In thermal wheel heat exchangers, the wheel itself consists of smooth ducts arranged around an axis, one half of which are cooled by the cold air and the other half are heated by warm air in a countercurrent.
The heat recovery rate can be adjusted by adjusting various features of this wheel, such as the thickness of the storage material, the cross-section and depth, as well as the thermal wheel speed.
Thermal wheel heat exchangers can transfer heat as well as humidity. The critical factor for this is the material or surface of the wheel.
There are three different versions:
- Firstly, there is the condensation thermal wheel, the body of which is made from smooth untreated aluminium. It only transfers humidity if condensation takes place on the warm air side and the condensate produced is partially absorbed by the cold air outside. This makes it best suited to winter operation.
- Secondly, we have the enthalpy thermal wheel or hygroscopic thermal wheel, the body of which has a capillary structure created by surface treatment. This results in humidity transfer by both condensation and sorption, whereby the sorption ratio is relatively minor. However, in summer mode it performs better than the condensation thermal wheel.
- Finally, there is the sorption thermal wheel, the body of which features a surface that effects humidity transfer purely by sorption. Consequently, the humidity recovery factor is almost independent of the condensation potential, in other words the humidity differential between the warm air and the saturated humidity level of the cold air.
Consequently this thermal wheel is suitable for summer mode, as the warm and humid outdoor air is not only cooled, but also dehumidified thanks to this form of sorption. This enables mechanical cooling capacity to be saved.
As a result of internal leakage and the rotation of the heat exchanger itself, there will always be a mixing of the different airstreams. The transfer from the exhaust air to the outdoor air side can be substantially reduced by means of a flushing zone. This routes outdoor air inside the bypass across the thermal wheel into the exhaust air. This method generally depends on the type of thermal wheel, the existing flushing pressure and the height at which it is mounted. For this, the arrangement of the fans is critical. Where possible, a combination of fans arranged on the pressure side of the extract air and the intake side of the supply air should be avoided.