Air flow measurement

[op9]

I hope this hasn't appeared before as I tried to post it as Combustion air flow but it didn't appear!!

I have been doing some tests on an Asphalt plant burner which is suffering from a lack of combustion air due mainly to the inability of the exhaust fan and system to draw adequate suction at the burner. I have taken pitot tube measurements in a straight exhaust duct area and come up with a flowrate of 7428 ACFM (measured at 162degF). To try and get some appreciation of how much additional exhaust fan capacity I need, I am comparing it to specs published by NAPA which state values of 12000 ACFM (at 290degF). How do I directly compare my reading on the same datum temperature?

I did a calculation based on the gas laws V/T = Constant using absolute temp values. This converted my reading to 8957 ACFM. Is this the correct procedure?

Basically I am saying that if if you measure flow in a duct at one temp, then further down the same duct the temperature is raised (or lowered)what flowrate will you measure?

Would appreciate any comments.
Rod.

 

[TD2K]

All other things being equal, the ACFM will vary as the absolute temperature.

Thus, your flow rate of 7428 acfm at 162F equals 8957 acfm at 290F, just as you came up with.

 

[25362]

TD2K is right. Kindly pay attention to his statement on "all other things being equal".

The standard conditions for dry air in the fan industry are 70^oF and 29.92 in. Hg. The density corresponding to these conditions is 0.075 lb/ft³. Please note that other industries have different standards.

 

[op9]

Actually where I measured the flow with the pitot tube was the only straight constant cross section duct available and was after the water sprays (dust removal), so the actual air would be heavily loaded with water vapour. The temp at that point was 162degF but I would expect maybe close to 100% saturated. What density would you estimate to be used in the pitot velocity calc?
Thanks,
Rod

Rod Nissen.
Combustion & Engineering Diagnostics
canded@iprimus.com.au

http://www.canded.com.au

 

[NeedAHoliday]

op9,

Without knowing the pressure at the point you made your measurements it is hard to state the density. However you can calculate the volumetric ratio of water vapour to dry air, the corresponding densities for water vapour and dry air at the set conditions, and from this data determine the density of the air/water vapour mixture.