Superheater Tubes - Spear Head or Concentric Type?

[greensands]

Hi - Can anyone offer some advice on which type of superheater tubes are to be preferred for fitting to a 5" gauge loco. Don Young specifies concentric tubes for his 5" Black Five but I notice that Tony Allcock's Jubilee gives the option of using spearhead types. First thoughts are that it is probably easier to buy ready welded off-the-shelf spearheads and cut them to fit rather than make up and weld the concentric type of superheater but views and opinions would be welcomed. Reg

[houstonceng]

The late Jim Ewins, who did a lot of work on gas flow in model Locos, including ratios of tube diameter to length, always maintained that the concentric type were NBG.

One argument is that, colder steam entering the front of the outside annular space heats as it travels towards the firebox, it then enters the inner tube where, as it travels back to the hot header, it is cooled by the incoming steam from the wet header.

Be that as it may, the basic problem is laminar flow. This causes the steam to cling to the inside wall of the tube, stopping the faster moving portion to be insulated from the heat.

Jim (and others) advocated that spear-point superheaters should be crimped alternately top/bottom and then left/right to make the gass swirl.

Unfortunately, Jim's writings on the matter are lost to me.

[andyhigham]

My superheater is a single 10mm stainless tube, from the wet header down one of the two flues, a nice gentle return bend over the fire and back up the flue on the other side of the boiler.
The superheater is fitted from the back, one of the advantages of a marine type boiler.

[jma1009]

hi reg,
andy beat me to it, and what he says about jim ewins is absolutely correct. jim was highly critical of the coaxial design of superheater element. don young used the coaxial type from about 1973 onwards on Mountaineer in response to suggestions from alec farmer of Reeves fame.
in addition to the science and maths, there are also practical reasons for not using the coaxial type as they restrict the free gas flow (not very good anyway on don's Black 5 boiler) and you cant get a small flue brush around them to clean the flue.
cheers,
julian

[Deleted]

Doug Hewson also did some tests on various types of superheater and found the concentric type to be pretty useless. If you want real superheat then make sure your elements extend over the fire. Elements that only go to the end of the flues are little more than steam driers. Also any return fittings/blocks fitted restrict the gas flow through the tube and collect rubbish.

John

[Roger]

Presumably concentric types are put forward because of the simplicity of making and attaching them. I can certainly see the appeal from that point of view. It would be good to have some proper scientific test data showing just how good or bad they are in comparison to the return types.
Andy seems to suggest that the flow is from the outside to the inside but I would have thought it thermodynamically better to put the cold steam on the inside. I can see why the spearhead type ought to outperform the concentric type simply from the point of view of the whole surface of the superheater being exposed to the flue gasses. Welding them together at points along their length seems counter productive, thermodynamically speaking, but I'm sure I've seen one like that.

[pault]

Concentric superheaters have the advantage that they can be produced without welding although welding makes producing them much easier. In my view they also put less stress on the Smokebox plumbing as with the hairpin superheater you have a hot and cold leg which are joined together and expand at different rates.
As far as working goes non radiant elements of any type will produce very low levels of superheat. Coax radiant elements defiantly produce significant amounts of superheat have a look at
modeleng.proboards.com/thread/7812/7-1-4-dyno-car
I still have not got around to the trials of the hair pin element yet but I will.
As far as the practicality’s go a small airline will pass between the element and flue which will clear out any ash. Welding the element together at points along their length would probably a set up a lot of stress in the element and also create a significant bend in the element as the Hot (long) and cold (short) legs would do an impression of a bi metallic trip. Radiant elements tend to bend upwards in any case.

[houstonceng]

Re concentrict SHs. As far as I understood it from Jim, either way round for the gas flow made no difference - why would it ?

[Roger]

Nov 2, 2014 17:44:59 GMT houstonceng said:

Re concentrict SHs. As far as I understood it from Jim, either way round for the gas flow made no difference - why would it ?

Because the surface area presented to the flue holds steam at a different temperature and flowrate. There's no way it would be the same. The temperature gradient across each part of the tube as you go from one end to the other would be different at each point. The net result may not be hugely different but you can't just assume it would be the same, the conditions are different. You'd have to model it using FEA to work it out or conduct tests, it's not as simple as it appears.

[pault]

there is also the question of what material to make the inner tube of a coax element, I have used both copper and SS but have never done any measurements to decide which was best.

[fostergp6nhp]

In French full size there was a type of superheater called a Hoolay type, spelling might be wrong. It was basically a normal return tube type that changed into a concentric as it approached the firebox. I believe they were used by Chapelon later on.

[pault]

do you have any drawings Foster?

[Roger]

Nov 2, 2014 19:26:51 GMT pault said:

there is also the question of what material to make the inner tube of a coax element, I have used both copper and SS but have never done any measurements to decide which was best.

This is a very interesting question. For the maximum temperature of superheat, is it best to feed through the middle and insulate it as much as possible from the gas in the outer tube if that's the return? Does the heat passed from the superheated steam lower its temperature more than is gained by the pre-heating effect on the incoming steam? There are no answers without modelling or data.

[GWR 101]

Hi Reg, firstly apologies for the intrusion on to your thread I will be where you are in a few weeks time and wondered if I am doing the right thing. My build has two 3/4" flues with 1/4" dia. super heater elements with a return about 3/4" from the fire box.

I have purchased the castings below which I believe are Spear Heads for this purpose, is this a good plan. Regards Paul

[pault]

In my experience it is best to make the elements entirely from the same materials. if you don't the failures tend to be on the joint between dissimilar metals, probably due to different coefficients of expansion causing fatigue failures after a fair number of years. For preference i would go for welded stainless steel,however for maximum superheat copper elements would be best.

[fostergp6nhp]

Sorry no drawings. All I know is basically what I mentioned above which is what was told to me by a friend of mine who was a friend of Andre Chapelon and he had done footplate runs organised by Chapelon when the SNCF were still running steam.

[miketaylor]

The use of the annular type superheater as a part of the Houlet system was really incidental. The point of Houlet's device was that it shoehorned a supplementary superheater tube into the front section of the flue in order to reheat part of the low pressure steam between the high and low pressure cylinders of a compound engine. Hence Chapelon's interest.

Mike

[miketaylor]

PS

I cheat - Google is my friend.

Mike

[greensands]

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?

[miketaylor]

I should think you would need to silver solder tiny spacers - like a spider, onto the inner tube. Probably oversize to start with then file them down.

They could also be given a twist to make the steam flow gyrate in the tube.

I suspect that a single set about 2/3 of the way down from the smoke boxe would be sufficient for support purposes. If you were also using them to break up the steam flow then a second or third set might be useful.

I think it is also possible that at 5" scale the whole thing would work just as well without bothering at all. Perhaps just put a slight wriggle in the inner tube so it didn't lie flat against the outer for the full length.

Mike

(no google this time)