Direct induction

Compared to central ventilation concepts, it is noticeable that decentralized supply air systems provide acceptable cooling and heating capacities at first glance. However, if the proportion of energy that has to be used to bring the outside air into the room in a thermally neutral manner is subtracted, the cooling capacity in the room for all manufacturers is barely more than 250 watts (at 18°C KWVL). The following consideration shows why this must be the case:

To avoid condensation on the heat exchanger for hygienic reasons, the water inlet temperature should be 18°C above the dew point temperature of the outside air, for example. If the fan conveys 120m³/h of outside air at a temperature of 32°C through the heat exchanger, a maximum cooling capacity of 490 W is achieved with a decentralized air conditioning unit. Of this, 240 W alone is required to cool the outside air from 32°C to 26°C. This means that only 250 W of cooling capacity remains in the room. The theoretical limit for the maximum output of such a system is a discharge temperature of 18°C. A maximum cooling capacity of 560W would then be achieved.

This physical limitation is an indication that this flow principle must be abandoned in order to increase the performance of the devices. In principle, there are four ways to increase the cooling capacity for the room:

1. Increasing the fresh air volume flow(not sensible from an energy point of view)
2. Lowering the water supply temperature (complex condensate network required)
3. Increase the supply air flow by adding recirculated air,
4. Direct induction

What is direct induction?

A significant increase in performance can be achieved if the volume flow conveyed through the heat exchanger is increased by induction. Here, the outside air is first mixed with room air and the mixed air is then conveyed through the heat exchanger. The described boundary conditions result in a caloric cooling capacity of 635W, leaving 395W cooling capacity in the room.