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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