30 May 2005

THEORETICAL DESIGN OF A HIGH-SPEED, OIL-FREE RADIAL COMPRESSOR FOR DOMESTIC HEAT PUMPS


The development 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 opens 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 clearances 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.