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02-22-2006, 09:10 AM
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#1
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Registered User
Trade:
Mechanical
Join Date: Feb 2006
Posts: 8
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CFM's on 6" oval pipe
Just to settle a bet here at the shop, does anyone know the CFM's that 6" oval pipe carries? Someone believes in only carries the CFM equivalent of 5" round pipe. Where could I find the proof. I have written Sealtite with no reply.
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02-22-2006, 09:47 AM
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#2
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Pro
Trade:
Residential Contractor
Join Date: Feb 2004
Location: Jensen Beach, FL
Posts: 10,376
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To roughly find an equivilent diameter add together the 2 radii.
Say the oval pipe is 6" X 4". r1 = 3" + r2 = 2" D = 5"
6" X 3". r1 = 3" + r2 = 1.5 D = 4.5"
__________________
You can't solve you're problems with the same level of thinking that created the problems.
Albert Einstein
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02-22-2006, 10:07 AM
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#3
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Member
Trade:
sub-contractor
Join Date: Feb 2006
Posts: 87
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6" pipe is designed to carry 100 - 125 cfms in a properly sized system. 
oval or round makes no difference as long as it is not damaged or dented, and has a proper takeoff at the trunk line.
Last edited by Aceinstaller; 02-22-2006 at 10:10 AM.
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02-22-2006, 10:18 AM
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#4
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Member
Trade:
sub-contractor
Join Date: Feb 2006
Posts: 87
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to use a formula as teetor has shown, you would have to use the actual dimensions of the oval pipe. in which you'll find that the area of the interior of the pipe is exactly the same between 6" oval and 6" round. because 6" oval is like 7" x 3.5".
To make it very simple to explain to your shop buddies. Just take a small piece of unsnapped oval, lay it flat and when you measure it youll find that it is 18". then do the same with the round. For every 1" of diameter in a cirle, you have to add 3" to the circumference. hence the 18" for the 6" pipe. 
Last edited by Aceinstaller; 02-22-2006 at 10:22 AM.
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02-22-2006, 10:43 AM
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#5
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Pro
Trade:
Residential Contractor
Join Date: Feb 2004
Location: Jensen Beach, FL
Posts: 10,376
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I would stick with the area formula. To go to an extreme, take a small length of 6" and run over it with a truck. Your outside dimensions will remain the same while your AREA will be significantly reduced.
__________________
You can't solve you're problems with the same level of thinking that created the problems.
Albert Einstein
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02-22-2006, 11:04 AM
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#6
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Member
Trade:
sub-contractor
Join Date: Feb 2006
Posts: 87
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OK, OK,
I decided to grab the little duct sizing chart from the shelf just to be sure.
The engineers have done all of the formulas for us already:
Max. cfm through runout ductwork:
6" - supply=100cfm return=75cfm
3.25" x 10" stack(6" oval) - supply=100cfm retun=75cfm
These are the maximum allowable cfms through 6" runout ductwork for heat load calclation purposes from the Carrier installation handbook.
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02-22-2006, 01:27 PM
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#7
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Pro
Trade:
underground
Join Date: Oct 2004
Posts: 3,045
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So - if the oval pipe is 3.25" X 10" then r1 = 1.63" + r2 = 5" D = 6.63". Right?
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02-22-2006, 02:24 PM
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#8
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Member
Trade:
sub-contractor
Join Date: Feb 2006
Posts: 87
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By the formula given above, that would be correct.
but, it is possible that either the duct sizes given by manufacturers, for the purpose of sales, are rounded to the nearest whole number. or the formula is incorrect, because when I need to convert from oval to round, the pipes will fit togther tightly. therefore they have the same circumferance.
Although flattening a pipe completely would definately restrict flow, it is stated that the 6" oval stack is rated to have the same maximum cfm capacity as the 6" round. This probably has something to do with the static pressure. And the formulas involved with something like this have way too many variables to list.
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02-22-2006, 07:40 PM
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#9
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Pro
Trade:
Residential Contractor
Join Date: Feb 2004
Location: Jensen Beach, FL
Posts: 10,376
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Although flattening a pipe completely would definately restrict flow, it is stated that the 6" oval stack is rated to have the same maximum cfm capacity as the 6" round. This probably has something to do with the static pressure. And the formulas involved with something like this have way too many variables to list.
You are correct, more or less, on all points. At a given pressure a stricture results in an increase of velocity giving you approx. the same CFM. For HVAC work this is close enough.
Rich, are you baiting me? I didn't think that getting highly technical was appropriate here.
__________________
You can't solve you're problems with the same level of thinking that created the problems.
Albert Einstein
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02-22-2006, 08:51 PM
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#10
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Pro
Trade:
underground
Join Date: Oct 2004
Posts: 3,045
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Quote:
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Originally Posted by Teetorbilt
Rich, are you baiting me?
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You're like the old fish in the pond Steve - always sniffin' the bate before he bites. 
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03-05-2006, 03:55 PM
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#11
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Registered User
Trade:
General
Join Date: Mar 2006
Posts: 11
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Trane offers (Gratis) a CFM calculator... it's a round dual screen tool. Call trane and ask them for one... It allows you to calculate "Oval Ducting"... easy to use.
Pipe comes in schedules: 40/80/120/xxxheavy. Good Luck
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03-05-2006, 03:59 PM
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#12
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Home Improvement Guy
Trade:
Renovations contractor
Join Date: Dec 2005
Location: toronto,Canada
Posts: 1,470
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My old '69 Charger had an 850 c.f.m. Holley double pumper on it. Does this help?
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03-05-2006, 04:17 PM
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#13
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Registered User
Trade:
General
Join Date: Mar 2006
Posts: 11
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Quote:
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Originally Posted by LWF
Just to settle a bet here at the shop, does anyone know the CFM's that 6" oval pipe carries? Someone believes in only carries the CFM equivalent of 5" round pipe. Where could I find the proof. I have written Sealtite with no reply.
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Trane offers (Gratis) a CFM calculator... it's a round dual screen tool. Call trane and ask them for one... It allows you to calculate "Oval Ducting"... easy to use.
Pipe comes in schedules: 40/80/120/xxxheavy. Good Luck
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