Basic Practices in Compressors Selection

Selection of each major type of Reciprocating, Rotary, Centrifugal and axial Compressor is briefly reviewed. It ls explained how the nondimensional scales of Specific Speed and Speclfic Diameter can be used to select any of the above configuration of Compression machinery at its optimum efficiency range. The paper presents the approach of Engineers Constructors in selecting compressors in petroleum, chemical and other lndustrles. The llmiting aero,thermoand rotor dynamic parameters in respect of each type of Compressor are dlscussed. It is shown how the inltial Capital Cost, Operating Cost, environmental and logistics considerations lnfluence the final selection of a Compressor for a given application. Future trends in the industry are addressed. Areas needing attention by the manufacturers and research workers are identified to further our knowledge and confidence in the selection of Compression machinery. INTRODUCTION The type of compressors used in the petroleum, chemical and other industries falls into two broad categories viz positive displacement type and dynamic type. Fig. 1 presents varlous subcategroies referred to in accordance with their deslgn, construction and or operation. It is outside the scope of this paper to present details of each of these sub-categories. Fig. 2 explains the broad range of capacity (actual CFM) and discharge pressures (PSIG). Note the overlap between configurations. Final selection of any type of compressor is influenced by economic, operating, logistics and environmental considerations as outlied in Fig. 3. In order to provide general understanding of the selection criteria following parameters are defined. DEFINITION OF AERODYNAMIC PARAMETERS Specific Speed Specific speed is an index number for the impellers or rotors of various types of pumps or compressors. It relates to the flow and head handling capabilities of a turbomachine at a given speed. It enables to relate the optimum design at peak efficiency. It is defined as: Ns ~ N 0 Where {'gH) 3/4 Ns ~ Spgcific Speed, non-dimensional N ~ Compressor Speed, Q ~ Inlet Flow, ft3/Sec. RPM 60 H = Adiabatic head, ft lbf/lbm