compressors
DESCRIPTION
TRANSCRIPT
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Compressors
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Goals
• Describe the two basic types of compressors and their typical applications
• Describe how the design equations for compressors are derived from the MEB and the total energy balance (TEB) (understand the assumptions)
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Types of Compressors
Gasses can be compressed in the following ways:
• Reciprocating piston compressors
• Low flow rates
• High compression ratios
• Rotating centrifugal compressors
• High flow rates
• Low compression ratios
• Several centrifugal stages may be used to obtain higher compression ratios
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Types of Compressors
Centrifugal Compressors
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Types of Compressors
Radial Flow Compressors (multi-stage)
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Types of Compressors
Axial Compressor
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Types of Compressors
Reciprocating Compressor
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Typical Compressor Installation
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Compressor Design Equations
Mechanical Energy Balance
f
p
p
hdp
gZu
W
2
12
ˆ2
What is the unexpected assumption?
Viscous dissipation is negligible!
hf will be accounted for via the efficiency of the compressor
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Mechanical Energy Balance
2
1
ˆp
p
dpW
Ŵ is the work done on the fluid by the compressor
must remain inside integral since changes with p.
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Total Energy Balance
cWm
QHgZ
u ˆ2
2
adiabaticcompression
Note that Ŵc in TEB includes efficiency while Ŵ in MEB does not include efficiency
12ˆ TTCHW pc
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Isentropic Work of Compression
As a first approximation, a compressor without any internal cooling can be assumed to be adiabatic. If the process is also assumed to be reversible, it will be isentropic.
.1
1 constpp
Solve for , substitute into MEB, and integrate
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Isentropic Work of Compression
Upon Integration
This is the isentropic (adiabatic) work of compression.
The quantity p2/p1 is the compression ratio.
11
ˆ
1
1
2
1
10
p
ppW S
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Compressor WorkCompression is not reversible, however. Deviations from ideal behavior must be accounted for by introducing an isentropic compressor efficiency such that the true work of compression is given by.
ad
Sc
WW
0
ˆˆ
How can ad be found?
12ˆ TTCW pc
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Isothermal CompressionIf sufficient cooling is provided to make the compression process isothermal, the work of compression is simply:
1
210 lnˆ
p
p
M
RTW T
For a given compression ratio and suction condition, the work requirement in isothermal compression is less than that for adiabatic compression. This is one reason that cooling is useful in compressors.
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Polytropic Compression
In actuality the S = 0 path assumed in writing the expression p/const. is not the true thermodynamic path followed by gases in most large compressors and the compression is neither adiabatic not isothermal. A polytropic path is a better representation for which:
.1
1 constppnn
Here n depends on the nature of the gas and details of the compression process.
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Polytropic Compression
11
ˆ
1
1
2
1
1n
n
p p
p
n
npW
where Ŵp is the work for polytropic compression
Again the actual work of compression is larger than the calculated work and:
p
pc
WW
ˆˆ
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Polytropic Compression
The polytropic efficiency p is often the efficiency quoted by manufacturers. From this efficiency useful relations can be stated to convert from polytropic to adiabatic results:
To get n the polytropic exponent:
p
pn1 pn
n
11
or
To get relationships between T or and compression ratio simply replace with n.
n
n
p
p
T
Tge
1
1
2
1
2..
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A “REAL” Impact of Efficiency
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Multistage Compression
Consider a two stage compression process p1→p2→p3 with perfect intercooling (temperature reduced to T1 after each compression)
11
11
ˆ
1
2
31
1
1
210
p
pRT
p
pRTW S
Now find p2 which will minimize work, differentiate wrt p2
312 pppopt
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Multistage Compression2
1
1
3
2
3
1
2
p
p
p
p
p
p
So the compression ratio that minimizes total work is such that each stage has an identical ratio.
This can be generalized for n stages as:
nn
p
p
p
p
p
p1
1
1
2
3
1
2
.
1
1 constp
pT
i
ii
When T is not cooled to T1:
i
ii p
pT 1
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Multistage CompressionP-H Chart Method Example
Natural Gas (methane)P = 100 psia
P = 300 psia P = 900 psia
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Isentropic Compression
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“Real” Compression
HIdeal HActual