cheap heat exchanger for genset exhaust (air/air)
cheap heat exchanger for genset exhaust (air/air)
(OP)
I am new to the forum and not an engineer, just a science guy, but did see several threads on HXs so thought this might be considered.
Scenario: Hot air (<150F & 1600-2000 cfm) various engines/brands from the radiator of ~20 KW genset flows through the HX, which then uses exhaust heat 150 cfm (750-1000F)to increase the temp and use for dryer air for sand in drum tumbler. Max bk press on exhaust is 3 in Hg but only 0.5 in water on fan so may have to use bypass and thus get less flow from fan across HX. Quotes were efficient and got air to 197F but can't use because of cost. Must get cost to less that genset or <$3-5K. Granted, throughput through dryer may be slower to get to same drynesss but cost is a real limit. Higher price would not be justified on a payback basis.
What best might be done to lower cost of a quoted $12-20K SS double pass air/air HX? Lose fins? What is maj cost components? labor? Should I try just several bends of exhaust pipe in a insulated sheet metal box? Any estimate of what gain (temp) might be obtained?
Thanks, Mike
Scenario: Hot air (<150F & 1600-2000 cfm) various engines/brands from the radiator of ~20 KW genset flows through the HX, which then uses exhaust heat 150 cfm (750-1000F)to increase the temp and use for dryer air for sand in drum tumbler. Max bk press on exhaust is 3 in Hg but only 0.5 in water on fan so may have to use bypass and thus get less flow from fan across HX. Quotes were efficient and got air to 197F but can't use because of cost. Must get cost to less that genset or <$3-5K. Granted, throughput through dryer may be slower to get to same drynesss but cost is a real limit. Higher price would not be justified on a payback basis.
What best might be done to lower cost of a quoted $12-20K SS double pass air/air HX? Lose fins? What is maj cost components? labor? Should I try just several bends of exhaust pipe in a insulated sheet metal box? Any estimate of what gain (temp) might be obtained?
Thanks, Mike





RE: cheap heat exchanger for genset exhaust (air/air)
RE: cheap heat exchanger for genset exhaust (air/air)
RE: cheap heat exchanger for genset exhaust (air/air)
The possible contamination of your sand is a determination you must make. If your exhaust is that dirty then a heat exchanger may be hard to maintain from fouling. Either way the exhaust gases eventually end-up in the environment where people will breathe it after much dilution.
RE: cheap heat exchanger for genset exhaust (air/air)
The evaluation I was doing was for a jacket that went around the outside of the exhaust stack (very low added dP) and the gas to be heated was inside the jacket (outside the exhaust stack). When I insulated everything I was able to reach my target 350F, but I had to cut flow way back to get enough retention time in the jacket. It looked like this arrangement was able to recover about 15% of the heat in the stack, which was about all I wanted to recover because I had a minimum exhaust temp to minimize condensation in the stack.
This stuff wasn't cheep. I think the jacket was on the order of $10k, and the labor to get everything installed was about the same. This was on a 500 hp gas-fired piston engine. For your $3-5k I would expect that you won't get much.
David
RE: cheap heat exchanger for genset exhaust (air/air)
Because a Diesel is not typically throttled, the water is carried in a considerable volume of hot air, but any interior exhaust surface that gets under ~212F _will_ condense water. There will also be some soot, so it doesn't make good tea.
Mike Halloran
Pembroke Pines, FL, USA
RE: cheap heat exchanger for genset exhaust (air/air)
So yes, I believe that an HX must be used because the evap of 6% H2O gfrom the sand down to a 1% level is a cooling effect and I do not want to increase the humidity of the drying air or condense water back into the sand as the temp drops. Direct fired kilns operate at much higher temps and dont worry about such condensation. Waste heat recovery units operate differently. Nor do I want to burn more fuel to effect the drying. Rather, recover the most possible that makes economic sense.
So I am back to the orig ?s, what improvement(s) can be done with the exhaust other than a finned HX that costs 2-4 times what the gen set costs.? It has to have a better payback? Anyone know the relative costs contributions? I don't think SS vs CS will be much difference. So it must be the fins or the labor? Should I learn how to weld SS?
Are there any cost modling air/air HX softwares that I could do a couple of what ifs ? Again that don't cost an arm & leg?
RE: cheap heat exchanger for genset exhaust (air/air)
Fabrication is expensive. You can learn to do it yourself, but you need serious tools; stainless fights back, hard.
Mike Halloran
Pembroke Pines, FL, USA
RE: cheap heat exchanger for genset exhaust (air/air)
RE: cheap heat exchanger for genset exhaust (air/air)
RE: cheap heat exchanger for genset exhaust (air/air)
Fitting and welding a hundred or so between a pair of tube headers could be a challenge, unless you hire or become an experienced TIG welder. I have drawn 'em up, and I have watched them welded; I don't think I see well enough to do it entirely by myself anymore. Not that I ever could.
Given the gas flows that are required and the allowable pressure drops, I don't think one long tube is an ideal solution for a gas to gas exchanger.
Mike Halloran
Pembroke Pines, FL, USA
RE: cheap heat exchanger for genset exhaust (air/air)
RE: cheap heat exchanger for genset exhaust (air/air)
Mike Halloran
Pembroke Pines, FL, USA
RE: cheap heat exchanger for genset exhaust (air/air)
1. He is using screw conveyer to move sand
2. Screw shaft is hollow
3. Screw shaft is inclined
4. Put engine exhaust in lower end of screw shaft and it exhausts at upper end. Allow for water to drain at lower end.
5. Heat is transferred from exhaust to shaft wall to the screw to the sand.
6. Is efficiency really that important when the diesel exhaust is normally dumped to atmosphere and wasted anyway? At least he is getting some benefit for his fuel cost dollar.
Dan Bentler
RE: cheap heat exchanger for genset exhaust (air/air)
The next interesting challenge is getting adequate bearing life, given the hot gas going through the screw core.
The challenge after that is mitigating corrosion of the screw core from condensed water flowing down inside it.
Mike Halloran
Pembroke Pines, FL, USA
RE: cheap heat exchanger for genset exhaust (air/air)
OK, here is another design question: How much pressiure is created by each "U" of a serpentine (Long) tube. You can tell I have never seen the inside of one of these HX's.
My idea now is to buy some std finned tube lenghts and bend them to be parallel for 18-24 " with u turns (not 2 x90) and thus only have a header with 16 welded fittings done at a local shop. I could then experiment with backpressures vs lenghts and diams, etc. What is the finned (od) diam for a 3/8 tube? Place a thermister here and there during initial tests and a P-gage too. OK, to be sure the header gives no backpressure, how many 3/8 holes is equal to the area of a 2 in exhaust? let's see now, where did I put my slide rule.....:)
RE: cheap heat exchanger for genset exhaust (air/air)
The fins are made by bending a strip the hard way, flanging or wrinkling the inner radius. A diameter over the fins of three tube diameters is probably pushing what can be done, but talk to the fin-maker about it.
Fin pitches of 8..12 fins/inch are common, but what's achievable is also dependent on the material, so again, talk to the fin-maker.
A smooth u-tube will have a pressure drop that you can calculate, or at least estimate, based on geometry and assumed mass flow, using equations in books you probably already have. Sometimes heat exchanger tubes will be dimpled, or have turbulence inducers inserted, in which case the equations won't give you accurate results. After that, you use the estimated pressure drop to compute a new mass flow, and iterate until the numbers stop changing.
It may exist, but I haven't seen stainless finned tube as small as 3/8", and I wouldn't want to deal with the number of branches you need to get an equivalent to a 2" exhaust pipe. I'd guess 4 or 5 tubes of 1" od would be about right.
The tubes have to be paralleled, so you need two headers, of say 2-1/2" pipe.
Mike Halloran
Pembroke Pines, FL, USA