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Post by andyhigham on Mar 5, 2019 17:00:23 GMT
OK, I know the subject splits people more than Brexit or Marmite.
My question is which is the best way about, wet steam down the middle and dry up the outside? Or wet steam down the outside and dry up the middle?
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uuu
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Post by uuu on Mar 5, 2019 17:48:37 GMT
You can consider the superheater as a "system". What you are interested in is heat exchange between the outside and your system - what happens internally, exchange between elements of the system, can be ignored.
You would get more heat transfer from outside into your system if the temperature differential between the combustion gas outside and the steam just inside the outer element was higher, rather than lower. So wet steam down the outside.
Wilf
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Post by andyhigham on Mar 5, 2019 17:54:55 GMT
That was my thinking too.
I probably won't get much heat transfer in the flues as space is limited. The bore of the flue is 14mm/9/16" and the OD of the superheater is 12.7mm/1/2"
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uuu
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Post by uuu on Mar 5, 2019 19:06:52 GMT
"Jessie" is drawn with the wet going up the middle - it remains to be seen if I change this or not. It's not shown extending into the firebox, so I do intend to offset/overcome any other disadvantages of the design by lengthening it, and getting a radiant effect.
Wilf
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stevep
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Post by stevep on Mar 6, 2019 9:54:03 GMT
I am sure that there are people on here who can go through the whole thermodynamics bit about this, but I would venture to suggest that wet (i.e. cold) steam up the middle would be best.
If you put the wet steam up the outside, it would warm up, but would also form an insulating layer between the heat of the flue and the inner pipe, and hence would restrict any heating effect on the steam in the middle.
However, by putting wet up the middle, there would be a logical temperature gradient with the coldest in the middle, the hotter (superheated) steam on the outside of that, and the hot flue gases outside of that.
Personally, I'd use side-by-side superheaters - they're easier to plumb up.
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Post by andyhigham on Mar 6, 2019 10:58:58 GMT
To fit side by side superheaters in a sweet pea boiler you need to flatten them beyond recognition. The correct pipe size is 8mm 5/16" OD. the flues are 14mm 9/16" ID.
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Post by delaplume on Mar 6, 2019 14:11:28 GMT
I am sure that there are people on here who can go through the whole thermodynamics bit about this, but I would venture to suggest that wet (i.e. cold) steam up the middle would be best. If you put the wet steam up the outside, it would warm up, but would also form an insulating layer between the heat of the flue and the inner pipe, and hence would restrict any heating effect on the steam in the middle. However, by putting wet up the middle, there would be a logical temperature gradient with the coldest in the middle, the hotter (superheated) steam on the outside of that, and the hot flue gases outside of that. Personally, I'd use side-by-side superheaters - they're easier to plumb up. Hello Steve------Just a moot point but the idea of Superheating is to add more ENERGY, into the steam that has just been created by the Boiler......This type of steam still has a certain degree of water moisture within it and is therefore known as "Wet" or Saturate Steam and is usually collected in a common header device ( Large drum or cylinder for example ) which is called the "Wet" header.. Looking inside the loco's smokebox this will be the one at the top and it might well be combined with a regulator as well ...thus keeping the "Dead-leg" regulator response effect to a minimum.... By passing this wet steam back through the original heat source several times the last of this water moisture is converted into steam, thus more ENERGY has been added to the overall output and this steam is thus known as "Dry" steam and is collected in the lower header or drum....and thus it becomes the "Dry" header..........From here the Superheated steam then flows direct to the cylinders etc... Returning to our Concentric Superheater elements we know that for a given constant length the outside surface AREA of any pipe or tube increases with increase in Diameter.........and as heat transfer is determined upon}--- a) Surface area and b) Temperature difference then clearly the steam requiring heating ( the "Wet" steam ) must pass as near to the heat source as possible, which in this case will be the outer annulus........The returning steam within the central tube will thus be "Dry" upon output into the "Dry" header...... I think the word COLD is inappropriate as everything is obviously hot----- maybe "less hot"  As far as I'm aware this is a "Constant Pressure" system with just the Temp. of the steam increasing................... The last batch of GWR Castles had a very high degree of Superheat to such a point that the original Steam Oil specs. couldn't cope and the oil was carburised in situ and thus failed to lubricate............A new, higher spec. of oil had to be devised.... I personally prefer the side-by-side arrangement ... |
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Post by suctionhose on Mar 7, 2019 10:56:15 GMT
I've not used concentrics but I would tend to put wet up the middle and draw dry off the bottom of the outside. Superheaters are only hot when the fire is hot and at other times condensate may accumulate. The main steam pipe should naturally drain in the direction of flow. Places for condensate to form and accumulate should be avoided.
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Post by andyhigham on Mar 7, 2019 13:56:23 GMT
Some very good arguments both ways.
Passing the wet steam up the outside does make sense from a heat transfer point of view and also lends itself to my smokebox layout. As Suctionhose pointed out condensate could be a problem, I may be able to get around this with a small drilling in the smokebox end block from the anulus to the inner tube
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Post by andyhigham on Mar 7, 2019 14:20:09 GMT
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Post by RGR 60130 on Mar 7, 2019 16:04:13 GMT
Remember too that a contra-flow heat exchanger is more efficient than a parallel flow heat exchanger.
Reg
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Post by andyhigham on Mar 7, 2019 19:27:04 GMT
A thought entered my head this afternoon, I will try to explain
Stainless steel tube, like the outer tube in a concentric superheater, a strip of stainless steel around 0.5mm or 0.8mm thick and the same width as the bore of the tube. This would be inserted into the tube dividing it in two stopping just short of the firebox end which is plugged and about 20mm sticking out of the smokebox end. A split block would clamp the tube and the strip separating the two halves each half having a steam pipe, one in and one out.
OK the two halves will not be completely steam tight but as there will be negligible pressure differential there should not be too much short circuit
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Post by andyhigham on Mar 8, 2019 9:05:06 GMT
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Post by RGR 60130 on Mar 8, 2019 9:25:06 GMT
You could put a twist in the dividing strip too to help give a bit of turbulent flow and improve heat transfer, probably no more than 360 degrees of twist.
Reg
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Post by delaplume on Mar 8, 2019 10:42:51 GMT
You could put a twist in the dividing strip too to help give a bit of turbulent flow and improve heat transfer, probably no more than 360 degrees of twist. Reg Hi Reg, Wasn't there / isn't there a form of steam tube with "Twisty bits" built in ??........I seem to remember these were discussed on the MECH a while ago now .. |
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Post by steamer5 on Mar 8, 2019 10:48:12 GMT
Hi Andy,
At the risk of been sent to the naughty corner...….
I looked into concentric superheaters some years ago....the "local experts" at the time said no, no real reason so went no further.
Although I do like the idea as its nice & compact, but don't forget that you need to support the inner tube to be somewhat central in the outer one.
I also spent some time on trying to figure out how, when I get to building the boiler to putting superheaters into K1...full size takes it from the dome back to the cab thru the regulator & down to the rear cylinders...who said building the smallest Garratt would be easy? I was looking at a concentric steam pipe that would take the steam back to the regulator feed it forward to the smokebox thru the superhetaers & then back to the inner steam pipe to the back to bypass the regulator & out of the boiler......it looked good on paper
Right now for the real banning statement....
You cant superheat wet steam!! Was told this when I got into playing trains some 40 years ago....being young & dumb accepted that as they knew better.
Roll on a few years & a career change which included playing with boilers & it became clearer!
In our size we try getting the steam from as hi as possible in the boiler & then its thru the regulator & on to the superheaters, this steam will be carrying water as nothing to minimize / remove it is done. The steam once hitting the superheater will then receive heat which will convert this water to steam, but is unlikely to increase the heat much (don's pith helmet & edges quickly towards large rock to hide under!) I've read quite a bit on guys trying to test what happens & although the indications are that we do get superheating (some reports say lots), I always come back to the question as to were the thermocouples are positioned, if they are surface mounted then that doesn't give the steam temp, as most of the testers are no longer with us we cant ask.
Don't get me wrong, if you can fit superheaters then do so, you do get the befit of converting the carried over water & its conversion to dryer steam, hence more volume & hence more efficiency
In industry a boiler will have cheverons…..a bunch of wavy plate stacks, set as hi as possible above the water level, these are, dependent on service, connected to cyclone separators (there will be several of theses sets in a steam drum to maximize water removal) to get the steam as dry as possible (steam temperature will be dependent on pressure) BEFORE it hits the superheater's. The temperature of the steam is then increased with the pressure staying the same after.
Andy your idea of putting a "wall" down the center & Reg's idea of a twist or two in it would be yet another twist to the idea.
My youngest son is involved in the design of once thru steam generators in Canada, he's just been home so I got to see some pictures. These are rated at 90,000,000 BTU! that's about 24.5 MW's The water is feed in one end via a dual pipe arrangement in a shell firing via one end. He tells me that the last part on the tubing is rifled to spin the steam to get the water to the outer wall so its converted to steam...…as these are used to recover thick crude oil the pressure will mostly be reasonable (next trip over I've GOTTA check this out) so another idea for us to look at! Anybody know somebody building rifles we could ask about how to do that in our sizes??
Right now's ducks under large rock & waits for the flak!
Cheers Kerrin
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Post by andyhigham on Mar 8, 2019 10:52:49 GMT
It is true, most model superheaters are in fact steam driers
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Post by RGR 60130 on Mar 8, 2019 10:58:31 GMT
You could put a twist in the dividing strip too to help give a bit of turbulent flow and improve heat transfer, probably no more than 360 degrees of twist. Reg Hi Reg, Wasn't there / isn't there a form of steam tube with "Twisty bits" built in ??........I seem to remember these were discussed on the MECH a while ago now .. I think you're thinking of Swirlyflo tubes. Reg |
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stevep
Elder Statesman
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Post by stevep on Mar 8, 2019 12:55:27 GMT
All I know is that, on a cold day, the exhaust of my Rob Roy, which has side-by-side superheaters running right to the back of the firebox, doesn't condense until it is many inches above the top of the chimney.
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Post by steamer5 on Mar 8, 2019 13:13:21 GMT
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