Boilers with flue tubes but no superheater elements.

From 1925 the GWR fitted a pair of 5 1/8 diameter flue tubes in the upper corners of untapered boilers that had belpaire fireboxes, pressure 165psi and above and no superheater. This is reckoned to have reduced cracking in the corners of the firebox. Tapered boilers like the 94xx never had this feature, but it was seen on all post 1934 designs and also on replacement boilers on smaller pre group pannier tanks and side tanks like 850s, 2021s and I think 517s. Did any other lines use this design feature?

Jim C

 

It’s something I’ve never come across, either with main line or industrial boilers. I’ve often wondered why the GW pannier boilers had this and I’d be interested to read the theory behind this idea.

 

I would imagine it was pragmatic, based on experience, rather than theory. Plenty of P class boilers on pannier tanks had been superheated before 1925, with a single row of flue tubes, and by this date the superheater elements were being removed. If those boilers were seeing a significant reduction in problems then it was an obvious thing to try. It wasn't done on the taper boilers that had more room to lay out the tubes at the firebox end.
Worth noting that although the boilers were standard and interchangeable on the outside there were any number of different tube arrangements tried on the inside during the Churchward era.

 

This sounds very odd from the point of view of good even gas distribution across the tube bank. Surely there must be a restriction orifice or similar at the firebox end in the empty flue tubes to balance the flows?


Sent from my iPad using Tapatalk

 

You'd need to consult someone who works on them, but I haven't heard of such. If my sums are right its only 2.5% of the total cross sectional area. In my ignorance I find it hard to believe the air flow is very even across the entire tube plate., especially in corners, but such subtleties are way above my pay grade. Might one suspect that a minor inefficiency would have been considered a good tradeoff for increased boiler life?

 

Do you know what the tube bank looked like on similar boilers without the flues?

(Disclaimer: I am not a boilersmith).

My thought is that in rough terms, one flue tube would take equivalent area to about three normal tubes in a triangle (slightly less in fact). At which point there are several possibilities - that in previous boilers there were between zero and three tubes in that zone. If one flue replaces three tubes, potentially you might save cracks forming between the webs in what is a rather rigid part of the firebox. Alternatively, if that zone had no tubes at all, there is some loss of longitudinal rigidity there.

So I wonder if there was a design evolution that went something along the lines of - three tubes in the corner - experience shows prone to cracking between tubes in the corner - remove one or more tubes- experience shows corner prone to distortion - add a single flue tube back into the space to restore the support for the corner but without the webs between tubes.

Just a thought, I may be way off beam.

Tom

 

> Do you know what the tube bank looked like on similar boilers without the flues?

Not in detail, and detail counts!

Here's the RCTS list of unsuperheated P class (large pannier tank) boilers with belpaire fireboxes. As you can see there are plenty of variations! I only have one drawing of tube arrangements for a GWR boiler, which is the big Standard 7 taper boiler. The arrangement is extremely complex. Very roughly speaking the fire tubes are spaced very slightly closer together horizontally at the firebox end and wider apart vertically, but there's more to it than that. The upper rows of tubes are horizontal at the smokebox end and slightly arched to follow the slight arch of the inner firebox at the firebox end. My impression is that considerable care went into the location of individual tubes.



The majority of superheated P class boilers (those built from 1917 on) had 195 1 5/8 tubes and 6 5 1/8 tubes.

 

I don't think that's the basis of Jimc's point, though. Taking LMS boilers as these are the ones I'm more familiar with, although the principle's the same for all boilers, the Class 3 boilers used tubes 1 3/4 in diameter or 1 7/8 in diameter, giving a cross-sectional area of 2.41 sq.in and 2.76 sq.in respectively. The 5 1/8 in flues would give a cross-sectional area of 20.63 sq.in but much of this would be filled by the elements inside, reducing the actual area available to gas flow to similar to the tubes'. The critical ratio is the Surface over Area (S/A, but often expressed as the inverse), the internal surface area of the tube (length x circumference) divided by the cross-sectional area. The ideal ratio for boilers of this size was found to be about 400, and going 50 above or below this could lead to steaming problems. If the tube is too small, the hot gas passes too slowly and is cool at the front of the boiler; if the tube is too large the gas passes quickly and too much of the heat is carried to the smokebox and away up the chimney.

In this case, little of the heat carried by an empty flue would be transferred to the water but also, an excessive amount of the combustion gases would take the path of least resistance and head down the flues instead of through the tubes. Were the flues fitted with a restrictor, as would seem a logical idea?

 

This page has a good few photos of the boiler of 6412 (a Standard 21) under repair, and there are various views of the tube layout. I've struggled to find good images of the firebox end.
https://www.southdevonrailwayassociation.org/6412.html

 

GWR 14xx boilers are similarly fitted with 2 flue tubes for info.

 

Even more surprisingly perhaps RCTS suggests that when superheating was removed from pre group pannier tanks like the 1854s around 1930 the elements were simply removed leaving the flue tubes unaltered and the boilers still capable of being swapped onto superheated classes like Dean Goods. There's no mention of anything to modify the air flow, but I've asked the question of someone who's in charge of a 57xx.

 

Very interested by this: I had been told by someone particularly well informed that even draught through all the the tubes was not a given but got the assumption here - in a reply - that it could not vary from the smokebox vacuum in any particular tube.

Judging from from experience with the large boilered Ivatt Atlantic when it ran after preservation with the superheater elements removed and the steaming ruined over and above no superheat, what would go in a GW boiler with two superheater flues elementless would be no turbulent flow - the Reynolds number not being attained - and hence poor heat transfer plus the effect of more of the hot gases from the grate being drawn through the large diameter flues and less through the general tube bank - in which turbulent flow and better heat transmission was therefore impaired.
Possibly the last effect was mitigated - or even outweighed - by a brighter fire and highly efficient radiant transmission of heat through the inner firebox walls encouraged by better draught through the increased free gas area between the tube plates, but while 2% might make all the difference it cannot be assumed.

 

If I have the right figures from lner.info then the Ivatt Atlantic had 32 flue tubes and with superheater elements removed would have almost 40% of cross sectional area in the flue tubes, as opposed to 2.5% in the GWR boiler. It doesn't seem a very useful comparison. Even a superheated P class, which has just 6 flue tubes, only had 9% of area in the flue tubes. (all numbers rounded).

May I suggest that as the GWR saturated boilers to this design are well known to steam more than adequately, to imply that they should not is an odd position to adopt. I also suggest that with some hundreds of boilers in service as they migrated from one design style to the other the GWR works was well placed to evaluate the concept. I don't doubt there was an efficiency loss of some degree, but if it were balanced by significantly increased boiler life then presumably they reckoned that the trade off was worthwhile.

Anyway, I seem to have my answer, that the configuration was unique to the GWR, in the UK at least.

 

I've had confirmation that there's no especial restriction in the corner tubes.
I've had sight of 1902 and 1932 drawings of tube layouts of 2301 (eg 1854, 57xx pannier tank boilers) . Can't really answer James' question about tube layouts from them. The 1934 layout is very different. Even the actual shape of the inner firebox has changed a little. My uninformed guess is that the layout was worked up from a blank sheet each time rather than modify previous.

 

Hi,

There's no modification to airflow on 1450 or 1466 to my knowledge.

Reg.