Development of a Comprehensive Thermodynamic Modeling System for Refrigerant Screw Compressors

This paper describes analytical and experimental efforts leading to a complete thermodynamic simulation of refrigerant screw compressors with emphasis on methods for inspection of experimental compressors and assembly analysis to determine actual clearances for specific test compressor. This work led to a better understanding of leakage and improved analysis in the thermodynamic model. The need for accurate modeling of all of the compressor components is also discussed, with brief descriptions of the models used provided. Introduction There have been many thermodynamic modeling systems for screw compressors (see /1/ and /2{). The thermodynamic analysis in the system described here shares characteristics with many of these. However, the model contains a collection of elements in addition to the compression simulation. The design system also includes comprehensive assembly analysis procedures which provide important clearance information to the simulation. FE Analysis A schematic of the system in Figure 1 shows the two main elements, a thermodynamic simulation and an assembly model. Both branches use generic solvers. The thermodynamic solver is used in a scroll compressor simulation and the assembly solver is used in assembly analysis and design programs. A detailed analysis of compression within the screw rotors is included in a broader simulation of a complete compressor where effects such as motor loss, pressure losses and liquid or vapor injection are added. The model also includes a cycle analysis. Separate programs supply rotor geomety data and shaft and housing deflections for clearance calculations. The compressor model programs provide geometric and gas load data to rotor proille tool design, shaft design and bearing selection programs. Clearnnces are an important part of the simulation. A branch of the modeling system has been set up to provide analysis of assembled t Tool Rotor Design--Profile Parts Database AssemblY..Solver: Model clearances Solver: energy continuity Shaft De,ign J Bearing Selection Figure 1 Screw Compressor Modeling System clearances. A database of information from measurements of critical parts is used to compute internal clearances. The procedure can include the effects of thermal and pressure loads, providing an accurate picture of the clearances in the running compressor. This information is important in interpreting test data and using it to improve the simulation model. Thermodynamic Model The thermodynamic model has three elementsmodeling compression within the rotor pair, modeling the compressor containing the rotors and modeling the cycle with the compressor. The compression process is represented by real gas fonns of the energy and continuity equations with explicit treatment of individual mass and heat flows. The resulting set of differential equations are solved as an initial value problem. The initial state in the rotors is dependent on suction side pressure losses and hermetic motor losses which are in turn affected by the mass flow and power characteristics of the rotors. An extended model includes analysis of these and other elements of overall compressor performance. In addition, an embedded cycle calculation provides a consistent solution for mass flow rates and pressures for economizer cycles and unloaded operation with relief. Compression process modeling can be carried out for a variety rotor geometries, discharge port options, rotor and housing clearances and oil and/or liquid refrigerant injection. Clearance options include variations in rotor-to-housing clearance as a function of position within the housing, variation in rotor-to-rotor clearance along the length of the rotor, separate rotor-to-slide valve clearances and separate rotor discharge end clearances. Porting options include separate radial and axial discharge ports with geometries computed based on specified volume ratio and automatic adjustment of radial port geometry for slide valve unloader geometry. Options for four mechanical unloader configurations are provided, including details such as 'clearance volumes' for piston type ports which do not close flush with the housing. Other elements included in the full model are: constant or variable speed open or hermetic motor performance; inlet and discharge pressure losses based on specified geometry; unloader gallery loss based on gallery geometry; discharge pulsation with