How Refrigerators Work

Cartoon of a man looking in a refrigerator with a puppy pulling on his pants

How Refrigerators Work
If you are reading this then you are most likely not a refrigeration technician. This information listed here was created to help the Do-It-Yourselfer fix their own appliance and save money. That being the case, the explanation of How Refrigerators Work here will be as basic as possible. No homeowner who has a refrigerator that's not cooling properly is interested in learning the Law Of Thermodynamics, or what Dichlorodifluoromethane is.

A refrigerator is a device that removes heat from one area, and releases it in another area. In the case of the refrigerator that is in your kitchen, it is designed to keep your beer cold. Granted it can be used to keep other things cold, but the most important thing the refrigerator does in your kitchen is keep your beer cold.

So how does it do that? It is impossible to explain how a refrigerator works without going into some science.

Movement of heat.
Heat always travels from an area that is hot or warm to an area that is cooler. The greater the heat differential, the faster the movement. When you take a cooler and fill it with ice and warm beer, immediately the ice starts to remove the heat from the beer, and the beer will remain cold, until the ice melts. This is because the ice in the cooler is at least 32 degrees. The heat from the beer cans is absorbed by the ice cubes, either by conduction, when the ice is in contact with the beer can, or by convection, where the heat is transferred to the ice cubes via the air. In almost all refrigerators manufactured today, heat is removed from the items in the refrigerator by convection. If you had a manual defrost freezer, you could place an item directly on the evaporator coils, and the heat from that item would be transferred to the refrigerant by conduction. The warm air in a refrigerator is moved around by the evaporator fan, and as that warm air passes through the evaporator, the heat is removed quickly because the evaporator is extremely cold.

Change of state.
If you take a container of water and put it in the freezer, that water will go from a liquid to a solid, in other words, a physical change of state. For our purposes here, matter can exist in three states, a gas, liquid, or a solid. You could take that same ice cube out of the freezer, and put it in a pan on the stove. When you start to apply heat to the pan, the ice cube will go from ice (a solid), to water (a liquid), and if you continue to add heat the water will eventually boil and turn to steam (a gas). This is important to understand when servicing a refrigerator, because in addition to removing the heat that is in the air, thereby removing the heat from the items in the refrigerator, one of the big jobs that a refrigerator does is remove the moisture out of the air. This is why a glass of iced tea starts to sweat on the outside of the glass when you place it on the kitchen counter. The ice and cold tea in the glass is causing the moisture in the air to have a change of state. As the air touches the glass, the moisture in the air goes from a gas to a liquid. This is also referred to as condensation.

What is refrigerant?
Refrigerator refrigerants have the ability to easily change their physical state, without freezing. Whereas the freezing point of water is 32 degrees, the freezing point of refrigerants has to be well below zero. There are many different kinds of refrigerants that have been used over the years in refrigerators, but repairing refrigerator seal systems is beyond the ability of the average homeowner. In fact because refrigerant must be recovered so that it does not leak into the atmosphere, all kinds of special equipment is required to repair refrigerator sealed systems. They are called sealed systems, because there is no access port readily available. Unlike your car that has an access port, refrigerator sealed systems are soldered shut at the factory. This is to prevent leaks and limit the access to the sealed system.

What is a refrigerator compressor?
A refrigerator compressor is a sealed unit consisting of a motor, and a refrigerant pump. Motors on compressors today can operate on direct current, or alternating current. In most cases the motor operates a crankshaft, just like the crankshaft in your car, and moves a piston up and down creating a vacuum to move the refrigerant around the sealed system.

What is an evaporator?
An evaporator on a refrigerator is the set of coils inside the freezer section. Most refrigerators today only have one cooling section, and that is in the freezer. The exception to that rule is mostly high end refrigerators that have multiple sealed systems. When the system is operating, the temperature of the evaporator is extremely cold, as low as -35 degrees for example. As the air passes through the evaporator, the heat is immediately pulled out of the air.

The moisture that was in the air has a rapid change of physical state from a gas to a solid. The moisture in the air freezes on the evaporator in the form of frost that looks like snow. Evaporators must be defrosted to prevent the buildup of this frost that would block the airflow and significantly reduce refrigeration. Refrigerators use an automatic defrost system to remove this frost buildup. An automatic defrost system on a refrigerator consists of a defrost heater, a defrost terminator thermostat, and either a defrost timer, or electronic control system. Once the frost is melted off the evaporator, it must exit the refrigerator through the defrost drain tube. This water usually goes down to the compressor area under the refrigerator. Some compressors have a pan secured to the top, where the water drains into. The heat from the compressor causes the water to have a change of state, and go from a liquid to a gas back into the air in the room. On some refrigerators, the drain pan is a separate unit with its own heater. While still on other refrigerators the entire base of the refrigerator is a drain pan.

What is a condenser?
A condenser is a series of tubes, usually with some type of fins attached, used to cool down the refrigerant. In the case of condensers that are mounted under the refrigerator, a fan motor is used to blow air through the condenser returning the heat to the room. While on some other refrigerators, the condenser is mounted up the back of the refrigerator. Interesting enough, on freezers both chest freezers and upright freezers, the condenser tubing is looped over the top and down the side walls. That's why sometimes when you touch a freezer wall or top it's hot. This is done to prevent moisture in the air from condensing on the freezer walls if cold from the inside migrated through the insulation. This prevents the freezer from sweating just like that glass of iced tea we talked about earlier.

To review, on a normally functioning refrigerator, heat and moisture in the air are removed by the evaporator and absorbed into the refrigerant. The refrigerant is circulated through the evaporator by the compressor, and through the condenser where the heat is given off by the refrigerant back into the room.