Measuring the Flowrate of Fans by Embedded Sensors

Test Rig & Instrumentation

In most ventilation applications a measurement of the flowrate allows a better control of the system (especially when the flow varies with time) and a possible reduction of the energy consumption by avoiding to oversize the fan. However, the actual flow performance of a fan in a system depends on the overall pressure loss of the system, which is often not accurately estimated, and on potential fan system effects. The use of the technical specifications given by the fan manufacturer (e.g. the fan curve) may be not sufficient in this case to reach the expected accuracy on the flow rate knowledge.
A way to measure the flowrate of the system may be to do it on the fan itself. Fans with embedded sensors can be a suitable solution for this flow measurement.
In this paper different solutions based on the measurement of the differential pressure on the bell mouth of a double Inlet forward curved centrifugal fan of 200 mm diameter have been investigated. The tests were performed at different speeds and fan operating points. They included measurements with 9 pressure taps on each inlet bell, then with a pressure probe patented by the fan manufacturer Nicotra Gebhardt. This probe was fixed on one of the fan inlet arms. The choice of the arm and the radial location of the probe have been investigated to estimate their influence on the flow measurement.
All these tests have been performed on a test rig using orifice plates which allows to know the actual flow rate with a good accuracy. A relationship between the known flow rate and the measured differential pressure has been established for the two configurations:
• pressure measurements on the bell mouth taking into account the number of pressure taps,
• pressure sensor on an inlet arm taking into account the radial location of the probe and the circumferential position of the arm itself.
Considering the measurement uncertainty of the instruments used the uncertainty on the established relationship has been estimated for each test configuration.
A simulation has then been performed to estimate the uncertainty on the flow rate measurement in case of on-site use taking into account the possible accuracy of the pressure sensor, the knowledge of the actual air density and the impact of some fan installation effects.