Isentropic efficiency is a common performance measure for compressors and is useful for modelling the behaviour of real compressors in relation to discharge temperature or required mechanical power input. However, it has a weakness in that a basic assumption in the calculation is that the real compressor is adiabatic. If significant heat transfer from the compressor to the outside environment occurs, then the adiabatic assumption is invalid, but the negative effects of internal entropy generation are reduced, and in principle it is possible to have a measured isentropic ‘efficiency’ greater than 100%, which, while counterintuitive, is highly desirable. We practically demonstrate this effect with an air-cooled swashplate compressor and propose that for cases of intentional compressor cooling, the simple definition for isentropic efficiency is retained as a performance measure but renamed to isentropic performance coefficient (IPC).

The design of compact, high performance electric swashplate refrigeration compressors demands a clear understanding of different physical phenomena and their interactions taking place inside the compressor. The dynamic characteristics of the compressor are associated with the start-up transients of the swash-plate mechanism and the time variation of suction and discharge pressures. An experimentally validated, easy to implement transient swashplate compressor model has been developed that can capture the essential physics, including inertia of the pistons and swashplate to evaluate the electric motor torque loading during compressor start-up. The effects of moment of inertia, bearing torque, viscous resistance to piston motion, and suction and discharge pressures on the torque and compressor mechanical input power are investigated. For model validation, the start-up behavior is tracked experimentally using a high-speed data logger to monitor the changing phase currents of the brushless DC motor, capturing both the instantaneous power and rotational speed. Rotational mass moment of inertia is found to have only a small effect on the compressor torque and power output and can be made negligible by changing settings in the start-up algorithm for the electric motor controller. Suction and discharge pressures during start-up are found to have the largest influence on the required starting torque. More than 95% of torque is found to be because of the line pressures. Predictions are in good agreement with measurements and show that depending on the starting refrigerant pressures in the supply lines, the starting torque can be lower than the operating torque for the compressor. The original contribution of this work is in deriving a transient swash-plate compressor model that includes the inertia of the swash-plate mechanism and in clarifying the relative importance of inertia, line pressures, viscous losses and bearing resistance on the required start-up torque for this type of compressor.

A real-gas, restricted-flow valve model is compared with an ideal-gas, ideal-valve model for a 10-cylinder swashplate refrigeration compressor. Real gas properties of R134a are evaluated using the NIST standard reference database. A minor-loss discharge-coefficient approach is used to model the refrigerant flow rate through reed valves while the ideal-valve model requires no pressure difference to open the valve. In contrast with the ideal model, the discharge temperature, refrigerant mass flow rate and volumetric efficiency as a function of rotational speed are predicted well by including real-gas properties and flow restriction on the inlet valve. The ideal-gas model significantly overpredicts the discharge temperature and shows no dependence on rpm. Heat transfer to and from the cylinder wall during compression and expansion is found to have only a small effect on predictions of compressor performance. The valve model for the suction side has the largest influence on compressor performance predictions as a function of rpm.

Recent advancements in the field of mobile air conditioning and refrigeration have witnessed an extensive use of the swash plate compressor due to its compact structure, continuous operation, small size, light weight and better thermal comfort inside the vehicle. The design of the swash plate compressor is complex so that it requires considerable contributions from different fields of engineering viz. engineering mechanics, heat transfer and fluid dynamics. An estimate of compressor performance through an analytical/ mathematical model at the early stages of design and development serves as a useful tool for the designer. The input power, refrigerant mass flow rate, compression ratio and volumetric efficiency are important parameters to characterise the compressor performance. This paper presents an analytical/mathematical model for a 10-cylinder swash plate compressor with the emphasis on predicting its performance in terms of shaft torque and mass flow rate for a given rpm. A kinematic model is developed to obtain the piston displacement as an explicit function of angle of rotation of the swash plate. The model of piston and swash plate dynamics is developed then by analysing the interactions between forces and moments. The compression process model is formulated to determine the temperature and pressure inside the cylinder during one revolution of the swash plate along with the total mass flow rate in and out of the compressor. A time-varying model for the compressor is developed by combining the above three sub-models. Some experimental validation comparing predicted and measured drive torque has been done to verify the analytical/ mathematical model The predicted torque is in close agreement with the measured value.