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.
FVD decentralised air conditioning system with direct induction
LTG Aktiengesellschaft has integrated the direct induction concept into its decentralised supplied air units for installation in double floors with a grid width of 200 mm – 300 mm. It has long been known that it is more thermodynamically efficient to mix the outside air with the room air before it passes through the heat exchanger. In the field of centralised air conditioning units, it is general engineering practice to pass mixed air through a heat exchanger.
When combined with induction technology, this practice can now also be consistently adopted for decentralised units, to provide all the benefits direct induction has to offer:
- Increased heating and cooling capacity
- High level of protection against freezing
- Improved thermal comfort, and
- Efficient use of a free source of cooling
LTG products for direct induction: Decentralised Facade Ventilation Unit FVD.