Brazed S.S. Superheater

[rickhann]

I have been advised to definitely use superheaters in my 3-1/2" locomotive that is now under construction. My next question is: Is it OK to construct the superheaters using stainless steel/brazed (not silver solder)) construction or must the superheaters be welded? Are the conditions in the firebox too severe for brazing? My reason for asking is that I have an oxy/acetylene rig that will easily braze the SS connections, but I have no SS welding capability. Any insight would be appreciated. Thanks in advance.

Rick
Cape Cod, MA USA

[ianengr]

Hi Rick,

You should find brazing quite satisfactory. Although I have only used copper superheater tubes I am quite certain it is the usual practice to braze S.S. with bronze. (Tobin) or similar. It has been shown some time ago that it is easy to obtain a degree of superheat that is excessive, even in the smaller size locomotives, and this would almost certainly be the case if the bronze brazing was to fail purely from overheating, with a melting point around 1200 deg. C. (from memory) This can be bad news for lubrication with little gain in economy over a moderate degree.

I made the point in a personal message with "brozier" a few weeks back that the use of an arch plate may help reduce the servere heat from the superheaters particularly when the locomotive is standing with the blower on, such as when raising steam etc. this is something you might like to consider yourself if you are concerned about overheating, it may also make the boiler a better/more efficient steamer.

I'd love to hear about the type of engine you are building.

Kind regards,

Ian.

[steam4ian]

G'day Ian, Rick et al.

I recently saw an article in "Model Engineer" (I think) that seriously questioned the value of super heat for model locos. It was written by a learned gentleman, not a Luddite as happed a century ago in full size practice. I was only thumbing through the mag. in the newsagent (how many can you buy?) so I am not sure of either the magazine or the date. Part of the line of argument was that the boiler is more efficient without the flues blocked of by the superheater elements. I don't think radiant superheaters got a mention.

It would be an interesting debate on this forum. It is incredible that models, whose prototype (if one existed) would not reasonably have superheat, are given miniature superheated boilers.

Regards,
Ian (nuther one)

[cotswold]

The theorists may be correct, but when it comes to "our sizes", it is still theory. When I hear arguments that include extrapolation from the very large to the very small I am reminded that conventional aerodynamic theory proves that bumble bees can't fly!

With the possible exception of stainless steel projecting into the fire box, it may be true model super-heaters do not super heat to any real degree. But I am convinced that when viewed as steam driers they should not be dismissed.

[baggo]

Although the mentioned article by D A G Brown did suggest that 'in theory' superheating is a waste of time in our smaller gauges, in practice it is accepted by most that superheaters do greatly reduce the coal and water consumption. The radiant type extending into the firebox are far more effective than the shorter flue only type and can reduce the restriction of the flues as the return bend is not inside the flue.

Incidentally, the article by D A G Brown was shortly followed by another series of articles by Dennis Postlethwaite which basically proved the opposite

The argument that a boiler without superheaters can be more efficient due to the flues not being obstructed by the superheaters suggests that the superheater flues are too small in diameter for the size of the elements. It is easy to size them so that the free gas area through the flue/element combination is the same as that through a plain flue.

On Helen Longish I used Silverflo 24 for soldering the return bends on the stainless elements and have had no problems so far. The returns are tucked away right at the back of the firebox over the firehole so probably escape the ravages of the fire to some degree.

It would be interesting to measure the steam temperature on Helen after superheating as it seems to be very high, perhaps a bit too high. Unfortunately I don't have a suitable thermometer. I know that the original silicon O rings on the steam connections to the cylinders turned to 'chewing gum' soon after the first runs and seals made from PTFE suffered a similar fate, suggesting a temperature of 250° C plus. The seals have now been replaced with threaded connections and copper olives.

John

[Shawki Shlemon]

Hi
So far , as far as I know no one has proved any of the arguments on super heaters . Practically engines without seems to emit showers from chimney more often than those with super heaters . Not so sure about the efficiency , I decided a long time ago to go in the middle of the road and have super heaters that do NOT protrude into the fire box and therefore SS is not required . I have had no problems at all with any super heaters and therefore sticking to my decision .

[steam4ian]

G'day Cotswald

I agree, the drying function may be more use than the superheat. I must admit to being surprised by the article because it was well supported by maths and logical thought.

Regards
Ian

[alanstepney]

I believe it was Jim Ewins that wrote several articles on locomotive efficiency, one of which covered superheaters.
His conclusion was that they are of benefit, and I have seen other articles that came to the same conclusion.

The one caveat, is that they should be properly designed for the purpose.

[havoc]

because it was well supported by maths and logical thought

You can be as logical and mathematical as you want, but if you start from a wrong premises you arrive at the wrong conclusion.

[cotswold]

May 18, 2008 16:38:47 GMT alanstepney said:

... The one caveat, is that they should be properly designed for the purpose.

Every time I see one of the concentric tube devices so beloved of the late Don Young, I am reminded of a biology lesson from my school days.

The teacher was explaining that when a wading bird is standing in freezing water, blood leaves the birds body at normal temperature but arrives at the bird's feet cold. Nevertheless it then arrives back at the birds body at at very nearly the temperature at which it left. The trick is concentric webs of capillaries that act as heat exchangers. Instead of attempting to draw the individual blood vessels he drew a diagram that many years later I remembered and recognised when I first saw the drawings of my Hunslet.

Needless to say, I have used bought in stainless steel super-heaters with welded spear points.

[baggo]

I suppose the only way to prove the benefit or otherwise of superheating would be to build two locos, identical in every respect except one had a superheated boiler and the other non superheated. The boilers would have to have the tubes sized to give the same gas area and heating surface. They would then have to be run on a rolling road against a known load and the coal and water consumptions measured over a period of time.

It is my assumption that 'efficiency' is measured as the power (drawbar pull) produced versus the water/fuel consumed. I presume this has been the idea behind the IMLEC competitions etc. but, to my mind, they are not a true test as an unknown 'variable' i.e. the driver, is introduced into the equation. A good driver may well get a better result on a poorly designed loco than a less experienced driver would get on a more efficiently designed one. It is interesting to note that some drivers have won the trials two or three times on different locos which suggests to me that the success lies with the driver rather than the loco.

As Alan has mentioned, the late Jim Ewins did a lot of research into loco efficiency which makes interesting reading and he was probably the only one who made any recent progress into this field. As Jim said, there has been little, if any, development work in model loco design since the pioneering work by LBSC.

John

[circlip]

I'm sure a lot of maths and original thought went into the adoption of a square steering wheel on the original all agro's.

[havoc]

but, to my mind, they are not a true test as an unknown 'variable' i.e. the driver, is introduced into the equation. A good driver may well get a better result on a poorly designed loco than a less experienced driver would get on a more efficiently designed one. It is interesting to note that some drivers have won the trials two or three times on different locos which suggests to me that the success lies with the driver rather than the loco.

Very good point. This was also raised when comparing efficiency runs on prototype engines in 1:1 scale. Often important reductions in consumption were observed that vanished once the "new and better" engine started commercial duty. This was due to the crew driving differently when running a new, shiny and observed engine.

[alanstepney]

Who was the CME who said that he could improve the efficiency of a (full size) locomotive just by painting the chimney a different colour, as the crew would know they were being observed?

[Tony K]

May 19, 2008 5:34:48 GMT @circlip said:

I'm sure a lot of maths and original thought went into the adoption of a square steering wheel on the original all agro's.

Sir Clip, I think nottle. I think it was more about having something different to sell a car that was just an 1100 with rounded corners. It got a bad press and died because of common sense - something that goes round is mostly better if it is round also - the square wheel could have been a success if there was not a need to turn it. I think the fact that it was fitted to a naff car did not help. Maybe it was to imitate an aeroplane and you might think it went nearly as fast! However, if my stomach keeps expanding (eating too many Mars bars at Northampton Club), I may have to fit one of these Smart Wheel

I digress. So far we have established the superheater may just dry the steam - IMHO a worthy feature anyway. You say it may impede the flow of gases through the flues - IMHO the flue is designed to cope with that. If not - make more flue area.
It is easy to do a half-way house - just insert a tube instead of a multi-folded superheater into a current loco - OK some superheating will take place, but much reduced and the gas flow through the flue should improve. Comparison of performance would give some idea as to the advantages/disadvantages of the current arrangements. You do it though - I'm too busy! ;D Anxiously awaiting to hear the results.

In answer to Rickhann's original question (he has decided to have a superheater), ring up Paul Gammon who will supply an excellent stainless superheater to a standard not achievable by me, at very reasonable cost.

[chameleonrob]

DAG Brown article started with the comparison of dry steam and superheated steam, he completely forgot that saturated steam boilers are called that for a reason, hence his faulty conclusion.
btw I have a fiend who has a superheated romulus which ha been measured at 50^o superheat.
rob

[chris vine]

I forget the way that Dag Brown went about his proof that superheaters were no good.

However the trouble with saturated steam is that it makes the problem of condensation far worse. And given the tiny size of our cylinders, condensation is a bad problem anyway.

As far as I understand it the trouble is that once a bit of steam starts to condense onto a cooler wall, then the heat transfer speeds up (much more than with a gas) because the steam vapour (gas) moves closer to the wall because the last bit of steam there contracted enormously as it turned to a very small bit of water. In other words there is a large flow of steam towards the cool wall.

I think this is why superheating or at least super drying is so worth while.

Another thing to remember is that it takes a huge amount of heat to bring water from ambient temperature to boiling point and vastly more heat to convert boiling water to steam. (kettle takes much longer to boil dry than to boil and the heat input rate is the same)

C

[baggo]

Another advantage of 'dry' steam is that it is more 'fluid' than saturated steam and will negotiate tortuous steam passages such as are present in our sized cylinders with much less resistance to the flow.

I've just found a reply to the D A G Brown article by Doug Hewson (ME Postbag issue 4324) in which he (Doug) suggests that from practical experience superheaters are an absolute must in a loco. His elements are bent from one piece of tube and so dispense with the return 'block' which I am sure causes a severe restriction to the steam flow in the conventional built up type.

His 2-6-4T has 18m of superheater elements and his redesigned Britannia boiler has 20m.

As Doug says in the final part of his letter:

"Whatever the theory might tell you, there is no substitute for practical experience out on the road."

Nuff sed!

John

[Tony K]

May 19, 2008 22:51:02 GMT John Baguley said:

As Doug says in the final part of his letter:
"Whatever the theory might tell you, there is no substitute for practical experience out on the road."
Nuff sed! John

Mr. Baggo, I agree entirely. I could even agree with him if he had told us what his experiences were!!!

[alanstepney]

Doug Hewson has had many articles published in ME & EIM, the first that I recall being about the rebuild of his Y4 (I think it was) some 20 odd years ago.

He is well known for his enthusiasm for ground level 5", and for accurate scale of almost everything.
Yes, he certainly has plenty of hands-on experience.