The formula for constant volume

Constant volume plays a major role in many appli­ca­tions, but in resi­den­tial venti­la­tion it’s essen­tial.


Daniel Gebert, Aero­dy­namics Devel­op­ment, and Valerius Schaaf, Basic Devel­op­ment, at ebm-papst Mulfingen

Effi­cient resi­den­tial venti­la­tion calls for exact control of the amounts of air supplied and extracted by the fans. Other­wise, under- or over­pres­sure can arise in a room, which can result in mois­ture in the walls or exhaust emis­sions from a chimney into the living quar­ters.

To deter­mine the exact air volume, the trans­ported air flow must be measured. With the conti­nuity equa­tion, it can be expressed as the inte­gral of the velocity distri­b­u­tion over the area through which the trans­ported mate­rial flows within the exhaust nozzle. This can be visu­al­ized as a net stretched over the exhaust opening, with the product of partial velocity and area being taken at every knot and summed over the surface.

For forward-curved centrifugal fans, the trans­ported air volume can be deter­mined rela­tively easily from a char­ac­ter­istic curve using the fan’s speed and current draw. But these fans have effi­ciency deficits when compared with those with back­ward-curved impellers. For back­ward-curved centrifugal fans, however, it’s impos­sible to unam­bigu­ously deter­mine the air flow from these para­me­ters.

Simple and effi­cient measure­ment with vane anemometer

To deter­mine the air flow for back­ward-curved centrifugal fans, manu­fac­turers use various systems. Frequently a differ­en­tial pres­sure measure­ment is used, for example. But here the measure­ment accu­racy is reduced for low air flow by the quadratic rela­tion­ship between air flow and pres­sure, and the measured value can be distorted by the condi­tions of the fan’s instal­la­tion in the customer equip­ment.

Velocity Curve in Outlet, Depending on Oper­ating Point

We sought a more exact solu­tion that could be inte­grated in the fan. Thanks to close coop­er­a­tion between our design and aero­dy­namics special­ists, we have devel­oped a simple and effi­cient solu­tion for the new RadiCal in a scroll housing with EC tech­nology. A vane anemometer posi­tioned in the outlet records the flow velocity – and thus the actual air flow — contin­u­ously across the entire cross section without producing disturbing noise or signif­i­cant losses. The preci­sion is much better thanks to the linear rela­tion­ship between air flow and anemometer speed and the aver­aging of the flow velocity over the cross section.

Velocity distri­b­u­tion

The anemometer trans­mits the data to the fan’s inte­grated central control elec­tronics, which adapt the speed of the EC motor and regu­late the air volume. With this method, the air flow can be regu­lated with a preci­sion of ± 1 percent. The EC fan can put its advan­tages to full use here as its preci­sion control and high effi­ciency are main­tained even in partial-load oper­a­tion.

Required fields: Comment, Name & Mail (Mail will not be published). Please also take note of our Privacy protection.


Additional product information can be found here:

RadiCal in a scroll housing

Smart, precise, compact