continuous spiral in copper tube?

It might be less trouble in the long run to wind all the spirals individually in pairs, one out-wind cw, one out-wind ccw, then braze them together to form the entire stack. At least this way you'd only be winding from the center out.
LLoyd

Could you bend the coils as pairs, starting in the inner coil and working outward, then join the coil pairs at the outer edge of the assembly?
Wassit for?
Cheers Trevor Jones

How about wind one spiral out, then take from 0 to 1/4 turn to get back to the center, then wind the next spiral out, etc? It wouldn't be as pretty, but it'd be a heck of a lot easier to wind -- I'm visualizing a jig made with stacked plywood rings on some 2" steel pipe.

"michael gray" wrote: (clip) I will have to braze two lengths together at some point to achieve my 10 "pancakes", not a problem. (clip) ^^^^^^^^^^^^^^^^^ If a braze joint is not a problem, then why not make five outward spirals by the method you have outlined, and join them? (BTW, I suggest silver solder rather than braze, and copper fittings for the close bends.)

Pardon my total ignorance of how steam is generated, but having built several fire-to-hot-water exchangers, that seems like a very long path of very small tubing. I'm trying _not_ to make steam, but if I'm not careful I can over-fire an exchanger with proportions maybe one-tenth that long.
I put "monotube steam generator" into Google and the first hit has an intruguing diagram at the bottom left (and lots of relevant-looking formulas). If you click on that tiny diagram it opens large enough to see, with arrows showing the water path. His "pancakes" are Vee shaped, with the cold ones at the bottom left and water progressing toward the top right. Seems like you could wind that around an appropriate form relatively easily if you started from the hot end and worked out.
The detour from the middle of the large tubing pack to the single Vee of small tubing is curious - have they maybe calculated their flow rate so that the flash to steam happens there? But the flow rate has to vary with the required engine output, right?
Loren

It seems to me you can wrap an inward and an outward spiral in one operation. Imagine having a 4" mandrel and three 13" disks to fit it. Disregarding practicality for a moment, imagine notching the center disk enough to pass a length of tubing between the mandrel and the disk. Now rotate the mandrel to wrap up the tubing between the three disks (on either side of the center disk).
Is that clear?
Of course, that description is not a practical approach, but from it I'm sure you could figure a practical approach.
In principal, you could make a device to wrap the whole thing in one fell swoop, but I doubt it would be worth the extra trouble.

Hi Mike;
Have you seen the formed copper tubing that was developed for heat exchange purposes? It has a helix in its shape, which gives more surface area compared to the inner diameter and hence more efficiency (they claim up to 500% better). Here's a link: I saw them at the Pacific Design and Manufacturing show in Anaheim last month (see the blog on my site for a short write-up on that).
As for wrapping the tube into neat spiral disks, I think you'd need to build a special jig for that. If I had to do it, I'd wrap tube onto an hour-glass shape that can be removed from each side, then collapse the facing tube cones vertically to make them disk-shaped. But aren't most of these steam generators and water heaters made with cylindrical wrappings of tubes, so that the outer cylindrical helices wrap around the inner ones, and the heat goes up through the core? That would be a bit easier to do...
Andrew Werby

April 1. is tomorrow! What's the date on the plan?
Nick

Now I know how it can be done by just winding outwards. Tricky!
First, I'll describe how you make one inward wound / outward wound pair: Take some (calculated) length of tube that has the right length (you can also experiment) for two spirals. Start winding outward at one and until you have one spiral (and consumed half of the length of the tube. Do the same on the other end. Now you have something 8-shaped. Bend one spiral over the other to get one pair of spirals.
Now as you have understood how to make one pair, I'll explain how to make as many pairs as you want: Cut the tube to the desired length. Put marks (masking tape, whatever) every length required for one pair. Start winding on one end and wind one spiral. At the mark, make a 90° bend (this way, the remaining tube won't come in your way and you'll need that bend anyhow) and wind to the direction of the first spiral. Now you have a pair. Start winding on the other side of the bend and proceed like for the first spiral.
Maybe you should try that with some cheap wire before trusting my idea on your precious copper tube. :-))
Nick

This is to be read: Bend 90°, so the remaining tube sticks upwards.
Nick

According to michael gray : Since you already know that you have to make one joint, why not make the spirals in pairs gripped at the center and both sides wound at the same time. This would require three plywood plates, and the middle one will have to be designed so it can be disassembled -- or at the very least, leave a slot for the crossover tube in the middle to pass out through so you can slide the middle plywood plate out from between the two coils.
You would have to measure out a bit more length than really needed, and clamp the middle before you start winding.
This way, you are winding two coils from the center out, and they can be joined to other coils to make the thing continuous. This has the extra advantage that the joint is out where you can inspect it at need.
How do you plan to join them? The Sil-Phos used for air conditioning joining? I'm not sure, but I believe that I have heard that it is bad for use with steam. How high a pressure are you after? Enough to run a small steam engine, or just enough to steam-clean an engine?
I would be tempted to join them using good stainless steel Swagelok splice blocks instead of brazing. Brass also can be a problem with high pressure steam. It de-zincs over time and then becomes not much of anything useful.
But -- since I have never actually *done* any of this, I would suggest that you wait for other answers to have as much information as possible.
Good Luck, DoN.

When I wanted to sleep, I found an even better way (than my other posting) to do it. On a mandrel. All in one piece.
Say you want 100 mm inner diameter and 200 mm outer diameter. Get some tube with 100 mm diameter. Cut out disks with 100 mm bore and 200 mm diameter, thickness as required. In each disk, cut a radial slot as wide as the tube's diameter and nearly to the disk's bore.
Prepare the tube: Calculate or try the required length for one spiral. Cut full length for all spirals and make marks for the length of one spiral. Now bend the tube in this way:
(Monospaced required) +-+ +-+ +-+ +-+ | | | | | | | | + +-+ +-+ +-+ +
The distance between the verticals is the thickness of the disks, length of verticals is length for one spiral.
Now put your disks onto the tube. Put your zigzag onto your jig. Every horizontal of one side of the zigzag goes into one slot in the disk. Grab one of the other horizontal ends and wind it around the mandrel. So you are making two spirals in one go. Take the next bend and wind it around, ...
The ASCII-"art" shown above would give you 8 spirals.
You could make the spacer disk a bit better. Cut a keyway into them (and the tube) so they don't rotate. Further better, if you don't cut a radial slot, but just a short slot starting in the bore. This way, the tube can't jump out of the slot. Threading all the pieces onto the jig is a bit more complicated. And getting the disks off your finished product is impossible. You will have to cut them in situ, but you can move them around quite a bit, so you might break them (wooden) or snip off parts like a beaver.
Nick Nick

Nick sez: "> April 1. is tomorrow! What's the date on the plan?"
Very perceptive, Nick ! I wondered about that, also. The OP read sorta like one of Iggy's "Let's see how many we can suck in with this one."
Bob (ATBAON. . .) Swinney

Y'all never looked at flash steam boilers?
The nest of coils approach is a pretty common one.
Cheers Trevor Jones

Sounds like a "still" condenser to me. Sorry, but I can't picture this very well. When I buy soft copper tubing, it already comes in a spiral wrap. Modify the plan to fit the material?
Pete Stanaitis -------------------
michael gray wrote:

Nick and Bob. you gentlemen credit me with WAY to much cunning!
May I refer you to Model Engineer, Vol.157, No.3790, pages 640-641. "A Sequel to Simple Steam Raising plus some hints for Experimenters" by Ted Joliffe and Prof. D H Chaddock .
and a blessèd April's Fool Day to you both.
Mike in BC

My first thought. But then, see my other two postings. It can be done, and it ain't too complicated (once you got the trick).
The problem was, how the question was asked. He said "wound outward" and "wound inward". Winding inward triggers the "April-fool-but-not-me". But "winding inward" is only negative thinking. All wound outward is positive thinking.
Think positive! :-)
Nick

Anyone have any brilliant ideas on how to Thank you all for your suggestions. Especially the "wind two coils at the same time" variations so that my joins would always be on the outside of the nest. I'll try with wire first then my $$$ copper tubing. regards, Mike in BC

Sorry Mike ! No slur intended toward you. It was a sort of private little joke Nick and I share from time to time.
Bob Swinney