Screw Compressor For Ammonia-Water Heat Pump Lubricated By The Process Mixture

A prototype compressor for application in high temperature compression-resorption heat pumps has been designed, manufactured and tested. The paper presents details of the compressor design together with test results obtained on the pilot heat pump plant with the prototype compressor. The compressor is of the twin-screw type and its stainless steel rotors operate without timing gears. There is no oil present in the compressor working space, the rotors are lubricated by the process mixture of ammonia and water. To make operation with such a lowviscous lubricant possible, a profile with low torque transfer has been selected and a special friction reducing coating has been applied to the rotors. INTRODUCTION Wet compression-resorption heat pump is a promising type of heat pumps that operates with non-azeotropic refrigerant mixtures. The main differences, when compared to the classical Rankine cycle, are the non-isothermal phase transition of the mixture in the heat exchangers and the compression of the two-phase mixture in the compressor, which works as a gas compressor and at the same time as a liquid pump (that is why the heat pump is called “wet”). Wet compression results in reduction of the consumed power, excludes the vapour superheating and is especially attractive for high temperature applications. For such applications estimated gain in COP is up to 20% when compared to conventional dry compression heat pumps. The problem, which up to now eliminates wide spread of the wet compression-resorption heat pumps, is the lack of the compressors capable to perform the compression of the liquid-gas working mixture rather effectively. Besides being tolerant to the liquid carry-over, the compressor must be oil-free, because it is difficult to separate the oil from the process liquid by simple means. A prototype compressor for application in high temperature compression-resorption heat pumps has been designed, manufactured and tested. The compressor is of the twin-screw type and operates without timing gears. There is no oil present in the compressor working space and the rotors are lubricated by the process mixture of ammonia and water. The paper starts with the basics of the compression-resorption heat pump cycle and goes on with details of the prototype compressor design. The experimental part of the paper presents details of the test rig together with results of compressor performance measurements and their analysis. WET COMPRESSION-RESORPTION HEAT PUMP A simplified scheme of the wet compression-resorption heat pump is shown in Fig. 1 after Itard [1998]. The main components of the heat pump are similar to those of the classic vapour compression machine. However, a resorber is used instead of the condenser and a desorber instead of the evaporator. The working medium is a twocomponent non-azeotropic mixture. In this study the mixture of ammonia and water is considered. Unlike the vapour compression installation, in the wet compression-resorption cycle the working medium exits the desorber (evaporator) in the two-phase state. This two-phase two-component mixture having passed through the intermediate heat exchanger enters the compressor, which operates as a gas compressor and at the same time as a liquid pump. In the resorber, the compressed vapour is totally absorbed by the weak liquid solution and absorption heat is transferred to the heat sink. This process is non-isothermal. Then the strong liquid solution is subcooled in the intermediate heat exchanger and via the expansion valve is supplied to into the desorber. In the desorber, heat is transferred from the source to the process medium and non-isothermal desorption occurs. The liquid solution becomes weak and together with the produced vapour exits the desorber. Figure 1. Scheme of wet compression-resorption heat pump Market study has indicated that compression-resorption heat pumps might be attractive in application for upgrading of waste heat for food processing industry. Heat pump operation conditions related to this application are listed in Table 1. Table 2 presents operation conditions related to the compressor for the selected application. Table 1. Heat pump operation conditions Sink inlet temperature, C 110 Sink outlet temperature, C 130 Source inlet temperature, C 80 Heating capacity, industrial scale, kW 250 Heating capacity, test scale, kW 50 Expansion valve Compressor Heat sink Heat source Resorber Desorber Heat excanger