A model application to study circuiting and operation in CO2 refrigeration coils

In this paper a previously developed model has been used to show that there are cases where refrigerant circuiting is necessary. Most hydro fluorocarbons can accommodate only limited tube lengths due to excessive pressure drop in refrigeration coils as is shown here with R507A, commonly employed in supermarkets. A study of the effect of circuiting on coil operation has then been performed for three different configurations of evaporation paths, using CO₂ as a working fluid. The basic unit consisted of a single circuit forming the whole coil unit. The other two configurations had, respectively, two and three circuits corresponding to the same total area and tube length as the basic unit. Comparison between these three units was made using the basic unit as a reference. The analysis was based on the numerical model that calculates incrementally parameter distributions for air across the coil and for refrigerant inside the tubes. Circuiting which consisted in defining refrigerant flow paths and varying flow rates within the coil was shown to affect performance and general operational coil behaviour, particularly the refrigerant pressure drop and the corresponding temperature glides. It is possible to use longer circuits with CO₂ (in comparison to other refrigerants), therefore reducing their number for a given capacity.