Like the iceboxes of the 19th and early 20th centuries, a refrigerator is basically an airtight, insulated box, but instead of a compartment for ice, it has a cooling system powered by electricity. In most refrigerators, the cooling takes place behind the freezer compartment, and a fan circulates the cold air through the refrigerator.
The Cooling Cycle
The pump that circulates the refrigerant is typically located on the back of a refrigerator near the floor. It compresses an inert compound called a refrigerant and pumps it through a system of coils. Under the pressure generated by the compressor pump, the refrigerant liquefies, and as it does, it radiates heat -- which is why the coils on the back of a refrigerator feel warm.
The refrigerant eventually reaches a tiny aperture and is forced through by the pressure generated by the compressor. As it passes through into the low-pressure environment of the coils beyond, it evaporates, and the coils into which it passes become cold. The process is exactly the same one that makes your hand feel cold when water evaporates from it. These evaporative coils are typically placed directly behind the freezer compartment.
The refrigerant continues through the coils and returns to the compressor, where the pressure generated by that device turns it back into a liquid, and the cycle begins again.
Types of Refrigerants
In the early 20th century, manufacturers used dangerous chemicals, such as methyl chloride and sulfur dioxide, as refrigerants, but in a the 1930s, a new class of safer, inert compounds called chlorofluorocarbons -- CFCs -- came into general use. These compounds -- often known by the brand name Freon -- were used exclusively until their use was banned by international treaty because they cause damage to Earth's atmospheric ozone layer. Contemporary refrigerants, known as hydroflurocarbons -- HFCs -- do not make elemental chlorine, the chemical responsible for ozone damage, available in the upper atmosphere
Inside the Refrigerator
The evaporative coils don't add cold air to the freezer compartment; instead, they draw heat out of the air that's already there. The process causes moisture to condense and freeze on the coils, which has two consequences. The first is that the coils ice over and need to be defrosted periodically, and the second is that the air inside the freezer becomes dry.
The temperature in the refrigerator and freezer are regulated by mechanical and electronic controls:
- The freezer and refrigerator compartments are connected by an aperture with a moveable barrier called a damper. When the damper is open, the freezer and refrigerator exchange air, and the refrigerator becomes colder.
- A circulation fan helps the cold air to circulate through the refrigerator compartment.
- A thermistor, which can generate a small electrical current that varies with temperature, monitors the temperature in the refrigerator.
- The thermistor signals the thermostat to open and close the damper as needed to maintain the temperature according to the setting you choose. The thermostat also controls cycling of the compressor, turning it on and off as needed to maintain the temperature in the freezer.
The Dual Cooling Option
If your refrigerator has dual cooling, it has two cooling systems -- one for the freezer compartment and one for the refrigerator compartment. This allows you to regulate the temperature and humidity in the freezer and refrigerator compartments more accurately and efficiently.