Correct use of Flowhood

[Riccardo66]

Hi All,
I am searching for advice on the correct use of 'Flowhoods' of the type manufactured by Shortridge Instruments and others. I have seen them used directly on the fan inlet/outlet to measure airflow. However, this has clearly put the fans into stall and resulted in incorrect measurements. The hoods are marketed as a 'back pressure compensated' 'air balancing' tool. My understanding of the theory is that it should be fairly simple to estimate back pressure on a duct register when the air handling unit does not significantly change its operating point i.e. if you were balancing multiple registers connected to a large AHU. However, to put a hood directly on the fan and put it into stall.....I can't see how the hood can accurately predict the back pressure for its correction. Once the fan starts moving up its characteristic curve, how does the hood know which curve it is connected to? Would appreciate if anyone else out there has wasted any sleep over this particular conundrum.....

[Brian_UK]

As you say, that is not the way to measure the fan duty.

A duct traverse is the only safe way to achieve accurate readings that I am aware of.

For terminal airflow measuring you, of course, need a hood of suitable size for the terminal itself.

[frank]

As Brian said, you could make sure that all dampers in the system are 100% open and that all grille/terminal devices are 100% open, then do a Pitot tube transverse of the outlet duct. This will give you the:
1) Velocity
2) Volume (V x A)
3) Static pressure
4) Total Pressure (Velocity + Static)

Then, armed with this information, you can plot the point on the fan manufacturers fan curve data to determine the fan duty.