Application of Controls for Hot-Gas Defrost of Ammonia Evaporators

To defrost ammonia evaporators, it is sometimes necessary to check the plumbing arrangement and the valves used to accomplish the task. To enable hot-gas defrost systems to operate successfully, several factors must be considered. There must be an adequate supply of hot gas. The gas should be at a minimum of 100 psig. The defrost cycle should be accurately timed. Condensate removal or storage must be provided. An automatic suction accumulator or heat reservoir should be used to protect compressors from liquid refrigerant slugs if surge drums or other evaporators are not adequate to handle the excess gas and condensates (see Fig. 10-7).

Controls must be used to direct and regulate the pressure and flow of ammonia and hot gas during refrigeration and defrost cycles.

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