Polytropic Efficiencies for Dynamic Compressors

1. Purpose

This procedure defines the tool to use when predicting polytropic efficiencies for centrifugal and axial compressors. The representation of polytropic efficiency is expressed exclusively as a function of actual inlet volume flow to the compressor.

2. General

In a polytropic compression process, heat transfer occurs and entropy increases.

There are two primary advantages to modeling a dynamic compression process as polytropic:

• The measured polytropic efficiencies from vendors are not dependent on the thermodynamic state of the gas being compressed.

• The sum of the polytropic heads for multiple stages is equal to the total polytropic head required to go from the existing or original state to the desired state of the gas.

The compression process in a reciprocating compressor is modeled as adiabatic, which implies no heat into or out of the system. This process is a special subset of the more general polytropic process defined above. Do not confuse it with an isothermal process, which results in the most theoretically efficient type of compression process.

Figure 1, Polytropic Efficiencies for Dynamic Compressors, is based on the capabilities of a cross section of global compressor manufacturers whose designs are in strict accordance with API Standard 617. Wherever possible, the data used to construct this figure was collected from guaranteed vendors' performance for Inflection Point Engineering Projects from 1993 to the present. A cross section of Inflection Point Engineering process unit compression services was also surveyed and the effect of process unit size was considered.

3. Curve Fit

The polytropic efficiency data has been curve fit and integrated into Inflection Point Engineering TZ-508-01. Also, to enable other departments or groups within Inflection Point Engineering to predict dynamic compressor performance consistently, these equations are listed below.

3.1 Centrifugal Compressors

Polytropic Efficiency, % = 0.3002 [Log(Qv)] 3 - 5.886 [Log(Qv)] 2

+37.577 Log(Qv) + 6.1925

3.2 Axial Compressors

Polytropic Efficiency, % = 0.9610 Ln (QV) + 78.724

where QV = Inlet Volume Flow, acfm

Figure 1: Polytropic Efficiencies for Dynamic Compressors