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Diesel generator exhaust flue within a building

Diesel generator exhaust flue within a building

Diesel generator exhaust flue within a building

The question concerns the design of emergency generator placed inside a proposed building. The exhaust flue must be placed inside the walls extending up to the top of the building. Because of a long equivalent length of the exhaust flue(about 40-50 FT and bends), the calculated pressure loss exceeds that suggested for the generator. One solution is to increase the size of the flue by making it rectangular and long on one side instead of circular to reduce the friction loss. However, I read that the the velocity must exceed a certain velocity to keep the diesel residue from falling out. Another is to place an expensive fan at the top of the building to draw the exhaust gas out, as they do with hospitals. Are these the only options? The generator is 200 kw.

RE: Diesel generator exhaust flue within a building

I don't understand the question.
If you red-flag this post, and repost your question clearly with more background information, in the Engine and fuel engineering forum, you'll reach a much wider audience of experts.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Diesel generator exhaust flue within a building

I think you may have misunderstood what someone told you.

What I'm guessing was said is that the exhaust backpressure was too high.

That simply means the flue has to get bigger in diameter.

That usually means a whole new flue.

Search locally for people who make marine Diesel exhaust systems. They should be able to figure out how much bigger it has to be, and make and install it.

Mike Halloran
Pembroke Pines, FL, USA

RE: Diesel generator exhaust flue within a building

It does not necessarily mean that the entire flue has to be bigger, just a portion of the existing flue. It is a type of calculation that a ME can do. Also try minimizing the number of bends if possible.

RE: Diesel generator exhaust flue within a building

How are you going to deal with oil (and paint, and etc.) residue burn-off, ozone production from the generator, and other associated noxious fumes/gasses from the genset?

RE: Diesel generator exhaust flue within a building

If there's one area that is familiar with diesels inside buildings, it would be NFPA. I note that NFPA20 has a statement that the exhaust back pressure should not exceed the engine manufacturer's limits, but there is no statement about a maximum size / min velocity. I would think that if it was a concern, NFPA would have addressed it. Some notes on here from Clarke Fire on exhaust piping:


RE: Diesel generator exhaust flue within a building

Please forgive me if I'm off base, but I didn't think this was allowed:
Ron757...The exhaust flue must be placed inside the walls extending up to the top of the building..

All exhaust stacks that I've ever seen, regardless of the purpose or fuel, are run with air space around them, clearance to all walls, penetrate walls and ceilings at near-90 degree angles, and so on.
Embedding an exhaust stack within a wall sounds like trouble.


RE: Diesel generator exhaust flue within a building

The Clarke Fire link does not load for me.

Addressing the newly edited version of the OP,

DO NOT EVER use square or rectangular tube for Diesel exhaust; it is guaranteed to crack from fatigue, and very soon.

DO NOT EVER put an uninsulated exhaust pipe within a wall. You can expect the pipe temperature to reach 900F for a Diesel with a turbo, and a few hundred degF more for a Diesel without a turbo, and a few hundred degF more than that for an SI engine.

I am not aware of a minimum velocity that would keep Diesel soot from 'falling out'. It lines the pipe whether you like it or not, regardless of velocity. The thickness of the deposit seems to be self-limiting; I have not heard about anything similar to creosote fires in woodstove chimneys.

OTOH, I suppose a pipe fire could happen without being noticed, because Diesel exhausts commonly use something much sturdier than the tin that's used in stove chimneys. My former outfit typically used Schedule 5S or thicker SS pipe for terrestrial engine exhausts, and 14gage 316L for our cheapest marine exhausts.

Nothing prevents you from splitting the exhaust with a wye and running two (or more) pipes to the outside to meet geometric constraints.

I wouldn't put a fan in a Diesel exhaust. The ones that might survive cost a fortune, and an AHJ or the engine manufacturer might logically insist that you meet the backpressure spec with the fan turned off anyway.

I would put some sort of rain cap on a pipe that points up.
I'm not a big fan of flappers, but a 90deg+ elbow (large radius) should be acceptable.

Mike Halloran
Pembroke Pines, FL, USA

RE: Diesel generator exhaust flue within a building

Quote (hemi)

I don't understand the question.
...If you red-flag this post, and repost your question clearly with more background information...
When I posted this, the question was different from what is now the OP. It was very terse and garbled. Now the question makes perfect sense. Well done OP. thumbsup2

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Diesel generator exhaust flue within a building

Why not go horizontally to an exterior wall within the generator room itself? Solves lots of problems. In addition to what's already been said:

I couldn't tell you where, but I'd bet code requires the generator to be in its own room. Said room being being a perimeter room just makes sense for so many reasons.

Code required 10' separation only applies to outside air introduced to occupied areas, not when used for combustion. More separation, the better though.

I'd also bet the generator room is not on the front of the building, architects like to put these rooms toward the back where louvers and exhaust pipes coming out of the wall is more acceptable visually.

A roof penetration is a lot more likely to leak. Since the stack would have to be in a chase (not actually in the "walls"), a leak would go a lot longer before it is detected.

Have you considered noise transmission through the exhaust pipe? Another reason to keep it all within the generator room.

Absolute best solution would be to get the generator out of the building altogether. Admittedly, site space constraits may be preventing this. Either in a separate structure or they do make generator to sit outside.

RE: Diesel generator exhaust flue within a building

I had an install where heat inside the building was going to be an issue as well as exhaust discharge.
I worked with the architect and we had a partition built across one end of the generator room. This formed a chase about two feet by the width of the generator room.
This space or chase was extended to the top of the building.
The radiator (with an out-flow fan) was butted up to the partition so that the hot cooling air was discharged into the space.
At the top there was a horizontal discharge with substantial louvres.
The exhaust was run in this space.
The exhaust discharged upwards at about 45 degrees with a 60 degree cut-back so as to avoid rain entering the exhaust.
This gave us good separation from the exhaust stack to the walls and cooled the exhaust stack and any wall surfaces that may be heated by radiation from the exhaust stack.
An added benefit was that in the eventual event that the exhaust stack started to leak, the fumes would be carried safely out of the building.
Also, and fumes from the set, such as paint burn-off and crank-case ventilation fumes would tend to be drawn by the air-flow through the radiator and discharged outside the building.
Ps; The walls and partition were non-combustible construction.

"Why not the best?"
Jimmy Carter

RE: Diesel generator exhaust flue within a building

I agree with all previous posts and would like you to consider Building and Fire codes regarding your original idea.
An enclosed long vertical exhaust may also bring these problems:
* Leakage of CO gases into building (codes may require sustained negative pressure in the annular space between duct and walls).
* Increasing temperature inside wall shaft (codes may require sustained ventilation of the annular space between duct and walls).
* Corrosion due to decreasing temperature of the exhausted gasses (heat transfer into wall shaft).
* Condensation to be collected at bottom end of duct.
* Access doors along the wall shaft for installation, demolition, maintenance and inspection(codes may require fire rated doors).

RE: Diesel generator exhaust flue within a building

I have no direct experience in this particular area but I think it would be essential for the plenum to be at a negative pressure relative to the rest of the building, which requires a suction blower on the roof. It might also be a good idea to exhaust the the generator into a duct in the plenum which is also sucking air from the generator room so that the exhaust is diluted with plenty of cold air near the generator to prevent condensation or heat radiation problems, much like the air gap at the top of gas water heaters. This pretty much echos what Lnewgban suggests.

RE: Diesel generator exhaust flue within a building

I can see a need for negative pressure in the flue. Ventilation of the of the annular space between the flue and walls might be another option. For safety reasons, it makes sense. I need to look at the fire and building codes for this application. Thank you so much for your help in this. I don't have much experience in this and you responses are great.

RE: Diesel generator exhaust flue within a building

Excellent advice from dbill74; first choice is to get the noise, vibration, heat and smoke outside of the building and to a far corner of the property. To that end, you can buy packaged gensets in a weatherproof and vandal resistant enclosure, sitting atop a double wall tank of Diesel fuel, installed on a nice concrete pad.

If you must put the genset in the building, run the exhaust out horizontally to an exterior stack running up the building. Be aware that the generator requires a substantial flow of combustion air, and gives up a fair amount of heat to its surroundings, so an exterior wall needs to have louvers for air to flow in, and louvers and an exhaust fan for heated air to flow out.

I.e., putting the exhaust in a flue or chase is far from first choice, and locating the generator room entirely inside the building, without at least one exterior wall, is even worse.

The engine supplier certainly puts out application handbooks and installation handbooks, and probably more, nowadays on DVDs or downloadable files. The generator supplier should be able to vector you to the right information.

Mike Halloran
Pembroke Pines, FL, USA

RE: Diesel generator exhaust flue within a building

You should consult the 2015 edition of NFPA 91.

"God will not look you over for medals, degrees or diplomas, but for scars." - Elbert Hubbard

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