Tube arrangements quesion

[cwr]

Hi

I’ve been reading up on how a pack of boiler fire tubes can be arranged, as I was wondering if any preference exists. In Martin Evan’s book (The Model Steam Locomotive) he says that in the horizontal diamond pattern you can get more tubes in, a vertical diamond patter offers better circulation, and better still in a square pattern.

K.N. Harris says in his book a diamond pattern with tubes in vertical rows (referred to as a horizontal diamond pattern by Martin Evans) is best for circulation, and a square pattern offers no advantage is not recommended.

Has anyone any thoughts or opinions on this?

[baggo]

I shouldn't worry too much about the layout of the tubes so long as there is a reasonable gap between them to allow water circulation. In our sizes the tubes contribute very little to the steam raising capacity of the boiler. Think of them as preheaters of the feed water. More important is the area of the tubes against the area of the grate so that sufficient air is drawn through the fire. The total cross sectional area of the tubes (based on inside diameter) needs to be about 15% of the grate area.

John

[steam4ian]

G'day CWR

For guidance the AMBSC Code requires 3mm spacing between tubes at the tube plate as designed with a manufacturing tolerance of 0.5mm. This implies that the water space between tubes will be about 3mm.

Regards
Ian

[Deleted]

John, can you confirm again when 'grate area' is stated, is this the total area of the grate (i.e. approximated by the internal area of the firebox) or the free air area (air gap area) between the firebars? Consider a boiler with 10 off 7/16" x 20SWG firetubes total internal CSA 1.01 sq ins, with two superheaters analogous to a 7/16" firetube each, total CSA is 1.22sq ins. Tube length 11 3/32", cylinders are 1 1/8" bore with 1 1/2" stroke. Total grate area is 9.33 sq ins but taking off the firebars and tiebars, free grate area is 3.19 sq ins . This means that the tubes are 13% of total grate but 38% of free grate. Incidentally, if slightly smaller, 3/8", tubes are used, whilst a few more will fit in, the total CSA is less. What are your thoughts? Thanks Dave

[baggo]

Hi Dave,

I've always assumed that the grate area is the total area of the grate and that the ratio stated takes into account the area blanked off by the fire bars. If you used the actual free area through the firebars then 16% of that would give you very little tube area.

The ratio stated is actually something like 12 to 16% so anything around those figures should be ok. A lot of boiler designs have ratios of less than this e.g. some of the LBSC designs have a tube area only 5% of the grate area. I can imagine such boilers need a very strong blast to drag enough air through the fire.

I've reached the conclusion that the two really critical issues in boiler design are the grate area compared to the cylinder size (steam consumption versus ability to produce it) and the area of the tubes versus the grate area (ability to draw enough air through the fire without a ferocious blast).

As only the first inch or so of the tubes actually contribute anything towards steam production, the length of the tubes is probably not particularly important i.e. the Keiller ratio of tube diameter to length perhaps does not matter much.

John

[Shawki Shlemon]

I agree with John on boiler design , the size of the grate ( fire box ) in most cases except free lance is decided by the locomotive design , the length of the barrel will assist in providing spare warm water for the fire box , I alway try to get tube grate ratio of better than 12% or as close as possible . I think anything above 10% ratio will give a good steamer .
This is from my experience and other may think otherwise .

[Deleted]

Thanks for that John and Shawki, your comments have helped clear up some doubts and confusion that I was having.
Dave