Long Exhaust System 10

[Balledone]

I am currently involved in putting together an exhaust system for a generator instal that has 2 x 30litre engines running into a single 450mm(18") pipe. This pipe has to run about 80m to clear the top of the building. The calculation on back pressure looks ok on paper. What happens to engine exhaust as it cools? Obviously it gets denser and the velocity will decrease. Will this have an effect on the pressure? i.e heavier gas harder to push? Any help would be appreciated

 

[GregLocock]

Excellent question. Buggered if I know. However if you get no useful answer I'll look it up, gut feel is that the numbers go your way rather than the reverse. How much higher is the outlet than the engine-the full 80m?

Cheers

Greg Locock


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[patprimmer]

If it does not net out to an acceptable number, could you insulate the pipe.

Regards
Pat
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[SomptingGuy]

The speed of sound will decrease with position too, so you may get the odd-sounding "cold rap" noise.

- Steve

 

[Lou Scannon]

If anything the cooling/reduction in velocity will reduce backpressure vs a constant temperature assumption. You may want to insulate anyway to reduce heat load inside the building.

 

[Compositepro]

I would guess that this pipe is running outside the building. There will be a "chimney effect". There will also be condensation to consider. You will definitely need a drain at the bottom. Until the pipe is hot all the way to the top you will get refluxing of condensate. You may need to insulate the pipe to keep it hot enough.

 

[JayMaechtlen]

- pipe outside - in winter?
Depending on locale, could the condensate freeze to the inside of the pipe and restrict the flow?
(imagining six inches of ice adhering to the inside of the pipe - then doing nasty things when it broke loose and fell!)


Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[L4189]

My experience with a 40 foot extension was that it actually reduced the back-pressure (chimney effect) but it is a double wall pipe without guy wires. You probably should use individual pipes for each engine or you could get cold back-flow down the walls.

 

[MikeHalloran]

NEVER NEVER NEVER NEVER NEVER NEVER NEVER NEVER NEVER NEVER NEVER NEVER EVER connect two engines to one exhaust pipe, not even if the shared length is very short.

Nasty parts of the exhaust gas from the running engine will condense in the engine that's not running, and destroy the cylinder bores, in a matter of hours.



Mike Halloran
Pembroke Pines, FL, USA

 

[Balledone]

Thanks for all of these thoughts.
The pipes are external and the stack outlet is about 55metres above the engine. The last vertical run is about 45m (11 storey building)
In regard to water, we understand we will be potentially dealing with 400 litres of water per hour so have made provision to drain this at a couple of points on the way.
In regard to the 2into 1, we are putting reverse flow protection on the intakes of the engines so if they are not running the exhaust cannot backflow.
Insulating the pipes for the full run would be a VERY costly exercise.
Can anyone comment on the effect of the denser gas being "harder" to push and increasing the back pressure???

 

[Compositepro]

In a horizontal pipe cooler exhaust will lower gas velocity and viscosity so you will have less pressure drop. In a vertical stack you will lose chimney effect which is probably more significant, depending on flow rate.

 

[BigClive]

I think the general answer is "no".
If you have a 45 metre long pipe it takes the same amount of work to pump air along it no matter what its orientation is.
Its hard for me to put into words but the amospheric pressure gradient is the same both inside and outside the pipe - so any pumping "uphill" effect cancels out.
Some years ago I was involved in a project where a submerged pump 20 feet down had to pump water to the surface of the water. There was much debate whether the pump would have to work against 20 ft. of "head". It turned out that it didn't have to pump against any "head".
Having said all the above - the hot exhaust gases would tend to rise under their own "steam" (so to speak) - so insulating the pipe to keep it warm might help.

 

[MikeHalloran]

>>>In regard to the 2into 1, we are putting reverse flow protection on the intakes of the engines so if they are not running the exhaust cannot backflow.<<<

I made no mention of backflow. The bad actor is condensation of acid and such from saturated gas onto cool surfaces. Preventing whatever backflow would otherwise exist might even make the problem worse.

Lemme see; 30 liter engines cost $100,000 or more per each, and even if you don't trash the entire engine, cylinder kits gotta run $10,000 each. An extra 80 meters of exhaust tubing, even pipe, has gotta be cheaper than trashing just one engine.


I am aware of one engine manufacturer who sold arrays of engines, with controls set up to bring them online singly with increasing load, alternating them for equal wear, etc., and used siamesed exhaust pipes in very neat, workmanlike installations, with nonreturn valves in the exhaust gas duct of each engine. Several generations of valves later, warranty costs forced them to require that the engine controls be hardwired so that all engines, or none, ran at once.

Don't believe me; ask your engine manufacturer to bless your 2 into 1 plan. Or just read the supporting data they provide.



Mike Halloran
Pembroke Pines, FL, USA

 

[Balledone]

Mike, I really appreciate your comments from experience. I was not entirely comfortable with the design, hence doing research in the form of this forum.
I understand what you are saying moslty, can you develop this a bit more
<<<<<The bad actor is condensation of acid and such from saturated gas onto cool surfaces. Preventing whatever backflow would otherwise exist might even make the problem worse.>>>>

If backflow is eliminanetd entirely and the connection between the 2 systems is post muffler and 5m from each engine, how does the concentrated gas get to condensated in the non-running engine?
The example you quote is very similar to what we are dealing with.

I am pushing this not to convince you but to get as much info as possible as I have to present this to a team.
Again your thoughts are most appreciated

 

[TheBlacksmith]

I used to work for a major engine manufacturer and the number one performance problem we saw in stationary installations was cobbled up, undersized exhaust systems. (Number two was a tossup between intake air and cooling air.) Definitely run separate pipes and consult the OEM; many have a calculator where you enter the engine/hp, total linear runs, number of bends, etc. and get a recommended minimum diameter. Did you know that one 90 degree bend has the resistance of up to 10 feet of straight pipe?

 

[Tmoose]

I would have bet the engine manufacturer has guidelines, at least for installations with more modest exhaust systems with shorter pipes.

Here is a link to a generator manufacturer's "INSTALLATION, OPERATION, and MAINTENANCE INSTRUCTIONS"

http://wincogen.com/winco_pdf_files/winco_aps5000_aps8000_opm.pdf

Included are recommendations for the correct pitch and drain locations to manage condensation, and pipe sizes for various length systems ( 50% bigger diameter when the length increases from 5' t0 50'). Also a reference to an NFPA document that may apply.

 

[MikeHalloran]

It may be online, or you may need to ask your engine dealer, but there is an 'application handbook' or 'installation handbook' or something of that sort for any OEM engine.

Somewhere in there the engine manufacturer will talk about exhaust systems, and will list things >not< to do; cross-connecting engine exhausts will be near the top of the list.

It will probably also appear as a line item in the list of stupid installation practices that will void the warranty.

Don't quote this forum as a source. Your engine distributor will probably be happy to send over an applications engineer or a service engineer to scream at the design team for going out of their way to guarantee an unsatisfactory installation.



Mike Halloran
Pembroke Pines, FL, USA

 

[chief]

Recommended exhaust gas velocity on a diesel stack is 35 -40 M/sec. Back press. allowance is 3kPa.
your exhaust trunk needs to be sized to stay withing these parameters.
As previously stated,never use a common exhaust in a multi-diesel installation.
If you need to clear a building or structure with the exhaust plume,look at LC.Eldridge exhust fan inducers to give you acceleration to a clear area.

Offshore Engineering&Design

 

[Balledone]

Hi Cheif,

Thanks for the info. The flow is about 35m/ sec and back pressure a theoretical 2kpa at the temp and volume leaving the turbo at full rating. As it travels it will cool, slow down and get heavier. Do you know if this will tend to increase back pressure by the denser gas being "harder" to push? The stack effect as mentioned earlier has a theoretical decrease of 15 pascals at 150c over the 45m stack height.

 

[Balledone]

Hi Mike

I appreciate your direct answers. I was aware of most of what you have said prior to asking the question. And yes the Cat and Cummins install manuals say don't do it. However, I am from New Zealand and we often are asking "why not" on the assumption that in getting to a full understanding of the problem, an answer may be found. Where I am at is a reliable isolating valve that guarantees 100% seal will maybe the solution. Know of any such valves?