Subcooling
Encyclopedia
In refrigeration
, subcooling is the process by which a saturated liquid refrigerant
is cooled below the saturation temperature
, forcing it to change its phase
completely. The resulting fluid is called a subcooled liquid and is the convenient state in which refrigerants may undergo the remaining stages of a refrigeration cycle. Normally, a refrigeration system has a subcooling stage, allowing technicians to be certain that the quality, in which the refrigerant reaches the next step on the cycle
, is the desired one. Subcooling may take place in heat exchanger
s and outside them. Being both similar and inverse processes, subcooling and superheating
are important to determine stability and well-functioning of a refrigeration system.
, thus, allowing the valve to work properly. If gas
reaches the expansion valve, in a refrigeration system, a series of usually unwanted phenomena may occur. These may end up leading to behaviors similar to those observed with the flash-gas phenomena: problems in oil regulation
throughout the cycle; excessive and unnecessary misuse of power and waste of electricity
; malfunction and deterioration of several components in the installation; irregular performance of the overall systems, and, in a totally unwatched situation, even ruined gear.
Another important and very common application of subcooling is its indirect use on the superheating process. Superheating is analogous to subcooling in an operative way, and both processes can be coupled using an internal heat exchanger. Subcooling here serves itself from the superheating and vice versa, allowing heat to flow from the refrigerant at a higher pressure
(liquid), to the one with lower pressure (gas). This creates an energetic equivalence between the subcooling and the superheating phenomena when there is no energy loss. Normally, the fluid that is being subcooled is hotter than the refrigerant that is being superheated, allowing an energy flux in the needed direction. Superheating is critical for the operation of compressors
because a system lacking it may provide the compressor with a liquid gas mixture, situation that generally leads to the destruction of the gas compressor because liquid is uncompressible. This makes subcooling an easy and widespread source of heat for the superheating process.
(as with an internal heat exchanger) is a good way of using up all of the condensing device's heat exchanging capacity
. A huge portion of refrigeration systems use part of the condenser for subcooling which, though very effective and simple, may be considered a diminishing factor in the nominal condensing capacity. A similar situation may be found with superheating taking place in the evaporator, thus, an internal heat exchanger is a good and relatively cheap solution for the maximization of heat exchanging capacity.
Finally, another widespread application of subcooling is boosting and economising
. Inversely to superheating, subcooling, or the amount of heat withdrawn
from the liquid refrigerant on the subcooling process, manifests itself as an increase on the refrigeration capacity of the system. This means that any extra heat removal after the condensation (subcooling) allows a higher ratio of heat absorption on further stages of the cycle
. It is to be noted that superheating has exactly the inverse effect, and that an internal heat exchanger alone, is not able to increase the capacity of the system because the boosting effect of subcooling is dimmed by the superheating, making the net capacity gain equal to zero. Nevertheless, some systems are able to move refrigerant and/or to remove heat using up less energy
because they do so on high pressure fluids that later cool or subccol lower pressure (which are more difficult to cool) fluids.
drop which is easily measurable, but it is possible to speak of subcooling in terms of the total heat being removed. The most commonly known subcooling is the condenser subcooling, which is usually known as the total temperature drop that takes place inside the condenser, immediately after the fluid has totally condensed, until it leaves the condensing unit.
Condenser subcooling differs from total subcooling usually because after the condenser, throughout the piping, the refrigerant may naturally tend to cool even more, before it arrives to the expansion valve, but also because of artificial subcooling. The total subcooling is the complete temperature drop the refrigerant undergoes from its actual condensing temperature, to the concrete temperature it has when reaching the expansion valve: this is the effective subcooling.
Natural subcooling is the name normally given to the temperature drop produced inside the condenser (condenser subcooling), combined with the temperature drop happening through the pipeline alone, excluding any heat exchangers of any kind. When there is no mechanical subcooling (i.e. an internal heat exchanger), natural subcooling should equal total subcooling. On the other hand, mechanical subcooling is the temperature reduced by any artificial process that is deliberately placed to create subcooling. This concept refers mainly to devices such as internal heat exchangers, independent subcooling cascades, economisers or boosters.
in refrigeration systems. This has led to a lot of research on the field. Most of the interest is placed in the fact that some systems work in better conditions than others due to better (higher) opearting pressures, and the compressors that take part of a subcooling loop are usually more efficient than the compressors that are having their liquid subcooled.
Economiser capable screw compressors are being built, which require particular manufacturing finesse. These systems are able of injecting refrigerant that comes from an internal heat exchanger instead of the main evaporator, in the last portion of the compressing screws. In the named heat exchanger, refrigerant liquid at high pressure is subcooled, resulting in mechanical subcooling. There is also a huge quantity of systems being built in booster display. This is similar to economising, as the compressor's efficiency of one of the compressors (the one working on higher pressures) is known to be better than the other (the compressors working with lower pressures). Economisers and booster systems usually differ in the fact that the first ones are able to do the same subcooling using only one compressor able to economise, the latter systems must do the process with two separate compressors.
Besides boosting and economising, it is possible to produce cascade subcooling systems, able to subcool the liquid with an analogous and separate system. This procedure is complex and costly as it involves the use of a complete system (with compressors and all of the gear) only for subcooling. Still, the idea has raised some investigation as there are some purported benefits. Furthermore, the United States Department of Energy
issued a Federal Technology Alert mentioning refrigerant subcooling as a reliable way of improving the performance of systems and saving energy. Making this kind of systems operationally independent from the main system and commercially possible
is subject to study due to the mentioned claims. The separation of the subcooling unit from the main cycle (in terms of design) is not know to be an economically viable alternative. This kind of system usually requires the use of expensive electronic control systems to monitor the fluid thermodynamic conditions. Recently, a product able of increasing the system's capacity by adding mechanical subcooling to any generic unspecific refrigeration system has been developed in Chile
.
The subcooling principle behind all these applications is the fact that, in terms of heat transfer, all the subcooling is directly added to the cooling capacity of the refrigerant (as superheating would be directly deducted). As compressors that are subcooling work on this easier conditions, higher pressure makes their refrigerant cycles more efficient, and the heat withdrawn by this means, cheaper than the one withdrawn by the main system, in terms of energy.
, the refrigerant undergoes phase changes from gas to liquid and from liquid back to gas. This enables to consider and discuss superheating and subcooling phenomena, mainly because gas must be cooled to become liquid and liquid must be heated back to become gas. As there are little possibilities of completing this for the totality of the flowing refrigerant without undercooling or overheating, in conventional vapor-compression refrigeration both processes are unavoidable and always appear.
On the other hand, transcritical systems
make the refrigerant go through another state of matter during the cycle. Particularly, the refrigerant (usually carbon dioxide
) does not go through a regular condensation process but instead passes through a gas cooler in a supercritical phase
. To talk about condensation temperature and subcooling under these conditions is not entirely possible. There is a lot of actual research on this subject concerning multiple staged processes, ejectors, expanders and several other devices and upgrades. Gustav Lorentzen
outlined some modifications to the cycle including two staged internal subcooling for this kind of systems. Due to the particular nature of these systems, the topic of subcooling must be treated accordingly, having in mind that the conditions of the fluid that leaves the gas cooler in supercritical systems, must be directly specified using temperature and pressure.
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
, subcooling is the process by which a saturated liquid refrigerant
Refrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...
is cooled below the saturation temperature
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
, forcing it to change its phase
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
completely. The resulting fluid is called a subcooled liquid and is the convenient state in which refrigerants may undergo the remaining stages of a refrigeration cycle. Normally, a refrigeration system has a subcooling stage, allowing technicians to be certain that the quality, in which the refrigerant reaches the next step on the cycle
Thermal expansion valve
A thermal expansion valve is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controlling the superheating at the outlet of the evaporator...
, is the desired one. Subcooling may take place in heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s and outside them. Being both similar and inverse processes, subcooling and superheating
Superheating
In physics, superheating is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling...
are important to determine stability and well-functioning of a refrigeration system.
Expansion valve operation and compressor safety
Subcooling is normally used so that when the cycling refrigerant reaches the thermostatic expansion valve, its totality is in its liquid formLiquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
, thus, allowing the valve to work properly. If gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
reaches the expansion valve, in a refrigeration system, a series of usually unwanted phenomena may occur. These may end up leading to behaviors similar to those observed with the flash-gas phenomena: problems in oil regulation
Polyolester
Polyolester oil, commonly called POE oil, is a type of Synthetic oil used in refrigeration compressors that is compatible with the refrigerants R-134a, R410A and R-12.-Use:...
throughout the cycle; excessive and unnecessary misuse of power and waste of electricity
Energy conservation
Energy conservation refers to efforts made to reduce energy consumption. Energy conservation can be achieved through increased efficient energy use, in conjunction with decreased energy consumption and/or reduced consumption from conventional energy sources...
; malfunction and deterioration of several components in the installation; irregular performance of the overall systems, and, in a totally unwatched situation, even ruined gear.
Another important and very common application of subcooling is its indirect use on the superheating process. Superheating is analogous to subcooling in an operative way, and both processes can be coupled using an internal heat exchanger. Subcooling here serves itself from the superheating and vice versa, allowing heat to flow from the refrigerant at a higher pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
(liquid), to the one with lower pressure (gas). This creates an energetic equivalence between the subcooling and the superheating phenomena when there is no energy loss. Normally, the fluid that is being subcooled is hotter than the refrigerant that is being superheated, allowing an energy flux in the needed direction. Superheating is critical for the operation of compressors
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
because a system lacking it may provide the compressor with a liquid gas mixture, situation that generally leads to the destruction of the gas compressor because liquid is uncompressible. This makes subcooling an easy and widespread source of heat for the superheating process.
System optimization and energy saving
Besides this, allowing the subcooling process to occur outside the condenserCondensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
(as with an internal heat exchanger) is a good way of using up all of the condensing device's heat exchanging capacity
Condenser (heat transfer)
In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant...
. A huge portion of refrigeration systems use part of the condenser for subcooling which, though very effective and simple, may be considered a diminishing factor in the nominal condensing capacity. A similar situation may be found with superheating taking place in the evaporator, thus, an internal heat exchanger is a good and relatively cheap solution for the maximization of heat exchanging capacity.
Finally, another widespread application of subcooling is boosting and economising
Economizer
Economizers , or economisers , are mechanical devices intended to reduce energy consumption, or to perform another useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, powerplant, and heating, ventilating, and air-conditioning uses are...
. Inversely to superheating, subcooling, or the amount of heat withdrawn
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
from the liquid refrigerant on the subcooling process, manifests itself as an increase on the refrigeration capacity of the system. This means that any extra heat removal after the condensation (subcooling) allows a higher ratio of heat absorption on further stages of the cycle
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
. It is to be noted that superheating has exactly the inverse effect, and that an internal heat exchanger alone, is not able to increase the capacity of the system because the boosting effect of subcooling is dimmed by the superheating, making the net capacity gain equal to zero. Nevertheless, some systems are able to move refrigerant and/or to remove heat using up less energy
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
because they do so on high pressure fluids that later cool or subccol lower pressure (which are more difficult to cool) fluids.
Natural and artificial Subcooling
The subcooling process can happen in many different ways; therefore, it is possible to distinguish between the different parts in which the process takes places. Normally, subcooling refers to the magnitude of the temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
drop which is easily measurable, but it is possible to speak of subcooling in terms of the total heat being removed. The most commonly known subcooling is the condenser subcooling, which is usually known as the total temperature drop that takes place inside the condenser, immediately after the fluid has totally condensed, until it leaves the condensing unit.
Condenser subcooling differs from total subcooling usually because after the condenser, throughout the piping, the refrigerant may naturally tend to cool even more, before it arrives to the expansion valve, but also because of artificial subcooling. The total subcooling is the complete temperature drop the refrigerant undergoes from its actual condensing temperature, to the concrete temperature it has when reaching the expansion valve: this is the effective subcooling.
Natural subcooling is the name normally given to the temperature drop produced inside the condenser (condenser subcooling), combined with the temperature drop happening through the pipeline alone, excluding any heat exchangers of any kind. When there is no mechanical subcooling (i.e. an internal heat exchanger), natural subcooling should equal total subcooling. On the other hand, mechanical subcooling is the temperature reduced by any artificial process that is deliberately placed to create subcooling. This concept refers mainly to devices such as internal heat exchangers, independent subcooling cascades, economisers or boosters.
Economiser and energetic efficiency
Subcooling phenomena is intimately related to efficiencyEfficient energy use
Efficient energy use, sometimes simply called energy efficiency, is the goal of efforts to reduce the amount of energy required to provide products and services. For example, insulating a home allows a building to use less heating and cooling energy to achieve and maintain a comfortable temperature...
in refrigeration systems. This has led to a lot of research on the field. Most of the interest is placed in the fact that some systems work in better conditions than others due to better (higher) opearting pressures, and the compressors that take part of a subcooling loop are usually more efficient than the compressors that are having their liquid subcooled.
Economiser capable screw compressors are being built, which require particular manufacturing finesse. These systems are able of injecting refrigerant that comes from an internal heat exchanger instead of the main evaporator, in the last portion of the compressing screws. In the named heat exchanger, refrigerant liquid at high pressure is subcooled, resulting in mechanical subcooling. There is also a huge quantity of systems being built in booster display. This is similar to economising, as the compressor's efficiency of one of the compressors (the one working on higher pressures) is known to be better than the other (the compressors working with lower pressures). Economisers and booster systems usually differ in the fact that the first ones are able to do the same subcooling using only one compressor able to economise, the latter systems must do the process with two separate compressors.
Besides boosting and economising, it is possible to produce cascade subcooling systems, able to subcool the liquid with an analogous and separate system. This procedure is complex and costly as it involves the use of a complete system (with compressors and all of the gear) only for subcooling. Still, the idea has raised some investigation as there are some purported benefits. Furthermore, the United States Department of Energy
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...
issued a Federal Technology Alert mentioning refrigerant subcooling as a reliable way of improving the performance of systems and saving energy. Making this kind of systems operationally independent from the main system and commercially possible
Commodity
In economics, a commodity is the generic term for any marketable item produced to satisfy wants or needs. Economic commodities comprise goods and services....
is subject to study due to the mentioned claims. The separation of the subcooling unit from the main cycle (in terms of design) is not know to be an economically viable alternative. This kind of system usually requires the use of expensive electronic control systems to monitor the fluid thermodynamic conditions. Recently, a product able of increasing the system's capacity by adding mechanical subcooling to any generic unspecific refrigeration system has been developed in Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
.
The subcooling principle behind all these applications is the fact that, in terms of heat transfer, all the subcooling is directly added to the cooling capacity of the refrigerant (as superheating would be directly deducted). As compressors that are subcooling work on this easier conditions, higher pressure makes their refrigerant cycles more efficient, and the heat withdrawn by this means, cheaper than the one withdrawn by the main system, in terms of energy.
Transcritical carbon dioxide systems
In a common refrigeration systemVapor-compression refrigeration
Vapor-compression refrigeration 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, offices, private residences, hotels, hospitals, theaters, restaurants and automobiles...
, the refrigerant undergoes phase changes from gas to liquid and from liquid back to gas. This enables to consider and discuss superheating and subcooling phenomena, mainly because gas must be cooled to become liquid and liquid must be heated back to become gas. As there are little possibilities of completing this for the totality of the flowing refrigerant without undercooling or overheating, in conventional vapor-compression refrigeration both processes are unavoidable and always appear.
On the other hand, transcritical systems
Transcritical cycle
A transcritical cycle is a thermodynamic cycle where the working fluid goes through both subcritical and supercritical states. This is often the case when carbon dioxide, CO2, is the refrigerant....
make the refrigerant go through another state of matter during the cycle. Particularly, the refrigerant (usually carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
) does not go through a regular condensation process but instead passes through a gas cooler in a supercritical phase
Supercritical fluid
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid...
. To talk about condensation temperature and subcooling under these conditions is not entirely possible. There is a lot of actual research on this subject concerning multiple staged processes, ejectors, expanders and several other devices and upgrades. Gustav Lorentzen
Gustav Lorentzen (scientist)
Gustav Fredrik Lorentzen was a thermodynamic scientist from Norway.Gustav Lorentzen was a professor at Norwegian Institute of Technology, and Norwegian University of Science and Technology. In the late 1980s, Gustav Lorentzen rediscovered how CO2 could be used as a refrigerant in heating and...
outlined some modifications to the cycle including two staged internal subcooling for this kind of systems. Due to the particular nature of these systems, the topic of subcooling must be treated accordingly, having in mind that the conditions of the fluid that leaves the gas cooler in supercritical systems, must be directly specified using temperature and pressure.
See also
- Heat transferHeat transferHeat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
- EconomizerEconomizerEconomizers , or economisers , are mechanical devices intended to reduce energy consumption, or to perform another useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, powerplant, and heating, ventilating, and air-conditioning uses are...
- SuperheatingSuperheatingIn physics, superheating is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling...
- EvaporationEvaporationEvaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
- CondenserCondenser (heat transfer)In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant...
- Screw compressors
- Refrigerant
- List of refrigerants
- Carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
- Thermal expansion valveThermal expansion valveA thermal expansion valve is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controlling the superheating at the outlet of the evaporator...
- Refrigeration cycle
- Vapor-compression refrigerationVapor-compression refrigerationVapor-compression refrigeration 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, offices, private residences, hotels, hospitals, theaters, restaurants and automobiles...
- EvaporatorHeat exchangerA heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
- CondensationCondensationCondensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
- Thermodynamics
- Gustav LorentzenGustav Lorentzen (scientist)Gustav Fredrik Lorentzen was a thermodynamic scientist from Norway.Gustav Lorentzen was a professor at Norwegian Institute of Technology, and Norwegian University of Science and Technology. In the late 1980s, Gustav Lorentzen rediscovered how CO2 could be used as a refrigerant in heating and...