Having Desiccant Wheel. Normally the heat transfer between airstreams provided by the device is termed as 'sensible', which is the exchange of energy, or 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 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 hygroscopic coating, and the use of this, or the use of porous synthetic fibre matrices, allows for the absorption 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 Desiccant Wheel

Having Desiccant Wheel. Normally the heat transfer between airstreams provided by the device is termed as 'sensible', which is the exchange of energy, or 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 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 hygroscopic coating, and the use of this, or the use of porous synthetic fibre matrices, allows for the absorption 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 Desiccant Wheel