Design of an Unloader System for Moderate Capacity Refrigeration Screw Compressors

This paper reviews the design of a combination variable I fixed-step port unloader system for moderate capacity refrigeration screw compressors. The general specification of an unloader system to meet application requirements and specific issues of port and flowpath design are discussed. The paper demonstrates use of a comprehensive compressor simulation and a commercial pipe flow analysis program in the design. Comparisons with experimental data are also presented. INTRODUCTION A slide piston I axial port unloading scheme for screw compressors used in multi-circuit refrigeration systems can provide modulated capacity control over a significant portion of the operating load line. For this application, use of such an unloader system has little effect on full load performance. In the following, design requirements, design methods and the selection of details for a specific piston I port system for a screw compressor for air-cooled water chillers are described. UNLOADER SYSTEM DESCRIPTION The unloader system consists of a modulating capacity slide piston in the male rotor housing (Figure 1) and a flush axial piston on the female rotor discharge end plane (Figure 2). A complete description of this concept may be found in the U. S. Patent by Linnert Ill. Unloader Slots Unloader Gallery