Superheater Tubes - Spear Head or Concentric Type?

[andyhigham]

I guess we can all agree, hot dry steam works a lot better than wet steam.
Even an inefficient superheater, if it dries the steam enough to stop clouds of condensation and spray emitting from the chimney will make the loco more pleasant to drive

[pault]

Interestingly the results shown in the article posted by Jim Scott, of Jim Ewins work show a maximum steam of 620deg F which translates to 326deg C which is 63 degrees hotter than the 263 I recorded. This drops to 312deg C in the higher power output run, 49 degrees hotter than the highest temperature I recorded. It could be argued that for something that “does not work” and “is inefficient” it comes surprisingly close to the results recorded by Jim.
There is no record it would seem as to whether Jim’s elements were copper or stainless, mine were stainless. If I had made the outer tube of the element from copper with its better heat transmission coefficient I’m certain the temperatures I recorded would have been much higher. I would however not recommend copper elements.
It should also be remembered that Jim’s trials were on a test bed where everything can be allowed to stabilise at its highest temperatures. The runs I did were “on the road” where loads speed etc. fluctuate with the gradients. This does however mean that the highest numbers are not necessarily seen as the engine may be eased, causing the temperatures to fall before they have reached their maximum possible.
I think that the ideal arrangement is the steam coming up the inner tube which would be made of copper, coming back down the outer stainless tube. This however is not the easiest arrangement to achieve from a practical point of view. My own reasons for going down the coax route are that it is relatively easy to make, robust and easy to repair, and it provides superheated steam. It is interesting to wonder what a trick coax superheater could do with the flow going the right way, and sine wave inner copper element.
It is perfectly true that non superheated locos will run happily and as a number of people of people have said we are not worried about a little extra coal or water. But something to consider is if you are on a say 45 minute nonstop run a superheated loco will be a more relaxing drive simply because you will need to fire less and put less water into the boiler.

[Roger]

That's a terrific piece of research for a complete system but does little to settle the question at hand. That would need a different sort of setup that didn't involve an engine or complete boiler like that. It would have to focus just on the different superheater types and try to keep everything else the same. It would be hard to get meaningful results from a coal fired boiler, it's heat output it too variable to compare like for like unless there were two identical flues each with a different type of superheater and a way of measuring their temperature and output energy. It's not at all easy to do in my opinion.

[steam4ian]

Nov 7, 2014 19:56:13 GMT jma1009 said:

can i just point out that the jim ewins table posted by Jim Scott above is one of THE most significant pieces of research ever undertaken on a 5"g miniature loco (jim ewins' famous 5"g 0-6-2T loco - which ive driven). many thanks Jim for posting same!

it deserves the most careful scrutiny and study.

cheers,
julian

I agree Julian and thank you Jim S for presenting it. There are just too many variables and it possibly raises more questions than it answers. What we do not know is cut off and regulator opening. One thing is obvious is the relatively low stream chest pressure even at maximum output. This means there is a high delta P across the regulator with lots of wiredrawing; would this wire drawing make superheat redundant.

I question the coal consumption figures, how were they measured? The ratio of O2 in the flue gases is interesting suggestion firing conditions were anything but the same 10% O2 suggests significant excess air.

300 deg.F superheat is quite reasonable and according to Charles Law would mean an increase in volume for a given mass. My grey matter is struggling to remember if the temperatures for calculating Charles Law are absolute. Thank you Google, it is, so the volume increase is significant but not great, that's all my brain is telling me. This backs up Prof Hall's assertion that for model sizes the benefit of superheating is higher temperature cylinders.

To round out Jim Ewin's observations there needs to be a careful analysis presented with it.

[jma1009]

hi ian,

it should be remembered that jim ewins' 0-6-2T was the LBSC MINX inside cylinder joy valve gear design of 1949 vintage with slide valves. jim went on to build far more efficient locos - but never won that elusive IMLEC cup! in fact despite his many years building far better locos he came no better than to be placed 3rd in IMLEC which is the same as i was on my first attempt when young and green in 1995!

jim's published data and comments actually run to 10 pages, Jim Scott's table being but an extract of same.

jim managed to acquire/borrow the most up to date equipment from Imperial College London available in 1967, and spent many afternoons in his garden on his loco test bed exhaustively testing his 0-6-2T loco. i agree the steam chest pressure is low, but i can assure you from personal experience that jim's 0-6-2T loco performed amazingly on the track. he tested a 'bog standard' LBSC loco albeit fitted with radiant superheaters.

there are lots of results from jim ewins' experiments that have influenced me and many others. note the low back pressure - this is where jim ewins was very critical of those who applied fullsize sam ell draughting, as in fullsize the exhaust approaches sonic conditions, but will not in miniature. note also the temperature of the firebed - strange as it may seem till jim's experiments it was always assumed that a miniature lump of coal burned at a lesser temperature than a fullsize lump of coal! note also the significant degree of superheat obtained by the radiant superheater elements, and the temperatures of the flue gases along the tubes.

cheers,
julian

[Jim Scott]

As Julian states, the table is extracted from a a fairly comprehensive article (which I thought I had to hand but can't find right now). I think some of the questions raised might be answered in the text, eg superheater material, but as Ian points out, the data does stand further examination.

Jim S