Numerical investigation on mass and heat transfer in an ammonia oil-free twin-screw compressor with liquid injection

Ammonia is one of the most frequently employed chemical materials, while oil-free screw compressors have recently been recognised as a quite suitable compression tool for an ammonia refinement system. In this paper, a heterogeneous model with mass and heat transfer was developed to investigate the characteristics of an ammonia screw compressor. The mass diffusion equation was solved based on the proper calculation of the physical properties of ammonia water mixtures. Then the model was validated by experimental data using a two-stage compression ammonia refining system. With validated simulation results, the impact of liquid injection on the compressor power consumption and temperature glide was then predicted and analysed. It was predicted that the isentropic efficiencies of the two compressors were 75% and 64% respectively. The simulation results revealed that the mean droplet diameter of the injected liquid should be less than 100 μm in order to guarantee sufficient cooling effect and improve the compressor performance. Finally, an optimization method for the location of injection nozzle and the amount of injected liquid was proposed. A second liquid injection nozzle is recommended to be added and located after the start point of compression process where the temperature glide is maximum. And the optimal flow rate of the new liquid injection nozzle is about 70% of the liquid amount injected to the suction port.