Simple exhaust heat-x

[AdeV]

I've just started constructing a very simple exhaust heat exchanger. It probably won't be particularly efficient, but - one variant at least - won't suffer from clogging...

It's all based around some 2.5" dia thick wall Ali tube. I've cut some threads in one end to match the various threaded couplings I have here:



Once I figure out how to hold t'other end without crushing the threads, I'll thread the other end as well. That will give me a unit I can assemble between two elbows, and will replace the vertical pipe that you can see in my avatar.

I've also got 10mtrs of 10mm "microbore" soft copper pipe, which I intend to wrap either:

a) around, or
b) inside of

the heat-x tube.

Option a) is the one that definitely won't ever clog up with crap, as the internal surface is smooth bore. That's the upside. The downsides are; less thermal capture (some of the exhaust will travel straight up the centre, not releasing its heat to the walls. I may be able to partially fix this by introducing some kind of turbulence-inducing vanes or baffles. The other downside is the only transfer of heat to water is via the copper which is in contact with the aluminium. To maximise this, I could cut a spiral groove into the ali, into which the copper pipe sits snugly. This will increase the contact patch to around 50%; the whole thing can then be wrapped in shiny aluminium foil, and then insulation, to try to keep as much heat inside as possible.

Option b) may clog up, I don't know... The idea would be to wrap the copper pipe more tightly, and introduce it inside of the exhaust pipe. The copper would be nearly 100% in contact with exhaust gases, and therefore much greater heat extraction should be possible. Downsides - otehr than the possibility of clogging, there's also the acids in the exhaust which may corrode the copper. It would need cleaning as the carbon deposits built up on the copper (irrespective of the clogging possibiloty), reducing efficiency over time.

I have 4x 1/2mtr lengths of aluminium, so I've potentially got 4 goes to get this right In a quick test, the aluminium heated up very rapidly, even with no load on the engine. I chose aluminium because I'd planned on wrapping it in copper & wanted the superior heat conductivity; putting the copper inside hadn't occurred to me at that point. I don't know what effect the engine exhaust will have on the aluminium - it may very well eat it. In which case, I'd be tempted to put a thin stainless steel pipe inside the aluminium pipe - rather than switching to an all-steel pipe.


Over to you clever chaps for you thoughts 

[Crofter]

AdeV, Aluminum and copper are better  materials  for battery plates than partners in a heat exchanger. They are a long ways apart in the activity series. The copper inside the aluminum and exposed to the exhaust acidity might be a problem even if you could roll it tightly enough to get it in there. Boundry layer insulation is a heat transfer balk, as you have identified, as is the layer of fur that the exhaust will soon deposit on the relatively cooler metal it contacts.

Winding copper in the grooves OD addresses contact area there to some extent but I think electrolosis corrosion film in the interface will hurt your game.

[mobile_bob]

Ade:

what engine are you building this heat ex to fit?

are you able to run it at a fixed load, or nearly so?

what exhaust input temps have you available?

how long are the engine run time anticipated to be? and how long between runs?


bob g

[AdeV]

Jens - Yep, some kind of sleeve is what I'm looking into, I'm not sure if I've got anything big enough unfortunately, and I'm currently not able to do internal threading at this size (no suitable tool). But.... I will find a way.

Crofter - interesting thoughts on the electrolysis, that hadn't occurred to me. I could, of course, use aluminium tube (if I had any), which would cure that issue & just leave the potential problem of exhaust gas acidity/erosion. The copper tube I'm using is 10mm microbore, and is fully annealed as supplied, so it should easily coil up to fit inside the pipe. I can coil it on the lathe, which will ensure a good tight spiral, plus I can see what's going on at all times. If push came to shove, and it didn't want to coil without flattening, I'd just fill it full of water & cap it at both ends

Bob, the answers to your questions are:

what engine are you building this heat ex to fit?
- It's going on a 6/1 with electric flywheels

are you able to run it at a fixed load, or nearly so?
- At the time of writing, I cannot load the engine at all. However, I anticipate having the alternator mounted and belted up within the next week or so (maybe today if my friendly welders have finished the bracket - but that's unlikely), so I'll be able to put a fairly constant load of anything between 0.5kW and 3kW on.

what exhaust input temps have you available?
- Unknown at this time; except for the experiment I did a while back, which suggests 310C will be available at full load. I will be making provision to insert a thermocouple at the inlet & outlet ends, although I only have 1 device suitable for the temperatures involved, at the moment.

how long are the engine run time anticipated to be? and how long between runs?
- I've not yet decided.

I expect the early runs will simply be proving out that it works, that there are no leaks, and other such mundane tasks. It'll also allow a good coat of filth to build up on the inside of the pipe, which should make it behave more like it will in the real world.

Once it's "broken in", and proven out, I can go about getting some efficiency readings from it, using known loads & runtimes.

[Curbie]

Ade,

I think your exhaust temperature estimate of 310C (590F) is "in the ballpark", but am not certain what collection temperature you're looking for, which makes sizing this idea anyone's guess.

I think I would trying to stay with mass produced stuff sold for like purposes because of cost, longevity, and safety reasons:

1)   2.5" corrugated exhaust tube (flex tube), because it's mass produced, intended for this purpose, the corrugations will increase turbulence/heatX, and the corrugations will better absorb engine vibrations that will increase safety during periods of unattended use.
2)   Coil your copper tube tightly around the outside of the corrugated exhaust tube using the corrugations as guides.
3)   Use high temperature tape over the copper tube as you're coiling the corrugated exhaust tube to both tightly fix the two and reduce some heat loss.
4)   Wrap the whole assembly with high temperature insulation.
5)   Water or propylene glycol as a heatX fluid circulated counter to the flow of exhaust

Size exhaust tube, copper tube, and selection of heatX fluid and circulating pump all seem to be collection temperature dependent.

Good Luck,

Curbie

[Crofter]

I am afraid that if the corrugated exhaust tubing is flexing at all, the copper tubing bound to the outside of it wont be happy for long. Not much fatigue resistance.
That small tubing is not good at picking up heat unless it is immersed. I have installed tank heater coils which get strapped to the outside of vessels and a heat transfer "mastic" gets slathered between to increase heat transfer. Whatever the material is, it does not run off when hot. It does not take much of an air gap to greatly reduce heat transfer. Often tubing used like that is seen soldered or brazed to the source or target.

[Ronmar]

If your lathe chuck is small enough, you may be able to grip the pipe from the inside pressing outwards...

I agree that small tube used like you describe is usually soldered for enhanced contact or completly immersed.  You could fit a larger outer pipe and just fill the gap with liquid...

[Curbie]

Crofter and Ronmar,

I agree for the finished version the copper tube should permanently affixed to the exhaust tube for heat loss reasons, but here's my thinking, without knowing the desired collection temperature, heatX fluid, and heatX desired fluid flow rate (IF), I couldn't say if you could achieve the results you wanted with a single copper tube coiled around the exhaust tube or it would take a multiple parallel winding.

Not unless someone is offering to do the heatX math for a home-made, one-off heat-exchanger this seemed like a "trial and error" design and I suggested the idea with multiple rebuilds in mind. Once you got close with the temporary attachments, knowing that may they will introduce heat loss, the permanent attachments should produce better performance in the finished device.

Curbie

[Crofter]

If you are trying to capture most of the exhaust heat of a fully loaded 6-1 that will amount to approx 5 kw hour. The first necessity is sufficient interface area between the exhaust and that side of the exchange material. While aluminum has a higher conductivity rate than iron the difference in effectiveness is less than apparent since the exhaust soot and boundary layer is the main detriment to transfer. 
You need lots of area and a greater number of smaller tubes is more effective at bringing the most hot gas in contact with the most surface area. Ideally the heat pickup liquid should be on the other side of the single transfer material.  You should be guessing I like tube and shell exchangers, Lol!

They can often be arranged to have a single inspection cover removed to brush the tube ID without having to disconnect any of the product lines but that is only a matter of convenience not effectiveness. I am not a fan of aluminum compared to steel because of steel being so much easier to find compatible fittings for or weld to, and cheaper to boot.

Certainly it will be necessary to size transfer area and your coolant flow through rate depending on whether you want it approaching steam temperature or 140F or so for heating or domestic water. You may not want your exhaust gas going out too cool either and condensing but that is probably not the most likely problem. I dont think you have oversized contact area.

[Ronmar]

Along with what Crofter said...  You biggest issue with a single half meter piece of 2" pipe is going to be time.  Thermal expansion not withstanding, a 6/1(4.5" bore X 5" stroke) at 650 RPM displaces just shy of 80 CU/IN X 325 exhaust strokes per minute = 25,844 CU/IN/Min of exhaust gas.  That is 431 CU/IN every second.  A 2" ID pipe X 24" long has an internal volume of about 75 CU/IN.  So that hot exhaust gas is going to pass thru your heatex in about 1/5 of a second, with only a very small fraction of it ever contacting the roughly 1.05 SQ/FT of internal surface area...  Internal baffeling or swirl enhancers are not going to have much effect in that short time, and anything that would would raise exhaust backpressure.  You start backing up the exhaust, and you might possibly damage the exhaust valves.   A half meter straight pipe heatex thing might make you a cup of tea every half hour or so, but I don't see you getting much heat out of it. 

In order to get some meaningfull heat out of it, you need enough internal volume to hold the exhaust gas for seconds, not fractions of a second without adding restriction, but yet still maintaining enough turbulence to breakup internal laminar flow.

Once you get some longer duration contact time between your fluids, you will get stable temp differentials.  Once you have these, the thermal math is not all that difficult.  Aluminum for instance has a thermal co-efficient of around 144 BTU per SQ/FT per hour, per degree F temp difference.   

[Geno]

Around 4 years ago I put in the starting ebay bid of $500.00 on this heat ex never expecting to win it. It was new and retails for over $7000.00  Well, there were no other bids and 2 weeks later its in the garage. What a monster. All stainless. It doesn't have any problem taking heat out heat of the exhaust.

Thanks, Geno

[billswan]

Geno
I see your heat exchanger is a monster are the internal tubes half inch? Does it clean up easily? 
I tried something similar with my 10/1 but this is what it looked like after about 150 hours + or - .



The problem was it was way to efficient and the gases were cooled down to the point that the exhaust was cold and dirty black rain out of the pipe outside of my shop was the result. And you know where that black rain ended up when the wind started to whip it up. On everything that it was not welcome on! Took a year to get it off of the side of the shop. Soap and water helped but only so much...............


Billswan

[Geno]

I get about 500 hrs. out of it before a cleaning. A pressure washer tip on the handle (no wand) makes it easy and quick. But even at low pressure it's very messy. If the tube plugs when I hit it with the washer it just sprays out the top, all over the engine room and me. Then I use a stiff wire to unplug it. Long sleeves, gloves, head protection and goggles are strongly recommended. I think the tubes are 3/8. I do get the black water in the bottom of my cinderblock muffler. I just put some lime on it from time to time. That supposedly neutralizes things. I doubt I'd go with this setup again. I wish somebody would come up with something better for us guys......

Thanks, Geno

[Crofter]

Has anyone ever tried injecting fuel ahead of the unit and thus flame cleaning the tubes? I think some catalytic converters deliberately program to do this. From time to time I have noticed my VW diesel really letting out a sustained blast of smoke and soot on the rare occasion that it gets hard accelleration and high revs.

[Ronmar]

Then the heatex would have to be all steel, or preferably stainless and designed to handle full exhaust temp and then some.  At what temp does the oily carbon exhaust residue breakdown? 

Another thought would be to steam clean it.  Drain the coolant and leave the shell open to the atmosphere, then with the engine loaded and heating the tubes to full exhaust temp, inject a fine spray of water mist into the exhaust at the top.  You would of course need some method in the outlet plumbing to collect/capture the debris.

I hear tell that diesel fuel is pretty good at breaking down the carbon deposits.  Another thought would be to have a diesel wash cycle that injects diesel or some other solvent at the top of the tubes and collects it at the bottom, on a non running engine of course.  The small quantity of diesel/solvent is allowed to settle and filtered at the bottom and then recirculated to the top.  Basically a turn it on and walk away affair.  Shut it off, blow down the tubes and collect any residue before startup.  the residue cooking off might make a little smoke, and of course the exhaust needs to be built to handle a possible burn-off of the residue...