Copper Boiler Fabrication

[sunworksco]

I'm getting ready to build a 7.5" diameter x 4.5" height all-copper boiler.
The wall thickness will be 1/8" and the working steam pressure of 150psi.
Will I need any stays? ???
I'm going to roll copper plate to form the tank shell and copper rivet the seam together then silver solder the overlapping seam.
Thanks,
Giovanni

 

[tel]

Stays will depend on the number and layout of your tubes. Generally speaking any area over about 3/4" x 3/4" will need staying, especially at 150 psi, which is above the general pressure for models. When you overlap the seam, make sure you put in some centre pop dimples to hold the plate/s far enough apart for complete penetration.

 

[sunworksco]

Tel,
Can you please post a drawing of that?
I am planning to flatten the rivets on the inside of the boiler, before soldering.
I am using rivets in the plates, as well.
I will use silver solder paste in between the plate seams.
Regards,
Giovanni

 

[Jasonb]

You may find it easier to to butt the ends of the ring together and then fit a "butt strip" behind, this will give you an easier job when it comes to fitting the flanged end plates as there will be no gap where the ring overlaps. The strip is kept short of the ends.

Basically if there is nothing supporting the end plates they will bulge so you need to fit stays, if you look at this pic of my Traction Engine boiler you will see the stays are spaced on 7/8" centres.

The act as stays on teh front tube plate but you can also see above the tubes on teh flat area there are 4 stays which run front to back.

Assuming you have done the calculations to arrive at your 1/8" thickness calculating the stays required should not be a problem.

Solder paste is not ideal as you may not get enough solder into the joint best to do as tell says and let it flow into the gap from one side.

J

 

[tel]

Jason (and his pics) have pretty much said it all, and better than I could.

Beautiful work on that boiler Jason - that could be used as a good example anywhere!

 

[Jasonb]

Can't take the credit for that one its made by Helen Verrall of Western steam

The brass bummy throatplate, manhole, washouts and door are the only bits I've done

J

 

[sunworksco]

Jason,
A beautiful textbook example!
Is that a stationary boiler?
Will I need any stays since I am filling the entire plate areas with 1/8" copper tubes?
Thanks,
Giovanni

 

[compspecial]

Giovanni, As I'm sure you know, Stanley boilers used a steel barrel reinforced by wrapping it with piano wire, but as there were so many tubes expanded into the end plates they acted as stays. I don't know if the same would hold true for a model copper boiler, but when the Stanley boilers were tested to destruction (at well over 1,000 psi) it was due to the tubes being flattened by the pressure!
Stew

 

[GWRdriver]

Will I need any stays since I am filling the entire plate areas with 1/8" copper tubes?

Wait! 1/8" tubes are FAR too small - they will severely restrict gass flow. Using an long-accepted formula for sizing flues [ID² = L/65] the optimum diameter for a flue of this length is approximately 0.260" so your flues could be .250" inside diameter but should not be less for optimized balance between free gas flow and heat absorption.

Have a look at the front end of Jason's boiler. Notice the proportion of flue diameter to barrel diameter. Now visualize what the proportion of 1/8" tubes to 7" diameter would be - far too small. If you are firing with coal you can use a slightly larger flue and accomodate slight changes in ID and increased resistance due to sooting. If you are gas firing this won't be a consideration. If you fill the flue sheets of your boiler with flues, similar to the way the bottom half of Jason's boiler has been "fillled" with flues, you will not need any longitudinal staying. It is also important to leave some distance between the flues for water circulation, in fact too many flues too close together can significantly impact the amount ot water and steam space a boiler needs. There are a couple of formulae for this space but generallay I use 1/8" minimum between nearest flue surfaces on small boilers, or boilers with small flues.

 

[Jasonb]

Without seeing the drawings I would not like to commit myself but if the whole surface of the two ends is being filled with tubes like the lower half of my boiler then probably not. Do you mean 1/8" wall as bore is a bit tight

No its only going to be stationary until I finish the rest, this was the state of play about 6mths ago.

J

 

[Jasonb]

If your boiler is going to have this many tubes then you won't have room for stays anyway

more pics here

 

[sunworksco]

Thanks, Guys!
I have learned a lot listening to you experts here.
I will use 1/4" ID copper tubes and keep them 1/8" distance from the walls.
Can I use Type-M copper tube?
I just got 3D CAD drawings for the Flying Tea Kettle body and chassis.
I can now make the wooden bucks to form the aluminum body sections.
Thanks again,
Giovanni

 

[arnoldb]

Hi Geovanni

Type M might be a bit too thin-walled for flue tubes; I'd strongly suggest you use type K.

I had a couple of questions on the calculations for flue tubes here and received excellent feedback; it might really be worth you while to go through that thread and do the calculations; there's also links to the copper.org site with some really great data sheets.

Jason, interesting post on that last image; I see they TIG those boilers!

Regards, Arnold

 

[sunworksco]

The laser programmer can set up the design for the tubing size and clearances with quick results.
Thanks,
Giovanni

 

[xo18thfa]

1/8" wall thickness for a 7.5" diameter shell at 150 PSI is way too thin. It needs to be 3/16" thickness. I site two references:

1. Kozo Hiaroka's "Building the New Shay" has an appendix on copper boilers. According to the chart on page 297, 1/8" wall is only good to about 100 PSI.

2. K.N. Harris. See excerpt below. The wall thickness should be .180" Max pressure on 1/8" wall is 104 PSI.

Also, I have heard, but never saw in writing, that the max pressure on any copper boiler is 110 PSI. The reason is that under pressure, water boiler boils at higher temperatures. At 150 PSI water boils at 366 degrees F, not 212F as it does at room temp. Copper looses strength quickly with elevated temps. A boiler may pressure test fine with cold water, things may be different when its hot.

If you are going to TIG weld, consider doing it in steel with copper flues.

 

[sunworksco]

Thanks!
Very good info.
I will keep the boiler at 100psi and use a 115psi pressure relief valve.
This amount of pressure should be fine in a 7.5" diameter x 4.5" height steaming a two-cylinder engine with 1-1/8" bores.
I will be using Anton gas/steam pressure regulators.
Is it possible to cast a copper shell?
I was considering rolling an 1/8" copper plate into a shell with tapered ends where the seam mates up and then silver solder.
I will be wrapping 316L stainless steel wire around the boiler shell similar to the full-scale Stanley boiler shell.
This method nearly doubles the strength of the boiler shell.
Thanks,
Giovanni

 

[xo18thfa]

Here is a link to K.N. Harris' book on model boiler building. The info is still relevant.

http://www.fastonline.org/CD3WD_40/JF/424/19-422.pdf

Rolling a copper barrel by tapering the ends is fine. That's one form of a coppersmith's joint. Strapping the joint is another method.

The dimensions you mentioned sounded like a Stanley Steamer boiler. The properties of steam and functioning of boilers sometimes don't scale down very well. The flues in model boilers need to be larger in diameter then just scaled down from an original. If the flues are too small, the fire won't draft through. It will just snuff out in the firebox. Don't ask me how I know that. Harris talks about flue size. He gives a range. Use the larger end of the range.

Harris also talks about heating surface, boiler efficiency and how much steam a boiler will produce. He also talks about how much steam a motor requires to operate at pressure and speed.

 

[GWRdriver]

Generally speaking 125psi is an absolute maximum for model boilers. As Bob's examples point out, in addition to the tensile strength of copper falling on a predictable curve as operating temperatures rise, the safe pressure capacity of a model copper boiler barrel is relative to it's diameter. My understanding of the behavior of copper is that once you go above 125psi the tensile strength curve drops off to such an extent that very quickly you come to a point where adding more wall thickness does not produce more pressure capacity. Now what, one might ask, is what about the many instances where British builders of full size locomotive boilers used copper plates for fireboxes? Many of those engines certainly ran at above 125psi. That's a question I've not seen asked and I don't know the answer.

 

[sunworksco]

I'm hoping that the wire wrap will help.
A 125psi rating would be great!
Thanks for the KNH book!
It is very nice. :bow:

 

[sunworksco]

Does anyone have info or experience with the Anton gas burner regulator valve???? ???
I'm very impressed with their products but not sure which design to use.