Using human comfort in cold weather to rethink low-temperature system design.

Last month’s column looked at some of the ways that refrigeration systems subvert our intuitive understanding of how things work. Another area of the system that is not as simple as it might at first seem is the defrosting cycle. Here are a few of the ways that Jack Frost can fool you.

At face value, frost buildup on an air cooler in a cold store is a bad thing, causing an insulating effect on the surface of the heat exchanger and restricting airflow. However, in the very early stages of frost buildup on a clean metal surface, the ice crystals form spiky fingers called “dendrites,” which have the initial effect of increasing the effective surface area on the air side and improving the heat transfer performance. In an ideal world, the air cooler would get slightly frosted and stay that way. Unfortunately this is a passing phase and soon the performance degradation is significant enough to require action.

There are several ways to achieve a defrost. The simplest, and cleanest in most climates, is to use electric heating elements. Like electric home heating, this can use a lot of energy and cost a lot of money. Even worse, the heat is not applied where the frost buildup originates, so more energy needs to be put into the evaporator to warm up the surrounding areas than is required simply to melt the frost. Electric defrost is not popular in large installations for this reason. When heater elements burn out, they can be difficult to replace so repairs can be expensive, too.

So-called “hot gas defrost” is probably the most common in industrial systems, but again, there are several ways to achieve this and some myths persist about the way that it operates. The gas doesn’t need to be hot, and the bulk of the energy transferred into the evaporator is the latent heat of condensation as the gas turns to liquid. One of the persistent myths is that the compressor discharge needs to be at least 85°F (about 30°C). This results in pipes and control valves being designed for this condition, and so the myth is perpetuated. In fact, the condensation will occur at about 50°F (about 10°C). The energy wasted by running compressors at the higher condition every time a defrost is required is massive.

All defrost methods share some common features. The energy that is put in to do the defrost needs to be taken out again. The energy absorbed in turning solid water to liquid flows out through the drain pipe, but the additional energy put in to heating the surroundings needs to be refrigerated.

It’s therefore worth assessing different defrost options in terms of a defrost efficiency: how much energy is needed in total versus how much is needed to melt the ice. Another common feature is that defrost methods are like baseball teams: everyone has a favorite and won’t hear a bad word said against it. More on this next month.