wet vs dry exhaust

FF, I was only asking Reefhunter's opinion on E/R temps with wet vs. dry, not assuming a wet exhaust system would create a cooler E/R.* In fact, in all physical world's other than RickB's special universe, the laws of thermodynamics determine how hot the E/R will be for a given set of conditions.* In fairness to RickB, if you sabotage the wet exhaust, the heat that would have been transferred out will in part be radiated in and the E/R temps will rise.* But in a* system not designed by an idiot or subject to sabotage by Rick, since the only propulsion source of heat is the engine itself, the exhaust system is immaterial to how hot the engine room gets.

Where I live and boat Heat is a good thing. I have to dodge ice most of the year. I have a dry stack the stack runs right up thru the head. Makes it nice and toastie when nature calls.

SD

"... since the only propulsion source of heat is the engine itself, the exhaust system is immaterial to how hot the engine room gets."


I think you'd better check for an exhaust leak on your boat.

When we bought the Eagle it was dry exhaust and very loud and hot.* We converted to wet exhaust and heat wrapped from the engine manifold to the raw water connection.* The wet made the engine room, and salon cooler and quieter.*


*
The important thing is the engine manifold is not raw water cooled, not what comes out of the tail pipe.

Phil Fill wrote:

*
When we bought the Eagle it was dry exhaust and very loud and hot.We converted to wet exhaust and heat wrapped from the engine manifold to the raw water connection.The wet made the engine room, and salon cooler and quieter.
*


The important thing is the engine manifold is not raw water cooled, not what comes out of the tail pipe.

*

Click to expand...

Phil, Could you post some pics of your engine room? So we can see your set up? It sounds like it was an interesting project.

Thanks,
John

*


-- Edited by Gamekeeper on Monday 3rd of May 2010 12:30:28 PM

RickB wrote:

"... since the only propulsion source of heat is the engine itself, the exhaust system is immaterial to how hot the engine room gets."


I think you'd better check for an exhaust leak on your boat.

Click to expand...

So some questioning assumptions from someone who admits to not knowing much about this topic.

An engine generates heat when it's running because of the internal heat of combustion and that heat's distribution thorughout the engine block from being transmitted via the metal, coolant cirulation, etc.* Right, wrong?

The temperature of the exhaust at a given power*setting and load*leaving the cylinders is going to be "x" regardless of what that exhaust gets fed into.* Right, wrong?

The exhaust manifold is going to radiate heat into the engine room.* Right, wrong?

If the exhaust manifold is water cooled, it will radiate less heat into the engine room than a similar but uncooled manifold.* Right, wrong?

If you have a non-water-cooled exhaust manifold and a non-water-cooled muffler and exhaust pipe, those components will radiate heat more heat into the engine room than the same components that are water-cooled.* Right, wrong?

The variable in the previous statement is the insulation on the dry*exhaust manifold, muffler, and exhaust hose/pipe runs.* Right, wrong?

If my assumptions are correct, logic would dictate there are two heat sources in the engine room, not just one.* The engine itself-- a big block of hot metal--- and the exhaust components.* So the type and construction of the exhaust system would seem to have a major effect on heat being radiated from the components.

All of which would lead me to believe that the exhaust system has a definite influence on the amount of*heat in the engine room.



*


-- Edited by Marin on Monday 3rd of May 2010 02:59:32 PM

Marin, all marine diesels I have seen lately have water cooled exhaust manifolds, whether they have wet or dry exhaust.* Open the hood of your Oldsmobile after a few miles and put our hand on its exhaust manifold and you'll understand why.* My CAT is cooled with coolant, yours might be cooled with sea water.* If sea water, you get the extra fun of hot salt water in your engine, zincs, lowered efficiency, etc., which is one of the reasons commercials usually go dry.* If by manifold you meant the pipe aft of the manifold(s) and turbo (if any), then that is either cooled with water with a wet muffled exhaust, or insulated if a dry.* Either way, there isn't a material difference in heat radiation between the two.* As you note, the source of heat is the engine, not the exhaust pipe.

The exhaust manifold on an FL120 is cooled with the engine coolant. I did not realize that the exhaust manifolds on dry-stack engines are always liquid-cooled, too. Also, while I know there is insualtion around the dry pipes and mufflers of a dry exhaust I assumed that was just so people wouldn't get burned on them, not to reduce the heat in the engine room. Thanks for the correction.

Marin, I don't know if all marine diesels have cooled manifolds - I just haven't seen any that aren't because the temperatures would get pretty toasty, as in glow in the dark, 3rd degree burn light your boat on fire toasty just like an automotive engine.* The wrap on a dry stack does keep you from getting burned, but it also serves to nearly eliminate E/R radiant heating.

"I just haven't seen any that aren't because the temperatures would get pretty toasty, as in glow in the dark, 3rd degree burn light your boat on fire toasty just like an automotive engine. '

Sorry but the exhaust temps of a gas engine are way (400-500F) above a good working diesel.

Most diesels will run 700 deg F or so , a far far distance from glowing in the dark.

Marin wrote:

I did not realize that the exhaust manifolds on dry-stack engines are always liquid-cooled, too. Also, while I know there is insualtion around the dry pipes and mufflers of a dry exhaust I assumed that was just so people wouldn't get burned on them, not to reduce the heat in the engine room. Thanks for the correction.

Click to expand...

Only the smaller industrial and automotive conversions use water cooled manifolds. Larger engines, especially turbo'ed versions use insulated exhaust lines. Heat in the exhaust is too valuable to throw away before it goes to work in the turbo.

The rules call for insulation to reduce the temperature of exposed piping to <180*F for crew safety and to reduce the fire hazard created by things like fuel and hydraulic leaks, and poorly stored items. The radiant heat from an exhaust can be as damaging as an actual fire and will start a real one rapidly.

You will notice that a wet exhaust below the spray ring does not require any special treatment. That is because adding water to the exhaust cools it below the level it can do any damage. If water is lost, the exhaust will burn and it will burn anything around it. This trashes the silly idea that all the heat in an engine room comes from the tiny amount radiated by the block itself. Go back and read the heat balance figures I posted earlier to see where the heat goes.

Ridlyme and proper design pretty well deal with any issues arising from seawater cooling of the exhaust manifold. My manifold temps (measured with IR gun) run about 20 degrees F cooler than the engine. Measuring these temps and others around an engine room are great for detecting a variety of early warning signs. As an aside, I am routinely comparing seawater inlet temps to PSS shaft log temps. There should be no more than a few degrees difference.

I*lament the fact that that my seawater cooled manifolds will outlive my remaining boating years. Is gin a good human consumption substitute for Ridlyme?? Hmmmm.

RickB wrote:Only the smaller industrial and automotive conversions use water cooled manifolds.

Click to expand...

John Deere describes their entire marine line in the Features and Benefits section of their data sheets as having Water Cooled Manifolds.* CAT indicates that most all of their engines this side of 1,500 hp as having either air shielded, water cooled exhaust manifolds, or like my 3306B as having water cooled manifolds.* The same can be said of Volvo, etc., etc.*

I'm not sure what your comment means, but it doesn't seem to map to the way marine diesels are manufactured and sold.



*

Sunchaser, your post actually highlights one of the engineering concerns of any cooled manifold, whether sea water or coolant, which is that it actually isn't optimum for engine efficiency for the exhaust to be anything other than blistering hot. I assume that's why no one cools the exhaust when safety can be maintained without the cooling.

Delfin wrote:I'm not sure what your comment means ...

Click to expand...

********* Obviously.

Delfin wrote:... *it actually isn't optimum for engine efficiency for the exhaust to be anything other than blistering hot.

Click to expand...

A "blistering hot" exhaust is a sign of inefficiency. The most efficient engine will have the coolest exhaust.

There is nothing to be gained by conserving any heat beyond the turbocharger or heat recovery devices other than that required to prevent condensation of moisture in the exhaust stack of a dry stack system.

RickB wrote:Delfin wrote:I'm not sure what your comment means ...

Click to expand...

Obviously.Actually Rick, I was attempting to be polite. I did understand what you wrote, so let me be clearer in what I meant.* Your statement that "Only the smaller industrial and automotive conversions use water cooled manifolds" would only be made by a lawnmower mechanic who retired in 1967.* It is contradicted by the manufacturers of these engines.* My suggestion is a subscription to boatdiesel.com.* They have a wonderful section that includes the data sheets on virtually every marine diesel engine made, and you can learn a lot about diesel engines there.*

*

Delfin wrote: Actually Rick, I was attempting to be polite.

Well, it was another failed attempt. Anything under the 1500 hp or so that you mentioned is a marinized design of an industrial engine or it is an automotive or industrial engine produced for the microscopic marine market.

It is painfully obvious that*your knowledge, such as it is, is limited to your trawler and similar boats but those boats*don't reflect the marine industry and marine propulsion in general. They are just the limit of your small universe and the generalizations you understand to be true for your limited experience don't "hold water" everywhere.

Click to expand...

*


-- Edited by RickB on Tuesday 4th of May 2010 08:55:37 AM

-- Edited by RickB on Tuesday 4th of May 2010 08:59:14 AM

Quite correct, Rick.* I will try to limit my comments to the small universe of those sites interested in trawlers and the type of marine diesel engines that are used in those trawlers.* You won't find me posting on any sites dedicated to oil tankers, diesel locomotives, gas turbines or pebble bed reactors.* Nosirree.

Delfin wrote:You won't find me posting on any sites dedicated to oil tankers, diesel locomotives, gas turbines or pebble bed reactors.*

Click to expand...

That is probably a good decision on your part. Since you don't understand the concepts which underpin the operation of the engine in your own boat, you would probably get*laughed*off those other sites pretty quickly.

If you weren't such a twit*you could probably learn something though.

*

Aww for crying out loud, would you two call a truce? I think you both have plenty of knowledgeable information to offer, and it seems to me that your arguing about two different classes of engines, and your both correct.* I think we all enjoy a spirited and RESPECTFULL discussion.* It's very possible to discuss, disagree, and still remain civil to each other.* I find both of you to be very knowledgeable, informative and entertaining, but cringe when things degenerate to name calling.** This is not a contest to see who is smarter, it's a free flow of ideas.* Some may hold more water than others, but thats how we all learn.* Feel free to disagree, but do it respectfully.* Keep up the postings though, we all stand to gain as NO ONE knows it all.

Arctic Traveller

Amen.

Arctic Traveller wrote:

It's very possible to discuss, disagree, and still remain civil ...

Click to expand...

Marinized forklift engines or 100,000 hp two strokes, the thermodynamics are the same, and the Earth isn't flat. There really isn't much to disagree with. Carnot figured it out, Diesel made it work and anyone who tries to argue otherwise is still a twit.

*

Well, whatever - on Penta as originally built (as a commercial fish boat) the exhaust (dry) consisted of a pair of 45 degree elbows from the manifold to the vertical stack which ran up the center of the aft wheelhouse cabin with the muffler above the cabin top. When I converted her I changed this system to the following; from the manifold (water cooled) through a "flex joint" to a short length of 3" steel pipe to a "Mac Muffler", then more 3" pipe to the port aft corner of the extended main cabin, to the vertical via 2 - 45's and up through the cabin top. The vertical run through the cabin is enclosed in sections of domestic stainless steel insulated chimney pipe that has an inner diameter of 8" which leaves a lot of air space around the pipe. From the manifold to the vertical run the system is wrapped in lagging (4 layers) so that the majority of the heat will stay in the pipe.
While I admittedly have not had a chance to run with this new system it has to be better than the old one which incidentally did not radiate all that much heat, sure the stack got hot but never enough to cause concern of a fire starting and the engine room here in the PNW was never that warm that it was uncomfortable to work in there while running.
The chimney pipe I am using is rated for zero clearance to combustibles with an internal gas temperature of 1400 degree F. so there will not be a dangerous heat problem within the cabin.
I have seen this sort of system in many wooden vessels here in this area as well as on the Great Lakes and none of them seem to have had any problems with excessive heat from the system and it sure is a lot simpler, coupled with a keel cooler, than what a "wet" system would be.
Just one goofy Canucks way of doing things !!
John Tones
Penta
Sidney, BC

Penta, since you have a water cooled exhaust manifold, yours must be one of those oddball marine engines less than 1,500 hp, like mine.* But even putt putt motors like ours have pretty big mufflers.* Did you wrap yours as well as the pipe and does it get in the way?

Delfin, et-al

Well for one thing my engine is a completely, specially designed marine engine from back in time when they did things like that. It is a Volvo, Model MD-47 - Series "Starke"
It produces 85 HP at 2500 RPM, natural intake and built in the late '60s and currently has just over 5000 Hrs since a total rebuild.
The exhaust run including the muffler is under the main cabin floor and is entirely wrapped in lagging. I built an insulated "duct" that the system runs through just to keep the air flow confined to the engine room and the exhuast "duct" so that I would have the maximum air flow up the insulated stack.
All this is in what you might call excess space and does not intrude in the engine room.
The way the boat is set up the tanks, fresh water and holding, are mid ships directly aft of the engine room and this exhaust system runs outboard, port side to the tanks, in what would be somewhat unusable storage as it is under the floor below the dinette.
I hope this all make sence to you but if you want I can try taking some pictures of it and send them to you.

John, Penta

Sounds nicely done, Penta.