tube spacing

[Johnny O]

You might try looking at The Golden Gate Live Steamers web site, I found some useful info there on boiler design.Also the Sugar Factory TEMA site is useful for tube sheet calculations such as Ligament sizes. I haven't been able to find a coralation to tube sheet thickness like min. thickness in relation to ligament thickness. I made a mock up for this, Im using 5/8" O.D. copper with .187 ligaments. Though I might change that. I know the D/4 rule stated in some ref.'s but the real limitation is due diligence in boiler water treatment and cleaning to fight corrosion. The Australian Safe Boiler Code is also a good reference. Which states maintaining a Min. of 1/4" of ligament or maybe it was 6mm, I forget. I found no distortion in the 1/2" thick tube sheet sample with 3/16" ligaments after rolling a few 5/8" tubes into it. I made a simple go/no go plug gage to size wall thickness reduction. The corrosion factor has me thinking about a new tube layout. Elliott Tube Roller site had other good stuff on it as well.
Hope this helps.

[rudd]

Thanks.
Getting back to what Fender was saying about the tube length calculation,
I went to the GGLS website and found Winton Brown's engineering data. It looks like the same info in the RR Supply book, right down to the typeface.
https://www.goldengatels.org/library/Te ... %20PDF.pdf
Are we thinking those rules were derived from full size?
I get differing results from those I get using formulas found at http://www.modeng.johnbaguley.info/Loco ... esign1.htm, but Baguley notes that those formulas came from analysis of 2 1/2" and 5" gauge engines (not 7 1/2")

[baggo]

I wouldn't get too hung up on the Keiller ratio for the size of tubes used in our small gauge boilers. Keiller did base his calculations on the tubes in full size locos where the conditions in the tubes are different to those in small diameter ones. In small diameter tubes the gas flow is mostly laminar instead of the turbulent flow in large diameter tubes. This laminar flow produces a layer of stagnant gas on the inside surfaces of the tubes which prevents effective heat conduction through the wall of the tubes. This is why turbulators are so effective in increasing the steam raising capacity of a boiler - they introduce turbulence in the gas flow which breaks up the insulating boundary layer. I know they are only normally used in gas fired boilers but I'm sure they would work equally well in coal fired boilers but keeping them clean would be a pain.

Jim Ewins did some quite comprehensive temperature measurements on a 5" gauge boiler using thermocouples on various parts of the boiler including in the tubes at three points - start, middle and end. From his results he concluded that most of the heat transfer takes place in the first third of the tube. After that, the rest of the tube just carries the waste gasses to the smokebox and contributes very little. With that in mind it follows that the tube length is not particularly important. It's far more important to have sufficient free gas area through the tubes to promote good air flow through the grate and the boiler can 'breathe'. If I'm designing a boiler I try to get the ratio of the tube area to be 12 - 15% of the grate area. However, it's often impossible to achieve that ratio with a wide firebox boiler unless you have a very large diameter boiler barrel as you just can't fit enough tubes in.

John

[Fender]

I would agree with Baggo on the tube sizes and flow area. All of my experience has been with coal, not oil or propane. From what I’ve read, most of the steam generation occurs around the firebox, and this makes sense to me. Maybe this is less so with oil or propane. But, in my opinion, the main thing is to prevent the tubes from restricting the flow of combustion gases through the boiler.