Modeling and performance study of a water-injected twin-screw water vapor compressor

Due to the reason of water injection, twin screw water vapor compressor can realize higher pressure ratio and saturated temperature lift of compressed vapor. Its application in mechanical vapor compression (MVC) heat pump systems has drawn much attention recently because of the great energy-saving potential and reliability. In this paper, a thermodynamic model of the working process in water injected twin screw water vapor compressor is established, in which heat and mass transfer between water liquid and vapor are considered. Its accuracy is validated by the experimental recorded p–V indicator diagrams. With the proposed model, the compressor performance is simulated and studied. According to the simulation results, water injection can increase the volumetric efficiency 5% and adiabatic indication efficiency 6%. Once the discharged vapor has been cooled to saturation, the shaft power of the compressor will increase while the adiabatic indication efficiency and the volume flow rate together with the volumetric efficiency change little with the continuous increase of injected water. Since the sensible heat of water is much smaller than latent heat, the temperature of injected water has little effect on the performance. Both the volume flow rate and shaft power increase linearly with the rotor speed. In addition, the volume flow rate of injected water should be adjusted with the regulation of rotor speed to guarantee a saturate discharge temperature. The volumetric efficiency will increase and the adiabatic indication efficiency will decrease slowly with the rise of rotor speed. Due to the cooling and evaporation effects of liquid water at the discharge chamber, the adiabatic indication efficiency does not decline when it operates at under compression condition. Water injection can greatly improve the compressor performance when the compressor operates at under compression or over compression condition, especially where a high saturated temperature lift of compressed vapor is in demand.