Tube area vs Grate area?

[steam4ian]

G'day all

A question. Is there an optimum relationship between the area of the grate and the total cross section area of the tubes & flues?

I am designing a boiler and can get things reasonably OK with respect to the formulae on Baggo's web site except that the tube to grate area is only 8%, this seems light on to me.

Comments are welcomed.

Regards
Ian

[baggo]

Hi Ian,

Can't give a definitive answer but I've got a table with the calculated details of about 60 loco designs and the tube to grate area ratio varies widely from 6% up to 15%. The average is probably about 10%. I take the area of a superheater flue as similar to a normal firetube as the area is considerably reduced by the superheater elements.

I do get the impression that it's often a case of cram in as many tubes as will fit into the available space on the tubeplates and that's near enough!

Of course, the 'free' area of the grate is not the same as the open bottom of the firebox due to the obstruction caused by the grate itself and the coal sitting on top of it. The firebars will reduce the actual area by anything up to 50% or maybe even more if you use grates made from wedgewire or cast grates which have wide bars and narrow gaps.

Perhaps the 'ideal' is where the area of the tubes equals the actual free area through the grate and coal?

John

[havoc]

Not so sure they should be equal. The area together with the amount of gas shifted through it determines the speed of that gas moving through the area. You don't want the speed throught the grate to be that high that the coal lifts But you do want it high enough through the tubes that 1) there is some turbulence 2) the temperature doesn't drop too much 3) the soot gets carried to the smokebox. Keeping the tube area smaller looks a way to have that.

[peterseager]

Our boiler inspector reckons the free gas area of the tubes to grate ratio should be less than 16%. He suggested a 5"G Dolgoch should be built with less tubes than the designer crammed in and it was successful. We also know of Dolgochs built to drawing that don't steam as well.

Interestingly, all the BR Standard locos were between 13.6% and 16.2%.

Having reread the previous posts, I should state that my figures are based on the grate size not free area.

Peter

[steam4ian]

G'day all

Thanks for your replies, at least there are still some engineers in the fraternity not just plan followers. I must admit that some engineering takes some time on the armchair, or at least the office chair.

I have spent some more time working through the formulae on Baggo's site and now have come to some understanding. There are three factors which influence tube vs grate area question. Obviously the number of tubes is important but so also is the diameter of the tubes and the length of the tubes. In my case the boiler barrel is relatively short at only 200mm (8 inches) so for the same resistance I can tolerate less tube area. My 8% of grate area corresponds to 10% if the tubes were 10" long or 16% if my tubes were 16" long.

Smaller tubes will have more resistance not just due to the reduced diameter but also due to the skin effects which will reduce the free passage. The Baggo formulae do not really account for this effect; I must talk to my mechanical engineering colleagues.

Peter's comments seem to agree with Baggo's comments about Helen Longish which in theory would have been better with two less tubes. the leaves a question; BAGGO, have you blocked of those two tubes yet? ;D.

Last night working on CAD I managed to get 3 flues of 3/4" and 13 tubes of 3/8" in to my tube plate with reasonable clearances all round. This gives me Kt = 78.6, Ee = 0.163 , Eb = 69.6 & Eo = 11.35; I think this will give a well balanced result. I have also managed to get much more clearance over crown sheet which will make the boiler easier to use on our steeply graded track.

I am chuffed. Now to get the plans finished and off to the boiler inspector for approval. More armchair time for a bit DrJ.

Regards
Ian

[peterseager]

Ian

The tube diameter went through my mind as I posted my response above. I came to the conclusion that the usual firetube formula took care of that

d = (L/65)^1/2

ie work you tube diameter from the tube length to make the tubes work properly and then the number of tubes from the required free gas area of the tubes to make the grate work properly.

Bill Hall had a boiler calculation program that had a link on the ME Clearing House site. That was useful to get an understanding of these things.

Re your comments about blocking off tubes. The Dolgoch I mentioned was one of a pair built by two friends. The second engine was never finished and the builder of the first engine took it over. The boiler of the second one was built to drawing so, on the recommendation of our boiler inspector, some tubes were blocked off. It too steams well.

Peter

Just checked, Bill's program is still there

www.modeleng.org/software.htm

[BHMEpete]

Hi guys,

Baggo, I've been using your spreadsheet to play with fire tubes for a standard Tinkerbell boiler as I am building one of these locos (I admire the Heywood patterned simplicity that seems to make many people cringe, lol!). I want the original marine boiler because it's quicker and cheaper to make, and it sits as low as possible thus keeping the centre of gravity lower for stability - and the original Tinks seem to fair alright from what I can tell on YouTube pulling freights at Moors Valley, and I've driven Captain Hook (one of the Sheffield club pair) which has the marine boiler and it seemed a good runner apart from some bad coal on the day. Despite this the tube layout as designed isn't great going by your spreadsheet and I'm contemplating substituting the four super heater flues for an extra six fire tubes; partly for ease of build and maintenance, and partly because the tubes and flues are so short at 113/8" that I question the usefulness of the super heaters themselves - plus I want to use an external dome-mounted gas valve for the regulator so life would be easier without them.

The question I have is how do the formulae and results fair for marine boilers since the fire depth is typically less than that of a conventional loco boiler? - Is it a case of the figures being the same but the driver has to look after the fire a bit more skilfully to keep the heat in? I know a Sweet Pea that steams like there's no tomorrow, goes like a rocket lap after lap, however the tube length to diameter is probably more correct for an SP boiler due to barrel length, whereas a Tinkerbell boiler is only 241/2" long altogether! - It's a short, tall, fat stumpy thing with a large dome for much needed steam space.

I can only get a KT of around 30 without going too silly on tube diameter and amount - the highest I've got is 45.5 with 40 tubes 3/8" ID - but this is a high number of holes to ream in thick steel plate, a lot of tube to buy (16swg copper) and the smallest expander my private boilermaker has is for 1/2" ID. I've currently got 26 tubes of 5/8" ID in the spreadsheet as a happy construction medium but KT is only 29 - this is the one part of the formulae I can't possibly win on due to boiler length, the rest is alright with my tweaks assuming they stand good for marine boilers.

As designed there are just 16 tubes at 3/4" OD and four flues at 11/4" OD, all 10swg steel - like I said the original locos seem to fair well enough in practise but the figures are well off with this original spec.

If I choose to go saturated steam as I want to, I might up the working pressure from 100 to 120psi subject to inspector approval - the barrel is 3/8" wall and end plates will be atleast 12mm, though the firebox and dome are meant to be 1/4" wall so perhaps 3/8" would be better for those too, subject to cost. We'll see! In any case the crushing strength of 16swg copper is suitably high enough and it's proven in the boilers my welder has built for his own locos; one a 7.25" Norfolk & Western Y6b that's been going 15 years. A first class engineer, how he achieves what he does is beyond me. He's not a 'minimum gauge' Tinkerbell fan by any stretch but he'll happily build a boiler, lol!

For such a loco the Tink pistons are only 2.5" but the boiler isn't a huge capacity as pretty much half the loco is taken up by the driver! A conventional boiler would allow larger pistons but for reasons already stated, I don't want one on my Tink. I'm going for simplicity and safety - don't want a high centre of gravity tipping me over, I know of a couple such incidents and one resulted in serious burns for the trapped driver. Plus, the Heywood eccentricity really does appeal as I said before! ;D

[steam4ian]

G'day Pete

Just a quick reply. I can't see how you are getting Kt so low with 1/2" tubes of 11-3/8 long. My design process I looked at 1/2" tubes 8-1/2" long and got Kt just under 50. Bigger tubes lower Kt and increasing length raises Kt.

BTW it is the tube Bore diameter which goes in the formula not the OD.

After playing around I realised that for shorter tubes you need proprotionally less tube area vs grate area for the same effect. Too low a value of Kt and you are losing heating surface; you also need a reasonable gas velocity to stop the build up of soot and ash.

Regards
Ian

[BHMEpete]

Hi Ian,

Could it be due to grate size? - It's a large grate for the tube length but I assume it's to get a reasonable fire since it's not deep. Grate area is between 70 and 75 inches. The grate is 11" long inside an overall barrel length of just 24.5". Not sure I'll be able to win on this one - 20 tubes at 0.542 ID as per original (substituting for the four flues, and correcting to 12swg not 10 as I thought) gives a KT of just under 39 but this seems to work ok in the original locos so perhaps it'll be ok?

I'll carry on playing with figures and see if I can improve...

Edit: Right, I've managed a Kt of 46 with 5/8" 16swg tubes and this amount of tubes (40!) definately precludes the fitting of super heaters, but as said I'm really not convinced of the benefit of elements so short on a loco of this nature so that's ok. Increasing the Kt any closer to optimum figures results in an unworkable tube diameter due to the minimum expander available in my situation, and the need for a ridiculous amount of tubes to keep the other factors within range. To get Kt dead on while keeping the other values within range would appear to require 1/2" OD at 16swg, and 80 of them - yes, 80!

It's certainly an eccentric loco, but that kind of fits me well...

Pete

[baggo]

Hi Pete,

Jim (Ewins) did say in his original development of these formulae that they may not hold for all gauges. He based his results on mainly 3½" and 5" gauge locos and the formula seem to work pretty well for those. They certainly go a bit pear shaped for smaller 2½" gauge locos and this seems to be because there is a practical limit to how small you can go with fire tube diameter. For these smaller boilers you need to enter tube diameters of 1/4" and 3/16" inch to satisfy the equations and obviously such diameters are not really practical in a coal fired boiler!

The Kt factor holds for any size boiler as it is based purely on the diameter of the tubes, but as you have found, for large boilers, it may not be practical to use the diameter of tube called for.

I am beginning to think that the really critical factor in how well a boiler steams is the size of the grate. The area needs to be matched to the size of the cylinders and the diameter of the wheels i.e. the grate area has to provide enough heat to match the steam consumption of the cylinders. This is of course the engine factor, Ee. If you get that right, then you should have a loco that steams well. Too small a grate and you will be struggling to maintain pressure without thrashing the fire. Too big a grate and you will be wasting steam and water. That is probably the preferable of the two though. I've driven a few wide firebox locos with large grates in 2½" and 5" gauges and they all have no problem maintaining pressure and just run all day with the safeties blowing most of the time. The boilers could probably steam cylinders twice the size!

John

[steam4ian]

G'day John

Based on a full sized loco which was basically unsuccessful I suspect that too big a grate with respect to the tube area may also cause steaming problems. There was a class 720 on the SAR that was not a success. The design was an extension of a successful class 700 but with the grate area increased from 40 sqft to 59 sqft without an increase in the number of tubes. The wheel arrangement was changed from 2-8-2 to 2-8-4, but wheel diameters and cylinder sizes stayed unchanged. Steaming of the new locos was difficult until oil firing was introduced but they were never economical. I suspect that had they been given a Kylchap or Porta front end to increase smoke box vacuum they might have performed better.

It the fear of having too high a resistance through the tubes that prompted me to start this topic.

Regards
Ian

[BHMEpete]

Hi guys,

The grate area matches the engine well which as pointed out is the main thing. I'll have to hope for the best with the tubes - and after driving a saturated Thomas II for a while yesterday I'm having second thoughts about the super heaters, I may just put them in afterall. Being a simple large 'minimum gauge' loco the Tink has slide valves but as long as lubrication is correct I don't suppose I should have trouble with them wearing with dry steam? The T2 while a lovely drive, did feel a little sluggish and wet somehow, and did drink a good amount. Tinkerbell boiler being a short stubby marine type, really needs all the help it can get so I guess super heating would be best afterall, stop it being too thirsty.

[steam4ian]

G'day Pete

I was asking questions about superheat earlier for a boiler rebuild. A saturated boiler would have been easier and would have guaranteed that I could meet my objective of more freeboard over the crown sheet. On the old boiler with only 8" long tubes superheat? did not stop me getting wet.

I noted that there were some successful 5" gauge designs without superheat, Sweet Pea and Blowfly for example. Looking at 7" gauge boilers on other sites showed many were not superheated. But asking around at my club showed great support for superheat including on 7-1/4" gauge locos. It is argued here that superheat is essential for small gauge locos. I doubt LBSC included superheating on Tich without it being of some practical use.

I was going to suggest radiant superheater elements but then I realised that Tinkerbell has a drum firebox so there is not much room in there.

What do other Tinkerbells have? If they generally are not superheated then don't lose the heating surface. A full size Tinkerbell would only be 3 times bigger and would not be superheated.

Regards
Ian

[BHMEpete]

Eyup Ian (I'm a Yorkshire lad, haha!)

The original Tink plans by Roger Marsh call for for superheaters, there are four 1.25" OD 10swg flues arranged in a square pattern, surrounded beside and below by 16 fire tubes 3/4" by 10swg. Am I right in thinking that the radiant design is the one with feed and return pipes brazed into a pointed end, in the ME field? They are designed like this but not looking at the drawing right now I can't remember if they protrude out the fire end or not - in any case there's actually a good couple of inches between the baffle plate at the end of the grate and the throat plate, so there's room for them to protrude.

I just found this page: www.atzz49.dsl.pipex.com/attic/superheaters/index.htm and I must say I'm tempted to use his method instead of the old brazed copper setup as drawn, providing I find out the correct grade of stainless to use as some grades just break down with changes of extreme heat, hot to cold etc.

The baffle plate itself baffles me somewhat (ha!) as it's a removable but snug 7.5" diameter fit within the fire barrel and has a 2 inch cut away at the top, which the gases have to go through to get to the tubes, including the lowest tubes, and ofcourse the cut away is only 2" in the middle and reduces to nothing either side with the circular nature of the thing - seems a bit counter productive but it's the nature of a marine type boiler I guess. I once experimented with someone's Sweat Pea baffle by making a full round plate, with holes corresponding to the tube layout. It was 5mm ordinary mild steel and burnt away in no time - and the original stainless one which was a plain plate that ended just above fire level lasted even less time, in the words of the builder it was 'any old stainless' hence gaining the knowledge that it must be a certain grade to stand this environment.

I think some Tinkerbells are saturated but those tend to have a conventional boiler that can handle the demand better in terms of overall capacity. The very first Tink which was an experiment by Roger Marsh, didn't appear to be superheated as the regulator was a gas valve mounted outside the dome, taking wet steam directly from inside - but the design was altered slightly when he made a batch of the locos, some mechanical dimensions were altered and superheaters were added so they are indeed meant to be fitted, and those stainless ones I've linked to certainly appeal over those as drawn... we'll see!

Cheers
Pete

[smr779]

A tinkerbell marine boiler made as per the drawings but without the superheaters (i.e. tube layout as drawn) steams well, no problems. I have driven Tink herself a few times, she has this layout (those silly superheaters went ages ago). Most tinks are saturated - the superheater's a pain to make and does very little to help things.
With a locomotive boiler, you could bore out the cylinders to 2.75" easily, the extra capacity of the boiler will supply these easily. And not a superheater in sight.
Regards
Peter

[baggo]

Hi Ian,

Yes, I agree that the tube area also has to match the grate area and that increasing the grate area without increasing the number or size of tubes may have counter productive results, as obviously happened with the 720.

Pete, radiant superheaters are the type that extend into the firebox over the fire and produce far higher steam temperatures than the ordinary fire tube type.

I will be interested to see how successful the threaded return bends work out in practice, especially if they are only sealed with Boss White. I've used Silverflo24 for silver brazing the return bends onto the tubes (all stainless) with no problems so far. The set in Helen Longish have been in service for over 2 years now and they get red hot. The return bends are tucked up in the top corner of the firebox just above the fire hole though.

I'm not sure what grade of stainless the usual ME suppliers supply for superheater elements as it's usually not stated but I think Doug Hewson uses 316L.

John

[BHMEpete]

Ahha hi Peter - I'm the chap that emailed you around Christmas with regards building a Tink! I'd love to visit Swanley sometime but it's a fair trek from here. I'd certainly like a taste of a proper 'minimum gauge' railway, lol! Not been to Moors Valley either yet, certainly looks fun. I've seen your loco ploughing round there and Swanley on Youtube. I've got my frames together but am building funds to turn them into a running chassis, and to get boiler material hopefully not too far in time. I'm making enquiries about it at the moment as I've got someone who'll make the boiler; a fellow Brighouse member who's built a handful of large ones himself (though funnily enough, he turned up today with his new broad gauge 7.25 loco, a tiny engine for the gauge ofcourse, especially compared to his big Americans - and it has a steel boiler, 1/4" plate... and it's smaller than my 7.25" copper Tich boiler!).

Anyhow, back on topic - so the original machine herself fairs well without the superheaters - is she quite thirsty or not too bad? In leu of flues I could put more fire tubes in as originally intended, since it's a new build - would seem silly to put the flues in if they won't be used. But, I suppose if I did put them in, then if desired at a later date I could make superheaters if I felt like it'd be beneficial, maybe just out of curiosity? - those simple stainless ones wouldn't be too taxing to knock up.

I realise I'm still off topic, oops! Bringing it back I've worked out via Baggo's spreadsheet that the original Tink tube to grate ratio is only around the 6% mark with superheaters, but without it's easily brought up to the right level. Perhaps this is the way to go afterall? If I need to talk any more about my loco I'll start a new topic, sorry guys! lol

Pete

PS - Sorry Baggo, you posted while I was typing! I get ya regarding the radiant type - my Tich superheaters extend right over the fire to the back of the box but curiously are a tube-in-tube type with no bends. I didn't build the loco so am not sure what they're made of but it's due a full overhaul, probably in winter, so I'll inspect then, if they last in the meantime! The poor little thing has been run off its wheels so to speak - the mileage it must've covered, before and after I bought it!

[steam4ian]

G'day Pete

To warn of the experience of a friend who took his superheaters out leaving the flues intact. The loco did not steam well.

I think the problem that you would find is the the hot gases will favour the superheater flues when there is no superheater inside and the tubes will be starved. My friend and I agreed that if the superheater came out turbulators would need to be put in the flues to increase the resistance to gases to match the flues. The turbulators would also improve gas to flue surface contact.

Regards
Ian

[smr779]

Is she thirsty? Well, not more than the injectors can keep up with. . . . I think that, in a perfect world, if you can get several smaller tubes in place of the flues, then that would be best.

[ewal]

I've built 5 Tinkerbells, 4 with marine boilers & 1 loco type, the difference was the loco type used 3 times as much coal but as it was driving 4 cylinders it was OK. I fitted the first engine with super heaters but none since. All copper tubes 3/4" o.d. & all with SS baffle plates as per plan fitted to the fire bars. All the steel boiler tube was 10mm same for the dome & fire tube, the plate was12mm except for the dome cap which was 15mm. The 1st had 10" wheels the next 2 had 6" & the rest 8". I've forgotten how to post photos but I have lots including boiler making, if you live close enough I don't mind visitors.

E.W.