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**Need secondary resources**

Refrigeration is the process of transferring heat energy from one location to another for the purpose of cooling an area. This process is modernly used in things such as refrigerators for food preservation and growth control, and air conditioning for reducing the temperature in buildings. This process goes against the natural process of heat energy flowing from hot to cold.

Non-cyclic refrigeration

Non-cyclic (no cycle) refrigeration is a one-way process which uses a cold substance to cool down other items and substances. The most common example of this is the use of ice or freezer packs in an insulated container.
Ice boxes were a formerly popular device which also used this method. Large sheets of ice were loaded into the back of an ice box, which would keep the contents of the insulated box cool for extended periods of time, while the ice slowly melted into a tray. An "ice man" would come around periodically to sell more ice, so that food could stay cold. Each winter, ice would be dug out of ponds or rivers, and stored in quantity, for sale throughout the following seasons. In many cases, frozen water is still used for refrigeration, but it some modern cases, dry ice (frozen carbon dioxide) is used instead.

Cyclic refrigeration

Cyclic refrigeration is a two-way progress which uses the properties of a liquid/gas to force heat from one space into another. This process requires a refrigerant, which is moved through the system to transport heat.

Vapor-compression cycle

A single-stage vapor compression refrigeration system

The most common kind of refrigeration uses the Vapor-compression cycle. This method, which uses the reverse Brayton cycle, is employed by household refrigerators, home air conditioners, automobile air conditioners, and other coolant devices. It is the most efficient refrigeration cycle currently available.

1. To begin, gaseous refrigerant is compressed (by the compressor) and pushed into the condenser. The refrigerant becomes a liquid in the condenser, which is fairly hot. Heat is removed from the refrigerant by some kind of heat sync, usually a radiator with a fan pushing air across it, but other systems can be used, including fluid cooled systems.
2. Next, the cooled liquid is passed to an expansion valve, which reduces the pressure on the refrigerant and allows it space to convert back to a gas.
3. Then, the refrigerant is moved to the evaporator, where it is converted into a gas. This decompression consumes heat, making the surrounding area very cold. A fan moves air (or in some cases, liquid) through the evaporator, which gives heat to the refrigerant as it changes states of matter, but also cools air (or liquid) coming from outside the system.
4. Lastly, this cool gas is moved to the compressor, where is is converted back into a hot liquid.

The simple science behind this entire system is that for a gas to become a liquid, more energy is required. As heat is taken from its surroundings, something placed nearby can be cooled.
Conversely, when a gas becomes a liquid, heat must be released. (Atoms and molecules need more energy to move freely as a gas than stay as a liquid.) For refrigerant to be useful over and over, it must be converted back into a liquid. This is performed on the "hot" side of the system, so that the liquid can again be moved to the "cold" side and decompressed.

Gas cycle

Gas cycle systems are similar to vapor-compression cycle systems, but use the reverse Rankine cycle. The main functional difference from vapor-compression is that the refrigerant remains a gas throughout the entire process. For this reason, a specially formulated refrigerant is not usually needed. Regular nitrogen-dominant air is usually sufficient. However, this system is also less efficient than vapor-compression, and also much larger.
Gas cycle refrigeration is used in some cases where high-pressure air is readily available, or chemical refrigerants (like ammonia) are considered too dangerous. One place this system is popular in aircraft, since both of these criteria are met.

Vapor absorption cycle

• low coefficient of performance
• "the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which raises the pressure and a generator which, on heat addition, drives off the refrigerant vapor from the high-pressure liquid"

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Dehumidifiers

In some cases, a cyclic refrigeration system (almost always the vapor-compression cycle) is used without a full separation of sides. Although this does not cool any large area, it still creates a small cold area within the system, in which humidity will condense out. Since there is some waste in every system, this setup as a whole produces heat, but it also removes some moisture from the air. Such systems are sold commercially as dehumidifiers, for use when humidity is higher than desired but a temperature increase in not a problem.