Exhaust Heat Exchanger

[br549]

Was wondering about how to build an exhaust heat exchanger that might be simple and easy to clean.  I have some 4" square tubing lying around and thought about putting a flat plate heat exchanger inside the tubing with four holes in the side for the ports of the heat exchanger and fashion an inlet toward the bottom of the tubing that will blow hot exhaust directly on the exterior of the heat exchanger.  I believe the ports on the heat exchanger could be tig welded to the square tubing.  Any ideas on how well this might transfer heat?
Thanks.

[Ronmar]

Not very well at all...  All the Surface area of a brazed flat plate heatex is on the stacked plates inside.  The fluid contact area with the outside case is very small.  They transfer very little energy to/from the outside case.  You would do better with a pipe or two inside the box to collect some heat, but again not a lot of transfer area.  There is a reason traditional heat exchangers have all those little tubes packed closely together, it is to increase heat transfer surface area. Unfortunately those little tubes are hard to clean/decarbon in an exhaust heatex application.

[br549]

Thanks Ronmar for the advice.  I think I'll go ahead and spend my energy somewhere else.  It seems that extracting heat from the exhaust isn't as easy as I thought it would be.  Hopefully somebody will come up with something that is simple and effective.  It would be nice after say, a four hour run to be able to generate enough hot water from my little air cooled five horse, for a couple of hot showers. 
Thanks.

[WStayton]

br549:

  If you want something quick and dirty, you might look at the design of a marine engine wet exhaust manifold.  They're just basically a single "tube" (the real exhaust manifold) run through a "shell" (the exterior housing of the whole appratus), with the space between the two filled with outflowing water.  I don't think that they are particularly efficient, since their objective was to keep the exterior of the apparatus at a temperature where they weren't apt to have a fire problem, so they are probably only capturing half of the heat, or maybe less, that you could capture and still have enough heat in the exhaust to not have condensation problems.  Somebody, somewhere, must have made some wet exhaust manifolds for Listers to be used in marine service, no?  If so, its quite possible that somebody already invented/solved this problem and if you could find one, you would be good to go.  Though, having said that, it occurs to me that many commercial type applications use just plain old bare cast iron exhaust manifolds and dump the exhaust up, like through a stack, or even a mast, to get rid of it without having to have a hole out the back of the boat to create a potential leak.  I guess if you spend a lot of time making a boat tight and leakproof, it IS kinda dumb  to the put a giant hole in it! <smile>

  Just a thought, and, like many of mine, worth about what you paid for it! <grin>

Regardz,

Wayne Stayton