Unfreezing a Freezer

How do you run your refrigeration hardware through the winter? How far does our industry go to keep a fridge running in the winter or a hospital chilled in a blizzard? There are a number of technologies at play, all working to manipulate the hardware to do their bidding. These are collectively called Defrost Controls.


Let Nature Take Its Course

The simplest defrost system is little more than a few switches and a timer. In many systems, the greatest concern is that the evaporator coil may freeze over. This is especially common with industrial freezers. This is a year-round problem and it can be caused by staff frequently entering and leaving a freezer. The freezer temperature increases and many refrigeration systems end up running for entirely too long. The evaporator coil builds up ice from being chilled too much.

In configurations like this, the simple solution is a time delay. The compressor shuts down, but the evaporator fan is run for an extended period, forcing relatively warm air over the coil. Whatever ice was there melts and the system can return to it’s regular cycling after the defrost cycle finishes. This can be set to run on timers or in more advanced systems, as necessary using temperature readings. In some installations, it can even be boiled down to a timer which triggers the cycle once every 24 hours and just turns off the refrigeration system for an hour to let things naturally defrost.


Reverse the System

In other situations, nature cannot come to the rescue. Some refrigeration systems are designed to create conditions below freezing. In these situations, there’s no effective way to shut down the system and wait for the evaporator coil to defrost without risking the environment rising above its desired temperature. There isn’t enough tolerance in those applications. Luckily, most of the refrigeration hardware is tolerant to hot gasses.

In hot gas defrost systems, the refrigerant is heated and pushed through the system. There are numerous additional controls and hardware to manage this, which vary by manufacturer. In general the compressor is inactive for part of the defrost cycle, hot gas is generated, and the compressor is used to push the hot gas into the system. The method of generating the hot gasses can vary. Electric heating elements are one common solution to the problem. Solenoid valves direct the gas flow to the relevant, frosted over hardware, but generally do not heat the entire system. As the hardware heats up, the frost and ice melt, leaving everything ready for the next cooling cycle.


Heaters in a Freezer

There are more straight forward systems for defrosting a refrigeration system. Some configurations just embed heating elements in the hardware. In these configurations, when a defrost cycle is triggered, the compressor and evaporator fan shut down while the heating elements are energized. These elements are pretty much gigantic resistors, similar in principle to those used in electric stoves and electric home heating. When power is pushed into the resistor, it reduces the current by giving off the energy as heat.

This approach’s greatest shortfall is that it doesn’t always heat the entire system. The heating elements are generally installed strategically. If a system is not designed properly, frost spots may develop and cause issues.


The End Result

With these defrost devices in place, near any temperature can be achieved, regardless of environmental temperature, without fear of hardware damage. Your refrigeration system will ‘just work’ as desired, and that’s the part that matters most.

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