Airflow in a long, cylindrical, perforated pipe
Airflow in a long, cylindrical, perforated pipe
(OP)
Hello -
First time poster, several time referencer.
My question:
I have air coming off a 3hp regenerative blower at 0.433psig(12 in H2Og) on the outlet of the blower. Flowrate = 300cfm.
This flows through a 6" pipe with 3 90's, then forks into 2 L=50' ID=4" cylindrical segments.
The purpose: provide evenly distributed airflow for a composting operation.
The question: what size hole (probably need to assume), # of holes, and distribution of holes needed to ideally achieve Q = uniform through each airhole.
What has been observed in the field: the purpose of the air is to cool the self-heating pile. With evenly longitudinally-spaced 2" holes the end nearest the blower has lower temps than the end further away. Makes sense.
My thought: have larger spacing between the air holes nearer the blower, tighter spacing further away.
I don't have any CFD resources, which make my life alot easier. I could do it experimentally, but if there is a convenient approximation.. well that's why I'm posting.
Sorry for the long post.
CN:
How do you calculate air flow distribution through holes spaced long-ways on the cylindrical pipe fed by a small regenerative blower such that volumetric airflow is approximately equal through each hole.
Thanks.
First time poster, several time referencer.
My question:
I have air coming off a 3hp regenerative blower at 0.433psig(12 in H2Og) on the outlet of the blower. Flowrate = 300cfm.
This flows through a 6" pipe with 3 90's, then forks into 2 L=50' ID=4" cylindrical segments.
The purpose: provide evenly distributed airflow for a composting operation.
The question: what size hole (probably need to assume), # of holes, and distribution of holes needed to ideally achieve Q = uniform through each airhole.
What has been observed in the field: the purpose of the air is to cool the self-heating pile. With evenly longitudinally-spaced 2" holes the end nearest the blower has lower temps than the end further away. Makes sense.
My thought: have larger spacing between the air holes nearer the blower, tighter spacing further away.
I don't have any CFD resources, which make my life alot easier. I could do it experimentally, but if there is a convenient approximation.. well that's why I'm posting.
Sorry for the long post.
CN:
How do you calculate air flow distribution through holes spaced long-ways on the cylindrical pipe fed by a small regenerative blower such that volumetric airflow is approximately equal through each hole.
Thanks.





RE: Airflow in a long, cylindrical, perforated pipe
Then stage the spacing or diameter of the penultimate hole, and recurse until you run out of flow or pressure at the blower.
Put it all in a spreadsheet to make modifications easy.
Mike Halloran
Pembroke Pines, FL, USA
RE: Airflow in a long, cylindrical, perforated pipe
A second method, which is much easier to calculate but which results in larger pipes is to size the pipe such that there is a negligible pressure drop along the pipe. In this way every hole is exposed to virtually the same pressure so they all have the same flowrate. This means you can use the same size hole evenly distributed along the length of the pipe. Easier to calculate and easier to fabricate.
You are in the fortunate position that your 4" pipe falls into the second category. But 2" holes seem way too big to me. My estimates for the number of holes per leg (assuming 2 legs) is
1/2" dia 15 holes
3/8" dia 26 holes
1/4" dia 58 holes
These holes should be evenly distributed along the part of the leg that is in the compost.
Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Airflow in a long, cylindrical, perforated pipe
You can assume each hole behaves as an orifice (at least this is what I do) and evaluate the pressure drop provoked by the hole. This involves a reiterating procedure until you find the size that meets your requirements.
RE: Airflow in a long, cylindrical, perforated pipe
Katmar -
I understand this reasoning. In fact, one point I left out was that in the field the piping was littered with 2" holes, certainly causing a significant pressure.
My question is: how did you come to this result? I see that the total cross-sectional area is equal for these results, but how did you come to determine this area?
By my calcs: at 150cfm (per leg) & CS Area = 0.0873 ft2 my velocity equals 1718.87 ft/min. For 150 cfm to go through your area your velocity equals 7352.94 ft/min through each 1/2" hole. Is this correct? Or am I forgetting something like compressibility?
I appreciate all the help.
RE: Airflow in a long, cylindrical, perforated pipe
The holes are calculated as simple orifices with a fixed average discharge coefficient. It is buried deep in a program I wrote a long time ago and I think I took something like 0.61 for CD. I made no attempt to correct CD for different pipe and orifice sizes so the total area should be the same regardless of the hole size.
The logic is to determine the flow through 1 hole with the given pressure drop and then use that flow rate to determine the number of holes required.
I agree with your velocity calcs. At these pressures you can safely ignore compressibility.
I took the basic method from an article by KS Knaebel in Chemical Engineering, March 9, 1981 p118 but Perry describes much the same procedure.
Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Airflow in a long, cylindrical, perforated pipe
RE: Airflow in a long, cylindrical, perforated pipe
Good luck,
Latexman
RE: Airflow in a long, cylindrical, perforated pipe
http://en.wikipedia.org/wiki/Orifice_plate
Or look for Darcy formula (not Darcy-Weisbach) in Crane TP-410.
RE: Airflow in a long, cylindrical, perforated pipe
I've checked this wiki page before and my question is: is it applicable to perpendicular flow, which would be the case in question.
RE: Airflow in a long, cylindrical, perforated pipe
David
RE: Airflow in a long, cylindrical, perforated pipe
RE: Airflow in a long, cylindrical, perforated pipe
Thanks for all your help. I have another question. The blowers will be VFD driven and operate at range of flow rates.
The highest flowrate and static pressure was what you previously calculated. I'm wondering how numbers change at our minimum flowrate.
That being: 100 CFM & 1.2" H2O.
I'd imagine that the hole size decreases... but now I'm wondering (if it's even possible) to have even air flow through the holes at a range of backpressures and blower flow rates.
Thanks --
Rob
RE: Airflow in a long, cylindrical, perforated pipe
RE: Airflow in a long, cylindrical, perforated pipe
Good luck,
Latexman
RE: Airflow in a long, cylindrical, perforated pipe
I about to get a section of pipe, plug 5 pressure gauges in it. And start drillin'.
RE: Airflow in a long, cylindrical, perforated pipe
RE: Airflow in a long, cylindrical, perforated pipe
Good luck,
Latexman
RE: Airflow in a long, cylindrical, perforated pipe
Distributors are not quite as tolerant on the high flow side because at high velocities the pressure recovery down the distributor can become significant when the decreasing flow towards the end causes the velocity to decrease and the static pressure to increase (Bernoulli strikes again). But you have to over-range your distributor quite severely before you get noticeable maldistribution. In this counter-intuitive situation you can get higher flows out of the holes further from the blower.
Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Airflow in a long, cylindrical, perforated pipe
RE: Airflow in a long, cylindrical, perforated pipe
Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"