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 (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 temporary refrigeration, but it some modern cases, dry ice (frozen carbon dioxide) is used instead.
Cyclic refrigeration is a two-way process 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
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.
- To begin, gaseous refrigerant is compressed (by the compressor) and pushed into the condenser.
- The refrigerant becomes a hot liquid in the condenser. Heat is removed from the refrigerant by some kind of heat sink, usually a radiator with a fan pushing air across it, but other systems can be used, including fluid cooled systems.
- 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.
- 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.
- Lastly, this cool gas is moved to the compressor, where it 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.
Vapor absorption cycle
The first cyclic refrigeration system used the vapor absorption cycle. Since then, it has become much less common in modern refrigeration, since it is less effective than vapor compression (it has a low coefficient of performance). The vapor absorption cycle does not use very much electricity compared to other methods, but does require plenty of heat. The key to this process is ammonia, which is used as the refrigerant. Ammonia easily dissolves into water, which is why this specific substance is so important. The main difference between this and the modern norm—vapor compression—is the compressor, or specifically for vapor absorption, the lack of one. Rather than a mechanical compressor, this method uses an absorber which contains water. Here is the process:
- Gaseous ammonia is first pushed into the absorber, where it dissolves into the water. It is somewhat pressurized by a pump and heated significantly before exiting.
- The ammonia is then moved under pressure to the condenser, where it is cooled. Pressure is also reduced at this point, as the liquid shrinks due to the temperature decrease.
- The refrigerant is next pushed through a throttling valve, which allows it to rapidly decompress. This also cools the refrigerant significantly.
- Lastly, the already chilled refrigerant enters the evaporator, where it takes heat from its surroundings and provides a cooling effect.
- As a gas once again, the ammonia is again moved to the absorber, so that the process can repeat.
Although less efficient than modern options, vapor absorption still has its uses. Electricity is required (by the pump), but much less is needed since it uses no compressor. Rather, a large amount of the energy required is used as heat. This system is very common in mobile homes and recreational vehicles, where propane or natural gas is readily available, but battery power is limited. It is also useful in some industries where excess heat can be put to good use.
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 is in aircraft, since both of these criteria are met.
Magnetic refrigeration uses the Magnetocaloric Effect and the law of entropy to produce a cooling effect. It is a new technology which has for some time been only useful in research and cryogenics. However, it is becoming more common technology as it is quite efficient.
Put simply, Magnetocaloric Materials heat up when exposed to magnetism. When the magnetic field is removed, they cool down again. When used for refrigeration, such a material is exposed to strong magnetism, so that it heat up, while a heat sink removes as much heat as it can. Then the material is removed from the magnetic field, but is also isolated (usually by a vacuum). The material now has a high heat capacity, and is therefore cooler than the heat sink it was just in contact with. This cold substance can now be used to cool the desired air or liquid, before the process is repeated.
Other refrigeration methods
Thermoelectric refrigeration uses the Peltier effect (discovered by Jean Peltier in 1834) to transfer heat out of a space to be cooled. In such a system, two plates made of different metals are pressed together (usually with semiconductor beads between them) and electricity (DC) is run through them. The negatively charged plate becomes cooler, while the positively charged one becomes warm. The negative plate is then used to cool an item or substance, while the positive side is cooled by a heat sink (which usually needs a fan).
Although not an incredibly effective method of refrigeration, this is can be a convenient, compact option which can use power directly from a battery.
Vortex tube cooling requires high pressure air to be forced through the vortex spin chamber. It rotates first through the outer section of the tube (at about 1,000,000 RPM), then some escapes through a control valve at the far end. The remaining air is sent back through the chamber, now in the inner portion. This returning air releases some of its energy in the form of heat, which goes into the outer, hotter stream. The now-cold inner stream exits at the same end it entered, where it can be put to use while the hot air from the far end is ejected.
Vortex tube cooling has not proven an efficient cooling method as of yet, but could still be used, especially as an addition to another system.
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.