Air flow measurement

[op9]

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.

 

[quark]

Your calculation procedure is correct and the value of [7428*(460+290)/(460+162)]8957 ACFM is also correct. There are excellant discussions on the validity of SCFM and ACFM in piping engineering forum which highlight the loopholes in the system. My suggestion will be to convert the flowrate to standard conditions or in mass units. This will give you correct indication about your flowrate. You also need to check the pressure at which the flowrates are mentioned.

PS: Have you checked the CO₂ and CO contents?

Regards,

 

[op9]

Thanks quark,
Yes I have checked those or CO and excess O2 to be exact. CO is very high as this type of burner relies on the exhaust system to induce as much as 60% of the combustion air. Herein lies the problem as there is insufficient suction at the burner. I need a minimum of -1/4" wg. but measuring zero or barely negative. To make matters worse in the past they have added another blower to blow dust into the drum to add to the asphalt mix. This added air of ~350cfm has also to be evacuated by the struggling exhaust fan.
Regards,
Rod.

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

http://www.canded.com.au

 

[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

 

[quark]

Download pmtherm from the following link

http://www.pmilligan.net/pmtherm.htm

(your system should have vbrun300.dll to run this, which can also be downloaded from the same site at the bottom of the page.


Regards,