Changing boiler design?

[davebreeze]

The boiler of the loco I'm building uses 13 x 0.5" fire tubes and 2 x 1.25" superheater flues. I've been advised at my club to leave out the superheaters. Now I don't really want to re-open the whole superheater debate again, but just say for the sake of discussion that I took the advice and left out the superheaters, what should i do with the 2 big flues? Would it be a good idea to replace them with more of the 0.5" tubes, and if so how do you work out how many - would you be looking to replicate the cross-sectional are of the 1.25" flues (12 more tubes) or perhaps the heating surface area (5 more tubes)?

I've also been collecting the articles in Australian Model Engineer about building the 'Phantom' 2-6-0 loco for a future project. The designer recommends simply replacing the 3 big superheater flues in Phantom's boiler with 3 more 5/8" firetubes if you don't want to fit superheaters. This obviously works, as he says boilers have been built successfully both ways.

And if I was to change the tube layout in the boiler, is this classed as a re-design - would I need somebody to check the calculations and give a thumbs-up for the design?

[havoc]

Would it be a good idea to replace them with more of the 0.5" tubes, and if so how do you work out how many - would you be looking to replicate the cross-sectional are of the 1.25" flues (12 more tubes) or perhaps the heating surface area (5 more tubes)?

You would need to replace the heating surface towards the water. The section of the superheater tubes is larger because when the superheater is in them they should have about the same cross section available for as flow as the other tubes.

Yes it would be a good idea because the superheater tubes without the heaters in them would have less resistance to gas flow. So less flow through the small tubes that is not compensated by the -only slightly- larger superheater tubes.

[jackrae]

Have you considered simply fitting dummy superheaters to partially block off the larger tubes. Same design - no sweat
Jack

[baggo]

Hi Dave,

I would replace them with 0.5" tubes to keep the cross sectional area of all the tubes the same. If some tubes are larger than others, the gasses will follow the easy route. As a rough check, the area of the tubes wants to be 10-15% of the grate area. That figure ties in with most designs. If the 15 tubes give you enough area, you could perhaps alter the layout and space them out a bit more to help circulation.

I wouldn't worry too much about the total heating surface of the tubes as they contribute little to the heat output of the boiler. Most comes from the firebox.

You shouldn't have to have resubmit the design as you won't have made any structural changes to the boiler e.g. material thickness, number of stays etc.

John (firm believer in high superheat ;D)

[dscott]

Some very good points, and with a recent look at the big prairie tank boiler with our club chairman who regretted using the many small 3/8" tubes in his boiler as per the drawing, as soon as it had cooled down.
7/16" he suggested, so with back of large (and to the post office with new rules EXPENSIVE to post) envelope, I had to pay extra on one sent to me. I worked in 16 tubes and 4 super heater tubes! now we are steaming.

I think most myths about boilers started many years ago and still go round in some clubs.

David.

[davebreeze]

Thanks for your help folks.

The cross-sectional area of the tubes works out as 10% of the grate area. That's not allowing for the superheaters being in place, so it's actually even less. So if I just replaced the 2 big tubes with 2 small ones I would be down to 8.4%. So if I fitted 5 small tubes in place of the 2 big tubes I would approximately replace both the heating surface area and the cross-sectional area.

Jack - your idea is definitely the simplest, maybe the way to go.

This all comes from advice from the club boiler inspector who is fed up with superheaters as the ones in his Dart GWR 14xx keep leaking and have twice had to be cut out of the boiler and replaced as they always get stuck fast in the flues. So I can kind of see his point, he reckons they do give a small improvement in economy and performance, but aren't worth the hassle they give. Another thing is that the full-size loco I'm modelling wasn't superheated, so while I know it isn't a perfect scale model I'd like it to be a reasonable reflection of the real thing.

[drjohn]

Mar 18, 2009 8:44:08 GMT John Baguley said:

....

I wouldn't worry too much about the total heating surface of the tubes as they contribute little to the heat output of the boiler. Most comes from the firebox....

John (firm believer in high superheat ;D)

I don't know how one can measure it, but I get the feeling that there is quite a bit of heat transfer from the flues. On Simplex with 13 flues 1/2" diameter about 20 inches long, there is a total surface area of around 400 square inches - in copper, I reckon that will transmit quite a bit of heat from the gases which afterwards, heat the smokebox to about 200 C ..... Unless of course it's a stainless boiler ...

DJ (Tolerater of moderate steam drying ;D)

[baggo]

Ah, but the temperature of the boiler at working pressure is nearly 170° C so the flue gasses will be at least that temperature

John

[simonwass]

Other than Johns sign off in a previous reply, I'm suprised that no one has asked why you dont want superheaters? I'd like to see what your advisor is doing wrong by constantly having his superheaters leaking?
Not only will they cut down your water & coal usage, they'll give you a better drive as you are not going to be clouded with condensed exhaust.

[44767]

Just to add a little bit. In rearranging the tube pattern, the only concern I can see is the size of the ligament between the holes in the tube plates. If you keep these as per the approved version of the boiler, I'm sure it'll be OK.

The main thing to look at after the cross sectional area of the tubes versus the grate area is the cross sectional area of a tube versus its length. Now, I'm not sure exactly what the ratio is but my boiler has tubes 13" long and are 7/16" diameter. It reputedly is a very good steaming boiler. This is an important ratio to do with gas flow and is why we don't make working model boilers with the correct number of tubes and flues. No gas would flow through a 3 or 4 mm tube that length.

With this in mind we told one of our members building the large GWR Prarrie tank to use 7/16 tubes.

[baggo]

Mike,

the Keiller ratio for tube inside diameter versus length is 65 to 70 although Jim Ewins' formulae suggest about 80.

(Should perhaps have said that the ratio is length/diameter squared)

John

[drjohn]

As Simonwass says, what is your club boiler inspector doing to claim to be a boiler inspector when he can't even make leakproof superheaters - not exactly rocket science?

Just do it to the plans Dave - the guy who drew them up is/was a lot smarter than your boiler inspector.

DJ

[davebreeze]

Thanks for your various replies - I didn't manage to read this yesterday so just catching up.

My boiler inspector's problem with superheaters is (I think) that various types of silver solder have been used to do the joints but he reckons they erode in the fire and start to leak, and then when he tries to remove them to repair them they are stuck fast in the flues. I don't know what the Dart design is like, but I would have thought that was a function of how close a fit the superheaters are in their flues.

The economy aspect is not really an issue at our track, it doesn't really matter whether it uses a bit more or less coal per lap.

And as I said before, the full-size loco wasn't superheated so while I know it doesn't all scale down exactly, there is a part of me that feels it's not quite right to add bits the real thing didn't have!

I think I'm most likely just to build it to the plans and then I have the option of fitting the superheaters or dummies. Also the boiler inspector reckons he doesn't have the knowledge to check it if it's not built to the design, so I am just making life difficult for myself if I change the tube layout. Overall I'm not too sure about the whole club boiler inspector business, it seems too open to the personal whim/opinion of the inspector.

[baggo]

Mar 21, 2009 22:16:04 GMT davebreeze said:

Also the boiler inspector reckons he doesn't have the knowledge to check it if it's not built to the design

Then I would suggest that he has no qualifications to be a boiler inspector in the first place

IMHO a boiler inspector should be well versed in both the construction of boilers and also in the design of such. They should be able to say if a non-standard design or a modified design meets the necessary standards and if not, what needs to be altered to meet said standards.

John

[Shawki Shlemon]

I think as long one keep to the minimum requirement of boiler code , these should include material thickness , staying , flanges , bushes etc.. the number of tubes and their sizes are optional within reason as long as the ligaments requirements are met .The performance may be affected but that has nothing to do with the boiler inspector as it does not affect the safety . As for the super heater , its a controversial subject and that is one of the reasons I didn't comment before on it , I like them and have fitted then in most of my engines . I think the engines perform better with them and I get mine tested with the boiler . if properly made they give little problems .