For the air handling units with air volumes up to 9.000 m3/h approx., it is more and more frequent to use directly-coupled high-efficiency fans, with EX brushless motors technology. This type of fans implies an electronic control system which automatically adapts the number of rpms according to the set air volume.
This means that the fan automatically changes its rotation speed to adjust to the system pressure losses, thus guaranteeing the pre-set flow rate. This also means that the required air flow rate is ensure at all times regardless of circuit load loss, as long as the latter is not higher than the maximum pressure value which the fan can provide (see Table). Therefore not even the filter clogging causes a reduction in the air flow rate and the consequent reduction in the system performance. Another important advantage, particularly in units with two-way air flows such as the URP, is that the problem of performance imbalance between return and supply has been eliminated. This imbalance, caused by different pressure losses, within the two circuits, results in a low heat recovery performances and, above all, unwanted pressure or depressions in the rooms where the unit is installed. The various air flow rates available for each fan are easily programmed using a terminal board located on the fan itself. Table 2 shows, for each size of unit, three available nominal air flow rates, to which the standard performance data refer, but each fan can be programmed for 12 different air flow rates.
TCF selection software allows for the selection of such fans, alongside with the traditional centrifugal ones with double inlet, with forward- or backward-curved blades, and directly-driven plug fans.