Exhaust heat recovery- Exhaust to water exchanger

[clytle374]

Any suggestions on where to get?  Or how to make?

[GuyFawkes]

1/ to be an efficient heat exchanger you need to maximise the area of heat exchanging surfaces, so thin and flat beats tubes.

2/ it is an INEVITABLE by product of reducing the temperature of the exhaust gas that you promote the formation of soot, so the more efficient your heat exchanger, the quicker it will soot up.

3/ if you know you are going to pull heat from the exhaust gas, you want the engine to produce as little soot potential as possible, so a clean fuel and maximised compression ratio, turbos are used as much for emissions as power in road vehicles, the turbo makes a good exhaust gas cooler, but the other side of the turbo changes the combustion process enough to make the exhaust gases less prone to sooting, vehicles with blown turbos aren't just down on power, they soot a lot.

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6/1 on full chat is 6 bhp, at approx 750 watts to the bhp that's 4500 watts, going by the rule of thirds you're getting 4500 watts or work, 4500 watts of heat to lube oil and coolant, and 4500 watts of heat in exhaust gas, those are MAX numbers, at 2 bhp load the numbers for the other two are different...

the same rate of fuel consumption in a good burner will produce 13500 watts of heat.

you are trying to pull heat out of 4500 watts of heat in exhaust gas, getting even half of it out will cost a lot of time, effort and money.

HEAT has always been piss easy to get, you can make a burner that will burn just about anything, even dirty waste engine oil, which is free.

turning HEAT into WORK has always been the engineering challenge.

put the same time, money and effort into making the HEAT into WORK stage, eg your engine and whatever it is driving, more efficient will pay you bigger dividends.

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the last exhaust gas heat recovery plant I saw had an actual operating efficiency of about 12% when the motor was at full load, and below 2% when it was on light loads, given the rule of thirds this is 4% of the heat energy in the fuel at full load, and less than 1% at light loads.

it's not that hard to boost overall fuel efficiency in an engine by 4%, because you're doing a specialist single application system, not a made for the market road vehicle that might find itself in a desert summer or arctic winter.

Flywheels have an energy conversion efficiency of better than 80%, often nearer 90%, so doubling your flywheel mass will smooth the loads on the motor and improve overall fuel efficiency for a given power output at fixed RPM,

everyone is playing with small high RPM flywheels, because they are trying to make compact stuff with high energy density, often for use in a vehicle, we, as stationary bods, don't have any of these limitations, and low speed flywheels only have rim speed to consider, windage and bearing losses are small.

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bottom line here is don't waste too much time and money chasing diminishing returns, and ignoring other areas where you really can make savings.

[MeanListerGreen]

I saw an interesteing heat exchanger on a website several years ago.  I don't remember which one, but there were 3 propane water heater tanks in series.  The Lister exhaust ran throught the center flu of each water heater tank.  The water was then linked between the three tanks.  The claim was the system worked very well.  I think I'll check with some plumbers to try and round up some of these tanks to give this a try.

[hotater]

The exhaust heat is easy to harvest for pre-heating bio fuels, but not worth the trouble for heating 'spaces'.  First off, the chance of an exhaust leak following heat into a living space is BAD.  The much more efficient way is by heating water, but the soot problem arises with any increase in extraction.

  Forget it and concentrate or the coolant loop.  That one is cheap to do, easy to transport, and safe to be around.

[mobile_bob]

i don't know what it is about this forum, or for that matter probably all forum's,
much like history, if one does not take time to study it (history or the forum) one is destined to have
to repeat many painful lessons.

As GuyF and Hotater have alluded to there just isn't enough heat energy available from a 6/1 to get all anal about
trying to harvest, while it is one thing to go after the coolant system heat (because of ease), it is quite another going after
the exhaust heat,,, save for maybe wrapping a preheat coil around it the header to preheat heavier grades of oil.

this topic (among others) has been researched and debated to death over at least the last year, but rather than
go back and pickup where the discussion left off, we as a group find it necessary to reinvent the wheel?

personally i have spent countless hours debating the merits of various subjects on this forum,
and i gotta tell ya all,,, it took me a long while to understand where folks like GuyF and Hotater were coming from.

they come from both education and experience!

for those of you that are convinced that they have missed the point,, by all means "march" on.

i just don't have the energy for it.

godbless both Hotater and GuyF for having the patience to continue to try to explain things to some of you folks.

(to clear the air a bit,, this post is a response to both this topic and the rubber mounting one.)

bob g

[hotater]

I thought the purpose of re-incarnation was to be redundant, again.   

[clytle374]

I remembered it being discussed but couldn't find the thread, too many refrences to exhaust and heat. The engine is a 16/1@1000rpm but running at ~750rpm so ~12Hp.
As usual compromises are part of life and in order to make other things more ideal, the exhaust is less than ideal.  To avoid exiting the exhaust between the house and garage, I need to run it 25' or go thought the roof, or exit low through the wall.  I have not found a solution I like for exiting through the roof. I'm not even considering using a exhaust to air exchanger due to the possibility of a leak.

The house is heated by a outdoor boiler (195F max)  with a line to be installed to the garage for heating and for a booster for the hot water heater. So i have a existant source of circulated water.   I plan to use the boiler water for cooling the motor but only when the boiler is off, to avoid heating the garage in the summer.   

SO my question really is.
Anyone see a problem with using a 2" exhaust pipe with a 3" water jacket on this 25' long run to recover some heat and minimize the chance of setting something on fire?

[hotater]

YES!  That's too long and too cool.  It would be better to have insulation around the exhaust to keep up the heat to keep it clean.

[MeanListerGreen]

I'd like to point out there was a guy on here claiming to have captured exhaust heat and was heating his house in, I think, Michigan and maintaining 60 degrees in winter.  He was using radiant baseboard heaters I believe.  I don't remember how he was capturing the exhaust heat though.  Maybe it is possible and worth looking into, again.

[clytle374]

YES!  That's too long and too cool.  It would be better to have insulation around the exhaust to keep up the heat to keep it clean.

Yeah I keep looking at the length thinking "damn that's way too far", but I can't find another good option.  I understand the length problem but what is the cool problem?  I mean if the exhaust gets cool, the exchanger is working good. What other than the carbon and soot problem?  I will be using a expansion tank and muffler, the first bend will be the expansion tank then into the muffler.  Then next would be a slow 90deg bend into the straight heat exchanger and through the wall.  It could be cleaned easily with a pipe cleaner on a drain snake.  What am I missing? 


 

[hotater]

It could be cleaned easily with a pipe cleaner on a drain snake.  What am I missing? 

You're missing *something* when you're playing chimney sweep.  It depends on how much work you want to do for a dollar's worth of 'free' heat.

I didn't move way out here to be a slave to my engine!  I have the engine so as not to be a slave to candlemaking.   

[clytle374]

You're missing *something* when you're playing chimney sweep.  It depends on how much work you want to do for a dollar's worth of 'free' heat.

I didn't move way out here to be a slave to my engine!  I have the engine so as not to be a slave to candlemaking.   

I hear ya on that.  I have heard the fact that you can flow more hot exhaust than cold, but have never understood it exactly.  Will back pressure be a problem?  With the expansion tank and muffler I think the resonances will be canceled enough not to effect performance, and can't imagine that the exhaust cooling and contracting would restrict the flow in a straight pipe to any measurable degree.   If performance wont be effected, I'm thinking I might try it.    I need the exhaust away from the house, and I don't trust insulating it, at least not enough too sleep very good.  I worked my way into a corner(literally) with the exhaust.     

Anyway I was checking prices for metal, When did stainless steal become a precious metal?? 

[hotater]

When did stainless steel become a precious metal?? 

When the idiots in Washington embargoed Rhodesia about 1971 and they had 80% of the world's chrome production.
The good thing about surplus, used, nasty, stainless steel??  It ain't rusty.     

Look for dairy and other food plant auctions.

[clytle374]

Good advise.  Setting here thinking a have probably handled millions of dollars in SS with all the food service equipment I have machined parts for.

So am I screwing my self on the exhaust?  If I use steel?  I guess it is spelled steel, maybe I was thinking the only way I could afford it.

[M61hops]

I'm intending to try to capture heat from the exaust of my 6/1 but haven't settled on exactly what I'm going to do yet.  Whenever you burn a fossil fuel (not sure about bio-diesel or waste vegoil) you make moisture and acids as some of the by-products.  If you have a long run of pipe that cools the exaust below the condensing tempeture of the water vapor you will rust out steel pipes in short order.  The manufacturers who make heaters make sure that the exaust gasses stay hot all the way to the outside of your house to keep the flue or whatever from coroding and sooting up.  I see lots of problems with trying to capture the heat from the exaust of my 6/1 and having any kind of reasonable maintenance and material times.  I'm thinking of making a type of water-lift muffler out of an old stainless steel pool filter that will trap most of the moisture, soot, oil and other crud and then route the gasses up through a stainless tube and shell heat exchanger.  I scrounged the parts or else this would never make economic sense and only time will tell if the heat gained is worth the maintainence hassel.  I'm hoping to scrub the smell as well as the heat out of the exaust.  Getting rid of the 5 to 20 gallons toxic waste water left in the filter tank will be a hassel but I think I can evaporate it down to a very small volume.  I'm just getting started putting all the pieces of my gen set together so I won't know how well it works for a long time yet, probably a lot of work for what is gained but I do this kind of thing for fun.