How Does a Freezer Work?

Throughout history, people have sought ways to keep food cool to prevent spoilage and protect their health. One of the earliest strategies, first used in India several hundred years ago, was to harness the cooling power of evaporating water. As long ago as the 11th century, techniques had been developed to condense water vapor in coils, pressurize it and send it through an aperture to make it evaporate. Water isn't a very good refrigerant, however, and these early cooling systems weren't very efficient.

Fast forward to the 18th century, when inventors began building refrigeration systems by compressing air, ammonia and other gases and then forcing them to absorb heat by reducing the pressure and allowing them to expand. Refrigeration didn't become viable on a large scale, however, until 1876, when German engineer Carl von Linde built a condensing system that could liquify these gases. Some 50 years later, General Electric ended the age of the icebox by marketing the first widely available refrigeration system, which used a proprietary gas known as Freon as the refrigerant.

Refrigerators and freezers are now commonplace and every household has one, although Freon is now obsolete. Most units have both a refrigerator and freezer compartment, but the cooling actually occurs in the freezer, and a fan circulates the cool air into the refrigerator compartment. Whether the unit is a small household appliance or a large commercial walk-in area, the cooling system works on the same basic principle of a deep freezer.

The refrigeration system you see on a typical freezer has two sets of coils, the condenser coils and the evaporative coils, and they're separated by a small aperture known as the expansion valve. If the freezer runs on electricity, which not all freezers do, an electric freezer compressor pump pressurizes the refrigerant in the first coil and that forces it to spray through the expansion valve into the second coil. Because the pressure in the second coil is much lower, the refrigerant evaporates, and that's what provides the cooling.

In the language of chemistry, evaporation is an endothermic process, which means it absorbs heat. The heat comes from the surrounding air, and it provides the energy the refrigerant molecules need to enter the gaseous state. A freezer doesn't so much add coolness to the air as it does subtract warmth from it, and that warmth has to go somewhere.

After turning into a gas, the refrigerant cycles back into the condenser coils where it gets pressurized by the condenser pump and turns back into a liquid. The pressurization generates heat, which completes the thermal cycle, and the heat must be dissipated away from the freezer compartment to maintain the low temperature in the freezer. Many units have a fan to facilitate this. The thermal cycle can continue indefinitely, provided the coils are sealed and none of the refrigerant can escape.

The deep freezer temperature can go down to 0 degrees Fahrenheit (-18 C) and even lower, but for this to happen, the compartment has to be well sealed. The evaporative coils are usually located behind the back wall of the freezer. As long as the compartment is sealed, the coils continue to absorb warmth from the freezer compartment and dissipate it through the condenser coils, and the temperature in the freezer continues to drop.

Most freezers have a thermostat that monitors the temperature. When the target temperature is reached, the thermostat signals the freezer compressor to shut off. If the compartment is well sealed and insulated, it takes a long time for the temperature to rise and for the compressor to cycle on. Poor door seals that allow warm air into the freezer compartment cause the compressor to cycle on more often, and this wastes energy.

One of the reasons you have a freezer is to make and store ice, but the ice should be in trays or bags, not on the freezer walls. When ice collects on the freezer wall next to the evaporative coils or the coils themselves, it interferes with airflow and reduces their cooling efficiency. As a result, the condenser has to work harder, and that wastes energy.

The working principle of a deep freezer is that the coils absorb heat from the surrounding air, but they can't do this if they're covered with ice or frost. This is why periodic defrosting is so important and why many freezers have an automatic defrost function. If a freezer doesn't have a defrost function, it must be manually defrosted by turning it off long enough for the ice to melt.

When a freezer does have a defrosting mechanism, it usually takes the form of a heating element attached to the evaporative coil. The defroster may come on automatically or you may have to turn it on manually. Either way, it melts the ice on the coils and the water goes through a system of drainage pipes to a pan from where it can evaporate.

Most refrigerators come with an attached freezer and they're in separate compartments. The ideal deep freezer temperature is around 0 F (-18 C), but in the refrigerator compartment, it's more like 40 F (4 C). To maintain this temperature difference, the compartments are separated by a vent opening and a fan blows cool air from the freezer into the refrigerator only when it's needed.

You regulate the temperature in the refrigerator compartment by adjusting the operation of the fan. If the door seals and insulation are intact and the freezer is at the proper temperature but the refrigerator is too warm or too cold, the reason is usually a fan malfunction. If the freezer temperature is higher than it should be, however, it's usually the condenser that's at fault, and that's a more serious problem.

When things go wrong in a refrigerator, you can often repair the fan or the controls yourself if you know what you're doing. It's a different story when you can't maintain the freezer temperature, though, because that usually means there's a problem with the refrigeration system. Federal law prohibits unlicensed individuals from servicing refrigeration systems.

One of the main reasons for the prohibition has to do with the volatility of the refrigerant. Although Freon (also known as R22 refrigerant) is no longer in use because it's a chlorofluorocarbon that damages Earth's ozone layer, some older systems may still use it. Moreover, some current refrigerants, such as hydrofluorocarbons (HFCs), can also harm the environment by contributing to global warming.

Some freezer systems, particularly propane ones, use ammonia as a refrigerant, which is a return to the early days of refrigeration systems. Ammonia is highly corrosive, and it causes burning of the eyes and respiratory system and can be fatal in large quantities. It takes skilled technicians and strict procedural protocols to work on refrigeration systems without releasing any of these hazardous gases.