Vapor-compression refrigeration: Difference between revisions

The cold refrigerant liquid and vapor mixture is then routed through the coil or tubes in the evaporator. Air in the enclosed space circulates across the coil or tubes due to either thermal [[Natural convection|convection]] or a [[Fan (machine)|fan]]. Since the air is warmer than the cold liquid refrigerant, heat is transferred from the air to the refrigerant, which cools the air and causeswarms the refrigerant, causing [[evaporation]] of the liquid, returning it to a gaseous state whilst absorbing heat. While liquid remains in the refrigerant flow, its temperature will not rise above the [[boiling point]] of the refrigerant, which depends on the pressure in the evaporator. Most systems are designed to evaporate all of the refrigerant to ensure that no liquid is returned to the compressor.

The [[working fluid selection|selection]] of [[working fluid]] has a significant impact on the performance of the refrigeration cycles and as such it plays a key role when it comes to designing or simply choosing an ideal machine for a certain task. One of the most widespread refrigerants is "[[haloalkane|Freon]]". Freon is a trade name for a family of [[haloalkane]] [[refrigerant]]srefrigerants manufactured by [[DuPont]] and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable at room temperature and atmospheric pressure, nor obviously toxic as were the fluids they replaced, such as [[sulfur dioxide]]. Haloalkanes are also an order(s) of magnitude more expensive than petroleum-derived flammable alkanes of similar or better cooling performance.

Newer refrigerants that have reduced [[ozone depletion]] effects compared to CFCs have replaced most CFC use. Examples include [[HCFC]]s (such as [[chlorodifluoromethane|R-22]], used in most homes) and [[hydrofluorocarbon|HFC]]s (such as [[R-134a]], used in most cars). HCFCs in turn are being phased out under the [[Montreal Protocol]] and replaced by hydrofluorocarbons (HFCs), which do not contain [[chlorine]] atoms. However, CFCs, HCFCs, and HFCs all have very large [[global warming potential]] (GWP).

More benign refrigerants are currently the subject of research, such as [[supercritical fluid|supercritical]] [[carbon dioxide]], known as [[R-744]].<ref>[http://www.r744.com/knowledge/faq r744.com – Everything R744] {{Webarchive|url=https://web.archive.org/web/20170724071330/http://www.r744.com/knowledge/faq |date=2017-07-24 }}, The Natural Refrigerant R744 (CO)₂, 2006–2012</ref> These have similar efficiencies{{Citation needed|date=September 2009}} compared to existing CFC- and HFC-based compounds, and have many orders of magnitude lower global warming potential. General industry and governing body push are toward more GWP-friendly refrigerants. In industrial settings [[ammonia]], as well as gasses like [[ethylene]], [[propane]], [[iso-butane]] and other [[hydrocarbons]] are commonly used (and have their own R-x customary numbers), depending on required temperatures and pressures. Many of these gases are unfortunately flammable, explosive, or toxic; making their use restricted (i.e. well-controlled environment by qualified personnel, or a very small amount of refrigerant used). [[Hydrofluoroolefin|HFO]]s which can be considered to be HFCHFCs with some carbon-carbon bonds being double bounds, do show promise of lowering GWP veryso lowlittle to be of no further concern. In the meantime, various blends of existing refrigerants are used to achieve the required properties and efficiency, at a reasonable cost and lower GWP.