**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.