Water Tube Boilers for 3 1/2" gauge locomotives

[philh1]

Guys,

I am currently reading a book by Martin Evans called 'Model Locomotive and Marine Boilers'. A chapter towards the back of the book shows a very simple gas fired water tube boiler for a 2 1/2" gauge locomotive and even suggests the design style could be modified for say 3 1/2". If these boilers use far less copper and silver solder is there any good reason why they are not becomming more popular e.g., is their performance simply not good enough?

PhilH

[chiptim]

Hi Phil,

I'm not sure why but one reason may be that water tube boilers are ideal for boats where there is a continuous supply of water. Firetube boilers are preferred in locos because they hold more water than the comparable water tube boiler so if you run out of water a firetube boiler gives you more time to address the situation??

Just a thought. I'm sure someone else will answer.

Regards Tim.

Regards Tim

[philh1]

Tim,

Thanks for starting the reply and yes I agree that water supply might be one of the issues but my understanding is that locomotive boilers are required to have two diverse, independant supplies i.e., an injector or pump and a separate hand pump. In addition, the boiler has pressure relief and a dump pin for the fire (coal) or you can turn the gas off for a gas supply. My feeling is that this is safe enough.

In addition, the boiler that I was looking at in the book was for a 2 1/2" locomotive and the article went on to suggest that there should be no issue for say 1 1/2" scale!!

4 things struck me as possible issues as follows;
1. The boiler design is not 'traditional' i.e., coal firing feels right for a 'proper' steam locomotive. So a fangled gas supply is simply not how a locomotive should be.
2. There would now be two pressure vessels i.e., the boiler and the gas can.
3. The current designs all show a boiler complete with loads of side stays and nobody has had a go at preparing new designs.
4. The ability of the boiler to generate steam is simply not good enough.

PhilH

[alanstepney]

1. Things become traditional because experience shows they are the best. (usually, but there are exceptions.)

2. Use of a commercial gas can would cover that, and wouldnt count as a pressure vessel (in our terms).

3. There has been numerous alternatives, one being the Briggs boiler, which eliminates most of the side stays. With a conventional boiler, the only way to avoid them is to make all surfaces curved, or to rewrite the laws of physics!









4. That is the main reason.

[steam4ian]

G'day Phil

I dont quite agree with Alan regarding the ability of a water tube boiler to make steam, all the large boilers in the world are some form of water tube boiler.
The real issue is the flexibility, ie, being able to cater for the rapidly varying demands of a steam loco. there is not the volume of water ready to flash into steam the moment the pressure drops.
The other issue is water feed which is a reflection of the volume of water in the boiler. For automatic level control on all but the smallest WT boilers the controller cannot simply rely on drum level. The water in is basically controlled by a feedback signal of the steam flowing out of the boiler with what is called (if I remember correctly) level reset.
Driving a loco and feeding the boiler would be an interesting exercise as the steam demand is a function of not just regulator position.
The alternative water tube arrangement is a flash boiler which I think has been tried on locos; it certainly has for boats. Flash boilers accept that some of the water tube circuit will be dry.

In our sizes a WT boiler might work if you could somejow get a large water drum with adequate steam space. Recently ME published a Yarrow design. See how small the top drum is relative to a loco type boiler and that translates to a very small surface for the steam to leave the water; carry over is almost inevitable.
I should coin a term here, " evapourating surface", I have learnt with my own loco that this is critical to successful operation. In my case the firebox crown is IMHO too high and the water has to be carried very high in the boiler. The boiler's round top reduces the evapourating surface rapidly as the water rises. A Belpare fire box with its straight sides hardly reduces the evapourating surface as the water level rises so priming should be less of a problem.

Regards
Ian

[domo1977]

How about the 'smithies' type boiler? like a water tube but with more 'drum' and less 'tube'! some LBSC and Greenly designed locos used them - greenly later added a wet backhead.

These are simple and I often wonder how big they could be scaled up? certainly would reduce the cost if my 7.25" Battle of Britian bolier could be done this way!

e.g.
www.sidestreetbannerworks.com/locos/loco114.html

[philh1]

Alan,

Thanks for your comments particularly the one about the gas. That is helpful.

However, I must agree with Ian regarding the ability to raise steam. This is how I started when I looked at the various different designs i.e., the heating surface (without a series of calculations) looks somewhere near the same and might even be greater. Also, the flow conditions of the heater gas stream across the heat exchange surfaces must be near enough the same or can at least be copied by additions of baffles etc (specially if they are only bits of steel sheet rather than expensive copper).

I think Ian's comments relating to flexibility and the available evaporating surface might be the key.

Ive also had a quick look at the Briggs boiler. Thanks for the name. That looks like an interesting design but I think is perhaps more relevant to much larger scale boilers. I reckon that building the various tube assemblies for the 'firebox' sides looks about as tricky as building a conventional boiler in smaller scale.

I think it might be worth doing a few basic calculations. They are no more than a 1 hour heat transfer exam question for a degree (after about 2 weeks of revision***!!!). Calculating the various areas and velocities for comparison is quite quick though. So I will do that and see what is revealed.

domo1977 - thanks for the link. The boiler design that I had seen was very very simple but still had a wet backhead.

Ill do a few sums and see what happens.

Philh

[alanstepney]

OK, I was simplifying the answer.

Yes, water tube boilers can be efficient. The Yarrow was one of the best and is probably the best known.
But, to just make a simple water tube boiler that fits in the same space as a firetube boiler, is far from easy.
There was only one full-size example that I am aware of.

In addition, making a Yarrow (or similar) boiler is quite a task.

Of course, it would make more sense if there was more height to play with so that the distance between top and bottom drums was greater.

But, do the sums and see how it looks.

I like the Briggs design, but agree it is probably best suited to larger loco's. The original was for 10_1/4" guage (and is now owned by Chichester Model Engineering society)


(The Smithies boiler is ideal in small sizes, but has little heat transfer capability.,)

[RLWP]

Mar 1, 2013 17:45:55 GMT alanstepney said:

But, to just make a simple water tube boiler that fits in the same space as a firetube boiler, is far from easy.
There was only one full-size example that I am aware of.

Two in the UK? Fury and 10000

Richard

[philh1]

Alan,

Is this a Smithies boiler?

PhilH

Attachments:

[steam4ian]

G'day Phil

Short answer is Yes! It is a wet back design, the simplest versions have the water tubes as longitudinal loops out of the boiler shell.

There is some arguement about whether the simple Smithies is better than a pot boiler with porcupine studs.

Regarding the Briggs, Bullied's Leader had a dry wall fire box and was therefore a Briggs boiler. The European stainless steel boiler designs are also Briggs variants.

Has anybody built a semi Briggs in copper? It would have a water surrounded firebox only in the portion normally enclosed by the drum but with dry legs below the drum. The dry portion could be protected by a stainless steel liner or by fire clay. It could give a wider grate between the frames. Call it a Foster Boiler.

Regards
Ian

[alanstepney]

Phil H, Basically, yes. The original Smithies boiler had the sloping tubes below the boiler shell, and into the firespace, but without an outer casing. It was designed (by Mr Smithies, in case anyone wondered) in the early years of the 20th century, for meths / spirit firing of small models.
(The original article is in one of the early Model Engineer magazines.)

The first Briggs boiler, designed by Mr Briggs, was fitted to a 10_1/4" loco named Winnie, that is now with Chichester Model Engineers, but has been re-boilered with a traditional boiler.
(I used to drive it, way back.)
Mr Briggs was an architect who, following a fire at a theatre in which several people died because the doors were locked, invented the "crash bar" that opens emergency doors when one pushes against them.
That made him enough money that he had a house built (Birdham? West Wittering? cant recall exactly)
and set up a railway on Bognor Regis sea front for his son to run.
Later, the railway was moved to the grounds of his house, and subsequently bought by Chi ME.


I realise that this doesnt answer the question, but some may find it interesting

[philh1]

Thanks Alan,

I have a similar thread on Model Engineer Forum and I have just asked if anybody is aware of any articles in the ME where alternative designs have been tested against eachother. Are you or anybody else aware of such testing?

PhilH

[philh1]

Alan/ All,

Just read an article on 'Battle of the Boilers' and a Wikipedia artilce on the same subject. These seem to give what I was looking for.

One example given was a 200lb pull from a conventional 'Stephenson' style boiler versus about 30lb from a spirit fired water tube boiler.

One of the articles also described an LBSC engine verus a Basset & Lowke. Both engines ran successfully - one ran longer the B&L but the LBSC ran with a heavier load.

Thanks for the input.

PhilH