Thermal wheel
Encyclopedia
A Thermal Wheel, also known as a Rotary heat exchanger, or Rotary air-to-air enthalpy wheel, or Heat recovery wheel, is a type of energy recovery
heat exchanger
positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the heat energy. Other variants include Enthalpy wheels and Desiccant wheels. A cooling-specific thermal wheel is sometimes referred to as a Kyoto wheel.
The heat exchange matrix is normally manufactured in aluminium, which has good heat transfer properties, but can also be manufactured from plastics and synthetic fibres. The heat exchanger is rotated by a small electric motor and belt drive system. The motors are often inverter speed controlled for improved control of the leaving air temperature. If no heat exchange is required then the motor can be stopped altogether.
Because of the nature of thermal wheels in the way that heat is transferred from the exhaust air stream to the supply air stream without having to pass directly through or via an exchange medium, the gross efficiencies are usually much higher than that of any other air-side heat recovery system. The shallower depth of the heat exchange matrix, as compared to that say for a plate heat exchanger, means that the pressure drop through the device is normally lower in comparison. Generally a thermal wheel will be selected for face velocities between 1.5 and 3.0 m/s, and with equal air volume flow rates gross 'sensible' efficiencies of 85% can be expected. Although there is a small extra energy requirement to rotate the wheel, the motor energy consumption is usually very low and has little effect upon the seasonal efficiency of the device. In addition, the ability to recover 'latent' heat, depending upon the materials and coatings used, can improve gross efficiencies by some 10% to 15%.
', which is the exchange of energy, or enthalpy
, resulting in a change in temperature of the medium (air in this case), but with no change in moisture content. However, if moisture or relative humidity
levels in the return air stream are high enough to allow condensation to take place in the device, then this will cause 'latent
' heat to be released and the heat transfer material will be covered with a film of water. Despite a corresponding absorption of latent heat, as some of the water film is evaporated in the opposite airstream, the water will reduce the thermal resistance of the boundary layer of the heat exchanger material and thus improve the heat transfer coefficient of the device, and hence increase efficiency. The energy exchange of such devices now comprises both sensible and latent heat transfer; in addition to a change in temperature, there is also a change in moisture content of the air streams.
However, the film of condensation will also slightly increase pressure drop through the device, and depending upon the spacing of the matrix material, this can increase resistance by up to 30%. This will increase fan energy consumption and reduce the seasonal efficiency of the device.
Aluminium matrices are also available with an applied hydroscopic coating, and the use of this, or the use of porous synthetic fibre matrices, allows for the adsorption
and release of water vapour, at moisture levels much lower than that normally required for condensation and latent heat transfer to occur. The benefit of this is an even higher heat transfer efficiency, but it also results in the drying or humidification
of airstreams, which may also be desired for the particular process being served by the supply air.
For this reason these devices are also commonly known as an Enthalpy Wheel
s for vehicle propulsion (around 1965), Chrysler
invented a unique type of rotary heat exchanger that consisted of a rotary drum constructed from corrugated metal (similar in appearance to corrugated cardboard). This drum was continuously rotated by reduction gears driven by the turbine. The hot exhaust gasses were directed through a portion of the device, which would then rotate to a section that conducted the induction air, where this intake air was heated. This recovery of the heat of combustion significantly increased the efficiency of the turbine engine. This engine proved impractical for an automotive application due to its poor low-rpm torque. Even such an efficient engine, if large enough to deliver the proper performance, would have a low average fuel economy. Such an engine may at some future time be attractive when combined with an electric motor in a hybrid vehicle
owing to its robust longevity and an ability to burn a wide variety of liquid fuels.
wheel is very similar to a thermal wheel, but with a coating applied for the sole purpose of dehumidifying or 'drying' the air stream. The desiccant is normally Silica Gel
. As the wheel turns, the desiccant passes alternately through the incoming air where the moisture is adsorbed
, and through a “regenerating” zone where the desiccant is dried and the moisture expelled. The wheel continues to rotate and the adsorbent process is repeated. Regeneration is normally carried out by the use of a heating coil, such as a water or steam coil, or a direct-fired gas burner.
Thermal wheels and desiccant wheels are often used in series configuration to provide the required dehumidification as well as recovering the heat from the regeneration cycle.
Matrices made from fibrous materials, or with hydroscopic coatings, for the transfer of latent heat, are far more susceptible to damage and degradation by 'fouling
' than for plain metal or plastic materials, and are difficult or impossible to effectively clean if dirty. Care must be taken to properly filter the air streams on both exhaust and fresh air sided of the wheel.
Energy recovery
Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another...
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...
positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the heat energy. Other variants include Enthalpy wheels and Desiccant wheels. A cooling-specific thermal wheel is sometimes referred to as a Kyoto wheel.
Description
A thermal wheel consists of a circular honeycomb matrix of heat-absorbing material, which is slowly rotated within the supply and exhaust air streams of an air handling system. As the thermal wheel rotates heat is picked up from the exhaust air stream in one half of the rotation, and given up to the fresh air stream in the other half of the rotation. Thus waste heat energy from the exhaust air stream is transferred to the matrix material and then from the matrix material to the fresh air stream, raising the temperature of the supply air stream by an amount proportional to the temperature differential between air streams, or 'thermal gradient', and depending upon the efficiency of the device. The airstreams must be flowing in opposite directions or no beneficial heat exchange can occur. The principle of course works in reverse and 'coolth' energy can be recovered to the supply air stream if so desired and the temperature differential allows.The heat exchange matrix is normally manufactured in aluminium, which has good heat transfer properties, but can also be manufactured from plastics and synthetic fibres. The heat exchanger is rotated by a small electric motor and belt drive system. The motors are often inverter speed controlled for improved control of the leaving air temperature. If no heat exchange is required then the motor can be stopped altogether.
Because of the nature of thermal wheels in the way that heat is transferred from the exhaust air stream to the supply air stream without having to pass directly through or via an exchange medium, the gross efficiencies are usually much higher than that of any other air-side heat recovery system. The shallower depth of the heat exchange matrix, as compared to that say for a plate heat exchanger, means that the pressure drop through the device is normally lower in comparison. Generally a thermal wheel will be selected for face velocities between 1.5 and 3.0 m/s, and with equal air volume flow rates gross 'sensible' efficiencies of 85% can be expected. Although there is a small extra energy requirement to rotate the wheel, the motor energy consumption is usually very low and has little effect upon the seasonal efficiency of the device. In addition, the ability to recover 'latent' heat, depending upon the materials and coatings used, can improve gross efficiencies by some 10% to 15%.
Energy transfer process
Normally the heat transfer between airstreams provided by the device is termed as 'sensibleSensible heat
Sensible heat is the energy exchanged by a thermodynamic system that has as its sole effect a change of temperature.The term is used in contrast to a latent heat, which is the amount of energy exchanged that is hidden, meaning it cannot be observed as a change of temperature...
', which is the exchange of energy, or enthalpy
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
, resulting in a change in temperature of the medium (air in this case), but with no change in moisture content. However, if moisture or relative humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
levels in the return air stream are high enough to allow condensation to take place in the device, then this will cause 'latent
Latent heat
Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. The term was...
' heat to be released and the heat transfer material will be covered with a film of water. Despite a corresponding absorption of latent heat, as some of the water film is evaporated in the opposite airstream, the water will reduce the thermal resistance of the boundary layer of the heat exchanger material and thus improve the heat transfer coefficient of the device, and hence increase efficiency. The energy exchange of such devices now comprises both sensible and latent heat transfer; in addition to a change in temperature, there is also a change in moisture content of the air streams.
However, the film of condensation will also slightly increase pressure drop through the device, and depending upon the spacing of the matrix material, this can increase resistance by up to 30%. This will increase fan energy consumption and reduce the seasonal efficiency of the device.
Aluminium matrices are also available with an applied hydroscopic coating, and the use of this, or the use of porous synthetic fibre matrices, allows for the adsorption
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
and release of water vapour, at moisture levels much lower than that normally required for condensation and latent heat transfer to occur. The benefit of this is an even higher heat transfer efficiency, but it also results in the drying or humidification
Humidifier
A humidifier is a household appliance that increases humidity in a single room or in the entire house. There are point-of-use humidifiers, which are commonly used to humidify a single room, and whole-house or furnace humidifiers, which connect to a home's HVAC system to provide humidity to the...
of airstreams, which may also be desired for the particular process being served by the supply air.
For this reason these devices are also commonly known as an Enthalpy Wheel
Use in Gas Turbines
During the automotive industry's interest in gas turbineGas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
s for vehicle propulsion (around 1965), Chrysler
Chrysler
Chrysler Group LLC is a multinational automaker headquartered in Auburn Hills, Michigan, USA. Chrysler was first organized as the Chrysler Corporation in 1925....
invented a unique type of rotary heat exchanger that consisted of a rotary drum constructed from corrugated metal (similar in appearance to corrugated cardboard). This drum was continuously rotated by reduction gears driven by the turbine. The hot exhaust gasses were directed through a portion of the device, which would then rotate to a section that conducted the induction air, where this intake air was heated. This recovery of the heat of combustion significantly increased the efficiency of the turbine engine. This engine proved impractical for an automotive application due to its poor low-rpm torque. Even such an efficient engine, if large enough to deliver the proper performance, would have a low average fuel economy. Such an engine may at some future time be attractive when combined with an electric motor in a hybrid vehicle
Hybrid vehicle
A hybrid vehicle is a vehicle that uses two or more distinct power sources to move the vehicle. The term most commonly refers to hybrid electric vehicles , which combine an internal combustion engine and one or more electric motors.-Power:...
owing to its robust longevity and an ability to burn a wide variety of liquid fuels.
Desiccant Wheel
A desiccantDesiccant
A desiccant is a hygroscopic substance that induces or sustains a state of dryness in its local vicinity in a moderately well-sealed container....
wheel is very similar to a thermal wheel, but with a coating applied for the sole purpose of dehumidifying or 'drying' the air stream. The desiccant is normally Silica Gel
Silica gel
Silica gel is a granular, vitreous, porous form of silica made synthetically from sodium silicate. Despite its name, silica gel is a solid. It is a naturally occurring mineral that is purified and processed into either granular or beaded form...
. As the wheel turns, the desiccant passes alternately through the incoming air where the moisture is adsorbed
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
, and through a “regenerating” zone where the desiccant is dried and the moisture expelled. The wheel continues to rotate and the adsorbent process is repeated. Regeneration is normally carried out by the use of a heating coil, such as a water or steam coil, or a direct-fired gas burner.
Thermal wheels and desiccant wheels are often used in series configuration to provide the required dehumidification as well as recovering the heat from the regeneration cycle.
Disadvantages
Thermal wheels are not suitable for use where total separation of supply and exhaust air streams is required, since air will bypass at the interface between the air streams at the heat exchanger boundary, and at the point where the wheel passes from one air stream to the other during its normal rotation. The former is reduced by brush seals, and the latter is reduced by a small purge section, formed by plating off a small segment of the wheel, normally in the exhaust air stream.Matrices made from fibrous materials, or with hydroscopic coatings, for the transfer of latent heat, are far more susceptible to damage and degradation by 'fouling
Fouling
Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...
' than for plain metal or plastic materials, and are difficult or impossible to effectively clean if dirty. Care must be taken to properly filter the air streams on both exhaust and fresh air sided of the wheel.
Other types of air-to-air heat exchangers
- Run around coilRun around coilthumb|right|300px|A run-around coil installation, serving [[air handler|air handling units]] on the roof of an office buildingA run-around coil is a type of energy recovery heat exchanger most often positioned within the supply and exhaust air streams of an air handling system, or in the exhaust...
- RecuperatorRecuperatorA recuperator is a special purpose counter-flow energy recovery heat exchanger positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the waste heat...
, or cross plate heat exchanger - Heat pipeHeat pipeA heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces....
See also
- HVACHVACHVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
- Energy recovery ventilation
- Heat recovery ventilationHeat recovery ventilationHeat recovery ventilation, also known as HRV, mechanical ventilation heat recovery, or MVHR, is an energy recovery ventilation system using equipment known as a heat recovery ventilator, heat exchanger, air exchanger, or air-to-air heat exchanger which employs a counter-flow heat exchanger between...
- Regenerative heat exchangerRegenerative heat exchangerA regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where the flow through the heat exchanger is cyclical and periodically changes direction. It is similar to a countercurrent heat exchanger. However, a regenerator mixes the two fluid flows while a...
- Air handlerAir handlerAn air handler, or air handling unit , is a device used to condition and circulate air as part of a heating, ventilating, and air-conditioning system. An air handler is usually a large metal box containing a blower, heating or cooling elements, filter racks or chambers, sound attenuators, and...
- Thermal comfortThermal comfortThermal comfort is a term used by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, an international body. It is defined as the state of mind in humans that expresses satisfaction with the surrounding environment...
- Indoor air qualityIndoor air qualityIndoor air quality is a term referring to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants....
- CCSICCSICCSI specialises in developing software for Air conditioning and engineering applications as well as virtual reality and 3D visualisation tools....
External links
- Enthalpy and heat wheels at the Cipco energy library
- Study on using Enthalpy wheels at airports from San Jose State UniversitySan José State UniversitySan Jose State University is a public university located in San Jose, California, United States...
- Enthalpy wheel data from the Live Building at Queen's UniversityQueen's UniversityQueen's University, , is a public research university located in Kingston, Ontario, Canada. Founded on 16 October 1841, the university pre-dates the founding of Canada by 26 years. Queen's holds more more than of land throughout Ontario as well as Herstmonceux Castle in East Sussex, England...