duct sizing for range hood vent to exterior

[stel]

I'm an Electrical Engineer in need of some HVAC advice. I need to install some duct work to vent a large gas oven/range to the outside. The range calls for a 800 CFM blower. This is an inline blower to be installed in the attic space above the garage, which is adjacent to the kitchen. The blower housing has a 10" round intake and ouput. I need to connect the 10" round intake to a wall stack and then connect that to the hood housing through the wall. What is the minimum sized rectangular wall stack I can use to maintain the 800 CFM? If I use too small of a rectangular duct, if I remember my mechanical principles, I will have a higher air flow velocity in the smaller ducts. I'm not sure how this will change the required 800 CFM of airflow. According to my calculation (using diameter = 1.3(a*b)^0.625 / (a+b)^0.25), a standard 3" x 10" wall stack is equivalent to roughly a 6" diameter round duct. According to the manufacturer, they say I need a 3" x 14" to maintain the 800 CFM. Due to the size contrainsts of the location where the hood is installed under the cabinets, it will be impossible to maintain this sized ductwork. There will also be a couple of 90 degree turns in there and I'm not sure how that will affect the airflow. Hopefully this makes sense. Any advice would be appreciated.

 

[Drazen]

As your main concern is to maintain 800cfm, you need fan characteristic diagram to show you available total pressure difference i.e. working point for 800cfm.

On the other hand you simply must have some recommendation for needed velocity at hood outlet, that is necessary for proper operation of the hood.

Velocity at hood outlet and fan inlet gives you data for calculating dynamic pressure difference. Static pressure difference you must find by choosing some duct dimension and then checking pressure drop (let us say that you can be conservative and add 30% for ellbows if you feel really bored of this). Totaling static and dynamic pressure difference gives you total pressure difference, which must be what you have found on fan characteristic (if not, you can go back with different assumption for duct size).

I didn't want to overweight you, but there is no other way to do proper sizing!

I am a little more lucky when I need to size power cable for squirrel cage motor, isn't it...

 

[stel]

Thanks Drazen. The information you mention is not provided by the manufacturer in their specifications sheet. I'm assuming I simply need to maintain the equivalent of the 10" round diameter duct work which might require some cutting of the cabinet, which will make most of it unusable. This should keep the air velocity thought the ducts equal, correct?

 

[bman289]

stel,

In addition to the post from Drazen, you'll need to calculate the resistance in the duct system and see if your fan will move 800 cfm at that pressure. If your duct resistance is greater than the highest pressure of the fan, then you will not get your 800cfm. Additionally, 800 cfm is fine, but for a hood ventilation problem, you need the capture velocity for the hood. This may or may not be 800 cfm. Review a copy of Industrial Ventilation book and that will point you in the right direction.

bman289

 

[stevenw]

Cut the cabinet and run the duct through it. You will give up a lot of storage space but a vent hood that works is worth it. Maintain the equivalent of 10" round.

 

[racingdan]

Fan curve can be obtained from the Technical Support Dept of the manufacturer ( or request from local vendor).

Check overall pressure drop:

1. Using L/D of ~30 for 90 degree elbows of 10" diameter duct.

Two 90D elbows - 60 x 10/12 = 50 feet of equivalent straight duct. Add ~ 25 feet of actual 10" duct.
75 Feet

800 CFM in 10" duct = 0.34" / 100 feet
Losses might be about .75 x .34" = 0.26"

2. Add entrance losses.
3. Add exit losses.

A reasonable total pressure drop might be in the 0.4 to 0.5" range.

Check where the final calculated pressure point is on the Fan Curve.

I suspect the air flow from the kitchen vent will not be significantly affected if the transitions are formed properly.

 

[Drazen]

Static pressure difference that is related to pressure drop in ducts is dependant on duct size, number of ellbows (and other fittings if existing, not to overcomplicate now) and duct lenghts. (as bman helpfully additionally pointed).

I presume what you have from manufacturer is the duct size for "direct" exhaust, with negligible lenght of ducting assumed. I think the most practical solution is to call manufacturer (which I often do when dealing with electrical stuff, haha).

They will:
1. confirm assumption regarding duct lengts related to the size you mention
2. possibly offer you simplified calculation, that is - which duct size you need for 800 cfm WITH actual size of your ducting to be installed.

 

[tombmech]

That must be some special Electrical calculation up there to determine the cross-sectional area of round duct. ;-) The area of a circle is PI*r^2. So, a 10" duct is 78.5 square inches. There's no way that a 3" x 10" duct is the same - it's only 30 sq. in. I don't even understand why the mfr would recommended 3" x 14" - that's still only 42 sq. in.

Forget all the calcs. Just remember the principle: higher velocities mean more friction and pressure drop, which means less flow for the same size fan power. So, you should choose a dimension of rectangular duct that will equal or exceed 78.5 sq. in. in cross-sectional area, or your velocities will increase. If you add elbows or an unusual length, then it should exceed that area. 4" x 20", or 6" x 14" is more like it.

 

[stel]

Thanks to all for the advice. I sucked it up and did what stevenw suggested - hacked up my cabinets and used 10" round elbows and straights. Lost some storage space, but I didn't have time to try to find someplace to make custom rectangular ducts. 10" rounds ductwork is more readily available. Thanks again.

 

[tombmech]

Yep - sounds like you did the easiest, but most acceptable, solution.