Characteristic diagrams show volume flow and the required work input for variable working conditions. They can be obtained either by measurement or by approximation calculations. The curves can be given either with absolute values or with values related to working conditions of volume flow, delivery pressure and power requirement. Alternatively, characteristic values such as pressure ratio, volumetric efficiency and efficiency are being used.
As a first example we look at a singlestage compressor with variations of suction and delivery pressures. For a natural gas compressor in boxer design the manufacturer supplies the diagram as shown (panhandle diagram). This diagram shows the changes in volume flow and power requirement in relation to the suction and delivery pressure, also the limits to operating conditions. A characteristic feature is the maximum of power requirement at average compression ratios. (The diagram was calculated with the technical data of the compressor and the gas composition.)
The volume flows of multistage compressors differ from those of the single stage compressors the more, the higher the number of stages. The enclosed graphs (related to parameters at working conditions) show as an example on a three stage compressor the variations of compression ratios and mass flow with the delivery pressure.
The compression ratio of the first stage hardly changes. Therefore the volumetric efficiency and volume flow of the compressor only changes to a very small degree. The increase of the compression ratio leads to smaller volumetric efficiencies in the higher stages. This is however compensated by the higher density of the gas in the suction duct of the respective stage.
This effect makes it impossible to influence the throughput of a multistage compressor by altering the delivery pressure.