Transient Thermal Analysis of Screw Compressors, Part I: Use of Thermodynamic Simulation to Determine Boundary Conditions for Finite Element Analyses

This paper is the first of a three part report on the transient thermal analysis of screw compressors. Knowledge of rotor-to-housing and rotor-to-rotor clearances is a fundamental first step in defining part dimensions that lead to a design with high efficiency and reliability, characteristics which must be balanced as they impose opposite demands on internal clearances. As reliability and performance are functional characteristics of the compressor, it is important that clearances be understood in the context of loaded operation. A proper assessment of the design can only be made if this understanding extends over the entire operating envelope of the compressor for both steady-state and transient operation. A finite element analysis procedure has been developed to allow computation of rotor-to-rotor and rotor-to-housing clearances for a refrigerant screw compressor. In this first part of the report, we describe a model for calculating surface heat transfer coefficients and refrigerant bulk temperatures using a one-dimensional thermodynamic simulation. The result of this step is a set of heat transfer coefficients and gas temperatures defined on the rotor and housing surfaces. This information is then passed on to serve as boundary conditions in a finite element model. Development of the finite element modeling approach, analysis of rotor-to-housing clearances and analysis of the thermal effects on rotor-to-rotor clearances are discussed in Parts II and III of the report.