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]

The backpressure per unit length of pipe depends on the volumetric flow rate. So as the gas cools the flow rate and the back pressure drop.

I can't get anywhere close to 3 kPa with 35 m^3/s and 450 mm pipe. 3 psi is more likely, say 20 kPa.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Balledone]

Hi Greg
This is the calc I used

Pipe Length inc bends allowance L 460 ft
Specific weight of gas S 0.029 lb/ft3
Exhaust gas flow Q 12600 cfm
Inside diameter of pipe D 17 inch

P (psi) = (L X S X Q2 / 5184 X D5)

Do you know if density influences back pressure?

 

[GregLocock]

As you can see from your equation the back pressure per unit length depends on Q^2. As the density increases Q drops, because the mass flow rate is the same (that is the mass of gas per second entering and leaving the exhaust is the same, ignoring condensate).

Your recent cfm numbers don't agree with your metric numbers, and the psi result is unbelievably low. I haven't checked your equation, L*Q^2*D^-5 is ok.







Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MikeHalloran]

>>> reliable isolating valve that guarantees 100% seal <<<
There ain't no such valve. ... not for any service, and especially not for engine exhaust.




Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

A poppet valve probably goes closest. I guess that's why they are used as exhaust valves,however they only need to hold pressure for avery short time.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[mtrider16]

I suppose this is an emergancy generator, so the valve will need to work automatically when the engine starts. I don't know of any that would be useful here.

How do the marine engines on a ship work, do they use individual exhasut for each engine?

David

 

[MikeHalloran]

I have done a few generator systems using high temperature rated (and damn expensive) ball valves to select between a barely submerged exhaust port for use underway, and a stack going all the way up the highest mast (to minimize soot on the yacht anchored alongside). All used wet mufflers to precool the exhaust gas. ... and two stacks for one engine is not the same as one stack for two engines, something I have never knowingly allowed.

I have worked on engines that use butterfly valves or plate valves (sort of like a swing check with an actuator) to 'stage' turbochargers. All were made from the very best materials available, and all were a constant maintenance headache, either sticking in position or leaking or both, virtually from the day they were installed.



Mike Halloran
Pembroke Pines, FL, USA