The Effects of Frost Growth on Finned Tube Heat Exchangers under Laminar Flow

Abstract

A study on the effects of frost growth on the performance of finned tube heat exchangers under laminar flow has been conducted. The study was both experimental and analytical.

The experimental part of the investigation dealt with different fin geometries: louvered, corrugated, wavy, flat plate fins and also spine fin types. The parameters which were varied include the air humidity, air and refrigerant temperature, air flow rate and fin density. A new term called the energy transfer coefficient has been defined to account for the combined heat and mass transfer processes occurring during frosting. The variables measured were the amount of frost growth, the energy transfer coefficient, the pressure drop across the coils, the enthalpy drop, and the effectiveness of the coils. The general trends were found to be consistent with the literature. It was found that the frost growth increases with humidity, air temperature, velocity and fin density due to an increase in the mass transfer. The dominant factor was the humidity content of the air. When the relative humidity was increased from 74% to 80 % at an air temperature of

32 degrees F, the frost growth increased by approximately 22 %. The energy transfer coefficient initially increases by 10 to 15% with the onset of frost formation due to increased surface area and surface roughness but then drops off due to the insulating layer of frost. The louvered fin type had the highest energy transfer coefficient in the neighborhood of 50 Btu/hr-F-sq. ft. as compared to the flat fin type which was in the neighborhood of 38 Btu/hr-sq. ft. • However, when these results were normalized with respect to dry co.1ditions, it was found that the flat fin type performed better than the louvered fin type.

A mathematical model was developed to simulate the experiments, based on fundamental heat and mass transfer principles. Due to the complexity of the problem, the model in its present form can handle flat plate fin types only. In addition to being able to predict energy transfer and frost growth, the model also performs a non-dimensional analysis on the heat exchanger performance in terms of NTU - Effectiveness, and Fin Performance. Comparisons were made between the experimental results and those predicted by the model for the frost growth, energy transfer coefficient, pressure drop and heat exchanger effectiveness. The results from the model were within 15 to 20% of the experimental values