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 to 12 hours of compressor operation it runs a defrost heater element for about 15 minutes to a half hour. The defrost heater, having a typical power rating of 350 W to 600 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.

Alternatively, some systems use the hot gas inside the condensor to defrost the evaporator. This is done by using a circuit that is cross-linked by a three-way valve. The hot gas almost instantly heats up the pipelines inside the evaporator that on it's turn defrosts. This system is primarily used in commercial applications such as ice-cream displays.

Application

While this technique was originally applied to refrigerator compartments, it was later used for freezer compartment as well.

A combined refrigerator/freezer which applies self-defrosting to the refrigerator compartment only is usually called "partial frost free" or semi-automatic defrost (some brands call these "Auto Defrost" while Frigidaire referred to their semi-automatic models as "Cycla-Matic" ). These refrigerators usually have a pan under them where the water from the melted ice in the refrigerator section can evaporate.

Freezers with automatic defrosting and combined refrigerator/freezer units which also applies self defrosting to their freezer compartment are called "frost free". The latter usually 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 older models have no air circulation between their freezer and refrigerator sections. Instead, they use independent cooling systems (for example: an evaporator coil with a defrost heater and a circulating fan in the freezer and a cold plate evaporator with another circulating fan in the refrigerator).

"Frost-Free" refrigerator/freezer units usually use a heating element to defrost their evaporators, a pan to collect and evaporate water from the ice that melts from the cold plate and/or evaporator coil, a timer which turns off the refrigeration and turns on the defrost element usually twice to 4 times a day for periods usually ranging from 15 to 40 minutes, a defrost limiter switch that turns off the heating element before the temperature rises too much while the timer is still in its defrost cycle. Some models also feature drain heaters to prevent ice from building up in the drains.

Other early types of refrigerators also use hot gas defrost instead of electric heaters. These reverse the evaporator and condenser sides for the defrost cycle.

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
• Better temperature management.

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.
• 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, especially if there is a light load in the freezer.
• 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.

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)