Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. A device that performs this function may also be called a heat pump.
The vapor-compression refrigeration system uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Figure 1 depicts a typical, single-stage vapor-compression system. All such systems have four components: a compressor, a condenser, an expansion valve (also called a throttle valve), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at temperature and pressure at which it can be condensed with typically available cooling water or cooling air. That hot vapor is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air (whichever may be the case).
The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated.
The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser.
To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor.
Note: Saturated vapors and saturated liquids are vapors and liquids at their saturation temperature and saturation pressure. A superheated vapor is at a temperature higher than the saturation temperature corresponding to its pressure.
The thermodynamics of the vapor compression cycle can be analyzed on a temperature versus entropy diagram as depicted in Figure 2. At point 1 in the diagram, the circulating refrigerant enters the compressor as a saturated vapor. From point 1 to point 2, the vapor is isentropically compressed (i.e., compressed at constant entropy) and exits the compressor as a superheated vapor.
From point 2 to point 3, the superheated vapor travels through part of the condenser which removes the superheat by cooling the vapor. Between point 3 and point 4, the vapor travels through the remainder of the condenser and is condensed into a saturated liquid. The condensation process occurs at essentially constant pressure.
Between points 4 and 5, the saturated liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure. That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes). The adiabatic flash evaporation process is isenthalpic (i.e., occurs at constant enthalpy).
Between points 5 and 1, the cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator where it is totally vaporized by the warm air (from the space being refrigerated) that a fan circulates across the coil or tubes in the evaporator. The evaporator operates at essentially constant pressure. The resulting saturated refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.
It should be noted that the above discussion is based on the ideal vapor-compression refrigeration cycle which does not take into account real world items like frictional pressure drop in the system, slight internal irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).
The most common compressors used in chillers are reciprocating, rotary screw, centrifugal, and scroll compressors. Each application prefers one or another due to size, noise, efficiency and pressure issues.
Rotary screw compressors are also positive displacement compressors. Two meshing screw-rotors rotate in opposite directions, trapping refrigerant vapor, and reducing the volume of the refrigerant along the rotors to the discharge point.
Centrifugal compressors are dynamic compressors. These compressors raise the pressure of the refrigerant by imparting velocity or dynamic energy, using a rotating impeller, and converting it to pressure energy.
The schematic diagram of a single-stage refrigeration system shown in Figure 1 does not include other equipment items that would be provided in a large commercial or industrial vapor compression refrigeration system, such as:
More details about the design and performance of vapor-compression refrigeration system are available in the classic "Perry's Chemical Engineers' Handbook".
The cooling capacity of refrigeration systems is often defined in units called "tons of refrigeration". The most common definition of that unit is: 1 ton of refrigeration is the rate of heat removal required to freeze a short ton (i.e., 2000 pounds) of water at 32 °F in 24 hours. Based on the heat of fusion for water being 144 Btu per pound, 1 ton of refrigeration = 12,000 Btu/h = 12,660 kJ/h = 3.517 kW. Most residential air conditioning units range in capacity from about 1 to 5 tons of refrigeration.
A much less common definition is: 1 tonne of refrigeration is the rate of heat removal required to freeze a metric ton (i.e., 1000 kg) of water at 0 °C in 24 hours. Based on the heat of fusion being 334.9 kJ/kg, 1 tonne of refrigeration = 13,954 kJ/h = 3.876 kW. As can be seen, 1 tonne of refrigeration is 10 percent larger than 1 ton of refrigeration.
An interesting history of the evolution of refrigeration technology is available on the Internet.
|Refrigeration application||Short descriptions||Typical refrigerants used|
|Domestic refrigeration||Appliances used for keeping food in dwelling units||R-600a, R-134a|
|Commercial refrigeration||Holding and displaying frozen and fresh food in retail outlets||R-134a, R-404A, R-507|
|Food processing and cold storage||Equipment to preserve, process and store food from its source to the wholesale distribution point||R-134a, R-407C, R-410A, R-507|
|Industrial refrigeration||Large equipment, typically 25 kW to 30 MW, used for chemical processing, cold storage, food processing and district heating and cooling||R-134a, R-404A, R-507, R-717|
|Transport refrigeration||Equipment to preserve and store goods, primarily foodstuffs, during transport by road, rail, air and sea||R-134a, R-407C, R-410A|
|Electronic cooling||Low-temperature cooling of CMOS circuitry and other components in large computers and servers||R-134a, R-404A, R-507|
|Medical refrigeration||R-134a, R-404A, R-507|
|Cryogenic refrigeration||Ethylene, Helium|
Many systems still use HCFC refrigerants, which contribute to depletion of the Earth's ozone layer. In countries adhering to the Montreal Protocol, HCFCs are due to be phased out and are largely being replaced by ozone-friendly HFCs. However, systems using HFC refrigerants tend to be slightly less efficient than systems using HCFCs. HFCs also have an extremely large global warming potential (GWP) because they remain in the atmosphere for many years and trap heat more effectively than carbon dioxide.
With disruption of the status quo already a certainty, alternative non-haloalkane refrigerants are gaining popularity. In particular, once-abandoned refrigerants such as hydrocarbons (HCs, such as butane) and CO2 are coming back into broader use. For example, Coca-Cola's vending machines at the 2006 FIFA World Cup in Germany used refrigeration utilizing CO2
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