Analysis of the oil return in a pair of scroll compressors working in parallel at part load

Condensing units working with a pair of scroll compressors working in parallel (tandem) could be an interesting product since this configuration allows one to use these compressors at higher outputs and to modulate load more easily and efficiently. Nevertheless, this configuration can present serious troubles linked to oil return, especially at part load. In this paper, an analysis of a condensing unit working with a pair of scroll compressors working in parallel is presented. First the test bench is described. The problems linked to the oil return when using two compressors in parallel at part load are described. The configuration tested was equipped with a special connecting pipe to avoid the use of an oil level regulating system. The tests carried out over the range of the condensing unit are discussed. Emphasis is put on the oil circulation in the refrigerant network at part load when one compressor is cycling.