Exhaust gas back pressure of a reciprocating engine CHP and free discharge lost

[YellowPencil]

The maximun back pressure of a reciprocating CHP engine exhaust gas is 400 mmWG. To calculate the pressure drop of the whole exhaust system, I have taken into account: elbows,straight sections, dampers, diverter,tee, branches, silencers, heat exchanger, gradual expansion, free discharge.
But do I need to consider free discharge of the exhaust gas into the atmosphere? The resistance coefficient of the free discharge will be Kdischarge=1, that means that all the velocity of the exhaust gas is lost, but, the 400 mmWG of back pressure considers also the energy lost in the discharge(dynamic pressure) or just the other elements?

 

[Latexman]

I've seen a lot of sources in the relief device and venting area that bake the exit velocity loss into their correlations/graphs/etc. Since the exhaust must exit to atmosphere at some point, it seems reasonable that it is included in the 400 mmWG. Someone with more experience in this may know and may reply, but it seems only the source of the data can answer for sure whether it was baked in or not.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[YellowPencil]

Thanks Latexman. You are right, the answer is in the manufacturer, but I was hoping someone would have experience in that. Taking into account the free discharge, it would add +20mmWG pressure drop (no little thing).

 

[saplanti]

Why don't you calculate the total pressure drop in the exhaust system? This way you should not exceed the allocated maximum back pressure, this limit is given to you for the exhaust piping arrangement pressure loss to consider, I believe.

 

[YellowPencil]

Well, I had. The total calculate pressure drop is 395 mmWG, including free discharge. I was considering reducing the final stack diameter, but this will increase the free discharge pressure drop. Thats why I would like to know if I must consider the free discharge in the 400mmWG back pressure data or not.

 

[LittleInch]

Do you need to consider it - Yes I believe you do given that you've broken down the system into it's respective parts.

5% shouldn't be a massive issue and I would have thought items such as silencers and heat exchangers represent larger uncertainties to gas flow as temperature and hence density changes along the exhaust.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[saplanti]

Yes, It was given for a reason. It may probably effect the engine efficiency or work load. If you must use lower diameter pipe you need to get engine manufacturers' approval or change the configuration with/without pipe diameter to reduce the loss.

 

[MikeHalloran]

You need to model all the losses, and 5mmWG is a breathtakingly small margin for error.

I don't do it much these days, but I worked on the exhaust of 457 yachts so far, counting overnight hot spot sprayers as one boat, and counting fleets of complete exhaust systems as one boat. Most of the boats were between 35 and 250 feet OAL, mostly Diesels, majority turbocharged.

Of those, I had to calculate backpressure for several hundred, and then measure the backpressure at the turbo flange at WOT under load, and have factory trained mechanics witness or certify the backpressure.

All of the engine manufacturers with whom I dealt would refuse to issue a warranty for any engine that could not meet backpressure spec by actual measurement.

According to my simple models, using ASHRAE duct models with specified exhaust flow and temperature, the biggest contributions to pressure drop are the pipes with the smallest lumens, usually the first two elbows right downstream of the turbo flange. Those are also the places where upsizing gives you the best bang for the buck.


At least you don't have to deal with 'wet' exhausts, where too much engine cooling water dumped in the exhaust magnifies the backpressure, or with boats, where the actual exit to atmosphere may be underwater.




Mike Halloran
Corinth, NY, USA