Superheater Tubes - Spear Head or Concentric Type?

[RGR 60130]

Nov 3, 2014 9:19:31 GMT greensands said:

Hi - Thanks for the interesting set of replies. A puzzle I have is that if building a coaxial deign of superheater with a stainless steel outer and a copper inner tubing, how can one ensure that the inner copper pipe remains truly concentric along its length given that it is only held at the front (smokebox) end?

As mentioned previously, don't mix the inner and outer materials, stick to stainless. To keep the inner pipe Central you get a length of stainless TIG welding rod and wrap it around the inner pipe. Only one turn is required. Hold it in place with a couple of tack welds. The wire will also break up the laminar flow and assist in heat exchange. This is the same principle as was used in 'Swirlyflo' boiler tubes in full size - good but expensive to produce.

Reg

[Roger]

Nov 3, 2014 9:19:31 GMT greensands said:

Hi - Thanks for the interesting set of replies. A puzzle I have is that if building a coaxial deign of superheater with a stainless steel outer and a copper inner tubing, how can one ensure that the inner copper pipe remains truly concentric along its length given that it is only held at the front (smokebox) end?

I don't think it matters to be honest, it won't make much difference to the way it works. Maybe something to stop it rattling at the end and half way in case it sags. Pault tells me they don't bother with anything on the 7-1/4" gauge ones he uses them on.

[Deleted]

A spiral along the inner tube would probably make a significant difference to the heat transfer due to the turbulence. The Americans use 'turbulators' (lengths of twisted stainless strip) in the flues of gas fired loco boilers and apparently they make a huge improvement to the steaming of the boiler. They would probably work in a coal fired boiler if you could keep the flues clean.

John

[pault]

As Roger said on the ones I have made the inner tube is not supported and just sags down against the outer element. It does not matter if the tubes are not concentric the areas of the tubes remain the same and it may break up the the laminar flow a little. I did make up a press tool to make a sine wave element but never got round to trying it.

[Roger]

Nov 3, 2014 11:31:14 GMT John Baguley said:

A spiral along the inner tube would probably make a significant difference to the heat transfer due to the turbulence. The Americans use 'turbulators' (lengths of twisted stainless strip) in the flues of gas fired loco boilers and apparently they make a huge improvement to the steaming of the boiler. They would probably work in a coal fired boiler if you could keep the flues clean.

John

I like that idea, it kills two birds with one stone. I guess you could wind either a single or pair of wires as if you were making a spring and then stretch it out. You'd need to wind them on an undersized mandrel so they gripped the tube.

Another way of doing that would be to drill a hole across the open end of the inner tube and thread the wire through so that half of it was either side of the tube and then put the whole thing up on the lathe and wind both ends onto the tube in a double helix. It would need a to be Silver Soldered or Welded near the smokebox end to stop it from unwinding though. I think that would work.

Of course this assumes that you actually want heat transfer between the two, I'm not sure that do, at least, not away from the firebox. Maybe just this feature could be used at the firebox end and maybe insulate the inner tube with a thin PTFE sleeve for the rest of the way. My thinking here is that you can't expect the temperature of the return steam to go on rising or stay hot if you're cooling it with wet steam on the inside.

[RGR 60130]

No need to use a lathe Roger. Tack one end of the TIG rod with a TIG welder, pull it round the inner tube by hand and tack the other end to hold it in place - simple - even I managed it!

Heat transferred from the outer annulus to the inner tube means that the steam is hotter when it starts its second pass. The loss of heat is balanced with a gain.

Reg

[Roger]

I think you would need to perform tests to prove whether the gain balances out or not. You can put together an argument for several different arrangements of this in terms of whether heat is transferred or insulated from the inside. What happens at the radiant end? Is it better to insulate the wet steam and have it rapidly heat up in the firebox area? It's far from simple even though I suspect it won't make a huge difference. It would be interesting to do some proper experiments though of the different arrangements.

[pault]

I would not put PTFE inside an element as it starts to break down at 250 degrees C

[jma1009]

if it is accepted that jim ewins' views on coaxial superheaters are correct, then any discussion of coaxial superheaters is completely irrelevant! (for what it's worth, i have complete confidence that jim was correct! i would never fit coaxial superheaters to any of my locos!)
cheers,
julian

[Roger]

Nov 4, 2014 20:55:07 GMT jma1009 said:

if it is accepted that jim ewins' views on coaxial superheaters are correct, then any discussion of coaxial superheaters is completely irrelevant! (for what it's worth, i have complete confidence that jim was correct! i would never fit coaxial superheaters to any of my locos!)
cheers,
julian

You may well be right Julian, but I for one would never take anyone's word for anything without proper evidence and I haven't seen what's been published.

[jma1009]

hi roger,
i have jim ewin's paper on same somewhere, plus jim wrote a few letters to ME about same which accord with his paper. jim was both a scientist and practical hands on builder and driver of many very fine locos.
cheers,
julian

[Roger]

Hi Julian,
I'd be interested to see that, there's nothing like evidence to settle a discussion like this.

[pault]

Hi Julian I would dispute Jim Ewins view on coax elements based on the fact I have measured the post superheater temperature from a radiant coax element at a maximum of 263 degrees C and a maximum amount of superheat, of 155 deg C. this was with stainless elements, obviously with copper elements these numbers would be higher.Do you know what Jim's elements were made from? Other tweaks as described above could increase the temperature further. Whilst a hair pin element may well be more efficient, coax elements produce a useful amount of superheat and should not be dismissed just because "someone said they don't work".

Were Jims "views" just his thoughts or is there any documented evidence of any trials of coax elements?

[jma1009]

paul and roger,
you will have to let me find in my archives what jim wrote and give me a day or two especially as i have quite a lot 'on' at the moment, but he applied his scientific theories to same and completely demolished the fact that the coaxial type showed any advantage over the spearhead type, and were nothing like as effective. from my rather 'basic' appreciation of these things (completely non scientific) it is pretty obvious that if on a coaxial superheater element the steam in one direction isnt exposed to the flue gas temperature this is rather conclusive - jim had other ideas and applied the maths and science conclusively as you would expect!
cheers,
julian

[pault]

If you consider a radiant hairpin element it would be best to have the elements side by side rather than the normal one on top of the other. in the normal one on top of the other element the top element is shielded from radiated heat from the fire by the lower one. As the outer tube of the coax element is much larger than a hairpin element it can have a much larger surface area exposed to the radiated heat than a hair pin element.

[Roger]

My instincts are similar to yours Julian, but the it's the total area exposed to the flue gasses than matters, and a coaxial one could potentially have the same area. It also might be that the superheater flues need to be different proportions to suit the different designs.
It's also worth thinking about whether it's a better option to have three of one type to get the same effect as two of the other if the advantages of simplicity of construction and maintenance prove to be an advantage.
I also imagine you could make very slender versions of the coaxial type that would fit into quite small superheater flues.
Anyway, it's all very interesting and an education for those of us new to the topic.

[jma1009]

hi paul,
i would agree with your above comment if a high level of superheat is required on a miniature loco. ive just tried out fitting the radiant elements on my Stepney (see the Boxhill thread). i think in miniature (due to our lower working pressures compared to fullsize) all that is required is to avoid condensation in the cylinders when the steam is expanded, and to ensure the exhaust steam hasnt lost fluidity and become treacly! if same is achieved then significant reduction in steam consumption and hence coal can be avoided and you can also get away with (assuming correct draughting) a much lower back pressure and freer running at reduced cut offs and softer exhaust avoiding 'forcing' the fire and creation of clinker.
cheers,
julian

[pault]

a few quick sums show that a 20mm coax element (the size of most of the ones i have fitted) will have about 2.5 times the heating area facing the fire than an 8mm tube vertical stack hair pin and about 3.1 times the area of a similar 1/4" element.

[steam4ian]

Gentle persons

A bit of science please.

The issues with coaxial are, incoming steam cooling outgoing steam, surface area exposed to flue gases vs steam volume passing through tube, turbulence of both flue gases and steam to maximise heat transfer and cleaning of flue and superheater surface.

From what I can see a spear type superheater has significantly less contact of wet steam with dry steam, more surface area compared with steam volume, more likelihood of turbulence and the flue is easier to clean.

Coaxial elements may well be easier to make and might better suit radiant duty because the return connect which is exposed in the fire box is easier to make and thus can be expected to be more reliable.

Ian

[Deleted]

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