For single stage compressors, especially those where cooling is not possible or does not make sense for process reasons ( for example refrigeration compressors ), an ideal compression without removal of heat ( isentropic process ) can be assumed. One then refers to an isentropic efficiency , which becomes higher the higher the ratio of the specific work of the isotropic and isothermal process.
The reduction of the delivered mass can be described by individual factors depending on its causes. The sequence in which these factors are listed reflects their importance.
The volume factor describes the reduction of the volume available for the suction process due to the reexpansion of the clearance volume. Fig. a shows calculated values for various amounts of clearance volume and polytropic exponents.
The temperature factor discribes the reduction in density at the beginning of the compression due to the temperature increase during the intake of the gas. Fig. b shows expected values, where the lower values relate to small working chambers and gases with high isentropic exponents.
The leakage factor describes the reduction of the delivered mass. On new and oil lubricated compressors the influence of leakage is small. With non lubricated compressors leakage of the piston rings and pressure packings can reduce the volume flow by 10 to 15 %.
The humidity factor describes the reduction of the volume flow through condensation of a part of the water vapour content of the gas after compression.
In order to evaluate the energetic quality of a compressor ( as with all energy machines ) an efficiency is calculated.
With piston compressors also a volumetric efficiency is being used, which is defined as the useable part of the piston displacement for gas delivery.
Energetic efficiency
The isothermal efficiency compares the achieved pumping work with the work input.
The pumping work is the product of mass flow and the mass specific work required at ideal isothermal conditions. The mass flow of the compressor can be obtained by measuring the volume flow and the inlet conditions. By also measuring the delivery pressure the isothermal work can be calculated.
The power required is the power input at the clutch , which is obtained by measuring the torque and the revolutions per minute.
Part of the power input is used up as the mechanical power loss ( friction in the drive mechanism and the sealing elements, also the work input of auxiliary aggregates such as lubricators, cooling pump ).
The remaining internal work, which is transmitted by the piston onto the gas, is larger than the pumping work by the amount of the internal power losses .
After the experimental evaluation ( indicating ) of the cylinder pressure , which varies with time and crank angle , or by simulating the thermodynamic processes in a calculation which comes near to the real conditions , the internal work can be derived by the areas of the p,V- diagrams of all working chambers .
The following diagram shows the basic dependence of the isothermal efficiency of the pressure ratio for air compressors with various stages. For each stage there is an optimal pressure ratio. With smaller pressure ratios the throttling losses become more important. With larger pressure ratios the additional work due to deviations from the isothermal compression rises. The maximum achievable efficiency rises with the number of stages, as the approximation to the isothermal process increases.
Volumetric efficiency
In the ideal case the complete piston displacement at the end of the suction stroke is filled with gas at inlet condition. It would contain the complete displacement mass. The actual delivered mass after a compression cycle is always smaller then the ideal piston displacement mass.
The ratio of the actual delivered to the ideal mass per compression cycle or the respective mass flows is called the volumetric efficiency.
This parameter does not have a direct indication of the energetic efficiency of the compressor.
If the piston displacement and the volumetric efficiency of a compressor is known, the volume flow ( flow at suction condition ) follows .
