The Miniature Boiler

The Miniature Boiler

Charles V. Arnold -- Tiny Power

The North American Live Steamer, Volume 1 Number 7, 1956

No Arguments with Facts Derived From Experience

Having noted the letter of our good friend W. Hazard in latest issue of NALS it seems the boiler problem is always with us. The boiler, of course, is the very heart of any steam project and the tyro or lesser experienced builders read of formulas, ideas, theories and this and that. Some ideas contradicting other ideas and so on.

Mr. Hazard asks "will someone help me out?" It would have been better, I believe, if he had stated the request thus -- "Will someone who has built and operated many boilers over many ears with proven results help me out?"

Tiny Power, having built well over 300 miniature boilers, operated many, many boilers (in miniature) of all types over some 32 years, might be called on for some answers.

LBSC (Curly Lawrence) of England, with over 50 years' experience and many hundreds of successful boilers built all over the world after his designs is most highly qualified to give the answers.

Harry Coventry, who has the many years of experience and is a real engineer can well give the answers, and so on.

First, concerning size of fire tubes in a locomotive type boiler. This is determined (we are speaking from experience and not formula or ratio or theory) by I.D. and length of shell disregarding smokebox. And regardless of the grate area. Also consider the I.D. of the fire tubes or rather the thickness of their walls as in miniature boilers we usually state firetube dimentions in the O.D. -- 20 gauge or 0.035 inch wall tube is ample and proper for tubes up through 3/4 inch O.D. Anything thicker is detrimental to the steaming qualities of the boiler for reasonjs I will explain later.

Boilers with shell diameter up through 4 inches can well use 3/8 inch O.D. by 0.035 inch wall tubes, but I have found that a combination of 3/8 inch and 1/2 inch tubes work out in many cases to better advantage in getting the most flue area in the space allowed. Do not use 3/8 inch tubes in any case in lengths of over 10 to 12 inches and not at all in any size boiler if a fuel that puts out an appreciable amount of soot is to be used.

Atlantics-Tankers or Switchers in 3/4 inch scale with short shells might use the 3/8 inch tubes but the combination of 3/8 inch and 1/2 inch (plus the two 3/4 inch for superheater) has worked out best for us in fast steaming.

But for the Hudson or Pacific and longer shell boiler such as 2 inch scale tractions, do not use 3/8 inch at all but the 1/2 inch O.D. by 0.035 wall plus the two or four 3/4 inch O.D. by 0.035 inch wall for superheaters and length up to 16 inch have proven in many, many boilers of this size by us to be fine steam makers. In all cases not less than 1/16 to 3/32 inch bridges to be used.

Short 1 inch scale boilers should use 1/2 inch by 0.035 inch wall tubes but if the length gets over 16 inch, go up to 5/8 inch O.D. and if ever over 20 inch long go up to 3/4 inch O.D. by 0.035 wall.

Though not built too often, boilers with combustion chambers are better for the simple reason that we hereby shorten the firetubes and gain the advantage of the higher evaporating surface as compared with tubes here.

However, by using a long firebox (as the Case Traction) we get the same result. Firebox length in locos are limited, of course, by the type loco being modeled.

Now, as to grate, I believe that what was meant by too much grate area really was intended to mean too much AIR space coming thru the grate area. Large grate area is much to be desired, and it is in the firebox that we really get the steam, BUT the arrangement and spacing of the grate bars is the critical thing that many times determines the success or failure of a boiler to steam. Seems a lot of people think that the more air you can get in the firebox the better -- but not so.

Efficient combustion of fuel, regardless of kind, is dependant on the proper admixture of air at the proper place and proper time. If in your automobile carburetor--regard the tiny jets and orifices, adjusting screws, etc., for the only purpose of this proper air to fuel mixture.

In our miniature boilers--the size of grate bars--the space between grate-bars--the size of draft openings and the control of draft openings is of most critical importance--to a lesser degree but important also is the blast nozzle opening as related to boiler size and a ring blower capable of fine adjustment.

LBSC has also pointed out the importance of the above.

1/8 inch thick bars with 1/8 inch spacing using 1/4 to 1/2 inch long pieces of the same bar as washers for spacing at each end of the grate has proven ideal for most loco fire boxes in 1/2 and 3/4 inch scale. Some bolt these together with a long bolt and it is good practice to make the grate in sections of a width that can be passed thru the fire door. We here simply place the parts on edge on a firebrick and weld all together right across the tops of the two rows of spacers.

IN our 2 inch CASE traction grate we supply a cast iron casting which is 4-3/8 inch wide by 7-3/8 inch long by 1/2 inch thick. The ends are solid for 1 inch at each end, leaving grate bars 5-3/8 inch long by 1/4 inch thick. The actual air space thru the grate is 8 slots 1/4 inch wide by 5-3/8 inch long. The efficiency of this boiler has actually been proven by the fine steaming qualities of many of these boilers now in use.

We have a draft door at either end of the ash pan which can be opened to any degree at will. When the engine is working hard, one of these doors is completely closed and the other only partly opened. This amount of opening is found by experiment by the operator in each individual case.

Think it through--when the blast is hard a large quantity of air is sucked through the grate--when the blast is softer or the engine not working so hard less air is drawn through. If too much air is drawn through the grate and on into the firebox and tubes it actually cools the tubes and poor steaming is the result--only enough air for the proper combustion of the amount of fire in there can be used at any time.

In big practice, the fireman regulates his draft openings to wuit the work being done. How can he tell? By watching the smoke from the stack. Black smoke means too little air to duel (in case of oil firing it means also that he might be opening the oil firing valve too much). No smoke at all but just steam from stack means too much air. The ideal is a grey haze. The loco operator or traction operator will learn in time just where to carry his draft openings etc., in relation to the amount of work his engine is doing.

Then, as Mr. Coventry so aptly pointed out in hsi excellent article in the No. 4 NALS (p. 11), the thickness of materials used in the boiler construction is of vital importance. I would like to say here that anyone building Mr. Coventry's boiler will get a good steamer, as he has described its construction very well.

Some people have the idea that the thicker the materials the better for safety's sake. Result is a poor steaming boiler, as they are heating a mass of copper (or steel though why one would build a steel boiler is anything up to 1-1/2 inch scale considering the rust problem) instead of water. The formula given by Mr. Coventry in the article referred to before is correct and experience proven.

Mr. Hazard says that his steel boiler will not make steam as it should. I would ask him first about thickenss of materials used. In his case with a shell of around 5 to 5-1/2 inch diameter --- if the thickness is over 0.125 to 0.135 inch, then it is too heavy. If the sheet and tube wall thicknesses are much over these, then they are too heavy.

3/32 inch thick materials for boilers up to 4-1/2 inch diameter is plenty ampl and the crown sheets should be 64 ounce or 0.086. (I'm speaking of copper). For boilers with 5 to 6 inch O.D. sheels, 0.109 to 0.132 shell walls are ample with sheet of 0.093 ito 0.129 inch and crown sheet of 0.086 to 0.093 inch.

Above all, based on 100 pound working pressure or less, and to be hydro tested from 200 to 250 pound. Any more is excessive and might weaken the boiler. All flat surfaces to be stayed at not more than 3/4 to 7/8 inch pitch both ways.

Just as in the cylinder of engine, power from steam comes from the HEAT. The fire must heat up and keep hot the materials that enclose the steam as well as heat the water. For instance, in talking with a friend one time, I mentioned that in figuring the horsepower of a wee loco we were discussing, the M.E.P. or actual push on the piston was somewhere around 40% to 50% of the gauge pressure on boiler. He was amazed as he had figured that a boiler pressure of say 100 point mean that same pressure on the pistons. This, of course, is not true because the steam has to heat and keep hot all the piping, all the metal in cylinders, etc., which causes the loss.

Therefore, the lagging necessary on all metal that encloses steam to keep from heating the atmosphere. IN short, the comparatively tiny fire in our miniature steamers has a tremendous job to do.

Last, along with LBSC and other real authorities on this subject and for the heat loss reasons just given above, by all means a really efficient steamer should be superheated. Obviously, anything we can do to add heat to our already overworked steam to help in that constant necessity of keeping the metals hot as well as supplying push is to be desired.

In our 2 inch scale CASE for instance, we use 7 foot of 3/8 inch superheater, part of which is ove the actual fire. Result--dry hot steam and at least a fourth more power.

To sum up, anyone approaching the building of a boiler or having built a boiler or so and perhaps not getting the results anticipated would do well to first get prings, etc., from an experienced and tested and proven designer who has actually built and operated many boilers, not just one or tw, then follow to a "T" the print. Just as one getting a prescription from his doctor would not think of adding to or taking from it. Do not use brass in boiler construction of any kind. Flange all sheets, clean and flux them before riveting them together. Use SILFOS in practically every place except around the thin firetubes. Use EASYFLO for these. SILFOS is more or less self-fluxing and will flow and bond where EASYFLO will refuse if the flux has happened to burn away.


 * Charvles V. Arnold
 * Box 218
 * Corvallis, Oregon

P.S. I cannot help but add, many times a poor running steamer gets its boiler cussed when the fault is not in boiler at all but in the poor construction and fits in the valve gear and running gear or both. An engine consuming twice or three times the steam it ought to should be fitted to a boiler twice or three times the size required to take care of poor work.