(1.) Do not use tins or thin sheet iron for boilers. One cannot tell
how far internal corrosion has gone. The scaling of 1/100 inch of metal
off a “tin” is obviously vastly more serious than the same diminution
in the thickness of, say, a 1/4-inch plate. Brass and copper are the
metals to employ, as they do not deteriorate at all provided a proper
water supply be maintained.
(2.) If in doubt, make the boiler much more solid than is needed,
rather than run any risks.
(3.) Fit a steam gauge, so that you may know what is happening.
(4.) Test your boiler under steam, and don’t work it at more than half
the pressure to which it has been tested. (See p. 220.)
In the present chapter we will assume that the barrels of all the
boilers described are made out of solid-drawn seamless copper tubing,
which can be bought in all diameters up to 6 inches, and of any one of
several thicknesses. Brass tubing is more easily soldered, but not so
good to braze, and generally not so strong as copper, other things
being equal. Solid-drawn tubing is more expensive than welded tubing or
an equivalent amount of sheet metal, but is considerably stronger than
the best riveted tube.
Boiler ends may be purchased ready turned to size. Get stampings rather
than castings, as the first are more homogeneous, and therefore can be
somewhat lighter.
Flanging Boiler Ends.—To make a good job, a plate for an end should be
screwed to a circular block of hard wood (oak or boxwood), having an
outside diameter less than the inside diameter of the boiler barrel by
twice the thickness of the metal of the end, and a rounded-off edge.
The plate must be annealed by being heated to a dull red and dipped in
cold water. The process must be repeated should the hammering make the
copper stubborn.
Stays should be used liberally, and be screwed and nutted at the ends.
As the cutting of the screw thread reduces the effective diameter, the
strength of a stay is only that of the section at the bottom of the
threads.
Riveting.—Though stays will prevent the ends of the boiler blowing off,
it is very advisable to rivet them through the flanges to the ends of
the barrel, as this gives mutual support independently of soldering or
brazing. Proper boiler rivets should be procured, and annealed before
use. Make the rivet holes a good fit, and drill the two parts to be
held together in one operation, to ensure the holes being in line.
Rivets will not close properly if too long. Dies for closing the rivet
heads may be bought for a few pence.
Soldering, etc.—Joints not exposed directly to the furnace flames may
be soldered with a solder melting not below 350 degrees Fahr. Surfaces
to be riveted together should be “tinned” before riveting, to ensure
the solder getting a good hold afterwards. The solder should be sweated
right through the joint with a blow-lamp to make a satisfactory job.
All joints exposed to the flames should be silver-soldered, and other
joints as well if the working pressure is to exceed 50 lbs. to the
square inch. Silver-soldering requires the use of a powerful blow—lamp
or gas-jet; ordinary soft soldering bits and temperatures are
ineffective. Brazing is better still, but should be done by an expert,
who may be relied on not to burn the metal. It is somewhat risky to
braze brass, which melts at a temperature not far above that required
to fuse the spelter (brass solder). Getting the prepared parts of a
boiler silver-soldered or brazed together is inexpensive, and is worth
the money asked.
[Illustration: FIG. 78.]
Some Points in Design.
The efficiency of a boiler is governed chiefly (1) by the amount of
heating surface exposed to the flames; (2) by the distribution of the
heating surface; (3) by the amount of fuel which can be burnt in the
furnace in a given time; (4) by avoiding wastage of heat.
The simplest form of boiler, depicted in Fig. 78, is extremely
inefficient because of its small heating surface. A great deal of the
heat escapes round the sides and the ends of the boiler. Moreover, a
good deal of the heat which passes into the water is radiated out
again, as the boiler is exposed directly to the air.
Fig. 79 shows a great improvement in design. The boiler is entirely
enclosed, except at one end, so that the hot gases get right round the
barrel, and the effective heating surface has been more than doubled by
fitting a number of water-tubes, aaa, bbbb, which lie right in the
flames, and absorb much heat which would otherwise escape. The tubes
slope upwards from the chimney end, where the heat is less, to the
fire-door end, where the heat is fiercer, and a good circulation is
thus assured. The Babcock and Wilcox boiler is the highest development
of this system, which has proved very successful, and may be
recommended for model boilers of all sizes. The heating surface may be
increased indefinitely by multiplying the number of tubes. If a solid
fuel-coal, coke, charcoal, etc.-fire is used, the walls of the casing
should be lined with asbestos or fire-clay to prevent the metal being
burnt away.
[Illustration: FIG. 79—Side and end elevations of a small water-tube
boiler.]
The horizontal boiler has an advantage over the vertical in that, for
an equal diameter of barrel, it affords a larger water surface, and is,
therefore, less subject to “priming,” which means the passing off of
minute globules of water with the steam. This trouble, very likely to
occur if the boiler has to run an engine too large for it, means a
great loss of efficiency, but it may be partly cured by making the
steam pass through coils exposed to the furnace gases on its way to the
engine. This “superheating” evaporates the globules and dries the
steam, besides raising its temperature. The small water-tube is
preferable to the small fire-tube connecting furnace and chimney, as
its surface is exposed more directly to the flames; also it increases,
instead of decreasing, the total volume of water in the boiler.
A Vertical Boiler.
[Illustration: FIG. 80.—Details of vertical boiler.]
The vertical boiler illustrated by Fig. 80 is easily made. The absence
of a water jacket to the furnace is partly compensated by fitting six
water-tubes in the bottom. As shown, the barrel is 8 inches long and 6
inches in outside diameter, and the central flue 1-1/2 inches across
outside solid-drawn 1/16-inch tubing, flanged ends, and four 1/4-inch
stays—disposed as indicated in Fig. 80 (a) and (b)—are used. The 5/16
or 3/8 inch water-tubes must be annealed and filled with lead or resin
before being bent round wooden templates. After bending, run the resin
or lead out by heating. The outflow end of each pipe should project
half an inch or so further through the boiler bottom than the inflow
end.
Mark out and drill the tube holes in the bottom, and then the flue
hole, for which a series of small holes must be made close together
inside the circumference and united with a fret saw. Work the hole out
carefully till the flue, which should be slightly tapered at the end,
can be driven through an eighth of an inch or so. The flue hole in the
top should be made a good fit, full size.
Rivet a collar, x (Fig. 80, a), of strip brass 1/4 inch above the
bottom of the flue to form a shoulder. Another collar, y (Fig. 80, c),
is needed for the flue above the top plate. Put the ends and flue
temporarily in place, mark off the position of y, and drill half a
dozen 5/32-inch screw holes through y and the flue. Also drill screw
holes to hold the collar to the boiler top.
The steam-pipe is a circle of 5/16-inch copper tube, having one end
closed, and a number of small holes bored in the upper side to collect
the steam from many points at once. The other end is carried through
the side of the boiler.
[Illustration: FIG. 81.—Perspective view of horizontal boiler mounted
on wooden base.]
Assembling.—The order of assembling is:—Rivet in the bottom; put the
steam-pipe in place; rivet in the top; insert the flue, and screw
collar y to the top; expand the bottom of the flue by hammering so that
it cannot be withdrawn; insert the stays and screw them up tight;
silver-solder both ends of the flue, the bottom ends of the stays, and
the joint between bottom and barrel. The water-tubes are then inserted
and silver-soldered, and one finishes by soft-soldering the boiler top
to the barrel and fixing in the seatings for the water and steam
gauges, safety-valve, mud-hole, filler, and pump-if the last is fitted.
The furnace is lined with a strip of stout sheet iron, 7 inches wide
and 19-1/4 inches long, bent round the barrel, which it overlaps for an
inch and a half. Several screws hold lining and barrel together. To
promote efficiency, the furnace and boiler is jacketed with asbestos—or
fire-clay round the furnace—secured by a thin outer cover. The
enclosing is a somewhat troublesome business, but results in much
better steaming power, especially in cold weather. Air-holes must be
cut round the bottom of the lining to give good ventilation.
A boiler of this size will keep a 1 by 1-1/2 inch cylinder well
supplied with steam at from 30 to 40 lbs. per square inch.
A Horizontal Boiler.
[Illustration: FIG. 82.—Longitudinal section of large water-tube
boiler.]
The boiler illustrated by Fig. 81 is designed for heating with a large
paraffin or petrol blow-lamp. It has considerably greater water
capacity, heating surface—the furnace being entirely enclosed—and water
surface than the boiler just described. The last at high-water level is
about 60, and at low-water level 70, square inches.
The vertical section (Fig. 82) shows 1/16-inch barrel, 13 inches long
over all and 12 inches long between the end plates, and 6 inches in
diameter. The furnace flue is 2-1/2 inches across outside, and contains
eleven 1/2-inch cross tubes, set as indicated by the end view (Fig.
83), and 3/4 inch apart, centre to centre. This arrangement gives a
total heating surface of about 140 square inches. If somewhat smaller
tubes are used and doubled (see Fig. 84), or even trebled, the heating
surface may be increased to 180-200 square inches. With a powerful
blow-lamp this boiler raises a lot of steam.
Tubing the Furnace Flue.—Before any of the holes are made, the lines on
which the centres lie must be scored from end to end of the flue on the
outside. The positions of these lines are quickly found as follows:—Cut
out a strip of paper exactly as long as the circumference of the tube,
and plot the centre lines on it. The paper is then applied to the tube
again, and poppet marks made with a centre punch opposite to or through
the marks on the paper. Drive a wire-nail through a piece of square
wood and sharpen the point. Lay the flue on a flat surface, apply the
end of the nail to one of the poppet marks, and draw it along the flue,
which must be held quite firmly. When all the lines have been scored,
the centring of the water tubes is a very easy matter.
[Illustration: FIG. 83.-End of horizontal boiler, showing position of
holes for stays and fittings.]
The two holes for any one tube should be bored independently, with a
drill somewhat smaller than the tube, and be opened to a good fit with
a reamer or broach passed through both holes to ensure their sides
being in line. Taper the tubes—2-7/8 inches long each—slightly at one
end, and make one of the holes a bit smaller than the other. The
tapered end is passed first through the larger hole and driven home in
the other, but not so violently as to distort the flue. If the tubes
are made fast in this way, the subsequent silver-soldering will be all
the easier.
[Illustration: FIG. 84.—Doubled cross tubes In horizontal boiler flue.]
The Steam Dome.—The large holes—2 inches in diameter—required for the
steam dome render it necessary to strengthen the barrel at this point.
Cut out a circular plate of metal 4 inches across, make a central hole
of the size of the steam dome, and bend the plate to the curve of the
inside of the barrel. Tin the contact faces of the barrel and “patch”
and draw them together with screws or rivets spaced as shown in Fig.
85, and sweat solder into the joint. To make it impossible for the
steam dome to blowout, let it extend half an inch through the barrel,
and pass a piece of 1/4-inch brass rod through it in contact with the
barrel. The joint is secured with hard solder. Solder the top of the
dome in 1/8 inch below the end of the tube, and burr the end over. The
joint should be run again afterwards to ensure its being tight.
[Illustration: FIG. 85.—Showing how to mark out strengthening patch
round steam dome hole.]
The positions of stays and gauges is shown in Fig. 83.
Chimney.—This should be an elbow of iron piping fitting the inside of
the flue closely, made up of a 9-inch and a 4-inch part. The last slips
into the end of the flue; the first may contain a coil for superheating
the steam.
A Multitubular Boiler.
[Illustration: FIG. 86.—Cross section of multitubular boiler.]
Figs. 86 and 87 are respectively end and side elevations of a
multitubular boiler having over 600 square inches of heating
surface—most of it contributed by the tubes—and intended for firing
with solid fuel.
The boiler has a main water-drum, A, 5 inches in diameter and 18 inches
long, and two smaller water-drums, B and C, 2-1/2 by 18 inches,
connected by two series of tubes, G and H, each set comprising 20
tubes. The H tubes are not exposed to the fire so directly as the G
tubes, but as they enter the main drum at a higher point, the
circulation is improved by uniting A to B and C at both ends by large
1-inch drawn tubes, F. In addition, B and C are connected by three
3/4-inch cross tubes, E, which prevent the small drums spreading, and
further equalize the water supply. A 1-1/2-inch drum, D, is placed on
the top of A to collect the steam at a good distance from the water.
Materials.—In addition to 1-1/2 feet of 5 by 3/32 inch solid-drawn
tubing for the main, and 3 feet of 2-1/2 by 1/16 inch tubing for the
lower drums, the boiler proper requires 22-1/2 feet of 1/2-inch
tubing, 19 inches of 3/4-inch tubing, 2-1/4 feet of 1-inch tubing, 1
foot of 1-1/2-inch tubing, and ends of suitable size for the four
drums.
[Illustration: FIG. 87.—Longitudinal section of multitubular boiler.]