# 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.