Steady-state and dynamic behaviour of plate-fin-tube direct expansion evaporators when using a zeotropic refrigerant mixture.

Both steady-state and dynamic simulations of the operation of plate-fin-tube air coolers, under dry and wet conditions, and also of themostatic expansion valve (TEV) controlled coils under dry condition, were carried out. The investigation aimed at improving our understanding as how the operation behaviour of the cooler coils, under various coil conditions, was influenced by the use of a ternary refrigerant mixture (R407C) when compared to a pure refrigerant (R134a). Based on practical coil configurations, a distributive computer model was implemented, with governing equations for air and refrigerant sides and for tube walls set up for individual coil elements. For the TEV, equipped with an external equalizer, energy and force equations were set up. To compare the two refrigerants, a reference scheme, obtained based on the steady state simulation, was set up: the same air coil-inlet conditions, the same refrigerant mass flow rates and vapour qualities at the coil inlets, and the same refrigerant temperatures at the coil outlet. The dynamic simulation was based on having a step change in the coil inlet dry-bulb (DB) temperature. The analysis and discussion focussed on the temperature gliding and many other inter-related parameters/factors, e.g. the heat transfer coefficient (HTC), the coil arrangement, the refrigerant superheat and the refrigerant type. It was observed that for steady-state, the temperature gliding affected the spatial gradient of the temperature and humidity ratio of the tube-wall, the outlet dry-bulb temperature of the coil-face row, and the row outlet humidity ratio (HR). The temperature glide, when combined with the refrigerant HTC and the coil arrangement, had a strong influence on the sensible and latent heat fluxes, i.e. suppressing the influence of other parameters such as the effective air-side heat transfer and mass transfer coefficients. For the coil dynamics, the refrigerant temperature gliding influenced the gradients of the time profiles of the DB temperature and HR at the coil-outlet tubes that were unaffected by the superheat. For the TEV-controllcd coils, only the temperature gliding-up of R407C, not the temperature drop associated with R134a, was shown to have an impact upon the rate of change of the superheat-initiation location. The combined effect of the temperature gliding and the HTC influenced the transit times between steady states of both the superheat-initiation location and external-equalizer pressure