Document Type

Theses, Ph.D

Publication Details

This thesis was submitted to University of Dublin for the award of a PhD, based on research carried out there by Jim McGovern in the Department of Mechanical and Manufacturing Engineering.

Abstract

The purpose of the work was to critically re-examine and investigate the evaluation of refrigerant compressors and to determine and discriminate between the factors which influence their characteristics. The objectives also included the investigation of techniques by which the characteristics can be established and the suggestion of ways in which compressor performance can be described and quantified. The particular compressor which was tested was of the reciprocating open type. Existing theories relating to the efficiency of displacement utilisation are extended to yield an overall framework in which the various influences can be described and quantified. Conventional parameters for the quantification of shaft power utilisation are reviewed critically and alternatives are presented and justified. These alternatives are rational efficiencies based on the Second Law of Thermodynamics. Load stands and specialised measurement techniques for testing refrigerant compressors are reviewed. A design rationale for the load stand which was built and used for compressor testing is presented. The load stand was a bypass type which involved full condensation of part of the flow and a mixing process. The operating conditions of the compressor were controlled by means of throttle valves only. Data illustrating the main characteristics of the compressor which was tested, i.e. mass flow rate, shaft power, discharge temperature and oil concentration in the discharge vapour, are presented over a range of speeds and over a range of suction superheat values. In addition, many parameters of the compressor which underlie its performance characteristics are presented graphically versus speed and versus suction temperature. Dynamic measurements of cylinder pressure and valve displacements are presented, analysed and discussed. The displacement utilisation efficiency of the compressor is presented and the losses in displacement utilisation are quantified discretely under five headings. The test data did not allow full discrimination between heat transfer, leakage and solubility effects within the cylinder and the measurement difficulties are discussed in detail. The concept of a 'phantom mass' of refrigerant which dissolves and re-emerges from solution in the lubricating oil is conjectured and discussed. The indicated rational efficiency of the compressor is presented graphically. Data describing indicated work, the suction pumping work and the discharge pumping work are also presented, as are data describing the overall heat transfer rate during compression and the time averaged plenum heat transfer rates. It is concluded that thermodynamic availability analysis is essential to the understanding of the factors underlying shaft power utilisation. The outputs of a compressor simulation model are compared with test results. The characteristics of the compressor which was tested were found to be generally favourable for the purposes of capacity control by speed variation. In conclusion, new ways of looking at the utilisation of shaft power and of volume displacement are presented along with many other new ideas relating to the evaluation of refrigerant compressors.

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