Theoretical Design of a High Speed, Oil Free Radial Compressor for Domestic Heat Pumps
The devlopment of domestic heat pump systems is mainly based on the use of volumetric oil lubricated compressors. Following the implementation of economizer based rotary compressors, which represented a major improvement step, one major opportunity to improve both efficiency and heat rate, in particular for high temperature lift heat pumps, is to use two stage cycles. However the reliability of those can be strongly impaired by oil migration, resulting in oil level unbalance when using oil lubricated compressors, unless more sophisticated auxiliaries including an oil pump are added. The need for this additional equipment, the requirement of higher vapour velocity for oil return and impediments to the efficiency of enhanced surfaces for heat exchangers are some of the incentives for the development of oil free compressor systems. As high speed bearings and electrical motors are gradually becoming available this open the way to consider the use of low power, compact, oil-free and high speed radial compressors. This paper describes the basic design of a high speed single stage radial compressor for high pressure ratios and a wide flow range aimed for the first stage of a two stage heat pump. A commercially available 3D viscous code has been applied to calculate the flow field through the impeller and the diffuser. The particular choice of the bearings allows using very small tip clearences resulting in low leakage losses. To complete the design a commercial 3D Finite Element code has been used for stress prediction as well as for the calculation of the blade critical frequencies and their corresponding modes. During the design process the impeller blade geometry has been continuously checked for machinability in order to ensure a feasible impeller using conventional 5 axis milling machines. This paper furthermore describes the setup of the test rig to be used for testing.