6.2. k-Factor

To determine the volume flow, so-called inlet measuring cones are used, which function according to the differential pressure principle (see also ISO 5801 A.4) and are used for test stands according to AMCA 210 or ISO 5801. In some systems, one would like to know which flow rate is currently being passed through during operation in order to be able to regulate. Here, a simplified version of the differential pressure principle has become established on the market. Radial impellers are usually always used with an inlet cone. Pressure holes and measuring nipples are inserted into this cone, at which the static pressure is measured (cf. Figure 6.2.). If the inlet cones has been calibrated with the impeller, the volume flow in systems can be calculated. This can be done, for example, using the so-called k-factor.

Definition of k-Factor (calibration factor): The differential pressure method compares the static pressure upstream of the inlet cone with the static pressure on the suction side at the inlet cone (dynamic pressure components are not desired). The differential pressure can be assigned to the volume flow via the law of conservation of energy.

Figure 6.2: Centrifugal fan in installed condition (inlet cone with pressure tap)

Calculation:

The volume flow is calculated using the following formula:

 V=k\times\sqrt{\Delta p}

This formula is valid for a standard density . If the density differs from  \rho_{std}=1,205\frac{kg}{m^3}, the following formula is applied:

 V_p=V\times\sqrt{\frac{\rho_{std}}{\rho}}