Compressor Modeling Technique for Realistic and Broad Range Simulation

The realistic simulation of refrigerant compressors for heat pump applications is difficult because the needed physical parameters like clearance volume, blow-by, valve losses, polytropic coefficient(s), ohmic and core losses, jacket losses, etc. are not readily available. We have developed a simple yet very effective approach to this problem for our HFROST heat pump simulation program, which only requires a reasonably low amount of computer time. It consists of a mathematical model of the physical performance of a compressor, including the above unknown parameters, which is tailored to a particular, real compressor by using measured data points as published by standard compressor performance maps and determining the unknown parameters by solving the corresponding system of linear and non-linear equations. The achieved successful simulations cover a wide range of evaporating/condensing temperatures, and have been used extensively in heat pump simulations. Major difficulties encountered in this work involved: 1. Experimental errors in available compressor data, 2. Simulation of compressor jacket losses Cup to 60% of input power) and 3. The feasibility to derive one set of compressor parameters that can be used for heating and cooling operation. · INTRODUCTION Heat pump and refrigerant cycle simulation capability is one of many process simulation capabilities which have been developed at Honeywell in order to provide a good understanding of the process under both normal and abnormal operating conditions, to define control strategies and set points, and identify implications of equipment design, load, weather, controls, etc. on the system performance 123 and energy consumption. In order to answer the broad range of questions mentioned above, we have built our simulation models on the basis of the physical principles of the involved processes , and augmenting these with empirical relationships as needed. We designed the model to be simple, yet predictive, using input parameters that can be derived experimentally and providing a simulation output that can be validated by comparison with measured data. Manufacturer's compressor data typically consist of plots of input power, heating/cooling capacity, and/or mass flow of the refrigerants such as R22, measured under steady state conditions for various evaporating and condensing temperatures. These data may contain some experimental and extrapolation errors, since one compressor may be used to generate the basic data and to extrapolate to other models, which results in the provided smooth performance curves. Data obtained for four commercially available compressors are discussed here, for which we were able to achieve good agreement between their experimental performance and simulated curves. DISCUSSION Compressor Model Assumptions The compressor model is based on an ideal Rankine cycle and comprises the following assumptions: 1. The equation of state of R22 gas in the pressure and temperature range of inte{rft.was derived from Dupont's data 1nto the form .... ::;) ~ i; a:: 0 "' VI .... n:: ~ ::10 0 (.1 .. 0