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.
Frosting performance of fin-and-tube evaporators with small copper tubes diameter.
In modern heat pump systems, the heat exchangers use enhanced heat transfer surfaces for both air and refrigerant sides. In air conditioning applications, conventional 9.5 mm (3/8 inch) tube diameter fin-and-tube coils are slowly being replaced by microchannel heat exchangers. However, during heating mode the energy performance of heat pump systems with microchannel outdoor coils are generally lower than those of fin-and-tube direct-expansion evaporators due to a higher frequency of defrost cycles. A different approach might be to utilize fin-and-tube technology, which has proven records of excellent water drainage characteristics and good performance in frosting operating conditions, and enhance the air side heat transfer rates by introducing a larger number of small diameter copper tubes. In this paper, six fin-and-tube coils with copper tube diameter ranging from 5 mm (1/5 inch) to 7mm (8/29 inch) were experimentally investigated in frosting operating conditions. The laboratory experiments were conducted in an air flow wind tunnel at Oklahoma State University. Experimental data on heat transfer rate and air-side pressure drop across the coils were measured and the fin density and the tube diameter were varied in a parametric fashion during the experimental campaign. The performance of the fin-and-tube coils were also compared to those of a conventional 9.5 mm copper tube diameter fin-and-tube heat exchanger and of a microchannel heat exchanger that had similar air-side frontal area and at similar operating conditions of outdoor direct-expansion evaporators in heat pump systems for residential applications. The trends of the data during frosting operation suggested that reducing the tube diameter was beneficial for frosting performance at low fin density while was harmful at high fin density. The data showed that increasing the fin density increased the capacity but significantly reduced the time for heating service for the evaporator. Small copper tube diameter resulted in about 11% higher initial capacity at dry start conditions and about 4% higher average integrated capacity when considering the entire frost period. The data discussed in this paper serve as basis for future research on direct-expansion evaporators for air-source heat pump applications, in which the frosting of the outdoor heat exchangers is still one of the major concerns.
Avian infectious bronchitis.
Avian infectious bronchitis (IB) is caused by the gammacoronavirus infectious bronchitis virus (IBV). The virus causes infections mainly in chickens and is a significant pathogen of commercial meat and egg type birds. IB is an acute, contagious disease characterised primarily by respiratory signs in growing chickens. In hens, decreased egg production and quality are often observed. Some strains of the virus are nephropathogenic and produce interstitial nephritis and mortality. The severity of IBV-induced respiratory disease is enhanced by the presence of other pathogens, including bacteria, leading to chronic complicated airsacculitis. Diagnosis of IB requires virus isolation or demonstration of viral nucleic acid from diseased flocks. Demonstration of a rising serum antibody response may also be useful. The widespread use of live and inactivated vaccines may complicate both the interpretation of virus isolation and serology findings. The occurrence of antigenic variant strains may overcome immunity induced by vaccination.