Model making tries to imitate reality, if someone is not able to do it, they can at least try. Boilers making is one of the challenges. The reason is very simple, we are unable to reduce the scale of physical properties of matter, such as: viscosity, density, flow speed, specific heat, thermal conductivity, surface tension, capillarity, and other properties that are inherent to the matter.
A scientist named “Reynolds” tried to explain the mechanics of fluids with a certain coherence, but the properties laws of fluids are very complicated in order for a common model maker to understand. Therefore model boilers cannot be a linear scale of the true one.
A model boiler only tries to imitate the real shape boiler, mainly in external aspect. With regards to the piping, fittings and many other items it only feigns. Reason being that the fluids don’t flow through the pipes as well as in between them, as it should be necessary. You must therefore exaggerate the diameters of the pipes and increase the distance between all of them.
The final result is the fact that, all original designs will be changed and another boiler will be designed instead of the original. The best thing to do is to never copy a boiler, but to estimate the steam you must build up and make your own project but always adhering to specific rules.
There are several kinds of boilers but first,
you must divide them into two main types. The water pipe and the flame
pipe boiler. The water pipe can be defined as pipes with water inside that
cross the flame, and the flame pipe type is the flame that flows through
the pipe that is surrounded by water.
In model making, both boiler types are used depending on the final purpose.
For locomotives, you should always use flame
pipes, this is inherent to the locomotive type boiler. In the case of stationary
engines you can make any choice. For marine use you can use two kinds,
but water pipe are more used and even more reliable for the job.
Normally flame pipe boilers are heavier than water pipe boilers.
Two examples of this kind of boilers are;
1st Locomotive boiler, 2nd Scotch boiler, mainly used in common ships.
Others of the same class are: Clyde, Scott, Center Flue and vertical boilers.
Marine boilers are mainly water pipe and the types of most common use are, Yarrow, Bab-Cock & Willcox, Cross Pipe, Thornycroft and other boilers that are a little bit modified from those mentioned above.
With that the following question arises: “why so many types of boilers?” The answer to that is: “the development and updating.” Real boilers and model boilers are valued on three items only, 1st by weight, let’s say, with so many pounds of weight how many steam does it generate? 2nd efficiency or performance, is the item regarding to how many ounces of fuel it burns in order to generate a certain amount of steam. 3rd thermal inertia, this means, the quantity of fuel that must be burned before it reaches the point of steam generation.
Examples: how many minutes does a model boiler
with 4 paints of water, need to build up pressure, departing from ambient
temperature (about 67º ) to reach 212º .How many ounces
of fuel does the boiler use in order to boil a paint of water and
to turn it into steam at a working pressure of 80 pounds per square inch?
Inertia is great with fire tube boiler than water tube boiler, this is the reason they normally use water tube boiler in man-o-war, and in freighters they used Scotch type, a kind of fire tubes boiler (flame pipe). The efficiency is a bit better but enough to be the reason for the choice. A mere question of economy.
Burners: There are several types of fuel used in model boilers. The most used in Locomotives boilers is coal, the best type being Cardiff. At the present moment the most used is LPG (liquefied petroleum gas) mainly a mixture of Propane and Butane, the proportion varies according the season. In cold weather the Propane reaches up to 30% of the content. Thus increasing the gas pressure.
In the summer of 1991, I introduced the Ceramic
Burner for model boilers in Nordfolk / Norwich, England, it was a
success and since then nothing generates more heat inside a model boiler
than a ceramic burner burning LPG.
Before this kind of burner, the common burner was some kind of torch, the problems were: 1st the noise, 2nd the localised flame, 3rd the malfunction.
Advice!!!! You shall not use liquid fuel inside
a model boat, it will leak and set your boat on fire. Use always LPG it
is safer and with best performance.In certain items you need to make a
good project of a model boiler.
To find the safe working pressure for a model boiler.
S x P x R x C x T x 2
WP= -------------------------------- lb. per squ. in.
D x F
Where S- Ultimate tensile strength of material in lb. per sq.
For mild Steel 60,00
For Copper 25,000
P- plate thickness in inches.
R-riveting allowance- 0.5 for single riveting, 0.75 for double riveting, 0.8 for welded, brazed or silver soldered joints.
C- corrosion allowance for steel boilers, 0.5 for plate thickness under ¼ in. and 0.75 for plate thickness of ¼ or over
T- Temperature allowance for copper boilers, 0.8 for pressure 50 – 100 p.s.i. and 0.7 for pressure 100 – 150 p.s.i.
D- Diameter of boiler in inches
F- Factor of Safety- Generally between 6 an 10, but 8 is most recommended.
To find Plate Thickness for model boilers
D x WP x F
P= -------------------- inches
S x R x C x T x 2
Steam temperature at various pressures
25 PSI 260 Fº
50 “ 300 “
100 “ 340 “
Boiler fittings: Boiler fittings should wherever possible be made of drawn gunmetal or hard bronze, with spindles, etc. of stainless steel. Brass or mild steel screws should never be used for attaching boiler fittings to bushes and etc. The ideal for a small model boiler is to be made of copper and all silver soldered . If you need any further information regarding to Model Boiler the better way to follow is to buy a book witch name is “ Model Boilers” by Martin Evans of Argus Book – England.