An evaporator should never be allowed to get so iced up that it looks like this. However, a frosted evaporator is the unavoidable result of having the air inside a walk-in cooler, which always has some moisture in it, pulled or blown through the evaporator’s cold metal coils and fins. If the air is moist enough, and the metal is cold enough, the moisture will not only condense, but also freeze. The physical presence of ice between the fins reduces the spaces through which the air must pass, and insulates the metal fins, which reduces refrigeration efficiency and necessitates periodic defrost cycles.
Melting the ice on the evaporator coils, and letting the water drain out of the cooler, enlarges the spaces between the fins of the coils so that air can move more freely and efficient refrigeration can be restored. Any level of icing or frost on an evaporator will cause it to operate less efficiently. Severe icing creates the risk that the space’s temperature will climb outside the desired range. In extreme cases, the refrigeration equipment itself can be damaged. In attempting to avoid these troubles, evaporator fans have traditionally never been turned off. But, evaporator icing would still often get out of hand.
The photo to the left shows the back of an evaporator unit in which the coils have become seriously iced, probably over several months, even though the evaporator fans have never stopped running. There is no space between the fins at all across the entire lower half of the evaporator, where the coldest coils are usually located. Unless the whole system is shut down to perform a complete defrosting back down to bare metal, this condition is likely to continue to worsen. The efficiency of the system will further decline and eventually the space will not be able to be kept cold enough even with constant compressor operation.
There are three main methods of defrosting an evaporator:
- Air defrost (or off-time defrost) keeps the compressor off, and the evaporator fans on, for the length of the defrost period, by which time the ice is presumed to be gone. Air defrost is not used in freezers or in coolers kept close to 32°F as the air is not warm enough to melt the ice. An air defrost system relies on an oversized compressor and a time clock to ensure that the compressor stays off long and often enough, when the compressor is not running, to melt all the ice on the evaporator coils.
- Electric defrost, in which the compressor and the evaporator fans are off, uses energy-intensive electric resistance heaters between the evaporator to melt the ice.
- Hot gas defrost is similar to electric defrost, but the heat comes from rerouting the hot gas from the compressor (which remains on during the defrost cycle) to the evaporator coils.
In all cases, the defrost uses some amount of energy to melt the ice and, with electric and hot gas, will add additional heat to the box that will need to be subsequently removed by additional cooling.
Whatever method is used to defrost, during a defrost cycle, the frost on the evaporator coils melts and drips into a drain pan on the bottom of the evaporator, where the water collects and drains by gravity (or is pumped by a condensate pump) to somewhere outside the cooler.
Using an electronic controller, like the Freeaire® Cooler Controllertm, is the best way to ensure the defrosting process is done as quickly, efficiently, and completely as possible. Through the temperature sensor, or sensors, located in the evaporator fins allows, the controller can determine if there is still ice. Once the ice is gone, the defrost cycle will end. This means that you know that it has run long enough to remove the efficiency killing ice, but no longer.