uuu
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Post by uuu on Feb 1, 2024 10:41:21 GMT
Hi Chris
I can see that friction in an ordinary pipe would be a bad thing. The walls of the pipe would be heated, and the heat lost to atmosphere. But in a superheater, bathed in hot gases, or in the raging furnace, where is the heat generated by the friction to go?.
I can still believe it's a bad thing. But maybe not for so obvious reasons.
Fluid flow is indeed tricky.
Wilf
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JonL
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Post by JonL on Feb 1, 2024 15:17:54 GMT
I was thinking about this today while charging nitrogen cylinders at work. Going to a relatively safe 250bar.... the fittings were getting warm, partially through friction and partially through the affects of adiabatic compression I assume. Either way, it gave me pause for thought....
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pault
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Post by pault on Feb 2, 2024 18:38:36 GMT
Hi The flow in a superheater element is further complicated by the expansion of the steam as it passes through the element. Potentially the flow rate of the steam is lower entering the element compared to the flow rate as it leaves. One of the few devices where you get more out than you put in.
Regards Paul
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uuu
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Post by uuu on Feb 2, 2024 19:05:57 GMT
I was trying to evaluate the effect of a superheater and the expansion you mentioned did complicate things. The steam tables work on the weight of the steam used, one cylinder-full of superheated steam weighs less than the same volume of wet steam. So the extra energy per unit is offset by that. But even if those two effects balanced, you still got a benefit in the reduced amount of water used.
Wilf
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tenor
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Post by tenor on Feb 3, 2024 17:06:33 GMT
Just an update on comparing the Hewson and Evans versions of the B1 boiler. It's still early days, and it has forced me to look at upgrading my computer program to cope with some of the issues raised, so more of that another day. To answer some of the queries about the value of superheat, I use the experimental results for steam consumption that Bill Hall got from a "Speedy" cylinder, Bill's results can be found here: advanced-steam.org/wp-content/uploads/2017/08/Measuring-Steam-Engine-Performance.pdfFig 5 is the important one which shows 80 C of superheat will reduce steam consumption by 50% compared to saturated. Fig 5 is how I derive a "useful" volume of steam from a mass flow and superheat value. I initially set up all the data runs with the same fire data and coal input, but the Hewson boiler was giving so much more useable steam volume that it was not realistic. I reckoned at 3 bar steam chest pressure and 40% cut off the Evans design would cruise at 6 mph. Under the same assumptions, the Hewson would get nearly 20 mph, which might be possible, but it is then a bit silly to criticise the design for excess steam velocity in the superheaters. So I am now setting things up so that all the data runs will produce the same steam output (as near as I can get it), so they would all be doing 3 bar, 40% C/O and 6 mph. So this is all getting a bit more complicated than I originally thought. However, I can tell you the Evans boiler is fairly horrible. The flues and superheaters are not at all balanced. Exit temperature from the firetubes is 420 C and exit temp from the superheater flues is 190 C indicating the superheater flues are constipated and the gas is taking the easy route via the firetubes. It is so bad that the smokebox half of the dry superheater leg is giving heat back to the flue gas - cooling the steam! The Hewson design suffers from the same problem (constipated superheater flues) but not so badly. The big problem is that the steam pressure drop through the superheaters is quite large at about 12 psi. The superheat is a wopping 148 C though and you only need to fire 2/3 as much coal as on the Evans B1 to maintain that 3 Bar, 40% CO and 6mph. So, I think I can answer the OP's question at least roughly. You would get quite a lot of benefit from putting 2 out and back radiant superheaters (NOT the 4 pass Schmidt type that Hewson uses) in each superheater flue of an Evans boiler shell. The superheaters can be the 5/32 tube that Hewson proposes and the Hewson manifold block should sit quite nicely in the top of the smokebox. Using the same 3 bar, 40% C/O and 6 mph measures as above, you would need to shovel about 70% of the coal that the Evans design uses. I reckon the superheater would lose about 8 psi of steam pressure - quite large but not ridiculous. For those building the Evans B1 boiler, the design using Evans superheaters needs the flues increasing to 1" tube. I reckon you would need to lose 2 firetubes to fit those in, but you get a much better balance of flue and you will only be shovelling 80% of the coal compared to the "as drawn" Evans design. As part of this, I have been looking at my next project which needs a 6" barrell boiler. Evans' "Holmside" fits the bill more or less but has just two 1" S/H flues and 14 5/8" firetubes. I shall be chopping that about a bit................. Hope that helps a few folk, further discussion welcome. Martin
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jo479
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Simplex, Pricess of Wales, Prairie, N24X, LNWR Jumbo, Jeannie Deans, 7 1/4 Lion
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Post by jo479 on Feb 3, 2024 19:28:11 GMT
I made a Martin Evans B1 boiler which is 3" longer than standard, still has 4 Flue tubes but I didn't fit superheaters (I kept the flue tubes in case) it steams like a witch without any problems, it obviously uses more coal and water but I've never had any problems, used to be in steam for 2 or 3 hours pulling passengers without difficulty.
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Post by simon6200 on Feb 4, 2024 2:38:47 GMT
I checked the Springbok articles. Evans specified 9/32 copper elements. I assume this is what Martin analysed. My boiler has the four 7/8 flues, but I had to drop two firetubes to comply with AALS 3 mm minimum webs between tubes. My superheaters are 5/16 or 8 mm stainless radiants that go to the back of the firebox. The engine steams superbly and does heavy passenger work. I am a strong advocate for superheating. Yet I understand why people convince themselves that it isn’t necessary as they are such fiendish things to make. Just an hour ago I finished new superheaters for my Simplex. With ridiculous 5/8 flues, 1/4” copper elements only just go in with no wiggle room. It has dominated the last month with a lot of frustrations. But I would not want to be driving a saturated Simplex.
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oldnorton
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Post by oldnorton on Feb 4, 2024 12:14:02 GMT
Hi Martin
Excellent to read your analyses, thank you. When you have tidied your numbers further it would be really good to put your thoughts down in a Model Engineer article. That tends to be the reference place for small loco theory.
You are illustrating to us how differences in flow balance between the flues, and the use of 5/16" spearhead vs. 5/32" mini-multiple (Hewson) superheaters, can result in relatively small changes in coal consumption (one being 80% of another). Question - will this translate into similar numbers for water consumption? one assumes so.
I think we had all expected bigger effects for superheating vs. no superheating and you might confirm - can you state what you think those might be?
Also, you have previously commented that the effect of radiant heat on long superheaters in the firebox is not a big as 'some think', beyond the effect of simply increasing the length in a hot gas environment. If you could quantify these effects it would be very interesting, and stop us thinking that the radiant heating effect is all important. I think this radiant emphasis comes from actual boiler experiments in ME articles by Jim Ewins (sorry, going from memory for the name here and would welcome other's correction).
Norm
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tenor
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Post by tenor on Feb 4, 2024 13:09:47 GMT
Thanks for the comments, I knew when I was writing my post that at least one B1 pilot would say "steams like a witch". My main point is that it could be better. To clarify, my Evans analysis used 9/32" superheaters. Please don't get me started on Jim Ewins' tests; when you start to drill into them they are iffy at best and positively misleading at worst. A test giving twice the best IMLEC efficiency and never since repeated - yes, of course that's possible..... Martin
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Post by davewoo on Feb 4, 2024 16:38:57 GMT
Thanks Martin, glad you got the boiler drawings ok, Very interesting, not sure I understood all of it, but I think I got the main idea, as stated above I'm already committed to a welded superheater system from Steam Technology. Very keen to push the build of this engine on, hence buying a part built with a finished boiler. I did know Jim Ewins he was a regular visitor to a club I belonged to years ago,at one time he used my Simplex as a test bed for his injectors, spent a public running on a very hot afternoon with a thermometer sticking out of the side tank filler, Jim taking readings every lap. I always tried to prise one of his little blue painted injectors from him but after every running session he carefully removed them and placed them carefully in a tobacco tin! I did drive a couple of his engines his 9F "Lode Star" and large SR shunter with his own design of valve gear. Both very powerful, but I found them quite boring to drive, a bit like driving a modern car, did everything efficiently and with ease, but lacking in any character, there seemed no sparkle to their performance. Obviously just my take on it, probably biased because everything I build seems to have a few odd quirks and questionable efficiency!
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jma1009
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Post by jma1009 on Feb 4, 2024 22:37:03 GMT
I tend towards an empirical approach.
(And perhaps with apologies to Martin, I disagreed with his advice to Roger in respect of his Speedy boiler).
I do wonder whether any of us have taken into account the deterioration in the quality of coal for use in miniature locos when compared to say the 1980’s and shortly into the 1990s.
I am firmly against the concentric design of superheater.
You ought to ask John Heslop, Alan Crossfield, and Dave Sutcliffe what their views are on superheaters arguably.
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Post by chris vine on Feb 4, 2024 23:01:36 GMT
Hi Julian,
Yes, maybe coal has deteriorated, I don't know.
However, for a given heat input it would be good to know that the engine as a whole is as efficient as possible. I am not sure that a reduction in the specific energy of the coal would make much difference to the decision as to how to design the superheater.
Certainly, there is a question over the efficacy of the concentric type? Maybe the numerical approach will give us some answers...
All best Chris.
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Post by ettingtonliam on Feb 4, 2024 23:31:42 GMT
Yes, the coal we are used to has certainly deteriorated. Over 30 years ago, the site I was working on exposed some abandoned 19th century coal workings. They had been 'bord and pillar' workings (I think) so we had to excavate the remaining coal pillars left when the pit was abandoned. The coal was supposed to go to a nearby Coal Board depot, which it did, Monday to Saturday, but on Sundays a certain amount made its way to our homes.
When I made up the open fire with that stuff, I had to move my chair to the far side of the room, it was so hot!.
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tenor
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Post by tenor on Feb 5, 2024 14:53:12 GMT
Picking up on a few recent replies: JMA1009 - nice of you to own up to not agreeing with Roger F's Speedy boiler design. I did warn Roger that it might be TOO hot for gunmetal cylinders, so I also have some reservations that we pushed it too far. Time will tell. The coal problem can be analysed up to a point - which the program will do for chemical analysis. But there are other factors such as Particle Size Distribution on the firebed (which is not the same as PSD in the sack) which are in the "too difficult" basket. I have watched the ASTT tests on alternative fuels with interest and note that the spread in performance is not solely down to the spread in calorific value which suggests other factors are at work. In general terms a poor coal will need more combustion gas pumping out the chimney (so says LD Porta), so certainly has implications for draughting.
Davewoo & JMA1009 - I agree that just how much superheat (or not) we design in is a matter of judgement. Those that are happy without seem to be happy shovelling coal and filling up with water at every opportunity. I like Model Engineering to be a broad church and if that is what lights your candle - crack on. Equally, I am perfectly happy disappearing down my own rabbit warren of calculations.
Oldnorton & Davewoo - I do have in mind that this would make a good ME article, because it will be easier with pictures to explain what is going on. I do have a Flickr account, but it is 3/4 full of building a Fowler lorry so I don't want to load even more stuff on there just now. I perhaps gave Jim Ewins a raw deal. I have used his safety valve ideas (as on the 9F) plus plenty of PTFE and my Fowler has 8 Ewins lubricators fitted! Sadly, his tests reported in M. Evans' book remain dodgy IMHO. Laurie Lawrence took the P**S out of his methods in a ME Christmas special IIRC, which tells you that his experimental method was questionable from the outset. The best test on a model boiler is by Busbridge (ME 1/8/64), there is another on a Rob Roy (early seventies) which is fair. A more recent one looking at superheat on a Hunslet quarry loco (can't find it just now) had to be the result of instrument error. So good test results on models are certainly rare! Mostly because they are difficult to do.
I have resolved that the next upgrade on my model will include concentric superheaters. Partly because they will be just as prone to getting the flow balance wrong which will mess up an otherwise decent superheater arrangement of any sort.
Martin
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