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The gas volume of the screw refrigeration compressor is proportional to the rotational speed, and changing the compressor speed can achieve the purpose of adjusting the gas volume. However, the change of the speed will affect the relative leakage and friction loss of the unit, which will affect the efficiency of the screw refrigeration compressor. The literature analyzes the effect of rotational speed on compressor performance: at lower rotational speeds, the volumetric efficiency and adiabatic efficiency drop due to leakage increases sharply. At high speeds, leakage has little effect on efficiency, and volumetric efficiency and adiabatic efficiency change very little in the range of 2000 to 6000 rmin.
This is because the volumetric flow rate of the compressor increases when the rotational speed is high, which is reflected in the increase in the adiabatic efficiency on the efficiency curve. However, as the rotational speed increases, the friction loss power increases, resulting in a decrease in mechanical efficiency. Therefore, in general, as the rotational speed increases, the effect of rotational speed on efficiency will gradually decrease. According to the above analysis, the change of the rotational speed has little effect on the leakage amount in a certain speed range. Therefore, when the rotational speed is adjusted within the range, the decrease of the power consumption of the compressor will be proportional to the decrease of the load. The machine maintains high efficiency and superior performance.
Therefore, it is very economical to adjust the load by the shifting method within this rotational speed range. However, each screw compressor has its own optimal speed, which is much lower than the optimum speed. At the same time, the speed regulation of the motor also has its shifting range. The too low speed will also make the motor work unstable and the efficiency will decrease. Therefore, the shifting adjustment is only suitable for the adjustment within the load range of 100 to 60. Both of the above methods have their own weaknesses when used alone, and do not consider the pressure loss caused by the load change, which makes the internal and external pressure ratio unbalanced, and increases the additional power consumption of the unit. Therefore, in order to achieve good performance of the screw chiller under partial load conditions, it is necessary to optimize and match the energy adjustment method of the unit and the changes of the internal and external pressure ratios.
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