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Chemical Process Technology

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Sunday, August 1, 2010

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Compressor is commonly used to compress gas and vapor to higher delivery pressure. Energy is supplied to the compressor to develop compression head. Part of the energy is lost when energy is transferred via shaft and part of energy lost due to compression activity. Energy lost via shaft will convert to vibration and noise. Energy lost due to compression activity (instead of carry out compression work) will turn to fluid internal energy of fluid. As fluid internal energy is increased, temperature of fluid will rise. How much energy is lost to compression activity ? How much internal energy is increased and how fluid temperature is increased ? All this relates to one well known parameter in compression field, Polytropic efficiency.

There are two paths compression is carried out :

1. isentropic reversible path - a process during which there is no heat added to or removed from the system
and the entropy remains constant, pvk = constant
2. polytropic reversible path - a process in which changes in gas characteristics during compression are considered, pvn = constant

One shall take note that most compressors operate along a polytropic path but approaches the isentropic. Most compressor will use polytropic efficiency to account for true behavior.
Compression following polytropic path,



Polytropic head 

where
Zavg = Average compressibility factor
Ts = Suction temperature (degK)
M = Molecular weight
n = polytropic exponent
Pd = Discharge pressure (bara)
Ps = Suction pressure (bara)

Polytropic exponent (n) can be calculated base on following equation


where
k = isentropic exponent
np = Polytropic efficiency

Gas Horse Power,


where
W = gas flowrate (kg/h)

Compressor discharge temperature


where
Td = Discharge temperature (K)
Ts = Suction temperature (K)


Above equations were extracted from GPSA section 13.

Recent compression studies using several cases to find compressor gas horse power and discharge temperature with specific polytropic efficiency. The studies have used
  • GPSA method (as tabulated above) 
  • HYSYS 
to estimate compressor gas horse power and discharge temperature. Results from several international compressor suppliers.
CaseItems Supplier GPSA HYSYS
1aDischarge temperature(degC) 117.6117.5 117.2

Gas Horse Power (kW)2696.02678.42690.1





1bDischarge temperature(degC) 121.2121.0 120.6

Gas Horse Power (kW)2877.02858.42871.4





2aDischarge temperature(degC) 117.2117.5 116.5

Gas Horse Power (kW)10828.01074010651.4





2bDischarge temperature(degC) 88.088.0 87.4

Gas Horse Power (kW)4628.04575.14532.6





3aDischarge temperature(degC) 124.1140.9 124.7

Gas Horse Power (kW)8966.09147.89039.0





3bDischarge temperature(degC) 85.0117.9 86.1

Gas Horse Power (kW)3682.03798.03736.6





4aDischarge temperature(degC) 122.5139.5 125.8

Gas Horse Power (kW)9090.49210.79162.0





4bDischarge temperature(degC) 86.3120.7 87.1

Gas Horse Power (kW)3829.63926.73859.7





5aDischarge temperature(degC) 123.2142.3 125.6

Gas Horse Power (kW)9104.09262.29149.5





5bDischarge temperature(degC) 95.7121.0 87.2

Gas Horse Power (kW)4293.03937.83844.0






Several observations :
i) HYSYS consistently predict discharge temperature similar to compressor supplier results.
ii) GPSA overpredict discharge temperature for several cases.
iii) HYSYS & GPSA predict gas horse power proximity to compressor supplier results with HYSYS in better prediction.


Above results give us some indication that
i) GPSA method may be used but shall keep in mind GPSA potentially overpredicted discharge temperature. This potential results over conservative design and excessive cooling required.
ii) HYSYS prediction is using rigorous method which adjusting prediction rigorously and within a good range of prediction.

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posted by Webworm, 9:59 AM

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