Use of a Fluidized Bed Heat Exchanger to Improve the Performance of a Heat Pump. Final Report

摘要

The outdoor evaporator heat exchanger of a York 60,000 Btu heat pump in the heating mode was replaced with a fluidized bed heat exchanger. Air temperature control was achieved by recirculating the conditioned air from both the evaporator and condenser heat exchangers through a chambered plenum. Candidate particles and distributor plates for the design of the bed were tested separately. A particle size of 245 microns (glass spheres) at a static bed depth of 1-1/2 in. and a fluidizing air flow of 2.3 fps gave a heat transfer coefficient of 550 W/m exp 2 - exp 0 C and the best heat transfer to pressure drop ratio. The overall design heat transfer coefficient was 43 Btu/h-ft exp 2 - exp 0 F, which is about 5 times that of the conventional heat pump heat transfer coefficient. The heat exchanger consisted of two 4 ft by 5 ft shallow beds with a static bed 1-1/2 in. high. Particles used were sand, predominantly 300 microns in dia. A distributor plate consisting of a wire mesh screen supported by coarsely perforated plywood was used to support the large bed area (20 ft exp 2 ). Even air flow distribution through the bed was achieved with a stilling chamber beneath the distributor. Design heat duty for the bed was about 33% greater than that of the existing evaporator. Refrigerant was carried by 320 ft of 3/8 in.-dia. copper tubing with 3/4 in. dia copper fins spaced at ten per in. Total heat transfer area was 1/3 that of the existing evaporator. Test results showed: excessive superheat attributed to the high heat transfer efficiency of the fluidized bed; refrigerant entering the compressor at too high a temperature and pressure; and steady state operation of the system was not achieved. Nevertheless, data obtained in slow-transient tests showed a substantial improvement in heat transfer. The modified heat pump, with fluidized bed evaporator, gave an increase of approx. 40% in the COP (exclusive of blower losses). Ice prevention and removal from the tube was accomplished.