Chapter 3 centrifugal comp

important for mechanical engineers

1. CHAPTER THREE
CENTRIFUGAL COMPRESSORS, FANS
AND BLOWERS

2. Contents 
•Introduction
•Components and operation
•Effects of impeller shape on performance
•Velocity triangle and work/energy transfer
•Slip factor
•Diffuser
•Chocking, surging and rotating stall
•Pressure rise and loading coefficient
•Mach number
•Characteristic curve

3. Introduction
•Power absorbing turbo machines
•Compressor, pump and fans are the devices used to i ncrease the pressure of
the fluid. But they differ in the task they perform
•centrifugal fan and compressor employ centrifugal e ffect to rise pr.
•
All compressor, fan and blower are used to pump gas , but their difference is on the achieved pr. rise.
•A fan cause s only a small rise (up to pr. of 0.07 bar) of the flowing fluid. It
consists of a rotating wheel (called the impeller), which is surrounded by a
stationary member known as the housing.
•Energy is transmitted to the air by the power-drive n wheel and a pressure
difference is created, providing airflow .
•In blowers(0.07 to 3 bar) , air is com pressed in a series of successive stages
and is often led through a diffuser located near th e exit.
•Compressor for absolute pressure of 3 bar or above
•Single stage centrifugal compressor have prratio of 4:1 to 6:1

4. comparing centrifugal and axial flow
compressor
Centrifugal compressor are smaller length
In centrifugal compressor, wide range of mass flow rates of gas
The best efficiencies are 3 to 4 percent bellow tho se obtained from axial
flow compressors designed for the same duty.
In centrifugal compressor working fluid can be cont aminating gas like
exhaust gas.
At higher mass flow rate values, axial compressors have better efficiency
but at lower flow rates centrifugal compressors hav e better performance.
The centrifugal compressor is not suitable when the pressure
ratio requires
the use of more than one stage in
series because of aerodynamic problems
Disadvantage is larger frontal area and lower maxim um efficiency
Centrifugal compressors found mainly in turbocharge rs, turbofan and
turboprop aircraft engines

5. Components and Principles of Operation
Theprincipalcomponentsareimpelleranddiffuser
Therotorincentrifugalturbomachinesisalsonamedasimpeller.
Energy is transferred from the impeller to the fluid by almost purely
centrifugal
effect. A rotating particle about some axis of rotation usua lly tends to escape
in
theoutwardradialdirection.
Flowentersintothecompressorthroughtheimpellereye.Therotating
impeller
drives the inlet flow to the larger radius exit by centrifuga l effect.
Subsequently,
low pressure region is created around the impeller eye which allows fluid to
be
suckedintothecompressor.
The part of the impeller that creates low pressure region is c alled the
inducer
Theflowisinducedthroughtheimpellereyeandisforcedatimpellerexit.
Inletguidevanesareusuallyprovidedbeforetheimpellereye.

6. The flow is accelerated through a nozzle inlet casing in some
centrifugal
compressorsbeforeitenterstheIGVs.
Thefluidwhichgetsoutoftheimpellerisagaindiffusesthroughdivergingarea
of
stationaryblades(diffusers)toconvertitskineticenergyintostaticpressure. Energy is imparted to the air by
the rotating blades, thereby increasing the static
pressure as it moves from eye radius r1 to tip radi us r2.
The remainder of the static pressure rise is achiev ed in the diffuser. The normal
practice is to design the compressor so that about half the pressure
rise occurs in
the impeller and half in the diffuser.
The air leaving the diffuser is collected and deliv ered to the outlet.
Volute casing surrounding the compressor stage coll ects the diffused flow
.rpe,1

8. •
Shroudedimpellerhastheadvantage
of
notipclearancelossbutithas
increased
frictionloss.
Fig: -Types of impeller construction
Fig: -Vane –less centrifugal compressor

9. Centrifugal compressors can be built with a double entry or a single entry
impeller.

10. BThree possibilities for impeller blade orientation : Radia l blades,
Backward
curvedblades,Forward-curvedblades
aBackward-curvedblades(β₂<90°),
AsshowninFig.thevalueofCw₂(whirlcomponent
atoutlet)ismuchreduced,andthus,suchrotors
havealowenergytransferforagivenimpellertipspeed
aRadialvanes(β₂=90°)
havesomeparticularadvantagesforveryhigh
speedcompressorswherethehighestpossiblepressure
isrequired.
BBetterstrengthanddesignandmanufacturing
simplicityareimportantjustificationsforchoosing
radialblading.
Effect of blade shape on performance 

11. )rckg
•Forward-curvedvanes(β₂over90°)haveahigh
valueofenergytransfer.
•Therefore, it is desirable to design for high
values of β₂(over 90°), but the velocity
diagrams show that this also leads to a very
highvalueofC₂whichwillcauseatremendous
frictionlossindeliverypipes.
•As a result, backward curved blades are
usuallyusedincentrifugalpumpdesign.
4/17/2015

12. 4/17/2015
Jimma University School of Mechanical Engineering
12
Fig: Effect of blade
shape on performance
of pumps when mass
flow rate increases
IFor the same rotor tip speed, the highest pressure ratio is at tained in forward curved
impellersandtheleastpressureratioisobtainedinbackwardcurvedblades.
IThetotalpressureorheaddevelopedbythecentrifugalimpellerdependsonthemassflow
rate in backward flow and forward flow blades, whereas it doe sn’t pose any effect on radial
bladeimpellers.
IThe curved blades tend to straighten due to centrifugal effe ct. Strength is least concern in
radialbladeimpeller.

13. Velocity Diagram
•Thedesignvelocitydiagraminvolvesapurelyaxialentryattheinletthrough
theimpellereyeandthebladeisradial atexit
•arepresentswhentheairentersthe
impellerintheaxialdirection.Inthiscase,
absolutevelocityattheinlet,C1=Ca1
•brepresents velocity triangle at the inlet
to the impeller eye and air enters through the
inlet guide vanes.
•Angle Ɵ is made by C1 and Ca1 and is known as
the angle of prewhirl. The absolute velocity C1
has a whirl component Cw1.

14. ydtrk/
•
In the ideal case, air comes out from the impeller tip after making an angle of
90degree (i.e., in the radial direction), so Cw2=U2 . (fig C)
But there is some slip
between the impeller and the fluid, and actual valu es of
Cw1 are somewhat less than U2
. This results in a higher static pressure on the
leading
face of a vane than on the trailing face. Hence, th e air is prevented from
acquiring
a whirl velocity equal to the impeller tip speed.
•
d represents the actual velocity triangle.
Slip factor From above we can say that, there
is no assurance that the
actual
fluid will follow the blade shape and leave the
compressor
in a radial direction. Thus, it is convenient
to define a slip factor σ (less than unity) as,
•
=
n
tr
o
r If radial exit velocities are to
be achieved by the actual
fluid
, the exit blade angle must be curved forward about 10–14
degrees.

15. Cont..
For radial vanedimpellers, the formula for
B
dis given by Stanitzas :
Where n is number of vanes.
A slip factor of about 0.9 is typical for a compres sor
with 19
–21 vanes.
Work done Theoretical work done
Wc=Cw2r2ω= Cw2U2=B-


(considering B). In a real fluid,
some of the power supplied by the impeller is used
in overcoming losses (windage
,
disk friction and casing
friction losses)
To take
account of these losses, a power input factor(values between 1.035 and
1.04)can be used. Thus the actual work will be
dv aeB-


(assuming Cw1=0,
although this is not always the case
.)
Temperature equivalent of work done on the
air and compressor isentropic efficie
T02 at
the impeller exit,As no work is done on the air in the diffuser, T03= T02,
whereT03’
is the isentropic stagnation temperature at the dif fuser outlet

16. Mollierdiagram for the complete centrifugal
compressor stage

17. Pressure ratio
=

a
gefa
glI •u


a
gefa
gl=
xws
C
C
P
O .
=
VyDMhVyE
VyDhVyE
=
VyE
!yDM
!yE
hl
VyDhVyE
=
!yDM
!yE
hl
VyDhVyEkVyE
=
OyD
OyE
#$
#
hl
VyDhVyEkVyE
%
&=
1+
)*
VyDhVyE
VyE
#
#$
%
&=
1+
)
*xws
C
C
POVyE
#
#$

18. Diffuser
•Diffuser in compressor used to convert the KE impar ted by the impeller to
pressure rise. the maximum permissible included ang le
of the vane diffuser passage is about 11degree. Any
increase in this angle leads to a loss of efficienc y due to
boundary layer separation (eddy formation) on the o n diverging passage walls.
Diffusers can be
a. Volute or scroll collector : it is circular passage of increasing X-sectional
area, as increment of discharge and results constan t axial velocity around
the volute, and equal praround compressor casing an d hence no radial
trust. It is low cost
b. Veneless diffuser: diffusion takes place in parallel side passage and
governed by the principle of conservation of angula r momentum of the
fluid. Vanelessdiffusers has wide range of mass flo w rate, but due to long
flow path friction effect are important and efficie ncy is low.
c. Vaneddiffuser: the vanes (surrounding impeller outlet) are used to diffuse
the outlet KE at much higher rate, In shorter lengt h, and higher efficiency
than vaneddiffuser.

19. Examples
Example 1
Also calculate 1)the rise in total temperature during compression
If change in KE is negligible. 2)external diameter of the eye if the
Internal diameter is 15cm and mass flow rate is 10kg /s.

21. Compressibility effect

22. Mach number in the Diffuser
Pre whirl is used to reduce pressure loss by restricting Mach at the inlet by adding
inlet guide vanes. It also used to reduce the curvature of the impeller at the inlet.
But it ha disadvantages like, reduction in work capacity o f compressor, introducing
additional part(adds weight)

24. Centrifugal compressor characteristics Performance characteristics curve is plotted
for compressible flows, the one we
encounterincentrifugalcompressorstage.
The pressure ratio

yD

yE
is plotted against the
value of+
,
V
yE

yE
relative to the design point,
for various speed
- V
yE
and constant
efficiencylines.
The surge points for each operating
conditions can be indicated on the plotted
surge line, which is a critical design
considerationincompressors.

25. Choking
Whenthevelocityoffluidinapassagereachesthespeedofsound
at any cross – section, the flow becomes chocked (air ceases t o
flow).Thechokingbehaviorofrotatingpassagesdiffersfromthat
ofstationary passages, and therefore it is necessary to make
separate analysis for impeller and diffuser, assuming one
dimensional,adiabaticflow,andthatthefluidisaperfectgas.
InletChoking

27. •Impeller choking

28. The above equation indicates that for rotating pass ages, mass flow is
dependent on the blade speed.
•Diffuser Choking
For choking in the diffuser, we use the stagnation conditions for
the diffuser and not the inlet, b/c stagnation cond itions at the
diffuser inlet are dependent on the impeller proces s.
ρ0 and a0 refer to inlet stagnation conditions, which remai n unchanged

29. Examples
Example 1:

31. Example 3

32. Example 4