Performance Analysis of Centrifugal Compressors

A Compact Life Generating Device !!!

P M V SubbaraoProfessor

Mechanical Engineering DepartmentIIT Delhi

Performance of centrifugal compressors

• Pressure Ratio

1

01

112

01

,03 1

Tc

rVUr

p

p

p

wcompact

The overall or total-to-total efficiency

Performance of Radial Vane Compressor

Performance of Backward Vane Compressor

Dimensional Analysis

• Experimental performance curves or maps provide a versatile data for compressor engineers.

• The data is translated into most general form.• Data in general form is minimally effected by air speed,

altitude etc.• The total pressure ratio of a compressor is:

RpATmfp

poo

acto ,,,,,, 1101

3

Using dimensional analysis & Buckingham Pi Theorem:

stp

o

o

stp

stp

o

o

acto

T

TN

p

p

T

Tmf

p

p 1

1

1

1

3 ,

0103

0103

TT

TT scomp

stp

o

oi

stp

stp

ocomp T

TN

p

p

T

Tmf 11 ,

Test Rig For Centrifugal Compressor

Compressor Characteristics

Compressor exit

Supply pipe

Maximum Efficiency pointZero Efficiency Points

Pressure Cycling During Surging

Description of Surge

• Surge is defined as the operating point at which the compressor peak head capability and minimum flow limit are reached.

• The compressor loses the ability to maintain the peak head when surge occurs and the entire system becomes unstable.

• Under normal conditions, the compressor operates to the right of the surge line.

• Due to occurrence of fluctuations in flow rate, or under startup / emergency shutdown, the operating point will move towards the surge line because flow is reduced.

• If conditions are such that the operating point approaches the surge line, the impeller and diffuser begin to operate in stall and flow recirculation occurs.

• The flow separation will eventually cause a decrease in the discharge pressure and flow from suction to discharge will resume.

• This is defined as the surge cycle of the compressor-

Surge Control System

The function of the surge control system is to detect the approach to surge and provide more flow to the compressor through opening the recycle valve to avoid the potentially damaging flow reversal period and surge cycling.

Depictions of Surge in Terms of Compressor Head, Flow and Speed.

Compressor Map

oi

stp

stp

o

p

p

T

Tm 1

01

3

p

p acto

stp

o

T

TN 1

Vane Geometry : Compressor Characteristics

Mass Flow

Pre

ssur

e R

atio

Variable Speed Operation

Efficiency –Slip Factor Relations

oi

stp

stp

o

p

p

T

Tm 1

comp

Remarks on Centrifugal Compressors

• High pressure ratio per stage (r0p= 3 to 4).• Less flow area per unit mass flow rate.• A multistage centrifugal compressors have shorter lengths.• They are best suited for low mass flow rates.• As the flow is turned by 900, the efficiency of centrifugal

compressor is relatively low.• The impeller is an integral unit of blades and a disk.• Even if one blade is damaged the entire unit is to be replaced.• Look for other fluid dynamic phenomenon for ingestion of life

into fluids.• Aerofoil Theory.

Evolution of Compression Phenomenon

Centrifugal Aerofoil

Compact & Less Efficient Bulky & More Efficient

GE T700

AXIAL FLOW COMPRESSORS

An Efficient Way to Ingest Life in Fluids !!!

P M V SubbaraoProfessor

Mechanical Engineering DepartmentIIT Delhi

Schematic representation of an axial flow compressor

Axial Flow Compressors

Hybrid Compressor

Axial Flow Fluid Machines

The power, P of a fluid Machine

inwoutw VVUmTP ,,

Va12/cp

p1

T1

p01

T01

p03 = p02

T03 = T02

inintoutoutt rVrVmTP ,,

Va22/cp

p2

Va32/cp

p3

s

T

Thermodynamics of An Axial flow Compressor Stage