Study on the operating performance of cross hot-gas bypass defrosting system for air-to-water screw heat pumps

Large-scale air-to-water screw heat pumps often operate with substantial frost formation on the air side of evaporators in winter, and the frost layer has to be melted away periodically. This paper presents a defrosting method for air-to-water screw heat pumps, i.e. the cross hot-gas bypass defrosting method, which uses two air heat exchangers for hot air from one air heat exchanger to melt the frost and absorb heat from another heat exchanger through the corresponding four-way reversing valve. The principle and process of this method are illustrated. The dynamic characteristics are experimentally investigated on a 359 kW air-to-water screw heat pump. The discharge temperature of the unit is at the normal range and the pressure varies little. A higher suction pressure is needed to achieve low pressure protection. The time period needed for the combination of heating and defrosting processes is short and it is not necessary to shutdown the compressor for defrosting so that a longer heating time period is possible. The inlet & outlet water temperature can be kept at around 30 C and high heating COP is obtained. The test results show that the units work as expected.