Question About Duct Airflow

[jyl]

Question for the aerodynamicists

Installed a commercial upblast fan on my exterior wall.
Fan "throat" is 8" diameter.
Duct from hood is 10"x10".
Stud spacing is 8" (?) so absent more work than I'm willing to do, I need to use a 10"x10" to 8" round reducer to get through the wall.

Q: should I . . .

Choice A: end the 8" duct at the wall exterior, so the air goes from 10"x10" duct to 8" duct to large space between duct and throat to 8" throat?

Choice B: extend the 8" round duct to meet the fan throat, so the air goes from 10"x10" duct to 8" duct to 8" throat?



I was thinking Choice B will have less turbulence, even if the air is confined in the 8" duct for an extra six inches or so compared to Choice A. I was also thinking it might make no practical difference so i should do whatever is easier.

[jyl]

This is what I mean by fan throat.

[908/930]

I would bring it right up tight, should help to keep it clean. Option B

[Scott Douglas]

From what I recall reading about air flow in the past, I think the 10x10 reducer to 8 round shape will have more influence on the whole system than the other side of the wall will.
I could be wrong about this, but that's what I recall.
Smooth walls without interruptions, ie corners, would be what I would try to achieve. And I'd go with Choice B for the fan side of the wall too.

[BK911]

Definitely option b.
Way less turbulence.

[908/930]

Not sure how to ask this, how much smoke do you need to extract? Any chance a 8" fan is available? Do you intend to control the speed of the fan? If that is a 110V ac motor they tend to hum quite loud as you try to slow them down, not a problem with DC or 3 phase AC.

[A930Rocket]

Going from 10 inch to 8 inch through the wall, you’ve created a Venturi effect! Those stinky kitchen smells will be blasted out of there!

Just kidding.

[island911]

I don't expect turbulence to be a problem with either solution.

I would be concerned about rodents getting through and that it is designed for roof mount - being that vertical mount has little side-loading on the bearings, it will likely have bearings designed for minimal side loads. OTOH, I doubt you will be using it as much as a commercial space.

[jyl]

The fan is already installed, so I have to live with it. Captive Aire DU33, 1/3 HP motor. 11 3/4” fan.

It is spec’d and configured for side blast installation. Rodents - hadn’t thought of that, but the fan is pretty high up with no trees etc to jump from. Yes, it will barely get used, compared to the commercial applications it is designed for.

Continuing Thread: What Are You Fix-It'ing Today?

Hoping to get up to 1000 cfm but the hood has a big capture area so I think it will be ok even if I don’t. Realistically I’ll very seldom need lots of air flow, but every now and then I’ll need to, let’s say, sear steaks for twelve people all at once or something equally smoky.

That’s a bummer about the humming, but hopefully it won’t be audible inside the kitchen. I think if necessary I could resort to a VFD?

[otto_kretschmer]

Better figure out what the Reynolds number is it going to be before you do anything.

Just kidding, just slap it together either way. If this was a submarine you would be concerned with noise but this looks like an exhaust fan for a kitchen. You will run it for a short period and then turn it off.

[island911]

From their website: "Units up to 24" nominal wheel can be wall mounted"
so looks like you are good there.

[island911]

Originally Posted by otto_kretschmer

Better figure out what the Reynolds number is it going to be before you do anything.

Just kidding, just slap it together either way. If this was a submarine you would be concerned with noise but this looks like an exhaust fan for a kitchen. You will run it for a short period and then turn it off.

Yep, and that thing won't need to be on long to move a kitchen worth of air.

[908/930]

JYI, If the fan is a single phase motor (most likely) they are not really intended to be run with a VFD, they really require a 3 phase motor.

[jyl]

ah so

i’ll live with humming i guess

[908/930]

Hopefully you won't be able to hear it. That is a lot of air you are moving. They do make that fan with a ECM motor and controller, any chance that is the one you have?

[jyl]

no

I went to the Captive Aire sales office, with a sketch of my layout and the model of my hood (also a Captive Aire) and they chose the fan for me. I guess I should have done more study :-(

[jyl]

This is the hood, a 4’ Captive Aire. herroberst helped me lift it up, we had to use a chain hoist. I haven’t figured out how to fill the gap between hood and ceiling, but will probably have a metalshop make a, not sure what to call it, a wraparound stainless sheet piece that I’ll bolt to stringers on the ceiling. I may have them include a hatch with hinged or bolted panel, to access the area above the hood. Right now some speakers are sitting up there.

Eventually I may cut a hole in the floor under the range, for makeup air through a duct through the basement. I figure if the air can come up, around the hot range, and up to the hood, it might not need to be heated. This 113 y/o house is so leaky that makeup air may not be needed, but it’s in the plan.

[jyl]

Here is the duct run. It is in a false ceiling of the side stairwell, and is pretty short. There is a 12”x10”xI forget” box above the hood with a 12” (?) collar that fits on the hood outlet, a 90 deg elbow 10”x10” comes out the side of that box (visible in the pic), a 35” straight run 10”x10” to the exterior wall, the 10”x10” to 8” round reducer, then about a foot of 8” round that goes to the 8” fan throat. The Captive Aire guy calculated the pressure drop and selected the fan. He didn’t know about the reducer, because I didn’t know about it then. Hopefully it will all still work.

[jyl]

Following up. The airflow seems very adequate. I can't measure cfm, but smoke from a candle held a few inches outside the hood, level with the top of the hood opening, is pulled down and into the hood. Hmm, maybe I could measure the speed of the smoke and guess at cfm. The airflow is enough to suck paper towels firmly against the filters. It is not silent - the moving air is a distinct but not unpleasant whoosh when the fan is on max rpm, and any motor hum is not bothersome (or clearly audible, to me) over the air noise at any rpm. I guess I'll call this good enough.