mel344 centrifugal compressors
TRANSCRIPT
7/27/2019 MEL344 Centrifugal Compressors
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Department of Mechanical Engineering, Indian Institute of Technology Delhi
MEL 344: Refrigeration and
Air-Conditioning
Amit Gupta
Department of Mechanical EngineeringIndian Institute of Technology Delhi
1st Semester 2013-2014
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Overview
• First commercial centrifugal compressor promoted byWillis Carrier in 1920
• Dominant type in large installations
• Serve systems in the range 200-10,000 kW of
refrigerating capacity• Evaporating temperatures as low as -50 to -100 °C
range
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Principle
• Pressure rise: angular momentum into static pressure
• Steady flow devices, unlike reciprocating compressors
less vibration and noise
• Series of impeller wheels mounted on a steel shaft,
enclosed in an iron casing
• Number of impeller wheels?
• 2-4 stages of compression are common
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Centrifugal compressor showing discharge scroll
Centrifugal Compressors Operation
• Low pressure, low velocityvapor (suction) drawn in inlet
cavity (‘eye’) along the axis
of the rotor shaft
• Vapor forced radially
outwards between impeller
blades by centrifugal force
developed by rotating wheel
Image Source: Eastop and McConkey
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Centrifugal Compressors Operation
• Vapor from blade tipsdischarged into housing at
high velocity and increased
temperature (and pressure)
• Vapor collected in specially
designed passages in casing
• Reduce velocity and direct
vapor to inlet of next stageimpeller (to discharge
chamber in case of last stage
impeller)
Centrifugal compressor showing discharge scroll
Image Source: Eastop and McConkey
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Centrifugal Compressors Operation
• Depending on presence or absence of inlet guide vanes,refrigerant enters with pre-rotation or axially
• Rotating impeller wheels only moving parts of the
machine
• Action of impeller is such that both static and dynamicpressures increase
• Centrifugal force exerted on vapor confined between
blades of the impeller wheels causes self-compression
of vapor
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Centrifugal Compressors
Width of impeller decreases progressively as the density of the
refrigerant increases
Image Source: Principles of Refrigeration by R.J. Dossat
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Impeller
• Impeller wheel consists of two discs – hub and cover disc
• Vanes or blades (backward, radial, forward) mounted
radially between them
Cutaway view of centrifugal compressor impeller wheel
Image Source: Principles of Refrigeration by R.J. Dossat
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Centrifugal Compressor System
Condenser
Water cooling evaporator
2nd Impeller
1st Impeller
Image Source: Stoecker and Jones
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Centrifugal compressor with flash gas
intercooler
High pressure liq.
drains from
condenser into
intercooler
Intercooler:
increases
refrigerating effect
per kg and reduce
flash gas inevaporator
Flash vapor from
intercooler taken into
suction of second-
stage impeller
less compressionpower required.
Cool vapor from the
intercooler reduces
temperature of
discharge vapor fromfirst-stage impeller
capacity and
efficiency increase
Image Source: Principles of Refrigeration by R.J. Dossat
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Analytical Description
• Tip speed to develop pressure can be estimated fromfundamental principles of turbomachinery
2 2 1 1t t T m V r V r
2
2
1
1
where
torque
mass flow rate
tangential velocity of refrigerant leaving the impeller
radius at exit of impeller
tangential velocity of refrigerant entering the impeller
radius at entrance
t
t
T
m
V
r
V
r
of impeller
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Analytical Description
• If refrigerant enters the impeller in radial direction,
• Power required at the shaft will be
• At low flow rates of the refrigerant and for radial blades,
V2t can be approximated with tip speed of impeller.
2 2t T mV r
1. . 0t i e V
2 2t P T mV r
2 2 2
2t P mr mV
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Analytical Description
• Another expression for ideal power can be derived fromisentropic work of compression
• Equating
• Provides an order-of-magnitude estimate of tip speed to
achieve a particular compression ratio
1000 J/kJi
P m h
2
21000 J/kJi t
P m h mV 2
2 1000t i
V h
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Example
• Calculate the tip speed in order to compress R-11 and
ammonia from saturated vapor at 10 °C to a pressurecorresponding to condensing temperature of 30 °C.
R-11 NH3
hinlet (kJ/kg) 393.9 1472
hexit (kJ/kg) 406.7 1560
Δhi (kJ/kg) 12.8 88
V2t (m/s) 113.1 297
I s e n t r o p i c
c o m p r e s s i o n
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Significance of blade angle
• From the velocity triangle, angle βvaries as:
• β<90° for backward-curved
• β>90° for forward-curved
• β=90° for radial
• Power required written as
• From velocity triangles
2 2t P mu V
22 2
2
cot1 n
t
V V u
u
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Significance of blade angle
• Input power
• Thus, for a given tip speed, power required increaseswith β
• Backward curved blades have low power requirements
2 22
2
cot1 n
V P mu
u
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Design considerations
• Two crucial impeller dimensions: wheel diameter andspacing between impeller faces
• Refrigerant
• Larger the wheel, larger will be the tip speed and hence
higher pressure ratio• If motor operates at 60 rpm, the wheel diameter for R-11
will be 0.6 m, while for NH3 is 1.58 m (impractical, both
from assembly and structural standpoint)
• If NH3 compression done in two stages with equalenthalpy change, tip velocity reduced to 210 m/s
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Design Considerations
• Width of the passage: – Capacity can be increased by increasing width between
faces of impeller, which also increases power requirement
– Low density refrigerant allows using large width impeller
for given capacity
• Efficiency decreases for machines of low capacity
– Impeller width becomes narrow, and hence higher frictional
losses
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Performance Characteristics
• Useful to find outefficiency, flow rate at a
given pressure ratio and
speed or vice-versa
• Iso-efficiency lines are
shown for various
speeds
• For a constant speed,
pressure build-up
reaches a maximum
and then decreases
with increasing flow rate
D i s c h
a r g e t o s u c t i o n p r e s s u r e r a t i o
Flow rate
Image Source: Stoecker and Jones
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Surge
• Occurs when refrigerating load is low or condensing
temperature is high
• For e.g., increase in heat sink temperature increase in
discharge pressure
• If pressure increase greater than design pressure
difference refrigerant flow reduces and finally stops
• Further increase in condensing pressure causes reverse
flow causes increase of evaporator pressure
pressure difference reduces compressor starts
pumping refrigerant in the normal direction pressuredifference increases again with eventual reversal of flow
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Surge
• Oscillation of flow in the compressor and rapid variation
in pressure difference is called “surging”
• Produces noise and vibration
• Bearings experience severe stresses leading to damage
• Surging can be tolerated occasionally, but must be
avoided in the long run
• Sometimes avoided by passing a small amount of
discharge vapor into evaporator increases the load
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Capacity Control
• Capacity normallycontrolled by adjusting
angle of guide vanes
• Adjusting the vanes can
provide a swirl, thereby
introducing an inlet
tangential velocity
component
• Efficient when vanes are
near fully open condition
• At low angles, vanes act
as throttling devices
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Comparison with reciprocating
compressor at constant rpm
E v a p
o r a t o r T e m p e r a t u r e
Tons refrigeration
reciprocating
centrifugal
Fixed Condensing Temperature • Large change in refrigerating
capacity possible with small
change in evaporating
temperature
• Centrifugal compressorsmaintain the evaporator
temperature at a fixed level
with changes in load as
compared to reciprocatingcompressors
C
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Comparison with reciprocating
compressor at constant rpm
C o n d e n s i n g T e m p e r a t u r e
Tons refrigeration
centrifugal
reciprocating
Fixed Evaporator Temperature
Pumping
limit
• Rapid reduction in capacity ascondensing temperature
increases
• Possible to control centrifugal
compressor capacity by
varying quantity andtemperature of condenser
water
• Change in capacity with
speed:• For reciprocating type,
proportional to ω
• For centrifugal type, proportional
to ω2
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Summary of compressor usage
• Reciprocating: small refrigerating capacities to about 300kW
• Centrifugal: refrigerating capacities 500 kW and higher
• Screw: 300-500 kW capacities; competes against large
reciprocating and small centrifugal compressors• Vane: competes against reciprocating primarily for
domestic refrigerators and air-conditioners