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Post by mutley on Mar 4, 2008 18:05:07 GMT
Thought I would seek the collective wisdom of the group. Ive got to put some superheaters in a 7.25" gauge engine . The flues are 1.125" bore. I reckon I can get four 0.375" elements down each flue tube. Would I get more benefit from super heating the super heated steam, ie put a return bend half way down the flue and feed the already superheated steam again by passing it back over the fire a second time?
Andy
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Post by chris vine on Mar 4, 2008 21:20:34 GMT
Hi Andy,
It is not what I did on my loco but I think your idea is very sound. You just have to make some more bends for the returns.
Assuming you have 3 or 4 superheater flues you will still have 3 or 4 times the bore of the superheater pipe which I should think is more than sufficient for a good flow. I guess it depends on the size of the cylinders. On Bongo the bore thought the regulator (industrial steam ball valve) is 13 mm and not restricted after that. It goes mental if you open the regulator fully!
As an added bonus if there is a bit of a pressure drop in the tubes then the steam will have a bit of extra superheat added because the energy remains the same but the pressure is a bit lower so more superheat IE less condensation.
I have always rather hankered after making an old timer say 1860ish locomotive and trying to make it all correct. Of course there is rather less to make correct than on an A3 or Duchess etc. One of the questions is would one use a superheater when the original would not have had one fitted. My thought is to make a boiler with the same working pressure as the original(rather higher than normal for a model) and then install a pressure reducing valve as part of the regulator so that the maximum pressure available would be a more normal 100 or 120 say. This would make it quite realistic to drive and it would be very authentic (except for the pressure reducing valve of course!!)
Chris.
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Post by Shawki Shlemon on Mar 5, 2008 9:23:31 GMT
A model is built to a scale say in this case it is 1:8 ,correct me if i am wrong shouldn't the pressure ,load and everything else be to scale ? Anyhow most clubs only certify boilers to 100 psi excluding one built and maintained by industry .100 psi is over scaled already . The hotter the steam the better but at what cost and I don't mean money but complication and space . I always use super heaters and they are good ,but in my opinion one round of heating is ample for the models . It is your engine ,your idea is sound and will improve the heating but I think you will find it very hard to fit in and hard for maintenance .
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Post by havoc on Mar 5, 2008 11:44:29 GMT
This type of superheater was done in full size as well so I don't see why not. Only thing to keep in mind is what temperature your seals can handle. Also I have a bit reservation about "passing it over the fire". Normaly superheaters don't go into the firebox. (I know it is sometimes done in smaller scales)
Easiest way to calculate scale pressure is to calculate the weight needed on full size safety valve to get the correct full scale pressure. Then just scale that safety valve: linear dimensions go by the scale factor, surfaces by the square of the scale factor and weight by the cube of the scale factor.
So pressure scales down.
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Post by albert on Mar 5, 2008 15:30:56 GMT
Helo Andy,
Working on my loco building past I would fit two sets of super heater tubes 0.375 dia. through the flue and over the fire about half fire box length made off and welded stainless steel ( they can be trade bought if necessary).Advantages:- no return bends filling the flue for ash to build up bye,and better superheating.I feel sure the cross section area of these tubes will be more than the area of the regulator valve and if so no loss of steam flow.
Bye Albert
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Post by mutley on Mar 6, 2008 8:50:51 GMT
Hi All
Thanks for the replys. To give you some background, Im helping finish a 7.25" gauge Jubilee . This engine has three cylinders and the boiler is already built and hydraulically tested and has 4 super heater flues in of 1.125" bore hence my thoughts of trying to get 4 off 3/8" diameter elements down each flue. Drawing it out in CAD it looks possible but am I actually going to get any benefit from the extra return and pass of the steam over the fire .Regulator is a ball valve and I am going to use the fully radiant elments not the tube in a tube method. All joints will be TIG welded.
If I dont put the second return in is the relatively large gas area around the super heater element have any detrimental effect on the rest of the system?.
What did you do on Bongo Chris? I'd be intrested to hear how you tackled the problem.
Andy
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Post by chris vine on Mar 6, 2008 10:37:18 GMT
Hi Andy,
Without going outside to look (I live in Scotland and it is raining) I think there are 3 (or maybe 4) flues. I think there are just two superheater pipes, there and back again, with return bends (tig welded) in the firebox. Almost at the back.
Like most Model Engineers, I have no idea what the superheat temperature is. However it certainly is poweful enough to give a thrill.
One of these days I must try to make a tiny temp probe to stick in steam pipe where I have take off for the steam chest pressure gauge. Don't hold you breath for the results though!
One thing which is interesting, when driving with the valve gear almost in mid gear, you get quite wet from the exhaust. If you give it a longer cut off you stay dry. So clearly the cylinders are extracting more work from the steam nearer to mid gear which is as it should be.
Whatever you do it will most certainly work. I think you have to balance the extra effort to make it (and perhaps more important to fix it if there is ever a problem) with the gain. This gain will probably be marginal and almost certainly you won't be able to measure it. Or even if you could you probably never will!!!
Chris.
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