Evaporator Calorimeter: The Study of Overall Heat Transfer Performance

The objective of this study was to develop a flexible facility for evaluating the to-scale thermal performance of heat exchangers. During this phase of the project, exchanger testing was completed on three refrigerator evaporators: an as-manufactured plain-fin heat exchanger, a geometrically identical heat exchanger with brazed fin-tube contacts, and a spine-fin heat exchanger. These exchangers were selected because they are used in residential refrigeration, they are relatively simple, and because they provide a useful comparative study. The evaporator calorimeter was constructed with several air-side sections. Temperature and humidity control are provided in the thennal-conditioning section, and the flow-conditioning section provides thennal mixing, flow profile and turbulence control. In the test section, air temperature, velocity, pressure and humidity are measured. The tube-side flow is supplied and conditioned by a chiller; temperature and mass flow rate of the coolant are measured. The apparatus provides air mass flow rates up to about 725 kg/hr (1600 lb/hr); approach temperatures from -23??C to 49??C (-10 to 120??F); and relative humidity from about 30% to 90%. The coolant mass flow rate can reach about 500 kg/hr (1100 lb/hr) at temperatures as low as -23??C (-10 OF). With this apparatus, air-side and tube-side energy balances within ??3% are typically obtained, with worst-case energy errors less than ??7%. The overall heat exchanger conductance, UAT, was found to be highest for the spine-fin geometry. The overall conductance for the plainfin exchangers was roughly half that of the spine-fin geometry for Reynolds numbers from 500 to 3000 based on hydraulic diameter. Interestingly, when air-side area and tube-side resistance effects were considered, the plain-fin geometry had an air-side heat transfer coefficient roughly equal to that of the spine-fin geometry. Under dry-surface conditions, the plain-fin exchanger with brazed fin-tube junctions had an air-side conductance about 20% higher than that of the unbrazed exchanger. This result is probably due to fin-tube contact resistance in the as manufactured plain-fin exchanger; unfortunately, it is unclear whether contact-resistance is important under frosting conditions.Air Conditioning and Refrigeration Project 4