The Measurement of Air Volume Flow Rate in Rectangular Ducts Using Vane Anemometers

The problems associated with the accurate measurement of air volume flow rates in ducts were investigated. Measurements of air velocity were made in two sizes of rectangular duct using three sizes of vane anemometer. The ducts were traversed at various distances downstream of a number of inlet duct arrangements. It was shown that the flow rate could be measured with a good level of accuracy in the situations tested, if the anemometer traverse was positioned between 0.4 and 1.0 equivalent duct diameters from the face of the test piece, using a correction factor which was close to unity. Alternatively, if a traverse was made at the face of a test piece, the flow rate could be obtained with the use of a correction factor which varied from 0.57 to 0.90 depending on the test piece and anemometer size. It was also demonstrated that, once calibrated, a single anemometer in a fixed position could be used to measure the flow rate.
It was required to measure the flow rates continuously in one of the Faculty's air-conditioning plants. A set of ducts with air flow measuring stations was constructed for this plant and calibrated using the technique developed. Tests were made using two fixed position anemometers in each measuring station and the accuracy of the measurement of the total flow rate was estimated as +/- 5.0% using one second pulse counts.

The study indicated that provided certain precautions are taken, the length of duct required for measuring flow rates can be between one and two equivalent diameters. This fact could be of great benefit to ventilation engineers concerned with the accurate measurement of air flow when a limited amount of space is available.