tube spacing

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rudd
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tube spacing

Post by rudd »

I'm laying out a tube nest, small 2 1/2" scale engine, will use either 1/2 or 5/8" od tubes. I've checked the sources I have, and searched this forum, and don't find the info I am looking for.
What is the minimum recommended distance clear between tubes? If it isn't enough, the holes will get egg shaped when the tubes are rolled. But then I've seen several sources that recommend silver soldering in copper tubes, which I think would eliminate the rolling.
I'm intending to have someone that knows what they are doing actually build the boiler, but I need to come up with a feasible design with regards to heating area/steam space etc. I understand that space for the steam to rise straight up off the tubes is required, but how much?
The example in the image shows 1/2" OD tubes.
Capture.JPG
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Greg_Lewis
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Re: tube spacing

Post by Greg_Lewis »

The Australian Miniature Boiler Code for steel boilers, which is quite thorough and carefully thought out, says, "Tube holes shall be arranged in tubeplates so that no ligament is less than D/4, where D is the diameter of equal size tubes, or (D+d)/8, where D and d are the diameters of unequal size tubes. In no case shall a tube ligament be less than six (6) mm."

I'm sure the experts here will have other comments.

If you are building a steel boiler, I very strongly advise you to NOT silver solder the tubes. If you ever should have to remove one or more of them it will be extremely difficult if not impossible. Properly rolled tubes will be just fine and removable if ever needed. Please pay attention to this. It cost me about $600 to find this out.

I have a design by John Bailie for a shop-made tube roller, if you need it.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 35 years of turning perfectly good metal into bits of useless scrap.
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rudd
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Re: tube spacing

Post by rudd »

I saw the reference to "ligament" - is that Oz speak for the width of steel between tubes? I.e., w/ 1/2" tubes, my .187" should be .25"?

I would not mind seeing the design for the roller if it is not too much trouble.

thanks
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Greg_Lewis
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Re: tube spacing

Post by Greg_Lewis »

Yes, that's the space between the tubes.
Send me a p.m. with your email and I'll send you the tube roller article.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 35 years of turning perfectly good metal into bits of useless scrap.
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Greg_Lewis
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Re: tube spacing

Post by Greg_Lewis »

By the way, you said you were considering 1/2" o.d. tubes. Those seem rather small for a 2 1/2 scale engine. The i.d. of such a tube seems like it would restrict air flow.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 35 years of turning perfectly good metal into bits of useless scrap.
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Fender
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Re: tube spacing

Post by Fender »

What is going to be the distance from the front tubesheet to the rear tubesheet?
This is more significant in calculating the best tube diameter than the scale of the locomotive. The AMBC recommendation of 6mm minimum for “ligaments” is slightly less than 1/4”, which seems reasonable. You can probably make this A little smaller by using thicker tubesheets, but bear in mind that the tube expander tool will need some space around the tube to do its function, and greater attention will be needed to keep the tubes from “scaling up”.
Dan Watson
Chattanooga, TN
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rudd
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Re: tube spacing

Post by rudd »

tube length between sheets is 16.5"
The 5/8" OD tubes work better for getting the required free gas area.
From the "Engineering handbook for Recreational Railroaders"
TE/500 = grate area.
Grate area/8 = tube area
I'm back checking those figures with Jim Erwin's calculations
http://www.modeng.johnbaguley.info/Loco ... esign1.htm
I'm getting a K/t of right at 60, a bit low. i.e., the tubes could be a bit longer or smaller, but this doesn't really work well with the design of the loke I am modeling
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Fender
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Re: tube spacing

Post by Fender »

The formula for tube length vs. diameter was originally calculated for full-size locomotives, and I suspect that it doesn't extrapolate well to our small boilers, resulting in tubes that are too small. The reason is that coal won't fully combust in our small fireboxes. What fuel will you be using? If coal, I'd suggest 5/8" or even 3/4" OD. If propane, maybe 1/2" or 5/8".
Dan Watson
Chattanooga, TN
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rudd
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Re: tube spacing

Post by rudd »

The formula is specifically for model locomotives. I'll be firing on oil or propane, not coal. I'd prefer propane, but I am not sure at this point if I will get enough heat out of it. Again, the 5/8" tubes give me the tube area I need, the 1/2" will not.
The boiler will be a wagon top, the firebox end will be the same diameter as an Allen Mogul boiler, say 8.5", front about 6.5
The wagontop is needed to get enough steam space.
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NP317
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Re: tube spacing

Post by NP317 »

Based on the dimensions you noted, my Allen Ten Wheeler boiler probably has more volume that yours will have.
And I have way more propane heat available than I required, even when working the locomotive hard, with passengers in tow.
I have never run out of steam when pulling those long grades at Train Mountain. Data.
My Mikado seems to be just as hot with propane firing. Pics attached of its propane burner.

Comparing my Ten Wheeler boiler maintenance with that of my oil fired steam launch boiler, I would take propane over oil any day.
Boats are not conducive to propane for safety reasons, because it can settle in the bilge.
Propane explosions in boats are exciting, and launch the crew around a wide area...

I've also fired many miles of full-sized steam locomotives with oil fired boilers. They are dirty, very dirty. But not as bad as coal.
I have spent literally months of my life cleaning full-sized fireboxes and smokeboxes. A skin-color changing experience.

I like propane fired boilers.
RussN
Burner 1 small.jpg
Burner 4 small.jpg
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rudd
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Re: tube spacing

Post by rudd »

Laff! Russ, thanks for that. Yup, I have fired oil on 12" to the foot scale as well, luckily we had some young pups to clean flues. Had one young man learning to fire that didn't *quite* have the "cut down the oil when you cut down the steam" idea down, now a big charred spot on the turntable decking.

I know for a fact this boiler will have less volume than the Allen 10 wheeler. Tiny little Porter logging mogul, 16 tons per the catalogue reprint I bought. Looking at your burner, I could get maybe 5 of the 9 tubes in the firebox, but maybe 1 1/2" to 2" longer.

I have prints for the Mogul, it shows mostly 1/2 tubes but has three superheater flues. I wonder how many have installed those superheater flues.

I hope, probably in vain, to make it to Train Mountain some day.
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NP317
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Re: tube spacing

Post by NP317 »

Good to hear more of your Porter project. That will be a delightful locomotive. I look forward to seeing more.

I have chosen to not use superheaters in either of my locomotives. My experience with full sized units was always lots of work to maintain their integrity.
And in miniature, I do not need the slight increased efficiency they might offer. Not to mention the potential damage from lubrication failures with the dryer steam.
I experienced that in my Ten Wheeler, and it slightly damaged the hardened valve gear. After 15 years of steaming I just finished rebuilding the valve gear to remove all the lost motion, and interestingly, it was mostly in the right side where the lubricator input failed. And that after nearly 600 real miles of operation. A little bit of squeaking, and it's rebuild time.

And my propane setup allows 8 full hours of steaming with one filing of the forklift-sized tank I use.
Propane...
My thoughts.
RussN
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