Designing the SIMPLE DIY Exhaust Heat Exchanger

[Horsepoor]

There several threats on the forum related to shell and tube exhaust heat exchangers and the associated soot / fouling problems. Extracting heat from the cooling system, is easy, well defined, and not a consideration here. Thinking about an exhaust heat exchanger and wanting to have an easy to build, easy to maintain, simple enough for a DIYer to build anywhere in the world, I began thinking about this concept.

Executive Summary: Crude concrete box with soapy water inside, exhaust gas flows through the soapy water from bottom to top, heat exchanged to a closed system copper coil submerged in the soapy water, wrap concrete box with insulation, install drain in bottom to change soapy water every couple days and clean out all the crap that will accumlate.

Concept:
•   Creating a small concrete rectangular box container, about 3 inches thick, is not difficult, a few ½ plywood pieces, a couple of bags of big box store concrete, cast with two inch exhaust inlet at  the bottom, a one inch drain pipe / valve cast into the bottom so the dirty soapy water can be drained out.
•   Submerge the closed circuit copper coil into the soapy water so that the hot exhaust gas can bubble up and around. There will be a high Reynolds number and lots of turbulence. A toilet float system could be added on the side to keep the water level constant or one could just add more water during periodic fueling.
•   The top of the container must be fully removable so a small boat brush can easily and quickly be used to scrub down the container and coil. The cleaning process must be a SIMPLE (I.e. Two minute process). This must be kept simple, fast, and easy.

What do you think? I know this is not going to be as efficient as a multi pass shell and tube heat exchanger, but it should capture perhaps 40% of the waste exhaust heat, be extremely easy to clean and simple to build anywhere in the world. Your thoughts?

[Thob]

I like the idea of simple and DIY.  As a bonus, this box gives a large heat storage unit (concrete plus water).

But -

Concrete is porous, water will soak right through it.  You'll need paint or something on the inside to seal it.

Concrete cracks under vibration, the point where the pipe enters the bottom of the box is a possible failure point.  I'd suggest bring the pipe in the side near the top and then elbow down to about 3" from the bottom.

You have to be absolutely positively certain that there is no way water can get back to the engine.  I.E. make sure the box is down hill from the engine.  Plus, you have to make sure that as the engine cools off, and the remaining exhaust gas cools, it doesn't draw water back into the engine.  I know this sounds far fetched, but Murphy says it will happen when you least expect it.

You need to be sure the exhaust, after leaving the water, goes out the box and out the stack - you don't want exhaust in the engine room.  Does the lid need a seal?

I assume the soap in the water is to keep the soot and other stuff dispersed in the water?  Would there be any use in a having a "sump" at the bottom where junk could collect?  I.E. leave 6" or so of "still" water at the bottom?  Does soot float or sink?

Should the pipe that carries exhaust into the box be capped, and a bunch of small (1/8" - 1/4") holes drilled in it?  Smaller holes, smaller bubbles, much greater surface area to transfer heat.

Just some random thoughts on my part, not intended to discourage or anything.  Someone needs to build one and try it!

[BioHazard]

My first thought is that would be a pain in the butt to clean all the time. It would almost be like having to clean up after a pet. On the other hand, if you were burning NG/propane, the exhaust just might be clean enough that you wouldn't have to...

[Lloyd]

search this site: pete moss, and hidding a diesle.

I have outlined how to use wet exhaust, into a concrete receptor.

Lloyd

[Ronmar]

Having given this particular scheme quite a bit of thought, I don't think it has to be concrete does it?  A drum shouldwork OK and be easy to clean.  The exhaust could run down a pipe to the bottom and pass thru a plate near the bottom.  This baffle plate would cause the exhaust gas to spread out, and would be filled with small holes thru which the exhaust could percolate up thru the liquid across the entire area of the tank.  I think the sudden decrease in temperature would cause the carbon to fairly quickly condense in the water.  I believe this carbon since it cannot mix with the water would quickly fall out of suspension if the water was calm.  Any oils would float to the surface. I don't think you would need any soap, and that might be a real problem with the percolating exhaust agitating the soap into excessive bubbles...  I envisioned all this occuring in a removeable top drum with inlets, outlet and heatex coil lines all running thru the top.  The drum could have an easilly fabbed EDPM liner.  I have a removeable topped drum to experiment with, but just don't have the time(maybe this winter)  I was also wondering how much water could be used and still maintain a reasonable backpressure to the engine...

The big problem I have run into so far when brainstorming this however was the thermal properties of water.  As you approach temperatures closer to 150F, I think you will rapidly start to loose as much heat thru evaporation as you are collecting...  This evaporated water will simply carry the heat away out the exhaust in the form of vapor unless you have a way to cool the final exhaust sufficiently to condense all that vapor.  The other possible issue when disolving diesel exhaust with sulfer in it, is how fast will the water become acidic?  I am still planning on experimenting with it, but I think these two things are going to be issues...