How many boiler tubes?

[Ian MunroParticipant @ianmunro50762]

[Ian MunroParticipant @ianmunro50762]

I was looking through Kozo’s “Building the new Shay” and noticed that the boiler only has three boiler tubes. This seems a far fewer than most boilers of the same size, and was wondering how it would affect steaming? I’m guessing that apart from making the boiler easier to construct, the large tubes allow the gases to flow more freely at the expense of heating surface reduction.

 

Does anyone have any views, or suggestions of reading matter that covers the optimum selection of boiler tube diameters vs. quantity for small boilers?

[WearyParticipant @weary]

Maybe of some help as a starting point:

 

Boiler Design:

(Excerpts from Model Engineer, Vol. 182, No. 4096 Page 748 18 June-1 July1999

 

….. ‘Conventional wisdom says that the combined tube area should be 1/5th to 1/6th of the grate area and that the tubes themselves should be based up on a ratio of their length divided by the square of their internal diameter.  The ratio is suggested to be around 80 and is denoted by the letter ‘k’.

 

Referring to coals high in ash and extra grate area thus being required:

……. ‘Experience also teaches that the ‘k’ ratio might be better reduced by some 25-30% for this situation.

 

………Some of the indifferent steaming stems from cramming too many small bore tubes into the barrel  which raises the internal resistanc eof the boiler to gas flow.’

 

 

Martin Evans in ‘Model Locomotive Boilers, their construction and design’ writes:

 

…An examination of the most successful full-sized locomotive boilers shows that the length of the tubes divided by the square of the internal diameter works out at between 50 and 70.  For model boilers of all scales therefore, the following formula is recommended:

d= square root(L/65)

where d is the internal diameter of the tube, and L is the distance between tubeplates, both being measured in inches.

 

 

As a footnote the ‘original’ 3.5″ Kozo Shay boiler seems to have 12 tubes, each 10mm outside diameter.

 

[Ian MunroParticipant @ianmunro50762]

Cheers for that!

I’ll have to give that some thought, at the moment it seems like a ratio dividing a number by the square of another number isn’t going to produce a dimensionless ratio, which I guess is why Martin Evans says to make the measurements in inches, but my brain isn’t up to working out quite why at this time of the evening

[WearyParticipant @weary]

Evans gives some worked examples, to take one as an illustration:

 Formula (as above)  d= square root (L/65)

 d= internal diameter tube,  L = distance between tubeplate.  All dimensions in inches

 

(Suppose)….. ‘the distance between tubeplates is 11inches.  Then

 

d= square root (11/65)

= 0.41inches

 

The nearest standard size tube in this example would be 1/2″ outside diameter x 20 swg.’

(thickness of tube walls is dealt with elsewhere in the book).

 

later in the book he refers to spacing of tubes, referring to drawing the firebox backplate to fit the tubes  

‘ …….. draw in the required number and size of superheater flues as decided beforehand along a horizontal line placed high on the tubeplate.  The ordinary tubes are then drawn in, and if the more usual horizontal diamond arrangement is adopted, the top row can be marked out on a horizontal line immediately below the flues and with a pitch spacing of Diameter x 4/3, the center-lines of each subsequent row of tubes being Diameter x 9/8 below one another.

A closer spacing of tubes than this is not recommended, or the circulation of water – never good in locomotive boilers – will be impaired.’

 

Note:  Superheater element and tube requirements are dealt with elsewhere in the text.

I’m afraid that I cannot illustrate the horizontal diamond tube arrangement, but if you experiment with the ratios given above,drawing the tubes as close as possible to each other, then I think you will see that they ‘nest’, each group of four tubes describing a diamond shape.

This is a ‘vertical diamond tube arrangement’

    O O O O O

  O O O O O O

O O O O O O O

The ‘horizontal diamond tube arangement would have the horizontal rows dropped down as close to as possible to each other.  In an illustration M Evans places a line drawn horizontaly through the lowest point of each tube in a row  below a line drawn through the upper edge of each of the tubes in the row below it.

You are probably best having a look at one of his boiler designs to get the layout he favours.

 

In the ‘original’ shay, K Hiraoka seems to have his tubes in a ‘vertical diamond tube arrangement’ as above.  Three tubes in top row, four in second row, three in third row, two in bottom row.

[Ian MunroParticipant @ianmunro50762]

Thanks for the comphrensive reply. Now I’m off to hunt down a copy of Martin Evans book.

[Paul White 3Participant @paulwhite3]

If I may add a slightly divergent comment to this interesting thread (interesting to me anyway).  Over the last few years there have been some very good articles in Model Engineer of an  experimental  nature using the modern stress analysis systems.

Aspects of the work relating to model boilers include the fact that some 80% of heat transfer that takes place, does so in the firebox (this has been claimed in past but confirmed latterly), another significant finding is the concentration of stress around and between stays.  To apply these two “findings” to model boilers is what has been done by Kozo in his design  of boilers on the,  Switcher, and Shay.

This development in design is, I believe, to be embraced, particularly as it reduces copper and silver solder cost, and eases manufacture.  Improved superheater possabilities also come with the different approach (see the article on a single flue boiler fitted to an Isle of Man loco used for passenger hauling).

Use of these latter findings must  move our modelling foreward, won’t  it?

[WearyParticipant @weary]

Thanks for the reminder about the vast majority of heat transfer taking place in the firebox.  Kozo states that “90% of steam is generated on the surface of the firebox or on the crown sheet” in ‘The Penn. A3 Switcher’ (but no source or evidence given).  This does beg questions about the ideal size of firebox of course, perhaps the problem in model locomotives is that the prototype dimensions decide the (exterior) firebox size, which might be significantly smaller than the ‘ideal’, thus often requiring more tubes (and tube surface area) than the three relatively large diameter tubesthat Kozo now seems to prefer.

 

K Wilson, ME Vol.200, No.4327, page 700, 6th June 2008, writes:  ……

“Length of firebox is settled from the prototype.   But if such does not exist, then firebox length could be about half the length of the barrel for a narrow box (Belpaire), or the equivalent square or a wide or Wooten box.’

 

Continuing to refer to the arrangements for boilers constrained in firebox design by the prototype Wilson writes:

“Tube length should work out at 1.5 to 2 times firebox length.  If longer than that a combustion chamber should be considered……….

 

He later continues to give a tube spacing of :

” 3/16 inch between neigbouring tubes in 7.25″, 1/8 inch in 5″, not less than 3/32 inch for smaller sizes’ (see I have got back to the thread title & tubes!)

 

I have just looked at the Kozo ‘switcher’ boiler in 3.5″ gauge… Firebox (4.25) is 1/3 of boiler length (12.75″), but combined with the three large tubes (0.78″ or 20mm outside diam.)

 

It is interesting that Kozo most recently uses fewer and larger ‘staybolts’ than ‘we’, (i.e. LBSC, Evans, Wilson, etc.,) would seem to use (six 3/16″(?) at 1″ centers each side of the firebox in the 3.5″ switcher), thus in combination with the fewer boiler tubes making the boiler easier to construct with less soldering etc.  Perhaps this approach would increase the numbers of people constructing their own boilers – refer to topic on the subject on this forum – whilst not significantly affecting the steam generation ability???

 

I would be interested in exploring this whole topic of tube/firebox sizes etc., both ‘ideal’ and how to fit to prototype constraints in the light of the most recent thinking.  Has anyone any sources of info’?

I realise that many (most?) model engineers might consider this to be a rather esoteric theoretical pursuit of little or no interest…….. However……. Maybe boiler simplification and updating is how some older designs could be ‘updated’ and made more easy to construct to satisfy requests for such updating.

 

 

[Paul White 3Participant @paulwhite3]

Weary, many thanks for your input responding to my post.

Apart from Kozo’s comments, an interesting set of results on the matter of boiler heat transfer, as I am sure you know, were produced by Jim Ewins and are published in the back of Evans boiler construction book.  It was the reading of these findings that got me started on seeking a” better mousetrap” Kozo’s designs and the article on “Peverill” in 5″ gauge with the single tube boiler, both fed my search for something new.

 

I think that 50 plus years  of no real movement in model boiler design is plenty, not just for efficiency  reasons but also ease of construction. ” Model engineers” used to be regarded with ” experimental” engineers.!

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