Free engineering help needed

[jheldatksuedu]

I've taught engineering thermo and heat transfer in college and also worked on everything from watches to building fiberglass highway bridges. The people are right that are telling you the heat transfer coefficient of the tube material normally has a very small effect in the overall temperature difference between the fluids on either side of the heat exchanger walls. The transfer of heat from the liquid to the tube and tube to the air tend to be the controlling factors in this problem. The different coefficient will mean there will be a slightly larger difference in temperature from the inside wall to the outside, but not enough to make a big deal. For example there might be a 10 degree difference between average liquid temp and inside wall, a 1/8 degree difference between inside and outside wall and a 20 degree difference between outside wall and average air temp. Changing from copper to stainless, might make the wall to wall temperature difference be 3 degrees, changing the total from 30 1/8 degrees from water to air to 33 degrees. Nothing to loose much sleep over.

[Nicholson58]

Originally Posted by Guy

I am trying to build a spare heat exchanger. I can't afford to buy one. I've built 2 from copper and could not control the leaks, so copper is out for me. Now I have the material to build one from stainless and I should be able to tig weld it up leak free.
Question: Copper has thermal conductivity of 400 and stainless is only 14. Does that mean the stainless heat exchanger needs to be 28x bigger than the copper job? I know this very simplistic but any WAG?

As noted by the others, the reduction in SS is not so bad. The heat transfer coefficient for a heat exchanger is a combination of the metal conductivity, thickness, fluid properties on both sides of the tubes, fluid velocity/turbulence. The over-all coefficient is usually noted as 'U'. If you do the research for similar units made of different materials but the same fluids and temperatures you will find U is only a little different for SS.

Consider used? Most radiator and exchanger service shops and re-tube an old one for well less than new. I had one of my old 1500 dollar units serviced for less than 100 bucks.

[model 10]

Originally Posted by Dennis.G

Ihr). Back to OP's question, NO the tube surface area is not proportional to tube material thermal conductivity in overall heat exchanger design, but is effected by it to a lesser extent.

The online calculator you posted shows that to be true. It seems counterintuitive to me and most others but there you have it.

[Reefmagnet]

Originally Posted by Zai

I tig weld copper an brass quite often and hadn't noticed it to be that difficult.
My suggestion is to try it.
I do use a filler rod though.

Originally Posted by leftbrainstuff

Copper is appropriate. But you say you cant stop it leaking. It sounds like you lack the skills necessary. Copper TIGs beautifully.

Yes copper does TIG weld beautifully. In fact copper can be fusion welded to aluminium - as is done in bus bar manufacturing - quite nicely. It's the heat conductivity that's the problem. Granted the design of the heat exchanger is simplified if fusion welding, but if the OP is having trouble brazing I'm pretty sure he will have trouble TIG welding copper to a standard where it won't leak. Of course he can switch to helium shielding gas and ensure he uses deoxidised copper filler rods (in lieu of old electrical wire) to help, but it won't be like welding stainless steel or even aluminium.

So not impossible, just difficult.

[model 10]

[IMG]heat exchanger[/IMG]
Cost $30 US and tested to 100psi. All kinds of baffles etc. inside. Thanks to all who gave the free help.