Recommended :
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.
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
| Case | Items | Supplier | GPSA | HYSYS |
| 1a | Discharge temperature(degC) | 117.6 | 117.5 | 117.2 |
| Gas Horse Power (kW) | 2696.0 | 2678.4 | 2690.1 | |
| 1b | Discharge temperature(degC) | 121.2 | 121.0 | 120.6 |
| Gas Horse Power (kW) | 2877.0 | 2858.4 | 2871.4 | |
| 2a | Discharge temperature(degC) | 117.2 | 117.5 | 116.5 |
| Gas Horse Power (kW) | 10828.0 | 10740 | 10651.4 | |
| 2b | Discharge temperature(degC) | 88.0 | 88.0 | 87.4 |
| Gas Horse Power (kW) | 4628.0 | 4575.1 | 4532.6 | |
| 3a | Discharge temperature(degC) | 124.1 | 140.9 | 124.7 |
| Gas Horse Power (kW) | 8966.0 | 9147.8 | 9039.0 | |
| 3b | Discharge temperature(degC) | 85.0 | 117.9 | 86.1 |
| Gas Horse Power (kW) | 3682.0 | 3798.0 | 3736.6 | |
| 4a | Discharge temperature(degC) | 122.5 | 139.5 | 125.8 |
| Gas Horse Power (kW) | 9090.4 | 9210.7 | 9162.0 | |
| 4b | Discharge temperature(degC) | 86.3 | 120.7 | 87.1 |
| Gas Horse Power (kW) | 3829.6 | 3926.7 | 3859.7 | |
| 5a | Discharge temperature(degC) | 123.2 | 142.3 | 125.6 |
| Gas Horse Power (kW) | 9104.0 | 9262.2 | 9149.5 | |
| 5b | Discharge temperature(degC) | 95.7 | 121.0 | 87.2 |
| Gas Horse Power (kW) | 4293.0 | 3937.8 | 3844.0 | |
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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