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'''Vapor-compression refrigeration'''<ref>[http://web.me.unr.edu/me372/Spring2001/Vapor%20Compression%20Refrigeration%20Cycles.pdf The Ideal Vapor-Compression Cycle]</ref><ref>[http://iehmtu.edata-center.com/toc/chapt_r/ch18s82.html Scroll down to "The Basic Vapor Compression Cycle and Components"]</ref> is one of the many [[refrigeration cycle]]s available for use. It has been and is the most widely used method for [[air conditioning|air-conditioning]] of large public buildings, private residences, hotels, hospitals, theaters, restaurants and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. [[Oil refinery|Oil refineries]], [[petrochemical]] and [[Chemical plant|chemical]] processing plants, and [[natural gas processing]] plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems.  
'''Vapor-compression refrigeration'''<ref>[http://web.me.unr.edu/me372/Spring2001/Vapor%20Compression%20Refrigeration%20Cycles.pdf The Ideal Vapor-Compression Cycle]</ref><ref>[http://iehmtu.edata-center.com/toc/chapt_r/ch18s82.html Scroll down to "The Basic Vapor Compression Cycle and Components"]</ref> is one of the many [[refrigeration cycle]]s available for use. It has been and is the most widely used method for [[air conditioning|air-conditioning]] of large public buildings, private residences, hotels, hospitals, theaters, restaurants and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. [[Petroleum refining processes|Petroleum refineries]], [[petrochemical]] and [[chemical plant]] processing plants, and [[natural gas processing]] plants are among the many types of industrial plants that often utilize very large-scale vapor-compression refrigeration systems.  


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]]''.
Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere.   


==Vapor-compression refrigeration cycle==
==Description of the vapor-compression refrigeration==
[[Image:Heatpump.svg|thumb|300px|A simple stylized diagram of the refrigeration cycle: 1)&nbsp;condensing coil, 2)&nbsp;expansion valve, 3)&nbsp;evaporator coil, 4)&nbsp;compressor.]]
 
In the [[vapor-compression refrigeration]] cycle, heat is transferred from a lower temperature source to a higher temperature heat sink. Heat naturally flows in the opposite direction, and due to the [[second law of thermodynamics]] work is required to move heat from cold to hot.  A food ''[[refrigerator]]'' or [[freezer]] works in much the same way; it moves heat out of the interior and transfer this air into cold normally by a blower that is located inside of the fridge or freezer e.g hot air being pumped through cold air which cyles .
 
This most common [[refrigeration]] cycle uses an electric motor to drive a [[gas compressor|compressor]].  In an automobile the compressor is usually driven by a belt connected to a [[pulley]] on the engine's [[crankshaft]], with both using electric motors for air circulation. Since [[evaporation]] occurs when [[heat]] is absorbed, and [[condensation]] occurs when heat is released, air conditioners are designed to use a compressor to cause pressure changes between two compartments, and actively pump a refrigerant around. A refrigerant is pumped into the low pressure compartment (the evaporator coil), where, despite the low temperature, the low pressure causes the refrigerant to evaporate into a vapor, taking heat with it. In the other compartment (the [[Condenser (heat transfer)|condenser]]), the refrigerant vapor is compressed and forced through another heat exchange coil, condensing into a liquid, rejecting the heat previously absorbed from the cooled space.  The [[heat exchanger]] in the condenser section (the heat sink mentioned above) is often cooled by a fan blowing outside air through it, or in some cases, such as marine applications, by other means such as water.
 
==Description of the vapor-compression refrigeration system==


[[Image:Refrigeration.png|frame|right|Figure 1: Vapor compression refrigeration]]
[[Image:Refrigeration.png|frame|right|Figure 1: Vapor compression refrigeration]]


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 [[gas compressor|compressor]], a [[Condenser (heat transfer)|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 [[Condensation|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).  
Vapor-compression refrigeration 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 [[gas compressor|compressor]], a [[Condenser (heat transfer)|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 [[Condensation|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 [[flash evaporation|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 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 [[flash evaporation|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.
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<u>Note</u>: 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.
<u>Note</u>: 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.


=== Refrigerants ===
==Thermodynamic analysis==
"[[Haloalkane|Freon]]" is a trade name for a family of [[haloalkane]] [[refrigerants]] manufactured by [[DuPont]] and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable nor obviously toxic as were the fluids they replaced. Unfortunately, these chlorine-bearing refrigerants reach the upper atmosphere when they escape. In the [[stratosphere]], CFCs break up due to [[UV]]-radiation, releasing their chlorine atoms. These chlorine atoms act as [[catalyst]]s in the breakdown of ozone, which does severe damage to the [[ozone layer]] that shields the Earth's surface from the Sun's strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule. CFC refrigerants in common but receding usage include R-11 and R-12. Newer and more environmentally-safe refrigerants include [[HCFC]]s ([[Chlorodifluoromethane|R-22]], used in most homes today) and [[HFC]]s ([[R-134a]], used in most cars) have replaced most CFC use.  HCFCs in turn are being phased out under the [[Montreal Protocol]] and replaced by hydrofluorocarbons (HFCs), such as [[R-410A]], which lack [[chlorine]].
 
Newer 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_a.php R-744 as a natural refrigerant - FAQs]</ref> These have similar efficiencies compared to existing CFC and HFC based compounds.
 
==Thermodynamic analysis of the system==


[[Image:RefrigerationTS.png|frame|right|Figure 2: Temperature–Entropy diagram]]
[[Image:RefrigerationTS.png|frame|right|Figure 2: Temperature–Entropy diagram]]
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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 irreversiblity during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).
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 irreversiblity during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).


==Types of gas compressors==
== Refrigerants ==


{{main|Gas compressor}}
"[[Haloalkane|Freon]]" is a trade name for a family of [[haloalkane]] [[refrigerants]] manufactured by [[DuPont]] and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable nor obviously toxic as were the fluids they replaced. Unfortunately, these chlorine-bearing refrigerants reach the upper atmosphere when they escape. In the [[stratosphere]], CFCs break up due to [[UV]]-radiation, releasing their chlorine atoms. These chlorine atoms act as [[catalyst]]s in the breakdown of ozone, which does severe damage to the [[ozone layer]] that shields the Earth's surface from the Sun's strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule. CFC refrigerants in common but receding usage include R-11 and R-12. Newer and more environmentally-safe refrigerants include [[HCFC]]s ([[Chlorodifluoromethane|R-22]], used in most homes today) and [[HFC]]s ([[R-134a]], used in most cars) have replaced most CFC use.  HCFCs in turn are being phased out under the [[Montreal Protocol]] and replaced by hydrofluorocarbons (HFCs), such as [[R-410A]], which lack [[chlorine]].


The most common compressors used in chillers are [[Reciprocating compressor|reciprocating]], [[Rotary screw compressor|rotary screw]], [[Centrifugal compressor|centrifugal]], and [[Scroll compressor|scroll compressor]]s. Each application prefers one or another due to size, noise, efficiency and pressure issues.
Newer 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_a.php R-744 as a natural refrigerant - FAQs]</ref> These have similar efficiencies compared to existing CFC and HFC based compounds.
 
===Reciprocating compressors===
{{main|Reciprocating compressor}}
Reciprocating compressors are piston-style, positive displacement compressors.


===Rotary screw compressors===
=== Refrigerants used in various applications
{{main|Rotary screw compressor}}
 
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===
{{main|Centrifugal compressor}}
 
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.
 
===Scroll compressors===
{{Main|Scroll compressor}}
Scroll compressors are also positive displacement compressors. The refrigerant is compressed when one spiral orbits around a second stationary spiral, creating smaller and smaller pockets and higher pressures. By the time the refrigerant is discharged, it is fully pressurized.
 
===Others===
{{main|Diaphragm compressor}}
{{main|Axial-flow compressor}}
{{main|Diagonal or mixed-flow compressor}}
{{main|Liquid ring compressor}}
{{main|Roots blower}}
 
==Other features and facts of interest==
 
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:
 
* A horizontal or vertical [[pressure vessel]], equipped internally with a [[demister]],  between the evaporator and the compressor inlet to capture and remove any residual, entrained liquid in the refrigerant vapor because liquid may damage the compressor.  Such [[vapor-liquid separator]]s are most often referred to as "suction line accumulators".  (In other industrial processes, they are called "compressor suction drums" or "knockout drums".)
 
* Large commercial or industrial refrigeration systems may have multiple expansion valves and multiple evaporators in order to refrigerate multiple enclosed spaces or rooms. In such systems, the condensed liquid refrigerant may be routed into a pressure vessel, called a receiver, from which liquid refrigerant is withdrawn and routed through multiple pipelines to the multiple expansion valves and evaporators.
 
* Some refrigeration units may have multiple stages which requires the use of multiple compressors in various arrangements.<ref>[http://www.engr.siu.edu/staff1/weston/thermo/Refrigeration/VCRefrigeration.html Schematic diagrams of  multi-stage units]</ref>
 
More details about the design and performance of vapor-compression refrigeration system are available in the classic "[[Perry's Chemical Engineers' Handbook]]".<ref>{{cite book|author=Perry, R.H. and Green, D.W.|title=[[Perry's Chemical Engineers' Handbook]]|edition=6th Edition| publisher=McGraw Hill, Inc.|year=1984|id=ISBN ISBN 0-07-049479-7}} (see pages 12-27 through 12-38)</ref>
 
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 [[pound (mass)|pounds]]) of water at 32 [[Fahrenheit|°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 [[tonne|metric ton]] (i.e., 1000 kg) of water at 0 [[Celsius|°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.<ref>[http://www.rogersrefrig.com/history.html Excellent historical background]</ref>
 
==Applications==


{| class="wikitable"
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|}


==Economic analysis==
==Other features and facts of interest==
===Advantages===
 
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:
 
* A horizontal or vertical [[pressure vessel]], equipped internally with a [[demister]],  between the evaporator and the compressor inlet to capture and remove any residual, entrained liquid in the refrigerant vapor because liquid may damage the compressor.  Such [[vapor-liquid separator]]s are often referred to as "suction line accumulators".  (In other industrial processes, they are called "compressor suction drums" or "knockout drums".)


*Very mature technology.
* Large commercial or industrial refrigeration systems may have multiple expansion valves and multiple evaporators in order to refrigerate multiple enclosed spaces or rooms. In such systems, the condensed liquid refrigerant may be routed into a pressure vessel, called a receiver, from which liquid refrigerant is withdrawn and routed through multiple pipelines to the multiple expansion valves and evaporators.
*Relatively inexpensive.
*Can be driven directly using mechanical energy (water, car/truck motor) or with electrical energy.
*Efficient up to 60% of Carnot's theoretical limit (as evaluated in [[ASHRAE]] testing conditions: evaporation temperature of -23.3 °C, condensing temperature of 54.4°C, and ambient temperature of 32°C) based on some of the best compressors produced by [[Danfoss]], Matsushita, [http://www.copeland-corp.com/ Copeland], [[Embraco]], [http://www.bristolcompressors.com/ Bristol] and [http://www.tecumseh.com/compressors.htm Tecumseh] compressor manufacturers. However, many refrigeration systems use compressors having lower efficiencies of between 40-55%, since the 60% efficient ones cost almost twice as much as the lower efficiency ones.


===Disadvantages===
* Some refrigeration units may have multiple stages which requires the use of multiple compressors in various arrangements.<ref>[http://www.engr.siu.edu/staff1/weston/thermo/Refrigeration/VCRefrigeration.html Schematic diagrams of  multi-stage units]</ref>


Many systems still use [[haloalkane|HCFC]] [[Refrigerant|refrigerants]], which contribute to [[Ozone depletion|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 [[Haloalkane|HFC]]s.  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]].
* More details about the design and performance of vapor-compression refrigeration system are available in the classic "[[Perry's Chemical Engineers' Handbook]]".<ref>{{cite book|author=Perry, R.H. and Green, D.W.|title=[[Perry's Chemical Engineers' Handbook]]|edition=6th Edition| publisher=McGraw Hill, Inc.|year=1984|id=ISBN ISBN 0-07-049479-7}} (see pages 12-27 through 12-38)</ref>


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 CO<sub>2</sub> are coming back into broader use.  For example, [[Coca-Cola]]'s vending machines at the [[World Cup]] 2006 in Germany used refrigeration utilizing CO<sub>2</sub><ref>[http://www2.coca-cola.com/presscenter/nr_20060605_corporate_hfc-free.html Coca-Cola news release].</ref>
*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 [[pound (mass)|pounds]]) of water at 32 [[Fahrenheit|°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.  


==See also==
** A much less common definition is: 1 [[tonne]] of refrigeration is the rate of heat removal required to freeze a [[tonne|metric ton]] (i.e., 1000 kg) of water at 0 [[Celsius|°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.


*[[Absorption refrigeration]] or [[Einstein refrigerator]] or [[Gas absorption refrigerator]]
* An interesting history of the evolution of refrigeration technology is available on the Internet.<ref>[http://www.rogersrefrig.com/history.html Excellent historical background]</ref>
*[[Air conditioning]]
*[[Flash evaporation]]
*[[Heat pump]]
*[[HVAC]]
*[[Magnetic refrigeration]]
*[[Refrigeration]]
*[[Refrigeration cycle]]


==References==
==References==

Revision as of 17:34, 26 February 2008

Vapor-compression refrigeration[1][2] is one of the many refrigeration cycles available for use. It has been and is the most widely used method for air-conditioning of large public buildings, private residences, hotels, hospitals, theaters, restaurants and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Petroleum refineries, petrochemical and chemical plant processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize very large-scale vapor-compression refrigeration systems.

Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere.

Description of the vapor-compression refrigeration

Figure 1: Vapor compression refrigeration

Vapor-compression refrigeration 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.

Thermodynamic analysis

Figure 2: Temperature–Entropy diagram

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 irreversiblity during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).

Refrigerants

"Freon" is a trade name for a family of haloalkane refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable nor obviously toxic as were the fluids they replaced. Unfortunately, these chlorine-bearing refrigerants reach the upper atmosphere when they escape. In the stratosphere, CFCs break up due to UV-radiation, releasing their chlorine atoms. These chlorine atoms act as catalysts in the breakdown of ozone, which does severe damage to the ozone layer that shields the Earth's surface from the Sun's strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule. CFC refrigerants in common but receding usage include R-11 and R-12. Newer and more environmentally-safe refrigerants include HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use. HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), such as R-410A, which lack chlorine.

Newer refrigerants are currently the subject of research, such as supercritical carbon dioxide, known as R-744.[3] These have similar efficiencies compared to existing CFC and HFC based compounds.

=== Refrigerants used in various applications

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[4] R-134a, R-404A, R-507
Medical refrigeration   R-134a, R-404A, R-507
Cryogenic refrigeration   Ethylene, Helium

Other features and facts of interest

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:

  • A horizontal or vertical pressure vessel, equipped internally with a demister, between the evaporator and the compressor inlet to capture and remove any residual, entrained liquid in the refrigerant vapor because liquid may damage the compressor. Such vapor-liquid separators are often referred to as "suction line accumulators". (In other industrial processes, they are called "compressor suction drums" or "knockout drums".)
  • Large commercial or industrial refrigeration systems may have multiple expansion valves and multiple evaporators in order to refrigerate multiple enclosed spaces or rooms. In such systems, the condensed liquid refrigerant may be routed into a pressure vessel, called a receiver, from which liquid refrigerant is withdrawn and routed through multiple pipelines to the multiple expansion valves and evaporators.
  • Some refrigeration units may have multiple stages which requires the use of multiple compressors in various arrangements.[5]
  • 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.[7]

References

  1. ↑ The Ideal Vapor-Compression Cycle
  2. ↑ Scroll down to "The Basic Vapor Compression Cycle and Components"
  3. ↑ R-744 as a natural refrigerant - FAQs
  4. ↑ [1] Schmidt, R.R. and Notohardjono, B.D., High-end server low-temperature cooling, IBM Journal of Research and Development, Vol. 46, Nov.6, 2002
  5. ↑ Schematic diagrams of multi-stage units
  6. ↑ Perry, R.H. and Green, D.W. (1984). Perry's Chemical Engineers' Handbook, 6th Edition. McGraw Hill, Inc.. ISBN ISBN 0-07-049479-7.  (see pages 12-27 through 12-38)
  7. ↑ Excellent historical background

External links

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