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Spray Booth Input Volume

Spray Booth Input Volume

Spray Booth Input Volume

(OP)

How can a measured duct volume flow rate be only 40% of that measured across the face of the input filter?

RE: Spray Booth Input Volume

Duct leakage comes to mind.

It would also depend on how flow was measured.  

RE: Spray Booth Input Volume

(OP)
Duct velocity traverse with hotwire on negative side of fan - duct dampered on positive side of fan - duct length of 4m - no leakage. Face velocity measured across the filter with a vain anemometer. Both test instruments recently calibrated. Greater air volume calculated across the filter than in the duct. What am I missing?

RE: Spray Booth Input Volume

Did you have a breeze blowing in the shop when you were measuring the face velocity?
B.E.

RE: Spray Booth Input Volume

(OP)
No, the booth is sealed - the input filters are in the ceiling and a series of readings were taken across the face in a matrix and the average recorded. The test has been conducted at least three times and the same results achieved.

RE: Spray Booth Input Volume

Velocity reading at the face can give false reading due to placement of velocity sensor and calculation of filter area. Did you get a good clean traverse of the exhaust duct. Fittings, obstructions and short straight run will corrupt that data. Can you traverse the discharge of the fan? Do you have strange numbers from the velocity profile across the duct? I see the same issue with kitchen hoods. The face is easy to read but the numbers on the black iron duct do not match. 40% is too big. Can your hot wire be used to read the filters?

RE: Spray Booth Input Volume


Is it possible that some air in the booth is being entrained with the supply air? That would not account for a doubling of the flow however.

What kinds of flows and velocities are we talking about?

RE: Spray Booth Input Volume

(OP)

The average input duct velocity was 6.94 m/s across the 650 mm diameter giving a flow rate of 2.304 m3/s. The average face velocity of 0.37 m/s was recorded across the filter with an area of 9.165 m2, giving a flow rate of 3.391 m3/s. The fan feeds air into a plennum chamber that distributes the flow across the inside face of the filter with laminar flow.

I'm sure I must be missing some fundamental airflow dynamic principles beyond my understanding as I can't understand how the one fan can appear to be drawing air in a one rate and pushing it out at another!

I am tempted to go back and re-record the duct velocities with a pitot instead of a hotwire just to ensure the accuracy of the results. Hotwires are after all only accurate if positioned correctly. Maybe that's all it is.
 
On the extract side, the average face velocity across the face of the extract filter was 0.66 m/s , the filter area is 4.449 m2, giving a flow rate of 2.914 m3/s. The duct readings were 6.77 m/s across the 0.332 m/2 duct area giving 2.246 m3/s flow rate.

I would suggest although the recorded values is slightly lower, the extract is roughly the same as the input (as measured in the duct). We know the fans are of a similar size and only the extract damper is adjustable. The test was conducted with this fully open and I understand the seperate design/spec issues with this.

RE: Spray Booth Input Volume

The only thing I am seeing here is that you are using  two different instruments to take your readings. Try using just a pitot/ static probe to take both readings.
 Watch out for the fact that the higher water gauge pressure of the duct can give you a high reading if the static is not right behind the Pitot, but outside in the room.
B.E.

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