Efficiency of a gas compressor package can be defined in several ways and it involves both the characteristic curves of compressor and driver engine and their match in speed and power across all operating range under different external conditions. A method of analysis is proposed and exemplified on compressor installations experiencing variable operating conditions. The method was used later in the design of an application robust to variations in operating conditions, this providing predictable distribution for efficiency early in the design phase. In this way the customer was fully advised how the performance will evolve during long term operation.


The nominal point of a gas compressor package is specified with the main operating parameters: suction pressure, suction temperature, discharge pressure, flow, gas composition and ambient conditions. This information is usually provided by the customer together with other required characteristics. In addition, a number of extreme conditions like min - max suction and discharge pressure and flow can be specified in order to verify the extended operation. Most of the time the compressor unit is accepted if it meets or exceeds industry standards at the nominal point while still being satisfactory at the off design points. The compressor efficiency is typically presented at the design point where it reaches its maximum. The driver engine efficiency is also presented and guaranteed at the nominal power knowing that it will change with load and speed. From the combined performances of the two machines, the overall efficiency of the turbo-compressor package varies significantly at off design conditions. The costs incurred in operation are strongly dependent on compressor performance at design point as well as at off design points. In this paper the overall efficiency of a gas compressor unit will be evaluated in terms of the fuel efficiency.

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