How do you determine the proper duct size for a fan duct?

[mtumbleson]

Does anyone know of an “on line” calculator I can use to determine the proper duct size to use with my draft fan in my oven (or just clue me in). I don’t know anything about air hydraulics, but I do know it’s important to get the duct size correct in order for the fan to operate properly.

I think I’ve come up with a slick idea for drafting the hot air from the top of the oven then depositing the air in the bottom over the elements. At this point I think I will mount my fan at the top of the oven. The inlet opening will be the same size as the opening in the fan, only having to travel 4 inches (The thickness of the wall).

It will circulate through the fan then into a duct made from sheet metal running down the inside of the oven. It will be reheated on its way down the duct before it is exhausted at the bottom.

I have a fan rated for this type of application I just need to determine the duct size. Can anyone clue me in on how to get this right?

Thanks

[JTW37]

I did some searching on the net to see if I could locate a calculator for this. None so far. BUT, I know of a place locally that I have dealt with for venting out dust from my sand blast cabinet and he was pretty sharp about what it takes, fan size and such to move air. So maybe he will be able to tell me something.

What is the size of your oven and the cfm/volume of your fan?

He did tell me this. If your air movement is to great, all you have to do is, close down the size of the intake and it will slow down the fan to lower the amount of air movement. What he showed me with the fan running was to take a piece of card stock and he held it over the intake to prove his point.

Don't know if this would be of any help, but if you think it will, let know and I will see what he says. And maybe he has some number and formula he can supply me with to figure this out. If I get anything worth while, I'll post it here for everyone.

[JTW37]

Was still searching and ran across this. I'm okay with numbers but not sure if this would help to figure it out. It is mainly for Industrial fans and ducting. But take a look.

http://www.tombling.com/cooling/industrial-fan.htm

[nuttyman]

if you have a strong fan you can always wire a trim-pot after the switch or just use a trim-pot switch. my fan is rated for ~ 200 cfm & my oven is 96 cu. ft.

[pgwns]

The only thing I know about ducting I found out when installing my kitchen fan and wanted to discharge outside. The ventilation company told me that when using corrugated ducting the actual flow is half of the duct diameter. In other words 6" corrugated ducting has the same flow as 3" smooth ducting. This has to do with turbulence created by the irregular surface in the corrugated ducting.
I also changed my bathroom duct to smooth ducting and noticed a significant change in air movement.

[Popeye]

Duct size will depend on the Cfm and Static pressure you want to maintain. If you've got the specs on the fan I can size the duct for you.

[mtumbleson]

Thanks for that info! I was concidering using the corrugated tin because that is all I can find at Home Depot. I'm trying to avoid the local sheet metal supply, they are almost 300% higher on their prices than they should! (compaired to a supplier 100 miles away)

[mtumbleson]

I'm not sure what all you need, but this is a start.

* My inlet duct will be about 3.5" in length. (Just the thickness of the oven wall)

* There will be (3) 90 degree turns after the air leaves the fan. All three will have smoth radius bends. The FIRST one will be at the exit port on the fan. the SECOND will be 3.5" from the first. The THIRD will be about 6' from the second bend.

* The fan is 216 CFM @ 0. 0SP (I don't have any idea what "@ 0. OSP" is but that was the info on the fan I purchased)

* My oven is 92 cubic feet

* I'm moving the air from the top of the oven to the bottom. Oven is about 7.5 ft. inside.

Thanks for your help.

[Popeye]

The fan is rated 216 Cfm at 0 Static Pressure.. which means no restrictions. Depending on the style of the.. as the Static Pressure increases the CFM decreases. Static Pressure is a way of measuring the restrictions (air friction)
of a duct system.
There should be more info on the fan or in the documentation that came with the fan. What style blade does it have?
Some fans aren't designed to handle much Static Pressure and they flat line. Corrugated pipe and sharp bends are air flow killers.. 6' of corrugated pipe is the equivalent of about 75' of pipe.. regular elbows are the same as 20' of pipe

216 CFM at .02 SP needs 11" pipe or 15/6 ductwork to maintain Cfm

Any other info on the fan would be helpful

[mtumbleson]

http://www.electricmotorwarehouse.co...c940_small.jpg

http://www.electricmotorwarehouse.co...dimensions.htm

Does this help? It's all the info that is listed..