Auto-defrost

Auto-defrost, automatic defrost or self-defrosting is a technique which regularly defrosts the evaporator in a refrigerator or freezer. Appliances using this technique are often called frost free, frostless or no-frost.

Mechanism

The mechanism on a refrigerator involves heating the cooling element (evaporator coil) for a short period, melting any frost that has formed upon it and having it drain through a collecting duct at the back of the unit. The cycle is controlled by an electric or electronic timer: For every six hours of compressor operation it runs a defrost heater element for about a half hour. The defrost heater, having a typical power rating of 350 W to 400 W, is mounted just below the evaporator coil and is protected from short circuits with fusible links. Older refrigerators ran the timer continuously, but to activate the defrost heater less often and thereby save energy, contemporary designs run the timer only when the compressor motor runs, so if the refrigerator door is left closed, the compressor and heater element will run less often. The defrost heater circuit also includes a defrost thermostat that senses when the cooling element temperature has risen to 40°F (5°C), or warmer, interrupting the current flow in the element and preventing excessive heating of the freezer compartment. The defrost timer is designed to run the compressor motor or the defrost heater, but not both at once.

Inside the freezer, dry air is circulated around the cabinet using one or more fans. In a typical design the cold air from the freezer compartment is ducted to the fresh food compartment, and circulated back into the freezer compartment. Air circulation helps sublimate any ice that may form on frozen items in the freezer compartment.

Instead of the traditional cooling elements assembled within the freezer liner, auto-defrost elements are compact and separated from the main cabinet space, allowing them to be heated for short periods to dispose of any ice that has formed.

Application

While this technique was originally and is mostly applied to freezer compartments, it can also be used for refrigerator compartments.

A combined refrigerator/freezer which applies self-defrosting to the freezer compartment only is usually called "partial frost free", while one which also applies it to the refrigerator compartment is called "total frost free" or "total no frost". The latter features an air connection between the two compartments, with the air passage to the refrigerator compartment regulated by a damper. This way, a controlled portion of the dry and fresh air coming from the dynamic cooling element located within the freezer can reach the refrigerator.

Some newer refrigerator/freezer models have built-in electronic sensors that monitor how many times each door is opened and could also average the door open time which will automatically adjust defrost scheduling, thereby optimizing power use.

Advantages

• No need to manually defrost the ice buildup hence the consumption doesn't increase with time
• Food packaging is easier to see because it's clear of frost
• Most frozen foods don't stick together
• Smells are limited, especially in total frost free appliances, since the air is constantly circulating

Disadvantages

• The system can be more expensive to run when the occupant usage is high, if the fan continues or starts to run when the door is opened^[1].
• A safety device is required to be connected with the heating element, due to the high instant-power values that can be reached.
• Increased electrical and mechanical complexity compared to a basic upright freezer or freezer chest, making it more prone to malfunctions.
• The temperature of the freezer contents rises during the defrosting cycles.
• On hot humid days condensation will sometimes form around the refrigerator doors.

In laboratories, self-defrosting freezers must not be used to store certain delicate reagents such as enzymes, because the temperature cycling can degrade them. In addition, water can sublimate out of containers that do not have a very tight seal, altering the concentration of the reagents.

References

1. ^ Estimates of Refrigerator Loads in Public Housing Based on Metered Consumption Data, section 3.3.3

External links

• Original 1927 Patent
• Frost-free page on howstuffworks
• www.appliance411.com — How does a frost free refrigerator's defrost system work? (explanation and animated diagrams)