Sweet sixteen traction engine boiler.

There was a very long thread about stainless boilers posted somewhere, and I recall there were a large number of folks who were adamantly against it for a number of reasons.
I seem to recall some sort of degradation of the stainless under the heat and pressure, which made it unsafe.
I will look for that discussion, and try to post a link.
.

A guy in Spain - Joan Llutch :
made his own design of Stainless steel boiler - and complete loco.
- Well, I think he assembled parts made by professional machine shops as well as making some parts himself.
Hi boiler for Gas firing was based o his expertise - designing plant for chemical works - where various fluids flow into and out of heat exchangers, etc....
So he is a really expert guy to ask.
One thing I learned from him was that due to pressure differences between firebox and smoke box, then limitations of the Chimney (of whatever form) are unsuitable for gas firing in many cases. Loco chimneys are designed to draw a vacuum in the smoke box, that varies with engine load, and when stationary the blower, so as to draw or otherwise control the combustion rate of solid fuel by varying the air-flow through the fire.
A gas burner is totally different in that the gas-air mixture is a pre-determined fixed value, made by the gas and gas pressure, the jet size, the air-admission hole CSA, the venturi and mixer tube CSA and length, the burner holes CSA and volumes of various plenums en route, and the post burner back pressure. So varying Smoke box vacuum with a blower or blast pipe can seriously disrupt the gas mixture. AND the smoke box exhaust via a chimney is probably far too small for natural flow of the exhaust gases.... so a gas-fired loco boiler would be a seriously different design to a coal fired or wood fired boiler.
Gas needs much larger flue tubes, larger chimney, shorter flue tubes, larger firebox, etc.
That apart, gas burners do not worry about boiler material.
However, coal which contains a lot of sulphur (a lot of the smell of coal smoke!) produces sulphuric acid fumes, which dissolve metal. But Copper less than any steel, I think? So ask a metallurgist about that one.
Hope this isn't too much off thread?
K2

Gedeon Spilett said:

I did this little traction engine years ago, at the original scale 16mm to the foot, (i.e. 1/19 ) from the Rex Tingey book, but without its valveless engine, that I have replaced by a conventional cylinder and Stephenson valve gears... the boiler that I did according to the plan is a poorly efficient design as regard the hot gases exhaust, even butane fired it require an auxiliary fan for starting the fire and a blower to keep the flame...and doubling its size would be worse IMO...
I share all the above comments on the problems associated with changing the scale of a plan...Len Mason's" Minnie " is certainly a better option.
https://photos.app.goo.gl/ZWdGPpu86aNjDWSd9
https://photos.app.goo.gl/WuV52YAkK7MYEo327

Click to expand...

I looked at the burner configuration. There is no way it could not choke. Not enough clearance to the furnace top. Probably would work very well if the nozzles were turned 90 degrees toward the stack. I suspect if it was modified to be larger and placed in a short venturi the gases would act as a a driving force and you probably would not need a fan. And its quite possible to use only one nozzle but larger in size to deliver the fuel. The small gas ports would remain the same diameter but you would have more of them in an larger size nozzle. It looks like a nice model. Basically keep everything the same but change the orientation of the fuel nozzles. I like the looks of those old steamer tractors.

GreenTwin said:

There was a very long thread about stainless boilers posted somewhere, and I recall there were a large number of folks who were adamantly against it for a number of reasons.
I seem to recall some sort of degradation of the stainless under the heat and pressure, which made it unsafe.
I will look for that discussion, and try to post a link.
.

Click to expand...

I know of at least one commercial boiler made of stainless. It is still in operation. Went in to a facility that supplied starch product to a major brewery. However, you still have to maintain could water treatment same as all the other boilers in existence. It is however a costly material to use.

Hi Gedeon,
The problem of the burner appears to me to be the gas jet TOO LARGE for LPG (Butane) in that size of burner. - see the yellow tips on the flames? = Too much gas for the air-hole and mixer tube set-up. I.E. the gas jet is supplying too much gas and prohibiting the air from getting into the burner.
0.45 written on the drawing... maybe 0.45 mm? or "77" size - presumably a no 77 drill?
Without doing all the calculations....
A no 77 drill is 0.018in according to my reference: I.E. 0.46mm.
For that size of burner I should be using a 0.35mm (No. 16 JET) or 0.30mm drill (No. 12 JET) for the jet.
There is a ratio of sizes for a "correct" burner, and if one size is wrong (too small) it dictates the real limit for the burner to work properly.
In the case of this engine, the flue tubes (total CSA) and Chimney (CSA) are probably the limiting factors. So you need the burner to supply the maximum amount of gas that generates the amount of exhaust that can naturally pass up the flues and chimney. - Or the draught must be forced with a fan or steam exhaust.
So the easiest improvement is to make a smaller gas jet and see what really works?
Gas burning badly doesn't make as much heat as a bit less gas burning properly.
It is also possible, that when drilling the gas jet, it ended-up a bit over-sized, but from my home made burner tables it is just too big for the air -intake, mixture tube, burner head, flue tubes, and chimney... So it chokes as you describe!
Hope this is useful?
K2

Gedeon,
I have done some sums - attached. - with suggestions to improve the boiler-burner configuration.
BUT some dimensions are simply guesswork, and proper dimensions will possibly change the "possible improvements" itemised.
But it may be of interest to you?
K2

thank you for this study on the boiler of this little Sweet 16’ traction engine.
I have posted this simply to confirm that this design of the boiler has to be improved or completely redrawn, particularly if the size is doubled.

As I drilled the jet myself, the dimension of this hole is not precisely known, there is only a mention in my notes of a 0.45mm hole...but as I still have a sachet of 0.3mm drills, and not of 0.45mm, I would say that the jet is a 0.3mm hole!
looking under the firebox shows a smaller (6mm) additional flue tube, also visible in the firebox.
As such, this traction engine runs nicely at club meetings, and have good power, I just start the heating as I use to do with my gauge 1 locos, with the help of an external fan to get good draught.

I'm glad you have a runner! Maybe it would be useful - to anyone who picks up this thread in future - who is making this model if you can itemise any variations you have from the original drawings. Like the jet size that actually works for you instead of as drawn?
But well done on a nice working model.
K2