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Centrifugal Compressor Efficiency

Centrifugal compressors play an important role within the process industry. In some cases compressor capacity may determine the throughput of a plant. If a compressor is used for refrigeration then any variation in compressor operation may directly impact the temperature control in key unit operations such as flash vessels and exothermic reactors. Also, the compressor drive is often major consumer of electric or steam energy. Thus, an on-line calculation of compressor efficiency can be useful in evaluating the impact of operating conditions on compressor performance and cost of operation. As illustrated below, the compressor suction and discharge pressure and temperature measurements are required to calculate dynamic compression efficiency.

The dynamic compression efficiency is calculated based these pressure and temperature measurements as shown below:

, where:

• h(p₂,s₁) denotes isentropic enthalpy for suction entropy s₁ and discharge pressure p₂,

• h(p₂,t₂) denotes enthalpy at discharge pressure p₂ and temperature t₂,

• h(p₁,t₁) denotes enthalpy at suction pressure p₁ and temperature t₁,

• s₁ means the entropy a suction pressure p₁ and temperature t₁.

It is estimated that there are over 80,000 centrifugal chillers in operation in North America. Thus, in the upcoming series of blogs on Centrifugal Compressor Efficiency will focus on the dynamic compression efficiency of a centrifugal compressor used in a plant for refrigeration. This efficiency calculation may be implement using common tools in a DCS and put on-line within the control system. By making on-line compressor efficiency available to the plant operator as a continuously calculated value, the operator can better assess the impact of operation changes on the compressor efficiency and use this knowledge to improve plant operations.

References

[1] Centrifugal Compressor Efficiency - Part 1, Modeling and Control Dynamic World of Process Control, 2012.