Conference Agenda

Twin screw compressors are widely adopted in chiller systems due to their high efficiency, reliability, and low maintenance. Increasing the speed of the compressor can reduce the manufacturing cost and the leakage. However, higher speed brings efficiency decay and noise increase. In our tests, increasing the speed of the original machine directly from 3000 rpm to 5000 rpm reduced the adiabatic efficiency by 7.5% with significant noise. To improve the performance of the twin-screw refrigeration compressor at a high speed of about 5000 rpm, 3D transient full-scale CFD simulations were performed with high-quality moving grids. The simulated results were validated by experiment results. The general non-reflection boundary condition (NRBC) and a damping method were adopted to speed up simulations. The pressure distribution inside the compressor chamber can be distinctly discovered and help guide the compressor design. The design parameters of the rotor, such as rotor configuration, wrap angle, and length ratio, were studied. With more lobes, the discharge pulsation can be reduced with slight performance improvement. The length ratio was optimized, and the relevance of the rotor tip velocity was discovered. Improvement can be made with a higher wrap angle at high speed, and the suction delay can also increase the efficiency at the same level. In addition, a parametric study of the design of the suction and discharge ports was carried out. It was found that a small depth of radial suction space can make the intake flow more directional and increase efficiency. Enlarging the suction port by using an unmatched wrap angle was also found to increase the efficiency. A parametric study of the discharge port with different built-in volume ratios was also performed, and a slightly enlarged port was found to be helpful for reducing the resistance. Utilizing the CFD method, additional improvement potential of screw compressors for chillers at higher speeds can be continually uncovered.

Pursuing sustainable and environmentally friendly cooling solutions has underscored the significance of exploring alternative refrigerants in various industrial applications. Previous publications showed that high-speed screw spindle compressors are a viable solution for water vapor (R718) applications. This paper introduces a design optimization strategy targeting the efficiency enhancement and mechanical stress mitigation of high-speed screw spindle compressors. As rotational speed increases, so does volume flow and efficiency, but mechanical stress becomes a challenge, especially on the root diameter of the spindle teeth. The research investigates this trade-off and proposes a design that allows higher speeds while alleviating mechanical limitations, increasing its mechanical safety.

The proposed design optimization is validated through analytical calculations and thorough FEM simulations, displaying substantial efficiency improvements and volumetric flow rate increments at elevated speeds. The observed outcomes underline the approach's potential in facilitating efficient, high-speed screw spindle compressors.

Recently, due to climate crisis and to reduce the dependency of fossil fuel, legislators in both Europe and North America are implementing policies to phase out fossil fuels for heating. This has resulted to a rapid expansion of demand for heat pumps. Even in low outdoor temperature regions such as Northern Europe, the replacement fossil fuel heating with heat pump heating is required, which necessitates a high compression ratio compressor that significantly exceeds traditional air conditioning compressor operating conditions.

Although a two-stage compression process is generally more efficient than single-stage compression one at high compression ratio, a compressor configuration that requires multiple compressor units to achieve the necessary compression increases production costs. Specifically, such traditional configurations include compressors arranged in series or two-stage compressors with two series compression mechanisms within a single casing.

Therefore, a compressor capable of achieving two-stage compression with the same number of parts as a traditional single screw compressor, thus leveraging the unique characteristics of the single screw compressor that forms independent upper and lower compression chambers with one screw rotor and meshing two gate rotors. A prototype compressor consisting of a two-stage semi-hermetic single screw compressor has been operated and tested. This paper reports the results of characteristics and challenges extracted from this investigation.