For heat collection from air in cold districts, an important problem is how to prevent and/or remove the frost layer deposited on the cooling surface of the evaporator, because the frost layer will increase the thermal resistance and the coil will become blocked. Frost removal tests were made in a bubbling fluidized bed using three kinds of fluidized particles to see their frost removal capability. It was found that fluidized silica sand had some frost removal capability but was not sufficient to remove the frost layer completely. To cope with this problem a solid-circulating heat collector was developed. The apparatus consisted of a riser and a moving bed, with solid particles circulating between them as heating medium. Cooling surfaces for the evaporator were inserted into the moving bed. There is little possibility of frost layer formation on the cooling surface since only a small amount of air and moisture exist in the moving bed. Furthermore, the pressure drop in this heat collector is much lower than that in the bubbling fluidized bed since there is no need to have a distributor in the bottom section of the riser. Based on experimental results with the small apparatus, a bench-scale plant combined with a 10 kW heat pump system was developed. Test runs were conducted under outdoor conditions during the winter season in Sapporo. From a six-day continuous test run it was confirmed that the system operated stably without frost formation. © 1990, The Society of Chemical Engineers, Japan. All rights reserved.
