How A Refrigerator Defrost System Works
How A Refrigerator Defrost System Works | In automatic defrost refrigerators or freezers, 100% of the air inside the unit circulates through the evaporator. The evaporator is extremely cold, as much as -35 degrees Fahrenheit. As the air passes through the evaporator, the heat is removed and when the air leaves the evaporator, it gets very cold.
In addition to removing heat, because the air is full of moisture, also known as moisture, that moisture is removed from the air. Moisture remains in the evaporator in the form of frost. If this frost is not removed on a regular basis, it will prevent the evaporator from allowing air to pass through it. This will decrease refrigeration, especially in the fresh food section.
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This is because almost all refrigerators today only have one evaporator. That evaporator is located in the freezer. After the air passes through the evaporator, it is divided by a gate in the air flow system. A significant portion of that air is forced into the fresh food section, and the rest circulates through the freezer. As you can imagine, if the evaporator is blocked with frost, no air can pass to enter the air flow system. Therefore, the temperature in the fresh food section and the freezer will rise.
Refrigerator manufacturers have incorporated a system to remove this frost from the evaporator. That system is called an automatic defrost system. It is called automatic because the refrigerator user does not have to do anything to clean the frost off the evaporator. A defrost system consists of a defrost heater, a defrost terminator or thermostat, sometimes a defrost thermistor, and an electronic timer or control. Timed refrigerators are much easier to diagnose defrost system problems for a person with no repair experience.
Most defrost heaters are made of basically the same material as the baking element in your oven. That is, there is a metal tube with an insulated resistance wire inside. At the factory, this tube is formed to fit snugly against the evaporator. That’s right, in most cases the evaporator heater is physically connected to the evaporator. Other refrigerator manufacturers use a sealed glass tube with a resistance wire inside.
The glass tube does not touch the evaporator, but it is very close to the evaporator and radiates heat when energized. Whether the defrost heater is connected to the evaporator, or near the evaporator, it is designed to do the same thing: melt the frost off the evaporator. Doing this regularly prevents the evaporator from clogging up in frost.
The defrost terminator
A defrost terminator is usually a bimetal disc that is sensitive to heat at a very specific temperature. It is designed to open the circuit to the defrost heater when the evaporator is frost free. It is also designed to prevent the defrost heater from turning on if there is no frost on the evaporator. For example, if you move the refrigerator to a new location and have had a chance to heat up completely, there is no point in the defrost heater turning on right after you plug it in for the first time.
If the refrigerator uses a timer on the defrost system, when it was unplugged it may have been only two minutes since defrosting. If the refrigerator did not have a defrost thermostat or thermostat when the refrigerator reached the new location and was plugged in, within two minutes the defrost heater would turn on and unnecessarily radiate heat from the defrost heater to the freezer.
If the defrost system is controlled by an electronic control board, a thermistor is emitted to read the evaporator temperature. The thermistor temperature is monitored by the electronic control board and if the evaporator is hot, the electronic control will not allow the defrost heater to turn on.
A defrost timer is a small mechanical mechanism that includes a motor, a series of cams, gears, and switch contacts. The defrost timer was the standard since the beginning of automatic defrost systems. Of course, many refrigerators today use electronic control boards.
The refrigerator defrost timer can be located almost anywhere. Refrigerator manufacturers have installed defrost timers behind the skid plate at the base of the refrigerator, inside the fresh food section somewhere, and in the past in older refrigerators, somewhere even on the back .
Regardless of the location of the defrost timer, they all do the same thing. The timer closes a set of contacts that allow the compressor to run when the refrigerator control thermostat contacts are closed. The timer controls the amount of time a refrigerator is defrosted. In the past, refrigerator defrost timers were set to run between eight and twelve hours of compressor time, with 20 minutes of defrost time.
After eight or twelve hours of compressor operation, the timer changes and closes a set of contacts in the defrost circuit. If the contacts on the defrost terminator are closed, the circuit to the heater will complete and the heater will turn on. That heater will run until the defrost terminator opens the contacts to the heater or the defrost timer return to compressor run time and this article tells us also how to no frost refrigerator repair.