Flex duct for grease hood exhaust 1

[trashcanman]

I have a project for grease exhaust from a hood serving electric griddles. It is an Army DFAC (Dining Facility) with a tent structure roof. When the wind blows, the roof moves up and down and sideways 6'+/-. Thus a flexible connection will be required in the duct which goes up to a new roof mounted fan. I don't like using the flex duct either, but see no way around it. Who makes such a connection and what length is required for a 20x20 or 20" dia. duct? The roof does have metal joists and rafters. Structural members will be added to mount the fan and anchor it to the roof.

 

[PatBethea]

The US Army has exhausted through the roof of tents for decades (centuries?). Surely there is a way to do it without flex duct. Talk to some service personnel about their experiences.

 

[MintJulep]

I hope you mean that the tent moves 6 inches, not 6 feet.

Put your flex between the canvas and the duct, don't try to make the duct the flexi bit.

 

[trashcanman]

My question was not how the Army did it.

Yes, the tent moves 6"+/- just as written.

 

[PatBethea]

Trashcanman,

I know what your question was and I don't think you are asking the right thing. I think you *should* ask how the Army does it. Like I said, they've been penetrating tent roofs for many years for cooking and heating purposes and I doubt they use any flex. For a grease hood exhaust, I think you can and should find a way to do it without flex duct. MintJulep has made a suggestion worth checking into. The place I would start, however, would be by looking into how it has been done historically and then evaluate things from there.

Also, you originally wrote 6'+/-, not 6"+/-, so to say "just as written" is wrong. Lighten up, man.

 

[gepman]

Perhaps you should use rigid duct and connect the tent to the duct with some type of flexible material. It has been a while since I have done a grease hood exhaust but I don't think that a flex duct would meet code at least here in California.

 

[KiwiMace]

Gepman, a star for common sense. Flex the duct indeed.

Also check fire safety precautions, so a duct or grease fire does not light the tent.

 

[CountOlaf]

CinciMace & gepman,

I'm all for giving stars, but didn't gepman just reiterate what MJ said earlier and Pat seconded?

 

[gepman]

CountOlaf
Take away my star!

You are right, after rereading what MintJulep wrote I agree I just repeated what he wrote. I am so used to the word "flex" being used for either "flexible duct" in HVAC or "flexible conduit" in electrical installations that when I quickly read his answer I thought his sentence stating "put your flex" meant that he wanted to relocate the flexible duct.

I think everybody agrees that flexible duct in this application is NOT a good idea.

 

[KiwiMace]

My bad. Props to MintJulep from whom I should have expected a quality response based on history, and reread.