A methodology for the performance characterisation of a variable speed CO2 compressor

Abstract

Heat pump cycles have a vast range of industrial applications. A refrigerant withdraws heat from a reservoir, transferring it to another fluid. In order to design and sufficiently size these cycles, accurate operating predictions of all the components are crucial. Amongst these components are the compressor. Some analytical formulations exist to predict the working parameters of a single speed compressor. However, in most cases measurements and specifications known only by the manufacturer, or are difficult to obtain, are needed. The addition of variable speed compressors widened the range in which predictions are made. This study defines a numerical methodology that allows empirical derivation of operating equations for a variable speed reciprocating compressor. Operational equations are derived for a reciprocating carbon dioxide multi-hertz compressor at between 40 and 60 Hz after applying the methodology to a specific compressor. Four out of the six operating parameters must be known to calculate the remaining two. These parameters are; Refrigerant mass flow rate Refrigerant compressor outlet pressure Refrigerant compressor inlet temperatures Refrigerant inlet pressure Refrigerant compressor outlet temperatures Compressor operating frequency During the methodology description, the adaptability, versatility and applicability of the methodology is evaluated and discussed. The methodology has internal decisions that can affect the accuracy and complexity of the end result. Due to the amount of data required and time constraints, experimental data with acceptable accuracies were used instead of actual test-bench values. From the comparison table and the accompanying plots it can be observed that the discharge temperature equation tends to predict values that are on average 0.43% above the experimental values. The absolute error average of 0.99% shows a low measure of inaccuracy. The mass flow equation tends to predict 0.06 % on average above the experimental value. The absolute error value of 0.43% is combined with the plain average to state that over a large sample group, the mass flow prediction equation will be the more accurate equation since the tendency of over- and under-predicted value are minimal. As discussed, these accuracies are between the experimental and predicted values. Comparison with the actual test-bench values for mass flow and discharge temperature will contribute a further around 3% and 5% inaccuracy respectively