Comparison of Frost and Defrost Performance Between Microchannel Coil and Fin-and-Tube Coil for Heat Pump Systems
This paper presents a comparison of frost and defrost cycling performance between a microchannel heat exchanger with louvered fin and a fin-and-tube heat exchanger with straight fins employed as outdoor coils of a 14 kW (48 000 Btu/h) heat pump system. In addition to temperature, pressure and flow rate measurements taken at various locations of the systems, the fin-base and tube wall surface temperature were also recorded by using fine-gauge precalibrated thermocouples on the coils. Further, load cells were used to measure the mass of frost accumulation during heating tests. Data showed that the frosting time of the microchannel heat exchanger is more than 50% shorter than for the fin-and-tube heat exchanger, which is chosen as the baseline system. The average heating capacity and system performance over a frost–defrost cycle are also lower for the system with microchannel heat exchangers. Higher frost growth rate was mainly due to augmented temperature difference between air and the surface of the heat exchanger, and preferential frost nucleation sites on the louvered fins and microchannel tubes. Removal of residual water in the microchannel heat exchanger did not improve the frost performance significantly. Blowing nitrogen on the microchannel coil after defrost removed any visible water retained in the coil after the defrost cycle but the cycle time increased only by 4% with respect to wet and frost conditions. The cycle time of the same microchannel coil starting with dry conditions was about 60% longer than the cycle time in wet and frost conditions.
Operating characteristics of an air-source heat pump under frosting/defrosting conditions
Abstract This paper presents an experimental investigation on the behavior of an air-source heat pump operating under frosting/defrosting conditions. Dynamic data of parameters, such as the frost layer thickness, compressor suction/discharge pressures, fin temperature, heating capacity, power consumption, and fan electric current, were acquired during the test. Based on these experimental data, the operating characteristics of the heat pump under frosting/defrosting conditions were analysed, and the influence of frosting/defrosting on the performance and reliability of the heat pump was investigated. It was shown that a critical frost layer thickness existed, below which the heat pump presented no noticeable parameter changes. When the frost layer grew above the critical value, the compressor discharge pressure increased rapidly with a slight decrease in the suction pressure, but the shaft power remained almost unchanged. However, due to the increase in airflow resistance, the airflow rate dropped and the fan electric current increased abruptly, leading to fan performance deterioration and a reduction in the system heating capacity. At 50 per cent nominal airflow rate, the frost layer formed later and grew slower than at the nominal airflow rate. Under defrosting conditions, the suction pressure experienced a large drop, which led to frequent on-off switching of the compressor due to pressures falling beneath the low-level limit. This could be avoided by recirculating a portion of the high pressure gas in the compressor receiver back to the compressor inlet through a pipe fitted with a solenoid valve controller.
Performance of finned-tube heat exchangers under frosting conditions: I. Simulation model
Abstract This paper describes an analytical model that was developed to predict the performance of finned-tube heat exchangers under frosting conditions. The method models the frost growth mechanism and heat exchanger performance in a comprehensive manner. The results include frost accumulation and its effect on energy transfer in relation to varying humidities, fin densities and ambient conditions.