Numerical investigation of an oil-free liquid-injected screw compressor with ammonia-water as refrigerant for high temperature heat pump applications

Abstract

• Thermodynamic analysis of liquid injection with varying ammonia-water mixtures. • Model of an oil-free twin screw compressor with injection, leakage, and heat losses. • Injection of lean solution with low ammonia content is best for oil-free compressor. • Injection flow of 10% of compressor's suction flow is enough for full wet compression. • Two injection ports are advantageous to ensure sufficient lubrication and sealing. This study investigates a numerical model of an oil-free twin screw compressor with ammonia-water mixture as refrigerant and liquid injection. The compressor was identified as main constraint to increase the heat sink temperature of absorption-compression heat pump systems past 120 °C. liquid injection can reduce superheating of the vapor during compression, while an existing liquid film can provide lubrication and sealing, enabling oil-free operation. A quasi-one-dimensional numerical model was developed using the Modelica language. It considers the effects of liquid injection flows and injection positions, leakages, heat losses, and variation in ammonia mass fractions. The results revealed a strong influence of occurring internal leakages, varying ammonia mass fractions of the injected solution and changing injection flows and positions on the evolution of temperature, pressure, and compression power. The injection with lower ammonia mass fraction is beneficial for reducing the discharge temperature, discharge pressure and compression power. The increase in injection flow led to a further reduction in the values obtained. Continuous wet compression, which is required for sufficient sealing and lubrication of the compressor, was achieved with injection at 360° at the beginning of the compression phase and an injection rate of about 10% of the compressor’s suction mass flow. By distributing the injection to two injection ports, wet compression can be supported and occurring under-compression and backflow can be reduced.