The Steamboating Forum

I'm planning a small boiler with a 4" drum and 4" diameter coils in schedule K copper for wood firing at up to 150 psi.

On all the Ofeldt builds I have seen with steel drums and copper tubes, there are always steel nipples welded into the drum with the copper silver soldered or brazed into the nipples. Would it impossible or inadvisable to silver solder the copper tubes direct into the drum?

I know that getting the heavy steel and light copper to the same temperature is a challenge - I was assuming that doing it this way would require careful pre-heat, probably in a refractory oven of some sort.

The only other objection I can think of would be strength of the joint, but I would think that with a schedule 80 drum, there would be plenty of surface for the joint. Perhaps I'm amiss here?

I am having all of the steel welding done at my local code boiler shop. If it's safe and feasible, I'd love to avoid having the extra welding involved with the 24 nipples for the coils.

Thanks,
Scott

Scott,

Sounds like an interesting project.

Re: Silver soldering copper coils in to the drum, I don't see any issue with doing that. If you're trying to keep it to code, I would seek the correct information from the ASME.

I think the reasoning behind the steel nipples is they can be welded in and then the steel to copper joint is just a slip joint that can be brazed with easy access, as you've probably noticed, it adds to the OD without any real benefit. And I would use brazing rod instead of silver solder, it's stronger and cheaper. If it were me, I would omit the steel nipples, and braze the tube inside and out. A big brazing torch on oxy-acet will get it up to temp.

You could make up a pry-bar with a 60 degree cone that could be used in the inside of the drum to flare the tubing inside after the outside is brazed. Then braze the inside.

My buddy has two gasoline fired Ofeldts built just as you're intending that he runs at 400 psi. I don't ride with him much .. Apparently it's ok, he's driven both vehicles a lot and he hard fires them and he hasn't lost a tube yet. One thing though, you want to make sure you never run out of water and with the small center drum, you're going to have to stay on top of it, especially with a wood fire that cannot be turned off, keep the hand pump accessible and working if needed.

Instead of copper, have you looked at steel hydraulic tubing? Cheaper and pretty easy to work with and can be brazed, and you don't have the 406 degree wall to worry about. Another buddy uses that for his Bolsover boilers. It's like half inch OD and .03" wall and seamless.

-Ron

Ron,

I like the idea of the hydraulic tubing and brazing. At this small size, I'm sure I could still wind them myself and braze them in.

Re: water level control: This will DEFINITELY be automatic. The current plan is to have a large bore, geared down piston pump with variable stroke. The stroke will be controlled by either a generator type or stanley type control. From my experience with my bigger plant (for which I'll eventually finish machining the casting set I bought from you some time ago), I know that the refractory will store and discharge a surprising amount of heat, even after the fire is dumped or burnt out. Ideally I'd like to build a little duplex for feed as well, and have that set to start at a slightly lower water level, in case the safety or whistle lowered the level too much when the engine is not running. There will be a hand pump too, of course.

Liquid fuel would make this all a lot easier, but firing with wood or charcoal is probably my favorite part of the whole experience.

Cheers,
Scott

SL Ethel wrote:I'm planning a small boiler with a 4" drum and 4" diameter coils in schedule K copper for wood firing at up to 150 psi.

On all the Ofeldt builds I have seen with steel drums and copper tubes, there are always steel nipples welded into the drum with the copper silver soldered or brazed into the nipples. Would it impossible or inadvisable to silver solder the copper tubes direct into the drum?

I know that getting the heavy steel and light copper to the same temperature is a challenge - I was assuming that doing it this way would require careful pre-heat, probably in a refractory oven of some sort.

The only other objection I can think of would be strength of the joint, but I would think that with a schedule 80 drum, there would be plenty of surface for the joint. Perhaps I'm amiss here?

I am having all of the steel welding done at my local code boiler shop. If it's safe and feasible, I'd love to avoid having the extra welding involved with the 24 nipples for the coils.

Thanks,
Scott

I built a Lune Valley boiler w/ copper tubes, silver soldered into steel nipples welded into the drum. Perhaps w/ only a 4" drum, you may be able to heat it sufficiently and w/ enough control to make this work... but I doubt it. 4" pipe is cheap enough - cut off a scrap piece, drill some holes and try it w/ some short lengths of copper tube. You'll need a lot of heat to get the steel to silver brazing temp, and the copper coils will not take overheating well. If you really want to avoid the nipples, you could furnace braze the whole thing - but that's not a cheap thing to have done, either.

You could just drill 5/8" holes in the drum, and weld in the steel nipples (turned from 3/8 sched 80) yourself. The welds have a trivial amount of stress on them (23 lbs/running inch); of course, that's not a Code solution. I welded in 108 of these on my Lune Valley design.

Failing that, you can simply drill and tap the drum for 3/8" nipples, and cut 2" schedule 80 nipples in half, boring the cut end .502" in diameter and about the same deep.
Screw those into the drum nice and tight, and then silver solder your tubes in.... This isn't Code either afaik, but with .337" wall on the 4" pipe, should work fine. Again, test on scrap to be sure.

If you do go with the nipples, silver soldering coils in is done as follows: 0) test fit coil into nipples, bending coil gently as needed 1) brush bored mating surface in nipples clean w/ brush made for copper fittings. 2) heat nipple w/ torch so it's 250F or so...3) brush dilute HCL into mating bore w/ acid brush, avoiding the resulting acid fumes 4) flux bore w/ Stay-Silv White or equiv. flux. 5) clean ends of copper coil in same manner (brush, heat, acid). 6) fit coil into nipples; retain w/ wire if it makes a bid for freedom. 7) heat joint, focusing on nipple and watch for proper flux melt. Note that heat will spread much more easily in copper tube than in nipple. Once flux has melted, apply silver solder and keep heat even; watch for quick melt and resulting even silver solder line.

Note that any minor weeps in threaded steel nipples will resolve themselves as they rust shut .

A bit about Otter's boiler is here: http://www.smaalders.net/bart/boiler.html

- Bart