1. Process Equipment Design and Heuristics – Gas Compressors

2. Types of Gas Compressors
Compressors are designed to handle gases and can be damaged if liquid is present

3. Operating Range of Compressors

4. Dynamic Compressors Axial compressors Can handle large volume flow.
More efficient than centrifugal compressors.
Can be susceptible to fouling and are generally less reliable.
Should be considered only for air, sweet natural gas or non-corrosive gases.

5. Dynamic Compressors Centrifugal compressors
Preferred choice assuming it can handle the required flow with good efficiency.
Reliable and can operate over a wider range of flows than axial compressor.
Has the potential to operate continuously for long periods.
Flow at discharge conditions should be > 300 m³/h.

6. Positive Displacement Compressors.
Reciprocating compressors
Used if required flow is too small for a centrifugal compressor, or required head is so high that an undesirably large number of stages needed.
Require frequent maintenance, a full capacity spare provided as a general rule in critical services.
Alternatively, three half capacity units may be specified, (two running in parallel with the third as a spare. -

7. Gas Compressor Drivers
Compressors and pumps require drives.
Pump cost generally includes an electric motor coupled to it.
For compressors the drives are costed separately
3 types of drives:
Electric Motor (EM)
Gas Turbine (GT)
Steam Turbine (ST)

8. Compressor / Drive Capacity Heuristics

9. Compressor Specifications
If a compression or expansion of a gas takes place with no flow of heat energy either into or out of the gas - the process is said to be isentropic or adiabatic
An isothermal process must occur very slowly to keep the temperature in the gas constant. The adiabatic process must occur very rapidly without any flow of energy in or out of the system. In practice most expansion and compression processes are somewhere in between, or said to be polytropic.
The theoretical power requirements are independent of compressor type. The actual power requirements vary with the compressor efficiency. In general the power is calculated by:

10. Compression Equations
For an isentropic (reversible and adiabatic) process:
Based on a polytropic process:
k = isentropic Exponent (Cp/Cv)
Za = average gas compressibility
n = Polytropic Exponent (PVn=constant)
n depends on fluid being compressed
η is the isentropic efficiency
ηp is the polytropic efficiency
P1 is the suction pressure
P2 is the discharge pressure
T1 is the suction temperatures
q is the gas volume flow rate at PS and TS
R is the gas constant

11. Compression Equations
The isentropic heads is calculated by:
The polytropic head is calculated by:
The isentropic discharge temperatures is:
The polytropic discharge temperatures is:

12. Drive Heuristics

13. Gas Phase Heuristics

14. Aspen Plus Compressor Input

15. Aspen Multiple Compressor Input