Process Equipment Design and Heuristics – Gas Compressors

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

Operating Range of Compressors

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.

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.

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. -

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)

Compressor / Drive Capacity Heuristics

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:

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

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

Drive Heuristics

Gas Phase Heuristics

Aspen Plus Compressor Input

Aspen Multiple Compressor Input