Compared to centralised ventilation concepts, it seems at first sight that decentralised air supply systems offer acceptable cooling and heating capacity. However, if one subtracts the energy that has to be expended in order to introduce outside air into the room at a neutral temperature, then the indoor cooling capacity is barely more than 250 W (at a cooling water supply temperature of 18 °C) irrespective of manufacturer. The reason for this is as follows:
In order to avoid condensation at the heat exchanger for reasons of hygiene, the water supply temperature should, for example at 18 °C, be above the dew-point temperature of the outside air. If the fan propels 120m³/h of outside air at a temperature of 32 °C through the heat exchanger, then the maximum cooling capacity achievable by a decentralised air conditioning unit is 490 W. Of this, 240 W are required simply to cool the outside air from 32 °C to 26 °C. The residual cooling capacity in the room is therefore only 250 W. The theoretical borderline case if a system of this type is to achieve maximum performance lies at an outlet temperature of 18 °C. In this case, a maximum cooling capacity of 560 W would be achieved.
This physical limitation is one indicator that this through-flow principle needs to be abandoned if we want to increase the capacity of the units. In principle, there are four ways to increase indoor cooling performance:
- Increase the volume flow of outside air, (not worthwhile for energy-related reasons)
- Lower the water supply temperature, (requires a cost-intensive condensate collection/evacuation system)
- Increase the flow of supplied air by mixing with recirculated air,
What is direct induction?
It is possible to achieve a considerable increase in capacity by using induction to increase the volume flow through the heat exchanger. To do so, the outdoor air is first mixed with the room air, and this mixed air is then passed through the heat exchanger. Given the parameters described above, the caloric cooling capacity is 635 W, resulting in a residual cooling capacity in the room of 395 W.