Auto-defrost From Wikipedia, the free encyclopedia 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. Contents [hide]
Mechanism The defrost mechanism in a refrigerator heats the cooling element (evaporator coil) for a short period of time and melts the frost that has formed on it. The resulting water drains through a duct at the back of the unit. Defrosting is controlled by an electric or electronic timer: For every 6, 8, 10, 12 or 24 hours of compressor operation it turns on a defrost heater for 15 minutes to half an hour. The defrost heater, having a typical power rating of 350 W to 600 W, is mounted just below the evaporator in top- and bottom-freezer models and below and sometimes also in the middle of the evaporator in side-by-side models. It may be protected from short circuits by means of fusible links. In older refrigerators the timer ran continuously. In newer designs the timer only runs while the compressor runs, so the more the refrigerator door is closed, the less the heater will be on and the more energy will be saved. A defrost thermostat opens the heater circuit when the evaporator temperature rises above a preset temperature, 40°F (5°C) or more, thereby preventing excessive heating of the freezer compartment. The defrost timer is such that either the compressor or the defrost heater is on, but not both at the same time. Inside the freezer, air is circulated by means of one or more fans. In a typical design 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 or frost that may form on frozen items in the freezer compartment.
Instead of the traditional cooling elements being embedded in the freezer liner, auto-defrost elements are behind or beneath the liner. This allows them to be heated for short periods of time to dispose of frost without heating the contents of the freezer. Alternatively, some systems use the hot gas in the condenser to defrost the evaporator. This is done by means of a circuit that is cross-linked by a three-way valve. The hot gas quickly heats up the evaporator and defrosts it. This system is primarily used in commercial applications such as ice-cream displays. Application While this technique was originally applied to the refrigerator compartment, 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," Kelvinator often named these models as "Cyclic Defrost" ). These refrigerators usually have a pan underneath where water from the melted frost in the refrigerator section evaporates. Freezers with automatic defrosting and combined refrigerator/freezer units which also apply self defrosting to their freezer compartment are called "frost free". The latter usually feature an air connection between the two compartments with the air passage to the refrigerator compartment regulated by a damper. By this means, a controlled portion of the air coming from the freezer reaches the refrigerator. Some older models have no air circulation between their freezer and refrigerator sections. Instead, they use an independent cooling system (for example: an evaporator coil with a defrost heater and a circulating fan in the freezer and a cold-plate or open-coil evaporator 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 frost that melts from the cold plate and/or evaporator coil, a timer which turns off the compressor and turns on the defrost element usually from once to 4 times a day for periods usually ranging from 15 to 30 minutes, a defrost limiter thermostat that turns off the heating element before the temperature rises too much while the timer is still in its defrost phase. Some models also feature a drain heater to prevent ice from blocking the drain. 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 a computer that monitors how many times each door is opened and uses this data to control defrost scheduling thereby reducing power use. Advantages
No need to manually defrost the frost buildup hence power 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 because the air is always circulating Better temperature management.
The system can be more expensive to run when usage is high and 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 instantpower values that can be reached. Increased electrical and mechanical complexity compared to a basic upright freezer or freezer chest, making it more prone to component failure. The temperature of the freezer contents rises during the defrosting cycles, especially if there is a light load in the freezer. This can cause "freezer burn" on articles placed in the freezer, from partially defrosting, then re-freezing On hot humid days condensation will sometimes form around the refrigerator doors. Defrosting may not be completed by the time the defrost timer cycles back into normal operation (esp. In Hot, humid conditions with frequent door openings), leaving ice/frost on the evaporator coils. This condition can lead to "icing up" which will interfere with the operation of the refrigerator.
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 evaporate 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)
How does a frost-free refrigerator work?
How does a frost-free refrigerator work?
If you have an old refrigerator or one of the small dorm refrigerators, you know all about the frost that forms around the coils that cool the freezer. If you let it build up long enough, the frost can get 6 inches thick and eventually there is no room to put anything in the freezer. This frost forms when water vapor hits the cold coils. The water vapor condenses -- turns to liquid water. Think of the water beading up on a glass of iced tea on a summer day -- that is an example of water vapor in the air condensing. The same thing happens on the ice-cold freezer coils, except that when the water condenses onto the coils it immediately freezes. A frost-free freezer has three basic parts: A timer A heating coil A temperature sensor Every six hours or so, the timer turns on the heating coil. The heating coil is wrapped among the freezer coils. The heater melts the ice off the coils. When all of the ice is gone, the temperature sensor senses the temperature rising above 32 degrees F (0 degrees C) and turns off the heater. Heating the coils every six hours takes energy, and it also cycles the food in the freezer through temperature changes. Most large chest freezers therefore require manual defrosting instead -- the food lasts longer and the freezer uses less power.
How does a frost free refrigerator's defrost system work? There are many separate components in a refrigerator's defrost system that must work in consort for a frost free system to work properly. We will attempt to explain the workings using simplified electrical schematics. The heart of the defrost system is the defrost control. The most common control is a mechanical defrost timer which is a motorized device that opens and closes several electrical contacts. Each contact can be thought of as a simple light switch but instead of a light, one connects the defrost heater circuit, another connects the cooling system. When one of these is switched on, the other is switched off. A motor on the timer (NOT illustrated) turns a cam that opens and closes these contacts at set
intervals (see below for other types).
During the cooling mode, the defrost timer closes a contact to the compressor circuit so it will run. The circuit to the defrost heater is open. While in this mode, the thermostat (a.k.a. cold control) cycles the compressor and fan motors on and off to maintain an appropriate temperature.
The defrost timer eventually switches into defrost mode and supplies power to the defrost heater(s) to melt any frost that has accumulated on the evaporator (cooling) coil. The cold control contacts remain closed but since the defrost timer is no longer feeding power to that circuit, the compressor does not run.
Once the defrost termination thermostat (a.k.a. defrost limit switch) senses a set temperature, it opens the circuit to the defrost heaters, shutting them off. The timer remains in the defrost cycle until the timer advances back to the cooling mode.
Since the limit switch is open, the heaters are no longer on for the rest of the cycle.
When the timer again advances back into the cooling mode, the compressor will start to run along with any air circulation fans. The defrost limit switch will remain in the open condition until it is reset by cold temperatures. Once a set colder temperature is reached, the defrost termination thermostat closes again. This is OK since the defrost timer is no longer supplying power to the defrost circuit, the heater does not get energized. When the defrost timer again advances into the defrost mode, the limit thermostat will already be closed and will allow power to be supplied to the defrost heater to melt any frost that has developed on the evaporator coil again.
Normally the interior evaporator and exterior (if present) condenser fan motors should run whenever the compressor is running and vise versa. If the timer is stopping operation of the cooling system, neither the fans norcompressor should usually be running at that time.
Defrost Problem Symptoms
The most common symptom of a defrost system failure is a complete and uniformly frosted (not iced) evaporator coil. Frost may also be visible on the panel covering the evaporator, usually in the rear of the freezer compartment.
Excessive frosting can be cause by the defrost heater or limit thermostat being open (ie. defective), a mechanical defrost timer sticking and never advancing into the defrost cycle or a problem in an electronic defrost control or one of its sensors failing to allow the defrost heaters to be energized. Sometimes (but fairly rarely) both heater and cooling system can be energized by the timer at the same time. This can result in thawing then refreezing of food in the freezer compartment often leading to freezer burn on that food. In most cases the evaporator coil will remain mostly in an unfrosted state. The defrost heaters will cycle on and off as the defrost thermostat opens and closed due to the temperature it senses.
Refrigeration System Problems
NOT a defrost system problem If the evaporator coil is only partially frosting (see illustration above) or a ball of ice develops on just a small area of it (see illustration below), it is usually a sign of a refrigeration system problem in which case a trained refrigeration technician will be required to determine the cause and correct it. These conditions are not caused by a defrost system failure.
NOT a defrost system problem
Defrost Component Locations
On most frost free refrigerators, the evaporator (cooling) coil is inside the freezer compartment covered by a panel. The freezer fan motor is usually in the same general area. The defrost heater is mounted onto or woven right into the evaporator coil in the freezer. The defrost termination limit switch is usually mounted on the side of the evaporator coil or on one of the connecting tubing.
The defrost timer can be in various places including behind the kickplate at the front of the cabinet, inside the fridge compartment possibly in a control panel along with the thermostat or on older models, at the back in the motor compartment by the compressor. Some strange Kenmore refrigerator models had the defrost timer built into the icemaker unit.
The Defrost Heater
The defrost heater is basically a wire filament enclosed in a quartz, glass, aluminum or other material, tube sheath which gets hot when powered. It will either have resistance (show continuity) and be good or will have infinite resistance (no continuity) and be defective. How much resistance it has is irrelevant as its resistance will not normally change except to being open (infinite resistance) when it fails.
The Defrost Termination Thermostat
The defrost termination thermostat (aka defrost limit switch) is basically a small SPST (single pole) electrical switch which is actuated by temperature. Depending on the temperature it is, it will either have no resistance (show continuity) and be good or will have infinite resistance (no continuity) and be defective. At room temperature it will usually be open (which is normal and not a sign of being defective) and only close when it gets cold. How cold it has to be to close will depend on its particular calibration but usually near or below freezing point.
Some newer model refrigerators (Amana and Frigidaire in particular) and some older models (GE included) run power for the evaporator (freezer) fan motor through the defrost heater element and defrost limit switch. If either of those components should fail, remaining open, the fan will not run which will stop the circulation of cold air throughout the refrigerator. On that design, the evaporator fan motor will not start running after a defrost cycle until the evaporator has had a chance to begin cooling again. While it is generally a good design idea so as not to blow the warm defrost air throughout the refrigerator, a failure in one part of the defrost system will usually render the whole refrigerator ineffective because of the lack of air flow.
Defrost Timers Types
Early production mechanical defrost timers would go into the defrost cycle after a set amount of time. Common timing periods were 6, 8, 12 and 24 hours. This meant that say every 6 hours, the refrigerator would go into defrost whether it needed it or not. The duration of time it would remain in the defrost cycle was fixed and could be anywhere from 18 to 30 minutes depending on the timer design but it would always be the same length of time. As stated above, the defrost heaters may not be on for that full length of time, thanks to the defrost limit thermostat, but the cooling cycle would not start again until after the complete defrost duration was ended. This original design was wasteful as the refrigerator would defrost regardless if it was necessary or not. A later design tried to help this a bit.
Cumulative Run Timers
The next design was called a 'cumulative run' timer. These timers were installed in a way were they would only count the time that the refrigerator (the compressor) was actually working. This makes sense since no frost could build up if the compressor was not running. These later mechanical model timers would only advance the into defrost when the compressor had actually been running for a certain length of time, usually 6 to 8 hours of accumulated compressor operation.
Adaptive Defrost Control
The latest, energy saving variation is electronically controlled and called an adaptive defrost control. Not only does the period between defrost cycles change but also the time duration of the defrost cycle itself. The device is programmed to keep track of the appliance usage and how long it takes for the evaporator coil to be thoroughly defrosted. It will then calculate the amount of time required and adjust itself accordingly.
GE's 'Mother Board'
Newer GE made refrigerators are almost totally electronically controlled. Their motherboard takes the place of both the temperature and defrost functions even controlling DC fan motors that can operate at several different speeds. On this system only the defrost heater is the same as described above. A defrost cycle is ended when a thermistor detects a temperature rise of the evaporator which is the signal to the main control to terminate the defrost and start the cooling. The termination thermostat on this appliance design only acts as a safety device to shut the heater(s) off in case of a malfunction before the plastic interior liner melts, otherwise it is never active. The control can not be manually put into defrost mode.
The mechanical timer types described above will usually have a screw on their underside which can be turned clockwise to manually change its present cycle. If running, slowly turning it until it clicks once should put the timer into 'defrost mode'. When in the defrost mode, turning it until it clicks once should put the timer into 'run mode'. It can be left in the defrost mode to see if it will advance by itself to the run mode (like it should in less than 30 minutes) or the defrost heaters may be able to be tested to see it they are receiving power or not at that time. NOTE: Once the compressor has been turned off, it should be allowed to sit idle for several minutes (5-10 minimum) before attempting to restart it. This is required to allow the internal refrigerant pressures to equalize so the compressor is able to restart again without putting excessive strain on it and damaging it. Test points and how to manually switch adaptive defrost controls into the different cycles vary from manufacturer to manufacturer. For these, check the appliance's 'tech sheet' for instructions. It is usually included in the
appliance's wiring diagram which is most often located behind the kickplate at the bottom front of the refrigerator or inside the control console along with the defrost control.