Thermic Syphon

I've been discussing the boiler build on the Britannia with my friendly club boiler inspector and he suggested that the thermic syphon design may just increase complexity for not much advantage. I can completely see his point from the point of view of reducing the complexity, and will probably take that advice, but I did wonder just how effective the thermic syphon is at our scales? (In my case 3.5 gauge). Some things seem to scale down and still work very effectively, such as superheaters, what are peoples opinions on the efficacy of the thermic syphon when scaled down?

Actually I suspect its not actually a thermic syphon in the Britannia, just diagonal cross tubes. Again this highlights holes in my knowledge!

Just wondering - how did it become known as thermic syphon in boilers while it is thermosyphon in everything else?

Russell

I have never seen the drawings for Britannia, but assuming that there is a D shaped combustion chamber like the LBSC/Martin Evans 9F wouldn't you need to keep the syphons in order to stay it?

My son bought an Evening Star "project" a while back and was advised by a boiler inspector to consider redesigning the boiler in order to abandon the combustion chamber completely as it cannot be tested before it becomes completely inaccessible. He said he would no longer accept the old solution of sloshing soft solder about inside the boiler in the hope of fixing a leak in the tubeplate, so the risk of building a scrap boiler was too great.

Perhaps I have misunderstood things, but there seem to me to be enough questions about older boiler designs under current inspection codes to justify a complete series in ME or even a book of revised drawings and instructions.

Brian

Read series of articles running in ME at present for full low down on how useful or not thermic syphons are in small sizes. If you decide (wisely) to not fit them then you will have to arrange some crown stays, but that is a lot easier than syphons.

There seems to be a strange reluctance to having a properly referenced design code in the UK. Plenty of people will quote KNHarris said this, Martin Evans/Don Young said that, but no mention of what basis these gentlemen had for these pronouncements. I'm not suggesting they are wrong, but the Australian code is referenced back to national standards. Contrary to popular myth it is not unduly restrictive. I took the time once to compare it with UK standards and it agreed well.

Seems to me that some boiler inspectors are making their own rules up. Boilers should be inspected exactly to the requirements of the Rule book. If the rules state no soft solder, fine, you cannot use it, but if nothing is mentioned about it you should be OK to use it. Imagine what our MOT tests would be like if testers deviated from the rules.

Let me display my ignorance (again):

A thermosyphon system relies on two compartments full of liquid, one of which is heated, the other cooled, with two connections following different routes. If the colder tank is above the hotter, hot liquid will flow up the higher connection to the colder tank, and cold will flow down the other pipe. All relying on the expansion of the liquid with temperature.

On a model locomotive there is scant room for the boiler, never mind an extra tank, and why the system would provide an advantage is not clear (to me - see note above re ignorance).

Please can someone explain this - or is the term Thermic Syphon misapplied, or what?

Thanks, Tim

Edited By Tim Stevens on 15/08/2018 14:34:48

That is a good point, as whilst volume 2 says construction "should" (importantly not "shall" be silver soldered throughout but permits an exception for threaded and nutted stays, volume 1 makes no reference to soft solder at all. It does however require that the inspector checks all joints penetration of "silver" solder which could imply that a soft soldered joint, or one that has been repaired with soft solder, is not permitted. To me as a layman, it doesn't seem sufficiently definitive either way.

Incidentally, whilst volume 2 prohibits phosphorous-bearing brazing alloys for coal-fired boilers, volume 1 doesn't mention brazing. Does this imply that Sifbronze is no longer permitted on boilers over 3 bar-litres, or that it is permitted because it isn't mentioned?

Brian

.

Boiler making/inspection is not my thing, but I have had a lot of experience with may/should/shall, and I applaud your effort to highlight the importance of the word chosen, Brian.

It's a great pity that the inappropriate smiley has jumped in.

... Will we never be rid of these ?

A final point: Given that, in this context, "silver solder" seems to have been deprecated as a description; it may be worth either locally defining it in the documents, or using current terminology.

[yes, everyone, this is a serious point]

MichaelG.

Edited By Michael Gilligan on 15/08/2018 15:16:14

"Just wondering - how did it become known as thermic syphon in boilers while it is thermosyphon in everything else?

Russell"

Could be because the guy who patented it in 1928 called it that. Google is your friend.

https://patents.google.com/patent/US1679051

Hello Jon,

From chief stirrer upper of boiler design dogma.

In my estimation, the thermic siphons and "combustion chamber" in the LBSC design are just complications without any great payback. In our small sizes, flame radiation is a lot less significant than in full size. Therefore, if a surface cannot "see the fire" it will not get the benefit of radiant heat from the fire. You will appreciate that the forward half of the thermic siphons and quite a lot of the "D" combustion chamber cannot see the fire.

If you reduce the design to an ordinary firebox, and make the tubes larger you will end up with similar performance. If you are lucky, you will be able to slot in an extra tube, having lost the extra flanges of the D combustion chamber.

I suggest you need to download my boiler design program and start exploring how much contribution all the complicated bits make. See

**LINK**

for a shortcut to my program and the technical description.

I already have a datafile for the boiler "as per LBSC" so P.M me for a copy. Will be happy to help in the analysis.

Martin

I'm sure ME would welcome a serious series on boiler design, from simple pot boilers working up to proper loco boilers and traction engine ones.

It would be particularly wonderful if (a) it was about designing to meet the codes noting any major difference in Aus or Kiwi land, for example, and (b) there was one or more persons willing to check over and sign off new designs, possibly for a suitable fee, as a thrid party sense-check before they get ripped to pieces by nervous boiler inspectors.

That said, who would be a boiler inspector? Taking on a huge H&S responsibility for no reward as a volunteer and having to tell people what they often don't want to hear; is it any wonder that many err on the side of caution in the absence of really clear and explicit guidance. Perhaps there should not just be rule books, but plenty of 'worked examples' and case studies.

Neil

More than anything else I think this highlights my lack of understanding of various principles! Back to the library for me.

It's raised some very interesting discussing, thanks all.

You're not going to get anyone to 'check over and sign off designs ...for a suitable fee'. That would leave them open to being sued by Flywheel Schyster and Co if anything went wrong with a boiler, unless MyTimeMedia covered the personal liability insurance, and I suspect that's as likely as me winning a beauty contest.

Having said that, ME recently re-published an ancient LBSC design boiler which I'm fairly sure would have given most modern boiler inspectors conniptions. There was a health warning but why publish in the first place.

Neil, when I get back from holiday I'll send you a pm about Aussy standards and BS

One assumes that Bulleid used thermic syphons in his Pacifics, with the object of improving water circulation within the boiler. Did any other Railway/road use them? Certainly, in UK the Belpaire firebox seemed to provide sufficient circulation for pretty high rates of steam production, (circa 30,000 lbs ) without the need to resort to mechanical stokers.

As a non steam man, adding thermic syphons to a boiler, which in prototype form did not have them, would seem to be introducing an additional feature which might, or might not be beneficial, or could even be a source of problems.

In the amateur radio world, the motto is often KISS, (Keep It Simple Stupid )

Also, although "Ye canna scale physics", how often does a model need steam to be produced at a maximum rate? Given that the grate, fuel, fuel size, draughting and driving/firing skills, may not be the same scale as the prototype, are they really needed? Are the loads, in a cubed scale, really of the order of hundred of tons?

Howard

Nicholson gained a US patent for his thermic syphon in 1928 and this is a link where some of its advantages are described. http://www.athra.asn.au/library/Thermic_Syphons.pdf

As to other locos tyhat used them Gresley designed the V4 2-6-2 in 1939 (his last design). The first member of this class, 'Bantam Cock' has a traditional copper firebox while the second had a boiler with an all welded steel firebox and thermic syphon. Why? Perhaps because it was felt that the increased efficiency would counter the lower heat conductance of a steel firebox compared to a copper one. Alternately Bulleid, who had been Gresley's assistant until he took up his position with the Southern, might have considered the case for the thermic syphon proved, but adding them to a copper firebox problematic so the welded steel firebox of the SR pacifics enabled their inclusion. It is difficult to determine exact reasons after this amount of time.

A final thought, while not professing to any expertise in boiler design or construction, the elimination of design features such as combustion chambers or syphons would surely make a boiler 'not to an accepted design' and raise more questions than it gives answers.

Edited By Nick Clarke 3 on 16/08/2018 11:42:31

Edited By Nick Clarke 3 on 16/08/2018 11:43:54

Edited By Nick Clarke 3 on 16/08/2018 11:53:28

The difference in thermal conductivity between steel and copper firebox makes no sensible difference to the heat transfer. The main heat drop is from the hot gasses to the metal, temperature drop through the metal is very small

Can I go back a few posts and pick up the point made by Brian G . I find it difficult to understand the inspectors comments viz :- " consider redesigning the boiler in order to abandon the combustion chamber completely as it cannot be tested before it becomes completely inaccessible "

Firstly if you are re-designing the boiler it is surely no longer a published design benefiting from " grandfather rights" You would need to get the altered design proven by calculation and as Duncan rightly points out who will sign it off? Not the boiler inspector I suspect!

Secondly having built a combustion tube boiler with cross stays for LBSC Uranus which is very similar size to a Brittania boiler in 3 1/2 G, one can either test the combustion chamber alone by blanking off both ends and pressurise from the combustion space outwards.

In my case I also fitted the end tube plate and the embryo firebox and throatplate, and it can then be tested up to full pressure by blanking off the firebox base, and the tubeplate ends and pressurising "inside out and upside down" so to speak. In my case I used an aluminium plate with a rubber gasket around the grate opening with a connection for the boiler test pump and a vent. I did it this way because all these joints were brazed with 750 deg silver solder and the remainder of the boiler is with 580 / 600 to avoid disturbing these joints at the later build stages. This will confirm the cross tube and the throatplate joints.

I agree that boiler inspectors take on a huge responsibility voluntarily on our behalf, and a proper set of standards for design and construction along the lines of the Australian code would at least give them all a standard to work to and avoid individual interpretation.

God forbid we should have an accident somewhere, then everything will be put on hold until we do get a code and a set of standards approved, which wont be a quick process; and the ensuing limbo will mean no one, not even the commercial builders will be in a position to build a boiler without knowing what the future standards will be. So perhaps the way forward is to be proactive and get it before the event.

I have been thinking about boilers at 3 1/2" gauge for a while now and I keep coming against the same question when people raise the issue and nobody ever answers i.e., 'Calculations - what calculations?'

From what I can see - there are just a few 'rules' relating to plate and tube thicknesses etc but for plates they always end up the same thickness anyway i.e.,3/32" or 1/8" thick copper!!!!

I'd prefer a few updated drawings with corrected features e.g., bushes in the plates for longitudinal stays rather than a threaded plate etc etc.

Phil H

Some recently suggested we should get ALL our designs signed off...

I'm sure a chartered engineer with appropriate experience and Professional Liability would do it happily - that's what their job is, after all. But I was thinking that boiler designers might take advantage of a reasonably costed service, suitable calcs have been published many times and an engineer would only be in trouble if they were negligent in choice of calculations and made mistakes in checking them. It's no different from designing your own extension and then getting an architect to check the drawings.

Come to think of it, surely every time a Boiler Tester advises on boiler construction then tests it, surely they are in the same situation - and all are happily protected by the club's insurance if they do the task with due care.

Neil

Edited By Neil Wyatt on 16/08/2018 19:04:22

I didn't express myself all that clearly. Yes a Chartered Engineer with professional liability insurance would sign off the calculations (assuming they are correct of course), but you are going to pay for his time and the insurance will be built into his hourly rate. This is going to get expensive, and I doubt ME or MEW is going to cover the cost for designs they publish. The sums are not terribly difficult, and are well set out in the Australian code. I don't think there is any requirement in UK regulations for independent checking, so do them yourself. What we don't have is a standard for acceptable stresses which is traceable back to national standards. LBSC said this or Martin Evans said that will not wash I'm afraid, even if they were correct. 

The chances of a reasonably designed copper boiler which has passed a 2 times pressure test and a steam accumulation test exploding in service are very small as copper is such a ductile material, so let's not get too carried away. There are loads of known good designs available, just adapt one of those and check the plate thickness and stay spacing.

Edited By duncan webster on 16/08/2018 22:35:07

Edited By duncan webster on 16/08/2018 22:53:05

Edited By duncan webster on 16/08/2018 22:59:11