Two approaches to hot gas defrosting

There are two approaches to optimize hot gas defrosting.

1.     Hot gas defrosting should be done at higher condensing pressure to minimize the length of defrosting.

2.     Hot gas defrosting should be done at minimum allowable condensing pressure.

The second approach requires a longer defrosting period than the first one and it will release more heat into the refrigerated rooms. However, the first approach will require higher condensing pressure and additional compressor energy will be spend. To compare these two approaches, the energy use of the whole system (refrigeration plant) should be estimated.

Example.

A refrigeration plant has 24 evaporators. Every hour, 1 evaporator will be defrosted.

Approach 1. Condensing pressure is 120 psig. Length of defrosting (hot gas supply) is 20 min. Every hour, this refrigeration plant operates 30 min at 120 psig (defrost pressure) and 30 min at 100 psig (minimum allowable condensing pressure).

Approach 2. Condensing pressure is 100 psig. Length of defrosting is 30 min.

Assume that the settings of the hot gas balancing valves and back pressure regulators are the same for both approaches.

According to W.F. Stoecker, every 10 min of hot gas defrosting reduces the efficiency of the plant by 1%.

 Hot gas defrosting of the first approach is 10 min shorter (30 - 20 = 10 min) than the second one, and plant efficiency in approach 1 will be 1% better. However, the compressors in approach 2 will operate at 100 psig of condensing pressure all the time. In approach 1 the compressors will operate 50% of the time at 100 psig condensing pressure and 50% of the time at 120 psig of condensing pressure. Compressors operating at 120 psig of condensing pressure require 10% more energy than at 100 psig. Usually, 10% of compressor power is equal to 8% of refrigeration plant power. Approach 1 requires 4% more energy than approach 2 because 50% of the time this plant will operate at 120 psig.

Total system performances can be compared. Approach 2 requires 1% more energy due to longer hot gas defrosting, but 4% less energy due to the continued operation at 100 psig of condensing pressure. Total energy use will be better by 3% for approach 2. This proves that hot gas defrosting should be done at minimum allowable condensing pressure.