Model Engine Maker
Supporting => Boilers => Topic started by: Dan Rowe on February 09, 2020, 05:03:47 PM
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I am working up a design for a small vertical copper boiler. It will be either 2.125" or 2.625" OD with submerged tubes. I want to roll the tubes into the tube sheet.
Checking my boiler design books K.N. Harris stated that for rolled tubes a thicker wall is needed. Page 135 states to use .032 wall for tubes smaller than 7/16" I have a tube roller that will work for a 5/16 x .032 wall tube that I can get from Online Metals.
Now for the needed advice. What is the thickess of the tube sheet required for rolling that tube?
I can not find the info in Harris. Martin Evans in Model Locomotive Boilers Pg 30 states that for expanded tubes the tube plate should be 20 to 25 percent thicker than the boiler shell thickness.
E. I. Pearce in Model Boiler Making 9th ed page 9 states the tube plate should be 1.5 times the boiler shell or 1/4 the diameter of the tube taking the larger figure.
One quarter of 5/16" gives me the most conservative tube sheet thickness at 2mm thick.
Does anyone have any other practical advice on rolling tubes in a copper boiler?
Thanks
Dan
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Hi Dan,
Sorry no experience with rolling copper tubes in boilers, have done a few in steel full-size ones, which is why with my small ones I silver solder the tubes in. Let us know how it works out, 2mm thick seams a bit on the thin side to me.
Gerald.
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Is this a steel shell with copper tubes or all copper? If its all copper I would go for silver solder.
On the steel locomotives I have worked on the tube sheets are usually 1/2" thick plate and the tubes seem to be a wall thickness of about .1 or so, so the tube sheet is maybe 5 x the tube wall.
Im not sure that is really a fair way to think about it, being steel not copper, but it seem like a reasonable start. So for copper that might translate to a tube sheet of .16. That feels a bit thick, but its a start.
I would look at the surface area of the rolled circle. If its to low you could be creating a shear action in the rolling and cutoff some of the tubes.
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The boiler is all copper. I realize that most people would silver solder a boiler made from copper, however, tube expansion is included in all books I mentioned.
K. N. Harris on page 132 talking about tube spacing says the minium bridge should be 1/4 the tube diameter and this can be increased to 33 or 40 percent of the tube OD for more strength to prevent cracking. "This is especially liable to occur where the tube expander is used with more enthusiasm than skill."
On page 148 Design No.1 "The boiler may be made on the riveting and soft solder caulking principal with the tubes fixed by expanding" This is a 6" OD 100PSI boiler with a 1/8" shell thickness I have to assume the tube plate is the same thickness as the round section as there is not a separate formula to calculate thickness of flat plates.
The boiler I am making is for a steam engine with a max of 30PSI so not a lot of pressure. I will make a small test piece and apply a hydro to it.
Dan
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I made up a test piece. I used 2.5mm copper sheet for the endplates and it passed the hydro test.
I pumped it to about 30 bar then closed the valve at 20 bar and it held for several minutes.
(http://www.7-8ths.info/gallery/6/213-100320085837.jpeg)
Cheers Dan
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I took the test piece up to 40 bar or 600 psi before I dumped the water. It held with no leak or deformation.
I used a 2.5mm plate as I thought 2mm sounded a bit thin. I think the advice in the boiler books mentioned is sound and the tube plate should be a bit thicker than the shell for tube expansion.
I am convinced that this will work for an all copper boiler. I am also planing a 6.25" copper scotch boiler that will be made from a 6" copper coupling. It will have about 30 tubes and will be very similar to the scotch boiler in the Harris boiler book.
Cheers Dan
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Dan, can you show how you are doing the tube rolling/expansion?
Are the tube ends just held by the forming, or is there a solder of some sort too?
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Chris, here is the setup I used to roll the tubes. The tube roller was designed for a much thicker tube plate so I used some 3/8" nuts as a spacer. The stopper is adjustable but it will not work if it covers the roller pins. The end of the roller pins are tapered for insertion in the tube so the tapered section has to be located past the tube plate for it to work.
The tube will try to slip out so I had to hold the far end when rolling the first side. No solder required.
(http://www.7-8ths.info/gallery/6/213-130320064047.jpeg)
(http://www.7-8ths.info/gallery/6/213-130320064140.jpeg)
Cheers Dan