Condensing pressure for hot gas defrosting

During winter operation, hot gas defrosting is one of the barriers to reducing the condensing pressure. When ambient temperature is low, optimum condensing pressure is below 100 psig. However, many refrigeration plants operate at higher condensing pressure to provide hot gas defrosting. What is the minimum allowable condensing pressure for adequate hot gas defrosting?

During hot gas defrosting, frost should be removed from the tubes and the fins of the evaporator coil. The tubes will have the highest temperature of defrosting. Fin tips will have the lowest temperature since the defrost heat should travel from the tubes to the fin tips. To get adequate defrosting, fin tips should acquire 5°C temperature or higher. We need outside tube temperature of 9-11°C to get 5°C at the fin tips. The colder the surrounding air temperature is the higher the tube temperature should be. To get the outside tube temperature of 9-11°C, the inside temperature should be 10-12°C (50-54°F) and corresponding pressure of ammonia will be 75-82 psig. It is necessary to acquire this pressure (or higher) inside of the evaporator coil to get good hot gas defrosting. However, this pressure is only one part of the hot gas defrosting process. Hot gas supply should also be properly adjusted, as undersupply of hot gas will lead to poor defrosting. Oversupply of hot gas will create a lot of blow by vapor which will travel to the compressor suction as a parasitic refrigeration load. Many people have concerns about the high pressure drop in hot gas line during defrosting at low condensing pressure. I do believe that this is not an issue.

Example.

A cold storage has 24 evaporator coils. During winter operation, every hour one coil will be defrosted. The hot gas line was designed for simultaneous defrosting of 8 coils (one third of all coils). A pressure drop in the hot gas main will not be significant, as only one coil at time is defrosted.

I believe that the majority of evaporator coils can be defrosted at 100 psig of condensing pressure or lower. However, this defrosting requires precise adjustment of hot gas supply, back pressure regulator setting and defrost time.