Wang Jingfu

Experimental study of dynamic characteristicsof single screw expander with compressed air and water steam as workingfluid

The research and development of novel high-efficient devices used for thermal energy conversion of low-temperature heat source (blow 230°C) is of great significance for energy saving. Single screw expander is a novel expander with a lot of advantages, such as a good axial and redial force balance, a long working life which is twice of double screw, a high pressure ratio which is up to 15 for single stage, a highest volumetric efficiency which can reach 92%, 4% higher than that of double screw, a relatively high efficiency under part load condition, a simple structure, a lowest noise which is 10dB lower than the double screw, and an oil free lubrication. In this paper, an experimental system used for testing the performance of a single screw expander whose diameter is 117 mm was designed and established. The number of screw head is 6 and the number of star wheel teeth is 11. The compressed air and the water steam were used as the working fluid to experimentally study the dynamic characteristics of single screw expander. In the experiment, the inlet temperature, the outlet temperature, the inlet pressure, the outlet pressure, the flow rate of the working fluid, the torque, the speed, and the power of the single screw expander were measured. When compressed air was used as the working fluid, the valve is fully open and the outlet pressure of the air compressor is set to be 0.75–0.85 MPa. From the experimental results, it can be seen that the single screw expander has an optimal power output working condition when the speed is about 2500–3500 r/min, and an optimal temperature drop working condition when the speed is about 2180–2700 r/min. The maximum torque was 4.4 kW when the speed of expander reached 2700 r/min. The maximum inlet flow rate was 32.9 m3/h. The initial inlet temperature of the expander was 22.7°C, and the final inlet temperature is 28.7°C. The variation of the inlet temperature is due to the inadequate heat dissipation caused by the long-time running of the air compressor. The lowest outlet temperature of the single screw expander reached - 20°C. The maximum temperature drop between inlet and outlet of expander reached 45°C. The lowest gas consumption rate was 55.2 kg/(kW h), the highest reached 187 kg/(kW h), and the average was 77.5 kg/(kW h). The maximum total efficiency of expander reached 58.8% and the average was 45.8%. Moreover, the irreversible loss increases first and then decreases with the increase of speed and with the increase of expansion ratio. The highest irreversible loss reached when the speed was 1200 r/min and the expansion ratio was 6.2. With the increase of the speed, the outlet pressure gradually increased and the expansion ratio gradually decreased. The gas consumption rate of the single screw expander first increased and then decreased with the increase of the speed. When water steam was used as the working fluid, the maximum power of expander was up to 3.9 kW. The lowest gas consumption rate was 22.5 kg/(kW h), and the maximum total efficiency of expander reached 66%.