mel344 centrifugal compressors

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




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




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




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





• 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





• 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




Centrifugal Compressors

Width of impeller decreases progressively as the density of the

refrigerant increases

Image Source: Principles of Refrigeration by R.J. Dossat




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





Centrifugal Compressor System

Condenser

Water cooling evaporator

2nd Impeller

1st Impeller

Image Source: Stoecker and Jones




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





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





• 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





• 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




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




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




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




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




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




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




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




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




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




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




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




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

mel344 centrifugal compressors

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