Experimental study of frost accumulation on fan-supplied tube-fin evaporators

Abstract

Compact tube-fin evaporators have been extensively used in refrigeration cassettes for light commercial applications. Such refrigeration systems are space constrained and, therefore, the heat exchangers (condenser and evaporator) must have a large area-to-volume ratio. In addition, such applications require a subfreezing evaporating temperature that induces the growth of a frost layer on the finned surface, which may block the evaporator if a proper defrost strategy is not used. Before completely blocking the evaporator, the frost layer depletes the heat exchanger performance by adding an extra thermal resistance and also by reducing the fan-supplied air flow rate. Understanding the way the frost forms on these compact heat exchangers and also the way the fan is affected by frost clogging is mandatory for the design of robust refrigeration systems and also to devise more efficient defrost strategies. In this study an experimental investigation on the frost accretion of tube-fin evaporators considering the fan characteristics is carried out. To this end, a specially designed, constructed and calibrated closed-loop wind-tunnel facility was used. Experimental tests were carried out with four different (three wavy-fin and one louvered-fin) evaporator coils under different conditions. It was found that the frost formation rate increases with the air flow rate, supercooling degree and the density of fins. A strict relation between accumulated mass of frost, air-side pressure drop and cooling capacity was also observed. It was also noted that the fan characteristics play an important role on the evaporator thermal performance, indicating that under frosting conditions the fan-evaporator pair must be designed as a coupled system. Furthermore, for the same operating conditions, the louvered-fin evaporator showed to be more sensitive to the frost formation effects than the wavy-fin coils.