Duct air flow measuring

Dear colleagues,

Can somebody tell me the explanation for following; We have AHU unit specified as 9900m3/h which is working at the moment 9000m3/h (value shown on AHU controller). When we measure the air flow in the main duct (600x600mm) our results show that the flow is 12000m3/h (and we know that this is wrong). We conduct measurement with the hot wire anemometer, which is 1 month old (and calibrated). Probe is placed at the side of the duct, value recorded at 5 different points, from center to duct wall, and afterward we multiple average value with the duct cross-sectional area. Does somebody can conclude what we did wrong? Is there a different method of measuring and determining average velocity?

Thanks in advance,

What size of AHU-ventilator (diameter) and what model of reference anemometer.

First we did check that AHU-ventilator and it’s characteristics and conclude that value 12000m3/h is not possible at all (maximum what ventilator can give is 11000m3/h). Used anemometer is Hot wire anememometer which has following characteristics:

Meas. Range +0.6 to +40 m/s
Accuracy ±(0.2 m/s +1.5% of mv)
Resolution 0.1 m/s
Operating temperature -20 to +50 °C
Telescopic handle max. 890mm

Please, cite meashured results air flow in m/s.

Results were: 10.5m/s, 10.3m/s, 9.4m/s, 8.4m/s and 7.8m/s

Thanks in advance,

Hi Del

What state is your system in?

Most fan’s have speed control systems, what is yours set to?

What back room pressures have in the system?

Have got your intake bag filters in?

Is the system in full recirculation?

Have you started to balance the system?

I would not look at a surplus in flow as a problem, AHU’s are made to certain stock sizes and it is always better to have a surplus. Balance it out and see if you still have a problem, I dont think you will.

Alex Kennedy