Free engineering help needed

[Reefmagnet]

Originally Posted by Guy

Re soldering vs tig welding; I can tig weld which is why I'm want to use stainless steel. I don't know if you can solder up a whole bunch of little copper tubes, roll up a body out of copper sheet, solder the whole mess together and have it not leak, I couldn't do it in 2 tries. Maybe the third but I'm moving on.
All this has me thinking back to fancy oil coolers in aircraft. Even the insides of the tubes or passages had little fins. It made them impossible to clean if you had a component failure because you could not get the dirt out. I can make little twisted vanes for each tube in mine..

Silver solder is easy-peasy but you will need good overlapping fitup for the capillary attraction and will of course need to clean up all the joints prior to soldering. The "secret" of soldering/brazing is to let the work melt the solder, not the flame. Copper, being such a good conductor, can make this a bit tricky but it is doable. I haven't checked where the alloys in silver solder and copper sit in relation to each other on the galvanic table, so you'd need to check that.

[Pauls]

Hello Sailor's post is accurate. You need to create turbulence in the liquid to eliminate laminar flow. You can build turbulators to install in the tubes. Some manufacturers form the tubing by pressing it in forms which form dents, bumps, spirals, etc into the tubing.

You also need turbulence outside the tubing for the cooling liquid flowing there.

Stainless is probably one of the worst metals you could use, it has poor corrosion resistance and lousy thermal conductivity.

Copper nickel is the standard metal for marine use. You can TIG copper nickel. But I'd recommend silver brazing. It is very easy to make hermetic silver brazed joints.

[Wotname]

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?

I can certainly do WAG . I can't help with heat transfer - I know JackS about that.

But I have never seen a professionally built SS HX, maybe there is a reason…

And maybe they do exist, just not around here

I can say that once you how to properly silver solder copper, it is as ReefM suggests, easy peasy - heck even I can do it but there was a small learning curve.

Have the copper clean, not necessarily see your face shine, but clean nevertheless. Brown scotchbrite is your friend!
Use MAPP gas.
I use 5% silver content, I find 2.5% not enough unless you have oxy gear.
Using a flux suitable for brazing brass doesn't hurt - not necessary once you are proficient but using it doesn't cause any problems IME. I forget the actual number of the flux right now and don't have any close by to check
The silver solder will flow beautifully and wick into the smallest gaps when you have the right amount of heat (about a very dull red for "normal" copper). If it isn't flowing "beautifully", the job is most likely too cold. Even dirty copper will flow with enough heat!!!!!

There could be better ways as I leant "on the job" with too much formal training.

I guess google has better detail and maybe better advice…

EDIT: +1 for what ReefM posted "The "secret" of soldering/brazing is to let the work melt the solder, not the flame" although unlike Reef, I think copper makes this easier - you can keep the flame further down the work away from the joint and as the copper transfers the heat to the joint, you control the amount of heat by moving up or down the work rather moving the flame into or away from the job. YMMV.

[exMaggieDrum]

Originally Posted by hellosailor

No offense meant, but if you can't solder copper

No offense taken. I'm not an expert on this stuff but I have "soldered" many a copper pipe in water pipes in houses, with acid flux. I did have a leak once but that was fixed after I got the thing totally dry.

I guess what I was trying to imagine is the rest of the fabrication required. There are people handy enough to do it but putting together a pipe to a bigger pipe requires pretty good hole cutting and fitting.

Silver soldering is definitely better quality. The OP may have the metal fab and soldering skills to do this. I wouldn't attempt it and don't think most people would. But that just my opinion. If the OP does it I'll give him lots of kudos. I would definitely be more worried that my home built unit would fail than I would one I built myself.

I do know there are shops that fabricate and refab HE's. One is in Bellingham WA. I don't remember the name. But I think they are much cheaper, and as good or better, than the OE units. Our boatyard sent them there all the time.

One thing I have found out is that there are a large number of very knowledgeable, skilled, and handy sailors out there. I know a few things about few things well. My sense was that may be the OP did not have these skills but I am probably wrong. You and the other posters seem to be totally up on the engineering, and perhaps the fab as well. But knowing one does not cause one to know the other. Seen too many engineers who are not good at hands on stuff. And - I do not mean to imply anything about that as I started out as one and know many engineers I respect the hell out of who are much better in a lot of things than I am, including fabrication. Hell, some of them have their own machine shops at home with DNC equipment.

So no offense taken.

[Seymore]

You've only considered building a tube type heat exchanger, identical to the stock one.

Consider building a plate type exchanger out of copper sheet. A plate type exchanger is a bunch of metal sheets bolted together with spacers. Two holes through the sheets alternately pass the liquids between the sheets. It's simple. The area of the copper plate exchanger would be the same as the area of the tubes in the tube type exchanger (I'd make it bigger, I'm very conservative). A picture is worth a thousand of my words, but I can't find an illustration of a plate exchanger to link to.

In the manufacture of tube type exchangers, the tubes are rolled/expanded into the holes in the end plates. One of your liquids is raw sea water, and I would not braze or solder the tube ends to the plate because of galvanic corrosion. If I were building a tube type exchanger I might consider machining a groove inside each hole in the thick end plate and seal the tube with an o-ring. I might, that is, but first I'd trundle on down to a radiator repair shop and see what they think--I know, they solder radiators together because the liquid inside is fresh water with anti-corrosion chemicals added, but they'll know a lot about heat exchange.

In the exchanger on my Yanmar 3GM30F here, turbulent flow is maintained by making three passes through the exchanger, each pass through only a few tubes.

Heat exchangers can be made of anything, copper or stainless steel or whatever. In a sulphuric acid manufacturing plant, they cool the sulphuric acid in heat exchangers with Teflon tubes, Teflon being just about the only thing that'll stand up to the acid. Teflon has very poor heat transfer capability, so there are a lot of very small tubes for a big surface area.

I'm in the camp that says 48X, assuming the same thickness of metals, because the heat transfer coefficients through the liquid films are the same in both applications. The only thing that changes is the metal. Again, if you do TIG weld the tubes to the end plates, the weld alloy must be the same as the tube/end plate metals. And generally, one of the end plates must be free floating, because of expansion from heating and cooling.

This is not my $.02. This is my free advice. Think of all the blather that would have issued for $.02!!

[Zai]

If you can TIG weld why don't you just TIG weld the copper?

[Reefmagnet]

Originally Posted by Zai

If you can TIG weld why don't you just TIG weld the copper?

Copper isn't easy to fusion weld. In fact it's quite difficult.

[Zai]

Originally Posted by Reefmagnet

Copper isn't easy to fusion weld. In fact it's quite difficult.

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.

[DougR]

Don't beat yourself up......Google "R&D Enterprises" in Michigan and see how they build OEM marine heat exchangers. They can give you good advice, build in several materials, customize to your requirements, etc.
Also, several other suppliers to the industry exist, such as Monitor Products, Sendure, and others.

DougR

[reed1v]

Originally Posted by exMaggieDrum

Good quality heat exchangers are expensive. But losing your engine, or your boat, is more expensive. Copper is one of the easiest materials to seal with solder or brazing but building a heat exchanger with internal and external tubing, and tubes going into the sides or other tubes is not the easiest project to do. I would save up my nickels and buy a used one if you can. If used get it inspected and serviced and put it in your spares. Make sure it fits. And cupro-nickel is a better material. But if you DIY good luck. And you may have to put in a zinc holder for Cu or Cu-Ni. I wouldn't recommend stainless but I'm not an engineer so FWIW.

Strange, because heat exchangers for small diesels usually run under $100 and most marine salvage places have them for around $25(used of course). Stainless makes no sense. All copper makes way more sense. Prevents internal fouling since copper is a poison.

[model 10]

Originally Posted by Zai

If you can TIG weld why don't you just TIG weld the copper?

Did not really think about it. I'll try it. Electrical wire for rod?
The problem with all the soldering was there were gaps between all the tubes and between the tubes and the body. This was not like your plumbing fittings in a house where things are a tight fit. It was not "easy peasy". A better design was needed.

[Zai]

Originally Posted by Guy

Did not really think about it. I'll try it. Electrical wire for rod?
The problem with all the soldering was there were gaps between all the tubes and between the tubes and the body. This was not like your plumbing fittings in a house where things are a tight fit. It was not "easy peasy". A better design was needed.

Try this:
https://www.airgas.com/product/Weldi...y/p/HAR03D0C50

[reed1v]

Just get an old heat exchanger and re build it. Or just build an external heat exchanger with copper tubing at the turn of the bilge. A lot of the lobster boats up here just use galvanized pipes.

[boater 86]

Originally Posted by exMaggieDrum

Good quality heat exchangers are expensive. But losing your engine, or your boat, is more expensive. Copper is one of the easiest materials to seal with solder or brazing but building a heat exchanger with internal and external tubing, and tubes going into the sides or other tubes is not the easiest project to do. I would save up my nickels and buy a used one if you can. If used get it inspected and serviced and put it in your spares. Make sure it fits. And cupro-nickel is a better material. But if you DIY good luck. And you may have to put in a zinc holder for Cu or Cu-Ni. I wouldn't recommend stainless but I'm not an engineer so FWIW.

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[leftbrainstuff]

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?

Stainless has poor thermal conductivity. You will need to slow the water flow down or upsize the heat exchanger which will take up much more space. Dont waste your time with stainless.

How many stainless heat exchangers have you ever seen? None. Im excluding monel and titanium extreme service heat exchangers which arent relevant to this discussion.

Copper is appropriate. But you say you cant stop it leaking. It sounds like you lack the skills necessary. Copper TIGs beautifully. Stainless wont help you. It will also move around so much during fabrication that the potential for leaks is much higher.

Why not just have your existing heat exchanger reconditioned on a regular basis. Local radiator shop will do it. I dont fix or recondition my own even though I can.

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