Refrigerants are working fluids that carry heat from a cold environment to a warm environment while circulating between them. For example, the refrigerant in an air conditioner carries heat from a cool indoor environment to a hotter outdoor environment. Similarly, the refrigerant in a kitchen refrigerator carries heat from inside the refrigerator out to the surrounding room. A wide range of fluids are used as refrigerants, with the specific choice depending on the temperature range needed and constraints related to the system involved. Refrigerants are the basis of vapor compression refrigeration systems. The refrigerant is circulated in a loop between the cold and warm environments (see figure). In the low-temperature environment, the refrigerant absorbs heat at low pressure, causing it to evaporate. The gaseous refrigerant then enters a compressor, which raises its pressure and temperature. The pressurized refrigerant circulates through the warm environment, where it condenses to liquid form and so releases heat. The high-pressure liquid is then depressurized and returned to the cold environment as a liquid-vapor mixture.
Refrigerants are also used in heat pumps, which work like refrigeration systems. In the winter, a heat pump absorbs heat from the cold outdoor environment and releases it into the warm indoor environment. In the summer, the direction of heat transfer is reversed. Refrigerants include naturally occurring fluids, such as ammonia, carbon dioxide, or isobutane, and synthetic fluids, such as chlorofluorocarbons, hydrochlorofluorocarbons, or hydrofluorocarbons. Many older synthetic refrigerants have been banned to protect the Earth's ozone layer or to limit climate change. Some refrigerants are flammable or toxic, making careful handling and disposal essential. Refrigerants are classified by safety using the ASHRAE Standard 34 system. Although refrigerants are most commonly associated with vapor-compression systems, they are also used for many other purposes. These applications include propelling aerosols, polymer foam production, chemical feedstocks, fire suppression, and solvents. Chillers are refrigeration systems that have a secondary loop which circulates a refrigerating liquid (as opposed to a refrigerant), with vapor compression refrigeration used to chill the secondary liquid. Absorption refrigeration systems operate by absorbing a gas, such as ammonia, into a liquid, such as water.
Requirements and desirable properties The selection of a refrigerant for a given purpose depends on a combination of factors. Different refrigerants, having different properties, are better suited to some applications than others.
Thermophysical property requirements In thermodynamic terms, refrigerants transport energy in the form of enthalpy, which increases or decreases substantially during evaporation or condensation. The difference between the enthalpy of the vapor and liquid phase is called the latent heat of vaporization. The latent heat of vaporization allows substantial energy to be absorbed or released, with minimal temperature change, in the evaporator or condenser. The temperatures in the evaporator and condenser are controlled by adjusting the refrigerant pressure in each component. A refrigerant must have a boiling point below the desired temperature of the cold environment. Heat will then flow from the cold environment into the refrigerant, causing it to evaporate. The boiling point is lower if the refrigerant pressure is lower. For this reason, the refrigerant in the evaporator (on the cold side) will have a low pressure. The evaporator pressure should be above atmospheric pressure to prevent air from leaking into it. Similarly, the refrigerant must have a boiling point above the temperature of the warm environment, so that heat will flow out of the refrigerant as it condenses. Since boiling point rises with increasing pressure, the refrigerant in the condenser (on the warm side) will have a high pressure. For most refrigeration systems, a critical-point temperature well above the condenser temperature is desirable. When the critical-point temperature is above the condenser temperature, the refrigerant can condense from the vapor to the liquid phase at nearly constant temperature; but if the critical point were below the condenser temperature, no phase change could occur. For fixed evaporator and condenser temperatures, increasing the critical-point temperature farther above the condenser temperature raises the energy efficiency of a refrigeration cycle. However, as the critical-point temperature rises, the vapor density at the compressor inlet decreases. A lower density raises the volumetric flow rate of vapor needed for a given amount of cooling (in other words, the compressor must be larger to do the job). Thus, a trade-off between energy efficiency and volumetric efficiency underlies the selection of a refrigerant. The refrigerant vapor's specific heat capacity also strongly affects performance. A lower specific heat capacity avoids liquid formation in the compressor, but too low a heat capacity can result in undesirably hot vapor at the compressor outlet. Optimization tends to favor refrigerant molecules with fewer atoms. A high latent heat of vaporization and a triple-point temperature well below the evaporator temperature are also desirable. A few refrigerants, like carbon dioxide, may operate in warm environments that are above the critical-point temperature. In these transcritical refrigeration cycles, the condenser must be replaced by a gas cooler operating over a wider temperature range. Refrigerants are sometimes blended to achieve a balance of desired properties. Pure refrigerants vaporize at a constant temperature when pressure is held constant (as it is in an evaporator or condenser). In contrast, blended refrigerants vaporize across a small range of temperature. This phenomenon is called temperature glide. For safety, an ideal refrigerant should be non-toxic and non-flammable. For environmental protection, the refrigerant should have no ozone depletion potential and a very low global warming potential. Refrigerants that are not naturally present in the atmosphere should have a short atmospheric lifetime and should decay into environmentally benign by-products.
… excerpt ends here. Continue reading the full article.
