Frozen Foods Warehouse Air Unit Defrosting with Inverted Buc
Frozen Foods Warehouse Air Unit Defrosting with Inverted Bucket Traps (1982)-In order to conserve energy and to avoid suction pressure jumps during defrosting, Safeway Stores, Incorporated has been including inverted bucket traps into its design for hot gas defrost valve assemblies. These valve assemblies were, in each case, installed in recirculated ammonia systems. Safeway has had good success in all of its +35 F. Warehouses with these valve assemblies; however, in the -10 F Frozen Food Warehouses and -20° F Storage Rooms for the Ice Cream Plants there was, on the first installations, a problem of leaving ice on the bottom tubes of the coils, with the heaviest ice build up being on the air entering side. the problem was defined as a failure to drain condensed liquid In the course of analyzing these installations, ammonia from the bottom of the mils, and the solution arrived at was to pass a small amount of gas around the bucket traps through an orifice or a hand expansion valve. The amount of gas passed into the downstream suction lines was calculated to be equal to less than 1% of the full load gas generated by the coils at the design temperature difference of 12 F. This amount of gas was small compared to the gas passed into the suction line by bypass pressure regulators in older Safeway systems and, thus, represented an increase in energy savings and other advantages of bucket trap defrosting over those older systems. Gas bypass orifices and hand expansion valves have now been installed in six locations in the U.S. and Canada and the air units are defrosting completely. This paper presents Safeway's observations and analyses of the systems as well as a design criterion for the system including the calculations for sizing the components. The calculations are all based on passing an amount of either liquid or gas that will give an average coil outlet velocity of 10 FPM per circuit being defrosted. The 10 FPM velocity figure is based on on-site observations of several installations incorporating different coil manufacturers and different circuit arrangements.
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