Small boiler with T flue

Hey there,

Currently, I'm designing my first gas fired, live steam locomotive for use on 45 mm track (1/22.5 scale). From my previous build, the “home made Dacre” I've learned that a lot of the energy generated by the poker burner is wasted (or used to remove the paint from the smokebox door ;D ).

To reduce this energy loss, I was wondering if a boiler with a T shaped flue would be possible. This has multiple advantages above a straight flue: more energy will be transferred between hot gas and water and the volume of water in the boiler is (marginally) larger.

I've attached some pictures with a rough sketch of the idea and dimensions (in mm!). As my experience in designing boilers is limited, comments and remarks are very welcome! The most important question for me is: is there a minimal distance between the “cross flue” and the end cap?

Cheers,

Mike

Hi Mike

I don't know what operating pressure you're aiming for, but one concern of the design would be that you lose any form of staying on the one end cap - besides acting as a flue in these horizontal boilers, the flue also acts as a stay for both end caps.

You might have a bear of a time soldering it together as well; the joint between the flue and "chimney" will have to be done in-place in the boiler shell, and will be difficult to inspect to check for pinhole leaks. If there is one, you'll only find it on pressure testing the boiler, and then it will be difficult to repair.

Just as a thought, have you considered cross flues in the flue pipe ? - these will slow down hot gas flow through the flue, as well as act as additional heating surface leaving the gasses reaching the smoke box cooler, and also marginally improving water capacity - basically meeting all of your requirements. The flue assembly can also be soldered up and properly visually inspected before soldering it into the boiler together with the end caps, reducing the possibility of finding pinhole leaks on pressure testing.

Oh, and welcome to HMEM ;D

Kind regards, Arnold

There were plans for a 3 1/2" dia Horizontal boiler plan by Sandy C in the download section, but I can't find them.

Regards,
Gerald.

You can always fit a baffle plate to the inside of the smokebox door if its getting too hot

As said by Arnoldb several cross flues would make for a better heating surface.

J

Hi
I did a very similar boiler some months ago, but smaller, for three Gauge 0 craker locos you can see there.
http://www.homemodelenginemachinist.com/index.php?action=dlattach;topic=13644.0;attach=17946;image
The boiler has no smokebox, just a copper elbow at the end of the flue tube, leading directly to the chimney; this increased somewhat the volume of a really small boiler. it works very well with a burner from a small butane pencil torch, you can put your hand above the chimney without being burnt.
the flue tube and the elbow are brazed first with a higher melting temp braze, inspected and cleaned, then put in the boiler tube followed by the end caps and bushes, the whole being brazed with 40% Ag brazing thread. tested at 9 kg/cm3 (ca 130 psi) for one night. quite easy to do right first time and gives a strong boiler.

cheers
Zephyrin



View attachment cracker4extracted.pdf

Hi guys,

To start: many thanks for the responses ! What I forgot to mention is the intended working pressure, which is just below 3 bar (or 43 psi).

@ Arnold: The soldering will be a bear, but not the grizzly you're expecting. Copper T sections are available (copper tubes for water). I was thinking to solder the flue to a T section, then solder the lower end of the T section in the boiler, solder the end cap in the flue and boiler, add a small piece of "cross flue" to connect the T section an boiler, an finish by adding the other end cap.

About the lack of stays: I haven't done a pressure analysis yet (I first wanted proof the idea wasn't that crazy), but using a curved end cap (like the bottom of a spray can) might be possible.

Adding cross flues in the flue pipe: somewhere I've read that this could affect the gas stream and functioning of the poker burner with that. No idea if that is true, but it does make me hesitate a bit. From the point of efficiency, an optimal design would be a T shaped flue pipe with cross flues. ;D

@ Gerald: Just look in the plans section (or click here)

@ Zephyrin: Great, that's almost what I was looking for. The only difference is that I"m using an overheater tube (tube from regulator fed through the flue to dry the steam), which needs to exit the boiler, hence the T section.


Concluding from the remarks: a boiler with T shaped flue will be hard to make, but not impossible. The question which remails: How do I calculate the minimum distance between the end cap and the"cross flue" (I'm building a prototypical locomotive, so the position of the chimney is set)?

Cheers, Mike

Hi Mike

OK - so it's a lower pressure boiler then, and that makes life simpler. I'd forgotten about the Ts and that should be fairly easy to use as you described in construction.

I had a quick dig through K.N. Harris' book, and the closest he comes to describing this particular situation is to say 1/2 of the flue diameter - which in your case would be about 10mm.

And a quick calculation for the un-stayed end cap gives a minimum thickness of 1.6mm for 43 psi - if you are going to use 2mm or thicker plate for it, it does not have to be stayed or domed; you can make it flat.

HTH

Regards, Arnold

View attachment fluespacing

View attachment fluespacing_thumb

Mike,
You're getting good advice, and questioning, from arnoldb and others who I concurr with. I fail to see the purpose of using a Tee rather than an El for the flue transition but there may be a purpous I don't see. If it's simply a matter of supporting the flue during soldering there are other ways of doing that. As mentioned earlier an El would not only make for a much smoother flow of gas (there would be turbulence at the Tee intersection) but it would add a bit more water space. I've built perhaps a dozen horizontal boilers with upturned internal flues although all of them were intended for marine use.

As for the thermic siphons, or cross tubes, I strongly endorse these, but with cautions. The first ones must be located far enough away from the burner flame so as not to disrupt or be directly in the flame. I also suggest that the first couple or three not be tubes but instead be solid copper rod or heavy wire in a "porcupine" arrangement, or if not then an extra-heavy walled tube made by drilling through some copper or bronze rod. The last few cross tubes can then be copper tube, and the more you can make as tubes the better, which is beneficial for water circulation. I use the heaviest wall copper tube available and always set my cross tubes to a 45° angle to the vertical, alternating L and R, and as close together as I dare. The reason for using solid or heavy walled cross tubes is to maximize the life of the boiler, to avoid so far as possible the possibility of a flue or solder joint burnout. The reason for 45° is that angle produces excellent water circulation.

It is important that you thoroughly pickle, scrub, and pressure test the flue assembly before final soldering in. Proving strength is one reason of course, but I know from experience that a flue assembly so made will be perfectly safe for its intended use. The reason is that if there is a leak you want discover and repair than now, because after it's installed a repair cannot be made and the boiler will be useless, unless a major surgery is done and for the work involved in that one might as well start all over. I learned this lesson some years ago when a single grain of hardened flux, gone undetected in the pickle and test, cause me to have to scrap a new boiler.

I would also suggest one additional thing, that you flange your end plates. This will make for a stronger boiler and is not nearly as difficult or time consuming as you might imagine.

GWR I think the reason for a Tee is so a superheater can be brought out of the flue and down to the valve chest as its not possible to bring it out of the smokebox.

The only difference is that I"m using an overheater tube (tube from regulator fed through the flue to dry the steam), which needs to exit the boiler, hence the T section

Click to expand...

Jason

Thanks Jason, I failed to notice that part.

@ Arnold: Thanks for the answer! From the mind (not at home right now, so I can't check) 10 mm was at the edge of what's possible from the point of the prototype...

Jasonb said:

GWR I think the reason for a Tee is so a superheater can be brought out of the flue and down to the valve chest as its not possible to bring it out of the smokebox.

Jason

Click to expand...

That's the reason Jason!

@ GWRdriver: Indeed, a T shaped flue will result in turbulence. The reason I'm using it is that I feed a superheater tube through the flue pipe to the cylinders. After a period of time this pipe tends to rupture, so it needs to be replacable, hence the T section. Alternatives to have a EL shaped flue and replacable superheater tube are very welcome...

Cheers, Mike

The plans for a 3 1/2" dia Horizontal boiler plan by Sandy C in the download section are not listed under Horizontal Boiler but as below, thats why I could not find them.
HMEM_H_Sheet_1.jpg
HMEM_H_Sheet_2.jpg
HMEM_H_Sheet_3.jpg
HMEM_H_Sheet_4.jpg
Burner_for_3.5_in_horizontal_boiler.pdf
Regards,
Gerald.

After a period of time this pipe tends to rupture, so it needs to be replacable, hence the T section. Alternatives to have a EL shaped flue and replacable superheater tube are very welcome... Mike

Click to expand...

Mike,
In 20+ years in Ga1 steam I've never seen an intra-flue superheater line rupture, but that doesn't mean it hasn't or won't happen. Either way I absolutely agree that if possible this should be made replaceable. I've been giving some thought to an alternative to the Tee but haven't come up with anything workable. The problem I see is to leave enough space to install the line, and then make the turn down towards the steam chests. I haven't yet come up with any bright ideas

As an addendum to my cross-tube description above, when I know I'm going to need to accomodate a steam line through the flue (I don't always know that) I offset the crosstubes a bit to make sure there's an unobstructed hole for the line. One fellow I did a boiler for failed to mention he would run steam line in the flue (and I failed to ask) and I had to make him a triangular shaped steam line in order to get through the flue. I did this by simply annealing a length of tube and squeezing it in a V-block.

BTW, to answer your question about how close the front head could be to the Tee . . . IMHO, since that part of the Tee will be as hot as any interior surface in your boiler and a point of vigorous boiling, I would allow for at least 1/4" and up to 1/2" of water space between the Tee and the front head.

Many thanks (again) for all the answers & helpful replies !!

Because there are lots of variables and unknowns (turbulence in T section, dimensions of T section, flow obstruction of cross tubes etc.) I intend to make a rig to test the flue with T section. I just started by making a new poker burner. I'll inform you guys about the results...

Cheers, Mike

G'day Mike

I have also considered a boiler which uses the smoke box volume to get extra water space.

In Oz we can buy unequal Ts as well as equal Ts at the plumbing stores; you may be able to do something creative with these.

Regarding staying you could double up the thichness of the end piece by first soldering an additional section to it. drill holes in the reinforcing section to make sure the silver solder joins the two surfaces.

Flanging has been mentioned and is a requirement of the Ozzie Boiler Codes. However an article apppeared in Australian Model Engineer where plain butt joints were demonstrated to be stronger than the tube; that is, the tube burst before the joint. AME Jan/Feb 2005, I happen to have the issue on loan from the Club at the moment.

Regards
Ian

Ian,
The joint test I think you are referring to was for sub miniature boilers which is AMBSC Part 3.
Here is a link to the test report.
http://sgcox.site.net.au/ritg/boiler-tests.pdf

Dan