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Safety -- Make sure you are safe! |
2011-10-17, 8:48am
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Senior Member
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Join Date: Nov 19, 2007
Posts: 1,078
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Status pressure calculation help needed
Hi,
I have determined, based on my vent hood, that I need a minimum of a 1500 cfm fan.
When looking for a quiet solution, I am finding that the cfm rating on ducted fans requires a static pressure measurement to know what the actual ducted cfm rating is. If I have about a six foot long static duct run with one 90 degree bend, can I calculate static pressure? I have a 4' x 3' restaurant vent hood to find a fan for. We are hoping to stay with 8" ducting but will increase if required.
ETA: this is the fan I am seriously considering. http://www.hvacquick.com/products.ph...all-Exhausters
Thank you, mathematical geniuses, in advance
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2011-10-17, 9:50am
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Senior Member
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Join Date: Nov 19, 2007
Posts: 1,078
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Ruh Roh. I have been reading elsewhere about ventilation needs and I think, given my multiple torches, I am way off base.
For a six torch setup, I am going to need a much heftier fan than I have been looking at. My big issue is that the outside view of the exhaust fan must *not* be unsightly!
So, given this new information, here is the deal...
We will have a central overhead ventilation hood mounted no more than 30" off the table. We will have a maximum of six torches running at any given time. The vent hood is 3' x 4'. The table will be centered beneath it. The ducting will have one single run of no more than 6' and a single 90 degree bend. I calculated my above post using the 125 cfm and vent face equation. Is the number derived from that now to be multiplied by the number of torches? If so, I am looking at one HUGE vent system!
Is it time to panic now?
FTR, the studio is located in an open air (eta, i do not mean outside but open to a larger space including 10 foot ceilings) facility of approximately 600 sq. Ft. But the studio itself is approximately 144 sq. Ft. There are no doors on the studio. It is open to an adjoining room with a small divider wall between.
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2011-10-17, 10:22am
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Curmudgeon Engineering
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Join Date: Feb 15, 2006
Location: Near Seattle, WA
Posts: 1,723
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I didn't download it but it may serve your needs. Go to,
http://www.bry-air.com/tech-center/r...c/default.html
and download the calculator for your answer.
Re 2nd post.
It is my opinion that your calculations are correct but with an open all around torching station I would err on the side of caution. The size and configuration of the 'space' does not enter into the calculations; it simply contributes to an omni directional source of makeup air. Your goal is to move contaminated air from your breathing area so the room is of no consequence. Where the makeup air enters the space may be a factor however. A duct run of 6' with one 90 elbow is equal to 8' of straight run and 8' is almost negligable(sp) for static pressure. Personally I would go up a few hundred CFM amd call it very good. Also the number of torches is a very very small factor in the system.
Keep in mind that the purpose of ventilation is to supply the people with fresh breathable air, the torches will take care of themselves.
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2011-10-17, 10:33am
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Senior Member
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Join Date: Nov 19, 2007
Posts: 1,078
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Thank you!
Now, I worry my vent hood is too small. Argh.
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2011-10-17, 11:03am
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Curmudgeon Engineering
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Join Date: Feb 15, 2006
Location: Near Seattle, WA
Posts: 1,723
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If I were doing it and my hood was fixed (you have it already) and I wanted 6 torch stations I would make the bench 6' X 7' and have 3 stations along the long side. Or 6' X 6' with 2 torches on two opposite sides and one on the ends. I would play around with bench height to get the most comfortable height while trying to shorten the distance of the hood above the bench but not interfereing with head room. Thirty inches sounds like a good starting point. Standing or sitting will be a consideration for bench height as well as hood height.
All these remarks are made without drawings or calculations and based on what I have done with several studio setups and tests I have done in the past as well as studios I have seen and worked in. It seems to me you are well in the ballpark with you plans.
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2011-10-17, 11:08am
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Senior Member
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Join Date: Nov 19, 2007
Posts: 1,078
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I am close, I have a 5.5' (diameter) round table. I was planning to "bump it out" with some stainless steel just for comfort sake. We may have to consider the rectangular concept after all.
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2011-10-17, 4:49pm
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Gentleman of Leisure
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Join Date: Jun 10, 2005
Location: A Little Bit West of Yosemite Valley
Posts: 5,200
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To avoid static pressure dilemma, use as large of diameter of ducting as possible....
To determine how much CFM you need if using a round hood, just use formula to find area of circle and take results and work standard ventilation formulas for CFM.
Dale
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San Francisco - A Few Toys Short of a Happy Meal
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2011-10-17, 5:27pm
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Senior Member
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Join Date: Nov 19, 2007
Posts: 1,078
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Thank you!
I just noticed my iPad changed the title to status pressure. LOL
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2011-10-18, 6:14am
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old fart
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Join Date: Apr 18, 2006
Location: st paul mn
Posts: 778
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do not get a hood bigger than you need. you can use a small quiet fan if your hood's face area (the width and height that faces you when you torch) is kept small and by enclosing your hood in the back, top, bottom, and both sides.
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Mark Wilson
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