
air finds its use
in
many industrial
applications.Some of its uses are
To operate pneumatic tools like drill, hammers, riveting machine etc.
i) Driving a compressed air engine
() Spray painting
(iv) Refrigerationand air conditioningindustry.
Gas turbine
power plants
(vi)
Supercharging of LC.
engines,
(V1)
Conveying the materials like sand and concrete along
a pipe line
Vii)
Pumping of water.
ix)
Driving mining machinery where fire risks are too
many.
() In blast furnaces and boiler furnaces.
(xi)
Cleaning the surfaces by air blast.
Basic Concept of
ThermodynamicCycle
for Compressorsand Efficiencies
8.4
Representationof Thermodynamiccycle of
compressors on (p-V)
and (T-S) diagrams for
compressors
ueneral: Schematicrepresentation of a compressor
is shown
in Fig. 8.4.1.

4. 8-4
Themodynamics(MU)
Heat loss to -Compressed
air
(p,V,.T,)
surToundings
(air/cooling
water)
Work supplied, W
(Ether by motor
or IC engines)
Compressor
Air in surroundings
(PV,T)
Fig. 84.1: Schematie representatlon of an
alr compressor
As shown in Fig. 8.4.1, the air is taken into compressor from the surroundings
P. V, T).
It is compressed
to pressure p, upto
state 2 (P, V2, T,)
on the
expense of work
oled
mlate
compreson
During the compression, heat may be rejectedcither to surrounding air in case of air cooled e
having fins or to cooling water in cylinder jacket in casc of watcr cooled compressors.
8.4.1 Representation of Compresslon Processes on (p-V) and (T-S)
Diagrams and Work Input
The theoretical air compression cycle is shown in Fig. 8.4.2 on (p-V) and (T-S) diagrams
representation of work and heat transfers.
with
Delivery pressure
T
2
2
Adiabatic, p.v=C
p.V= C
p.v
Polytropic, p.v-c
T T2 Isothermal
p(pV=C
a Suction pressure
P.V C
d s
a) (p-V) diagram
(b) (T-S) diagram
P
Compression
process
V.dp
Compression
V
(C)Work,W= area (a-1-2-b)
(d) Heat transfer
Q area (c-1-2,d)
Fig. 8.4.2:
Representation of compression processes on
(p-V) and (1-S) diagram with work and heat transfers

5. 7Themodynamics(MU)
8-5
Cornpressors
Usual
lly the compressors
are
high speed machines due to which the ratc of hcat transre
be negligible,
thus the
process is
essentially fadiabaticl If friction is
ncgccld
cess becomes reversible adiabatic or
isentropic and follows the law pV=
shows the thermodynamic cycle involved in compression. In this (a-1) shows the
aSSued
the
compressio
Fig. 8.4.2(a)
sess at constant pressure, followed by reversible adiabatic compression suctoue Dressure andtemperatures riseto P2»T2 dueto worksupplied fromexternal source andfinally
on process(1-2) during
which
the
air
is
deli elivered
in process 2-b) at constant
pressure, P2
Fig. 8.4.2(c) 8.4.2(c) represents the workdone required during the cycle on (p-V) diagram. In case the
in kinetic and potential energies are neglected then the work, W = [- V. dp. This work
changes
by the area under
the
compressionprocess on the ordinateic
epres ented by
the
Work done, W = V
dp
=
area (a-
1 -2-b)J
The heat transter dunng polytropicthe process of a (1 -2) cooled compressor is shown in
8.
s42(d) on (T-S) diagram. The area under the compression process curve on abscissa represenisthe
Fig.
heat transfer during the process i.e. area (1-2-c-d).
Based on the above discussion the work transfer and heat transfer in various processes can be
marized with the help of Fig. 8.4.2(a) and Fig. 8.4.2(b) respectivelyas follows:
Sr. No. ProcesSS
Work transfer, W
Refer Fig. 8.4.2a)1 Refer Fig,
84.2(6]
Heat transfer, Q
1. Adiabatic process, p . V=C
Area (a-1-2-b)
Zero
2. Polytropicprocess, p V =C Area (a-1-2'-b)Area (1-2-d-c)
Isothermalprocess, p . Area (a-1-2"-b)Area(1-21 e - c)|
3.
It is evident from above table that the work required is maximum with adiabaric process and
work required is minimum with isothermal process.
As a designer our aim is supply minimum enefgy input during compression process.
Therefore, isothermal compression process is considered as an ideal process because the
work input required is minimum. Thus the best value of index of compression isn = 1. m
However, the isothermal compression is not possible in practice since the heat needs to be
dissipatedcorrespondingto infinitesimaltemperaturerise during infinitesimal compression process.
This heat transfer will require sufficient time. In other words, an air compressor needs to be run at an
extremelyslow speed to achieve approximatelyan isothermalprocess. It will reduce the mass flow rate
of air which can be compressed. Whereas, the practical requirement
is to compress the air with high
massflowrate.
Generally,compressorsrun at sufficienthigh speed to obtain sufficientmass flow rate of
compressed air, the process of compressionwill be nearly adiabaticwith index n = y.
However to approach the isothermal compression process, the air or water cooling is done during
pression process or cold water is sprayed during compression, so that the adiabaticcompression
ges to polytropiccompressionwith index n <y. The value of index n varies between 1.25 to 1.35. chang
Due to continuouscooling of compressed air, it's specific volume reduces. It results in decrease
d work inputwhichequals to area (1 -2-2) as shown in Fig. 8.4.2(a) and Fig. 84.2(b) in
polytropic process and area (1 -2-2") in case of isothermalprocess.

6. Themodynamics(MU) 8-6
Compreseo
The actual work supplied at the shaft called shaft work or motor work will he.
compression
work if the mechanical friction is considered.
be more than
8.4.2 Isothermal, Polytroplc and Isentroplc Efflclencles
Isothermal work input
Isothermal eficiency, Tr Actual work input
84)
Polytr pic work input
Polytropic efficiency, n, Actual work input 842)
Adiabatic work input
Adiabaticor Isentropicefficiency,n,= Actual work input .(843
8.5 Uncooled Rotary Compressors
In case of uncooledrotarycompressors, the ideal T
process is isentropic process (1
-
2) and actual process
is
represented by polytropic compression process (1
-
2)
with
index n >y after considering the fluid friction. Thermodynamic
cycle is represented in Fig. 8.5.1 on (T-S) diagram.
Adiabatic,
p.VC
-p.V=C
(n
2
Note that cooling
of rotary compressors
cannot be
carried out due to inherent practical
difficulties. The additional
work needs to be supplied
to overcome the friction. Since
friction work converts into heat,
as a result the specific
volume
Fig. 8.5.1: Uncooled compressor cycle
during compression process
increases.
The additionalwork required in uncooledcompression will be the sum of -V dp work equal to
area (1 2 2) due to increased specific
(c 1-2 -d)
volume of air and the frictional work equal to area
Isentopic work done
Ideal or Isentropic efficiency,n Actual work done
...(8.5.1)
Syllabus Topic: ReciprocatingAir Compressor-Single Stage Compressor
and Computation of Workdone, Isothermal Efficiency
8.6 Reciprocating Air Compressor
MU-Dec. 16
UniversityQuestions
Define following terms for reciprocating compressors.
(1) Mechanical efficiency (2) Indicated power
Explain construction and working of single-state, double-acting reciprocating air compressor
neat labelled diagram
(Dec.16)
(Dec.16

7. Themoaynamics (MU)
8-7
Cornpressors
Description
Fig.
6.1 shows the sketch of a
I- intet Vat
reciprocating air
consists of a piston
D Ootvery Vatre
conventona/
1npressor.
Oyflinder
jch
reciprocates in a cylinder andit
dnve iven
through
the connecting rod
and crank
The rankshaft is driven
h
a prime
mover. The inlet
O
Connecting-
Piston
rod
the deliveryvalve (D) are
alue (I).
nted
in
the cylinder head. The
Cranik casa
mounted
Fa/Tesa
aves
are plate
type
and spring loaded
called
presure
differential type i.e. Crank
the
valves
are automatically opened
and
ciosed depending uponthe
sSure
diiference across the valves
Crank-Shaf
cen
outsideand cylinder
PTessures.
)Assumptions
Foliowing are the assumptions made in consideing the cycle of operation:
6)
There is no clearance.
) Working fluid is a perfect gas.
Fig. 8.6.1: Reciprocating air compressor
(i)
There are no friction losses.
iv)
There are no wire-drawing effects in the valves or pipe line.
(v)
The cylinder is well insulated.
(o) Working
An ideal (p-V) diagram for a single stage reciprocatingair compressoris shown in Fig. 8.62
Reversilble.x
adiabatic(pV'=C)
-pv"- Poyropio)
Isothemal (pV=C)
Stroke Volume
(a) (b)
Fig. 8.6.2: (p-V) and (T-S) diagrams for single stage air compressor

8. 8-8
Themodynamics (MU)
Comptenaa
The air is sucked inside the cylinder
at pressure p
When the inlet valve
conditions as represented by
the process (a-b).
The air is then compressed
adiabatically
and reversibly upto pressure pa
lve
opens al
atmosphed
epresented by
Curve (b-c), the
law of compression being p
V= Constant.
Now the dclivery
valve opens
and the compressed
air
of
the cylinderis dischar a
at constant pressure, p, represented by
the process(c
-
d).
The anca (a
be d) ropresents
the work required
to compress
the air from pregeq
discharged to a
roueve
pressure p, to
p,h
cquals to -V dp work.
(d)
Calculations for work of compresslon
and efficlencles
6)
Reversible adiabatic work
Workdone on the air per cycle,
PVa+ PVPp, V, =(T V,-P, V)
.
W Area (a bcd)= Area (o dc e) +Area (e c b)-Area (o a bf
But, PV
=
p2 V,
, for compression process (b-c)
-(
or, --
From Equations (i) and (i) we get,
W "- 8.6.1)
Also, p V, = mRT,
1
mR 1 gan
.8.6.)
Isothermal and polytropic work of compression
Howeverthe slope of an isothermal compression curve is less than the adiabatic curve. Theretor
in case the air is compressed isothermally it would follow curve (be"). The area (a b c" d woad
represent the work of compression which is less than the reversible adiabatic compression work by a
amount equal to the area (bc c'). In other words the isothermal process would be the most d
process but such a process is difficult to achieve in practice because it would need the compressor
run at an extremely slow speed consequently reducing the mass flow-rate of the air compressed
In order to save the work of compression, the practice is to reduce the index of compression
high speeds by cooling the cylinder.This is done either by spraying water on it or by water jacketing
cylinder in case of single stage compressors so that the law of compression becomes,
pV= Constant
Where the value of index 'n' is less than . It is represented by the curve (bc).
In
tnis
work of compression per cycle with the help of Equation (8.6.1) can be written as,
ase tne
A

9. 8-9 Cornpressors
Themodynamics (MU)
(n1yn
Polytropic work,
W,= .v. ..(8.5
In
case
of
isotherma isothermal compression the work of compression reauired per cycle would be gv y
by.
1sathermal work, WP Vlog. v PV, logmRT, log.D | P2 Isothermal work,
...(8.6.4)
P
Indicated power (L.P.)
as air power.required
to drive the compressor
i5 Thereforethe indicatedpower (L.P.),also known a
given
by
the
equation,
W n
LP.60x 1000 60000
W KW
8.6.5)
n Number of strokes/min.
completed by the
compressor
N Speed of compressor in r.p.m.
n N, for single acting compressor
n 2N, for double acting compressor
where
Let,
o)Isothermal efficiency
Isothermal work input
Actual work input
Isothermal efficiency, n .(8.6.6)
(o)Polytropic efficiency, n
Itis defined as the ratio of polytropicwork to actual work input.
Polytropic efficiency,
n
Polytropic work input
..8.6.6(A))
Actual work input
Note: In Equations (8.6.6 and (8.6.6(A), the actual work input may be taken as isentropic work input
in case the actual work inputis not given in a problem a
() Mechanical efficiency: It is definedas the ratio of indicated power to the power required to run
the compressor)The power required to drive the compressor is called the brake power (B.P.) or
shaft power or the motor power which, in case of compressors,is more than the indicated
power (1.P.) because of the extra power required to overcome the friction and other losses of the
compressor.
L.P
Mechanical efficiency B.P. 8.6.7)
Adiabatic efficiency = o
Adiabatic power
B.P.
.(8.6.8)
86.1 Methods of Improving Isothermal Efficiency T A
Use of fins over cylinder for faster heat dissipation from inside of compressor to outside.
0By providing water jacket around compressor cylinder and cireulatingthe cooling water through
waler jacket. Thus it cools the air during compression.
By spraying water at the end of injectionprocess.
However this method is not used since
) Alr gets mixed with water which has to be separatedbefore use.
) It contaminates the lubricant film on cylinder surface which may cause corosion.
un Special arrangementsneed to be made in compressor.

10. gpThermodynamics (MU)
8-14
Cornpke 0.88 0.885.8305 kw
Isothermal powe
0.7103 or
71.03%
Actual power, P 5. 1309
Actual power5.8305 U.7103 or 71.03%
(it)
Cylinder dimensions (D and L)
V, = V, xN =% D'LN;
I
=
D x 1.8
Dx 240 D
=
0.1434 m and L= 1.8 D = 0.258 m
(iii) Raiing of drive =
Actual
power
= 5.8305 kW
Syllabus Topic : Effect of Clearance Volume,Volumetric EfficieAir Delivery (F.A.D)
,Fre
8.7 Effect of Clearance Volume in Compressors
MU Dec.1 University Question
Define following terms for reciprocating compressors
(1) Volunmetric efficiency (2) Free
Air Delivery
(Dec. 18 Practically speaking,
a
certain amount
of clearance has to be
providedbetweenthe piston
nd
on
cylinder
so that the
piston does not strike with the
cylinder head. Also, a certain
space between nit
and
cylinder has to be
provided to accommodate valves.
The ideal (p- V)diagram for a
single stage air
compressor is shown in
Fig. 8.7.1 with clearance
volume
A small
quantity of air in clearance at
delivery pressure P2 of volume V. expands
polytropically along the curve (3-4) till its
pressure becomes
equal to the suction P2
pv =c
pressure.
At
point-4 the inlet valve
opens and the air is
drawn into
cylinder at
atmospheric pressure
represented by the
process (4-1). Therefore,
the volunme of air drawn (V,
-
V) is less than,
the stroke
volume (V,-V).
It follows that the
handling capacity of the
compressor is reduced due to the clearance
space between the piston and
cylinder head.
For this reason in case
of compressors the
clearance
volunme is kept asSmall-as
possible
The measure of
handling capacity in case of
compressors, is defined as the
vO efficiency.
P
-..
Effective volume
Total volume
b d
Fig. 8.7.1: Air
compressor cycle with clearance
UMIMluM
Fu

11. MU)
8-15
Thermodnamics(M CompressorsJsadd
ctually compressed and delivered at the inlet pressureand temperature
Therelore.
free air actuall
Volumc of Piston displacement
A.D)represents therateof volume of surrounding airwhich is sucked by
delivered (FA.D)
Noto:
the
compressor and
delivered atdischargeepre
Ahernati vely
Mase equivalent prsten-dspiaccment aýinketpressure andtermperature
1lvol.
Mass of actually compressed air and delivered air
ed that the workdone on the air delivered is not affected by the cleararnce ) asthe
work equire
epans ansion
from
point
4 to point 1.
should
be
noted
that t
d
to
compress
the
82)
of air in clearance volume is
theoretically regained during its
Volumetric Efficiency 87.1 alculations for
Eauation (8.7.1),
the volumetric efficiency is given by, (Vs +Vo)
V
where
the stroke volume
and V. represents
the clearance volume.
For
ihe polytropic
process (3-4)
we have,
PaxV = P4x V or, V,= Vs
Also, PPa
PP1
and V= Ve (Refer Fig. 8.7.1)
1/n
xVo
P1
Substituting for V, from Equation (ii) in Equation (i),
..(ti)
v-
Pi
-
=1+V, V,
-)
.8.7.3)
V Let C = Clearance volume ratio = V ()
Tp Pressure ratio
=
Pi
ASndoo The Equation (8.7.3) can be rewritten as:
1, = 1-IG,-11C
P e seen from the Equation (8.7.4) that the volunetric efficiency reduces with the increase
Sure ratio, T, and the clearance volume ratio, C.
...(8.7.4)
odvallg onnrebd on

12. 8-16
Compress
Thermodynamics (MU)
8.7.2 Other Factors Affecting the Reduction in Volumetric Efficiene y
Apart from
reduction in volumetric efficiency
due to increased pressure ratio.
volume ratio, C, other factors affecting the reduction in volumetric efficiency eeclea
compressors are:
aranoe
Teciprocaling
(a)
Increase in temperature of free air drawn from atmosphere due to the heat transter
Pressure drop
in the inlet passages
and across the inlet valve.
b)
cylinderwalls.Due to this heating the specific volume of air increases, hence, the m
actually present in the cylinder compared to the conditions of free air is reduced,
Leakagethroughthe valves or past the piston because this decreases the mass of air delive.
livered.
(c)
(d) Inertia effects in opening the inlet valves.
ee At
8.7.3 Volumetric Efficiency Referred to SurroundingConditionsor on Frea
Deliveredd
Since the condition of air at point-1 does not represent the conditions of free air delivered FA n
(at atmospheric pressure and temperature) due to heat transfer between the cylinder walls and D
and due to pressure drop past the valves, it is necessary to apply the correction factor in the expr
intake air
esion
of volumetric efficiency
as follows
Let, Vo= Volume of free air delivered at surrounding pressure po and temperature T
PoV
m RTo Mass of free air delivered,
Pi V-V4
RT
But, m
Since mass of air sucked remains constant, it implies from above,
PooP (V-V
RT RT
Po T (V-V)
Hence the volumetric efficiency of Equation () expression referred to surrounding conditions can
be modified as,
PT
PoT
..)
(V-V
V,
Substituting the value of
from Equation (8.7.4) in Equation (iv), the modified
expression for volumetric efficiency based on free air delivered becomes:
..(8.7.5)
-c
Note: Expression given by Equation (8.7.5) should not be used for calculating the dimenslons or
on
cylinder Only the expression of
volumetric efficiency given by Equation (1.74 bas
suction conditions should be used for calculationof cylinder dimensions.

13. Themodynamics (MU) 8-17 Compressors
Workdone
8.7.4
Workdone per cycle,
W Area (1 -2-3-4) = Area (a-1 -2- b) -Area (3-4-a-b)
n-1y1
w () ) -J-()»v
In case the index of expansion and compression
is same.
-1/1 (n-1n
W (
()»-V|)-
(n-1/n
8.7.6)
For a single stage single acting reciprocating air compressor,
actual volume of air
taken in is 10 m/min. Initial intake pressure
is 1.013 bar and initialtemperature
is
27
C. Final pressure
is 900 kPa clearanceis 6% of stroke.
Example 8.7.1
Compressor runs at 400 rpm.
Assume: LD
= 1.25 and index of compression
1.3
Determine: (1)
Volumetric efficiency
(2)
Cylinder dimensions
(3)
Indicated power
PA
Solution: Refer Fig. P. 8.7.1
(V-V.) = 10m/min,
P2Pa
-p.v=c
P
1013 bar
=
101.3 kPa
T
= 27°C
= 300 K,
P2 900 kPa
C 0.06=V -1.25,
P1P
Vs
n = 1.3, N= 400 rpm
Fig. P. 8.7.1
) Volumetric efficiency, n»
1=1-0.06T01.3 1|=0.738 or 73.8%
Ans.
1-
i) Cylinder dimensions, D and L
V,=D'LN
10
= Dx 1.25Dx400;
V, = V, x N
D = 0.3256 m Ans.
0.738
L
= 1.25 x D
= 1.25 x 0.3256
= 0.407 m ..Ans.
ndicated power (L. P.)
I.P. = ( v-v.) (v,-v.)( (-y