One Size Doesn’t Fit All When It Comes to Defrost Duration and Cycle Times
To keep walk-in coolers and freezers operating efficiently, it’s essential to determine their appropriate defrost cycles. However, local climates and the way in which the unit is used greatly affects the optimal defrost cycles and duration times. Let’s take a look at some factors to consider.
Refrigeration systems not only remove heat from their interiors but dehumidify the air as well. As humid air enters a unit, a heat exchanger known as the evaporator removes moisture b condensing on the evaporator coil. Due to the evaporator’s low surface temperature in freezers, moisture freezes on its coils. As frost and ice accumulate, it interferes with air flow and degrades the efficiency of the evaporator, making it difficult to maintain box temperatures which can result in spoiled goods. It is essential that the evaporator unit be periodically defrosted to remove the ice from the coils.
The number and duration of defrost cycles varies and is an adjustment that should be programmed during the unit’s initial set up. Depending upon the region and application, normal compressor shut off periods for coolers may provide adequate time to keep the evaporator coil clear of ice.
In the majority of applications, a timed or auto-defrost cycle is necessary to keep the evaporator coil clear of ice. During the defrost cycle, the compressor is shut off and a small heater in the evaporator is then used to melt accumulated frost from the finned coil surface. As the ice is melted from the coils, the resulting liquid water is caught in a drain pan and removed from the unit via a condensate drain. A p-trap should be used on the drain line so that air cannot enter the unit. In addition, it is critical to optimize the duration of the defrost cycle so it completes once the ice has cleared the coils. If the defrost cycle runs too long, excess heat will be added to the box in the form of water vapor. This water vapor can condense on the ceiling and will freeze, creating pebbling of ice on the ceilings. In successive cycles, this moisture will drip down from the ceiling to the floor, resulting in a dangerous, slippery frozen floor.
The region of the country can greatly affect the required defrost cycle times and durations. A humid area, such as the Southeast, for example, may require more frequent and longer defrost cycles than an arid region like the Southwest. The use of the unit will also affect defrost cycle requirements. If the doors are frequently opened for long periods of time to load the unit, moisture can enter and remain inside. This moisture builds up as frost and ice on wall, ceiling, evaporator and product surfaces. Installing strip or air curtains to keep warm air out can help, as well as programming the fan to cut off whenever the door is opened.
It can be challenging to determine the optimal defrost cycle times and duration for your commercial walk-in cooler or freezer. Working with your local refrigeration expert to determine this will be key to ensure that units are operating at peak efficiency.
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