Chapter XV: (the most elaborate mechanism included in the book) with (4)

Now mark out accurately the centres of the discs on both sides, and make marks to show which face of each disc is to be outside.

On the outside of both scribe circles of the size of the bearing tubes, and other circles at the proper radius for the bolt hole centres.

On the outside of D scribe two circles of 2-inch and 1-11/16-inch radius, between which the steam pipe will lie.

On the inside of D scribe a circle of 1-27/32-inch radius for the steam ports.

On the outside of E mark a 7/8-inch circle for the exhaust pipe.

On the inside of both mark the circles between which the ring must lie.

Bolt Holes.—The marks for these, six or twelve in number, are equally spaced on the outside of one plate, and the two plates are clamped or soldered together before the boring is done, to ensure the holes being in line. If the bolts are to screw into one plate, be careful to make the holes of the tapping size in the first instance, and to enlarge those in D afterwards. Make guide marks in the plates before separating, between what will be the uppermost holes and the circumference.

Bolts.—These should be of brass if passed inside the ring. Nuts are not necessary if E is tapped, but their addition will give a smarter appearance and prevent-the bolts becoming loose.

Bearings.—Bore central holes in the discs to a good fit for the bearings, and prepare the hole for the exhaust pipe. This hole is most easily made by drilling a ring of small holes just inside the mark and cutting through the intervening metal.

For A, B, and C cut off pieces of bearing pipe, 1/2, 1/4, and 3/4 inch long respectively, and bevel the ends of B and C as shown, to minimize friction if they rub. File all other ends square. (Lathe useful here.)

Bore oil holes in B and C, and clear away all the “burr.” Make scratches on the bearings to show how far they should be pushed through the case.

Now assemble the case, taking care that the edge of the ring corresponds exactly with the circles marked on the discs, and clean the metal round the bearing holes and the bearings themselves. The last are then placed in position, with the lubricating holes pointing upwards towards the guide marks on the discs. Push the spindle rod through the bearings, which must be adjusted until the rod can be revolved easily with the fingers. Then solder in the bearing with a “Tinol” lamp.

The Wheel.—Anneal this well by heating to a dull red and plunging it in cold water. Mark a circle of 1-1/4-inch radius, and draw radial lines 1/4 inch apart at the circumference from this circle to the edge. Cut out along the lines, and twist the vanes to make an angle of about 60 degree with the central part, and bend the ends slightly backward away from the direction in which the rotor will revolve. (The directions given on p. 189 for making a steam top wheel can be applied here.)

Bore a hole in the centre to make a tight fit with the spindle, and place the rotor in position, with piece B in contact on the C side. Get everything square (rotation will betray a bad wobble), and solder the three parts together with the blow-lamp.

Mount the rotor squarely by the spindle points between two pieces of wood held lightly in the vice, and, with the aid of a gauge fixed to the piece nearest the wheel, true up the line of the vanes. (Lathe useful here.)

The Steam Pipe is 15 inches (or more) of 5/16-inch copper tubing, well annealed. To assist the bending of it into a ring one needs some circular object of the same diameter as the interior diameter of the ring round which to curve it. I procured a tooth-powder box of the right size, and nailed it firmly to a piece of board. Then I bevelled off the end of the pipe to the approximately correct angle, laid it against the box, and drove in a nail to keep it tight up. Bending was then an easy matter, a nail driven in here and there holding the pipe until the ring was complete. I then soldered the end to the standing part, and detached the ring for flattening on one side with a file and emery cloth. This done, I bored a hole through the tube at F to open up the blind end of the ring.

Attaching the ring to disc D is effected as follows:—Tin the contact faces of the ring and disc pretty heavily with solder, after making poppet marks round the guide circles so that they may not be lost under the solder. The ring must be pressed tightly against its seat while heating is done with the lamp. An extra pair of hands makes things easier at this point. Be careful not to unsolder the spindle bearing, a thing which cannot happen if the bearing is kept cool by an occasional drop or two of water. A little extra solder should be applied round the points where the ports will be.

The Steam Ports.—These are drilled (with a 1/32-inch twist drill), at an angle of about 30 degrees to the plate, along the circle already scribed. If you have any doubt as to your boiler’s capacity, begin with one hole only, and add a second if you think it advisable. As already remarked, pressure must not be sacrificed to steam flow.

Lubricators.—These are short pieces of tubing hollowed at one end by a round file of the same diameter as the bearings. A little “Tinol” is smeared over the surfaces to be joined, and the lubricators are placed in position and heated with the blow-lamp until the solder runs. To prevent the oil flowing too freely, the lubricators should be provided with airtight wooden plugs.

Escape Pipes.—The pipe for the exhaust steam is now soldered into disc E, and a small water escape into the ring at its lowest point. This pipe should be connected with a closed chamber or with the exhaust at a point lower than the base of the turbine case.

Stirrup.—Fig. 69 shows a stirrup carrying a screw which presses against the pulley end of the spindle. This attachment makes it easy to adjust the distance between the rotor and the steam ports, and also concentrates all end thrust on to a point, thereby minimizing friction. The stirrup can be fashioned in a few minutes out of brass strip. Drill the holes for the holding-on screws; drill and tap a hole for the adjusting screw; insert the screw and centre it correctly on the spindle point. Then mark the position of the two screw holes in E; drill and tap them.

Feet are made of sheet brass, drilled to take the three (or two) lowermost bolts, and bent to shape. Note.—A side and foot may be cut out of one piece of metal. The difficulty is that the bending may distort the side, and prevent a tight joint between side and ring.

Assembling.—Cut out two rings of stout brown paper a quarter of an inch wide and slightly larger in diameter than the casing ring. In assembling the turbine finally, these, after being soaked in oil, should be inserted between the ring and the discs. Put in four screws only at first, and get the ring properly centred and the bearings exactly in line, which will be shown by the spindle revolving easily. Then tighten up the nuts and insert the other bolts, the three lowest of which are passed through the feet. Affix the pulley and stirrup, and adjust the spindle longitudinally until the rotor just does not rub the casing. The soldering on of the cap of A completes operations.

To get efficiency, heavy gearing down is needed, and this can be managed easily enough with the help of a clockwork train, decreasing the speed five or more times for driving a dynamo, and much more still for slow work, such as pumping.

A More Elaborate Turbine.

The turbine just described can hardly be termed an efficient one, as the vanes, owing to their simple formation, are not shaped to give good results. We therefore offer to our readers a design for a small turbine of a superior character. This turbine is shown in elevation and section in Fig. 70. The casing is, as in the preceding instance, made up of flat brass plates and a ring of tubing, and the bearings, BG1, BG2, of brass tube. But the wheel is built up of a disc 3 inches in diameter, round the circumference of which are 32 equally-spaced buckets, blades, or vanes, projecting 5/8 inch beyond the edge of the disc. The wheel as a whole is mounted on a spindle 3-1/8 inches long, to which it is secured by three nuts, N1 N2 N3. One end of the spindle is fined down to take a small pinion, P1, meshing with a large pinion, P2, the latter running in bearings, BG3, in the wheel-case and cover. The drive of the turbine is transmitted either direct from the axle of P2 or from a pulley mounted on it.

CONSTRUCTION.

The Wheel.—If you do not possess a lathe, the preparation of the spindle and mounting the wheel disc on it should be entrusted to a mechanic. Its diameter at the bearings should be 5/32 inch or thereabouts. (Get the tubing for the bearings and for the spindle turned to fit.) The larger portion is about twice as thick as the smaller, to allow room for the screw threads. The right-hand end is turned down quite small for the pinion, which should be of driving fit.

The Blades.—Mark out a piece of sheet iron as shown in Fig. 71 to form 32 rectangles, 1 by l/2 inch. The metal is divided along the lines aaaa, bbbb, and ab, ab, ab, ab, etc. The piece for each blade then has a central slot 5/16 inch long and as wide as the wheel disc cut very carefully in it.

Bending the Blades.—In the edge of a piece of hard wood 1 inch thick file a notch 3/8 inch wide and 1/8 inch deep with a 1/2-inch circular file, and procure a metal bar which fits the groove loosely. Each blade is laid in turn over the groove, and the bar is applied lengthwise on it and driven down with a mallet, to give the blade the curvature of the groove. When all the blades have been made and shaped, draw 16 diameters through the centre of the wheel disc, and at the 32 ends make nicks 1/16 inch deep in the circumference.

True up the long edges of the blades with a file, and bring them off to a sharp edge, removing the metal from the convex side.

Fixing the Blades.—Select a piece of wood as thick as half the width of a finished blade, less half the thickness of the wheel disc. Cut out a circle of this wood 2 inches in diameter, and bore a hole at the centre. The wheel disc is then screwed to a perfectly flat board or plate, the wooden disc being used as a spacer between them.

Slip a blade into place on the disc, easing the central slit, if necessary, to allow the near edge to lie in contact with the board—that is parallel to the disc. Solder on the blade, using the minimum of solder needed to make a good joint. When all the blades are fixed, you will have a wheel with the blades quite true on one side. It is, therefore, important to consider, before commencing work, in which direction the concave side of the blades should be, so that when the wheel is mounted it shall face the nozzle.

To make this point clear: the direction of the nozzle having been decided, the buckets on the trued side must in turn present their concave sides to the nozzle. In Fig. 70 the nozzle points downwards, and the left side of the wheel has to be trued. Therefore B1 has its convex, B2 its concave, side facing the reader, as it were.

The Nozzle is a 1-1/2 inch piece of brass bar. Drill a 1/20-inch hole through the centre. On the outside end, enlarge this hole to 1/8 inch to a depth of 1/8 inch. The nozzle end is bevelled off to an angle of 20 degrees, and a broach is inserted to give the steam port a conical section, as shown in Fig. 72, so that the steam may expand and gain velocity as it approaches the blades. Care must be taken not to allow the broach to enter far enough to enlarge the throat of the nozzle to more than 1/20 inch.

Fixing the Nozzle.—The centre of the nozzle discharge opening is 1-13/16-inches from the centre of the wheel. The nozzle must make an angle of 20 degrees with the side of the casing, through which it projects far enough to all but touch the nearer edges of the vanes. (Fig. 72.) The wheel can then be adjusted, by means of the spindle nuts, to the nozzle more conveniently than the nozzle to the wheel. To get the hole in the casing correctly situated and sloped, begin by boring a hole straight through, 1/4 inch away laterally from where the steam discharge hole will be, centre to centre, and then work the walls of the hole to the proper angle with a circular file of the same diameter as the nozzle piece, which is then sweated in with solder. It is, of course, an easy matter to fix the nozzle at the proper angle to a thin plate, which can be screwed on to the outside of the casing, and this method has the advantage of giving easy detachment for alteration or replacement.

Balancing the Wheel.—As the wheel will revolve at very high speed, it should be balanced as accurately as possible. A simple method of testing is to rest the ends of the spindle on two carefully levelled straight edges. If the wheel persists in rolling till it takes up a certain position, lighten the lower part of the wheel by scraping off solder, or by cutting away bits of the vanes below the circumference of the disc, or by drilling holes in the disc itself.

Securing the Wheel.—When the wheel has been finally adjusted relatively to the nozzle, tighten up all the spindle nuts hard, and drill a hole for a pin through them and the disc parallel to the spindle, and another through N3 and the spindle. (Fig. 70.)

Gearing.—The gear wheels should be of good width, not less than 3/16 inch, and the smaller of steel, to withstand prolonged wear. Constant lubrication is needed, and to this end the cover should make an oil-tight fit with the casing, so that the bottom of the big pinion may run in oil. To prevent overfilling, make a plug-hole at the limit level, and fit a draw-off cock in the bottom of the cover. If oil ducts are bored in the bearing inside the cover, the splashed oil will lubricate the big pinion spindle automatically.

General—The sides of the casing are held against the drum by six screw bolts on the outside of the drum. The bottom of the sides is flattened as shown (Fig. 70), and the supports, S1 S2, made of such a length that when they are screwed down the flattened part is pressed hard against the bed. The oil box on top of the casing has a pad of cotton wool at the bottom to regulate the flow of oil to the bearings. Fit a drain pipe to the bottom of the wheel-case.

Testing.—If your boiler will make steam above its working pressure faster than the turbine can use it, the nozzle may be enlarged with a broach until it passes all the steam that can be raised; or a second nozzle may be fitted on the other end of the diameter on which the first lies. This second nozzle should have a separate valve, so that it can be shut off.

XVII. STEAM TOPS.

A very interesting and novel application of the steam turbine principle is to substitute for a wheel running in fixed bearings a “free” wheel pivoted on a vertical spindle, the point of which takes the weight, so that the turbine becomes a top which can be kept spinning as long as the steam supply lasts.

These toys, for such they must be considered, are very easy to make, and are “warranted to give satisfaction” if the following instructions are carried out.

A Small Top.—Fig. 74 shows a small specimen, which is of the self-contained order, the boiler serving as support for the top.

[1]

[Footnote 1: Spirit lamp shown for heating boiler.]

For the boiler use a piece of brass tubing 4 inches or so in diameter and 3 inches long. (The case of an old brass “drum” clock, which may be bought for a few pence at a watchmaker’s, serves very well if the small screw holes are soldered over.) The ends should be of brass or zinc, the one which will be uppermost being at least 1/16 inch thick. If you do not possess a lathe, lay the tube on the sheet metal, and with a very sharp steel point scratch round the angle between tube and plate on the inside. Cut out with cold chisel or shears to within 1/16 inch of the mark, and finish off carefully—testing by the tube now and then—to the mark. Make a dent with a centre punch in the centre of the top plate for the top to spin in.

Solder the plates into the tube, allowing an overlap of a quarter of an inch beyond the lower one, to help retain the heat.

The top wheel is cut out of a flat piece of sheet iron, zinc, or brass. Its diameter should be about 2-1/2 inches, the vanes 1/2 inch long and 1/4 inch wide at the circumference. Turn them over to make an angle of about 45 degrees with the spindle. They will be more easily bent and give better results if holes are drilled, as shown in Fig. 75.

The spindle is made out of a bit of steel or wire—a knitting-needle or wire-nail—not more than 1 inch in diameter and 1-1/2 inches long. The hole for this must be drilled quite centrally in the wheel; otherwise the top will be badly balanced, and vibrate at high speeds. For the same reason, the spindle requires to be accurately pointed.

The steam ports are next drilled in the top of the boiler. Three of them should be equally spaced (120 degrees apart) on a circle of 1-inch radius drawn about the spindle poppet as centre. The holes must be as small as possible—1/40 to 1/50 inch—and inclined at an angle of not more than 45 degrees to the top plate. The best drills for the purpose are tiny Morse twists, sold at from 2d. to 3d. each, held in a pin vice rotated by the fingers. The points for drilling should be marked with a punch, to give the drills a hold. Commence drilling almost vertically, and as the drill enters tilt it gradually over till the correct angle is attained.

If a little extra trouble is not objected to, a better job will be made of this operation if three little bits of brass, filed to a triangular section (Fig. 76 a), are soldered to the top plate at the proper places, so that the drilling can be done squarely to one face and a perfectly clear hole obtained. The one drawback to these additions is that the vanes of the turbine may strike them. As an alternative, patches may be soldered to the under side of the plate (Fig. 76, b) before it is joined to the barrel; this will give longer holes and a truer direction to the steam ports.

Note that it is important that the ports should be all of the same diameter and tangential to the circle on which they are placed, and all equally inclined to the plate. Differences in size or direction affect the running of the top.

Solder the spindle to the wheel in such a position that the vanes clear the boiler by an eighth of an inch or so. If tests show that the top runs quite vertically, the distance might be reduced to half, as the smaller it is the more effect will the steam jets have.

A small brass filler should be affixed to the boiler halfway up. A filler with ground joints costs about 6d.

A wick spirit lamp will serve to raise steam. Solder to the boiler three legs of such a length as to give an inch clearance between the lamp wick and the boiler. If the wick is arranged to turn up and down, the speed of the top can be regulated.

A Large Top.—The top just described must be light, as the steam driving it is low-pressure, having free egress from the boiler, and small, as the steam has comparatively low velocity. The possessor of a high-pressure boiler may be inclined to make something rather more ambitious—larger, heavier, and useful for displaying spectrum discs, etc.

The top shown in Fig. 77 is 3 inches in diameter, weighs 1 oz., and was cut out of sheet-zinc. It stands on a brass disc, round the circumference of which is soldered a ring of 5/32-inch copper tubing, furnished with a union for connection with a boiler.

The copper tubing must be well annealed, so as to bend quite easily. Bevel off one end, and solder this to the plate. Bend a couple of inches to the curve of the plate, clamp it in position, and solder; and so on until the circle is completed, bringing the tube snugly against the bevelled end. A hole should now be drilled through the tube into this end—so that steam may enter the ring in both directions-and plugged externally.

By preference, the ring should be below the plate, as this gives a greater thickness of metal for drilling, and also makes it easy to jacket the tube by sinking the plate into a wooden disc of somewhat greater diameter.

Under 50 lbs. of steam, a top of this kind attains a tremendous velocity. Also, it flings the condensed steam about so indiscriminately that a ring of zinc 3 inches high and 18 inches in diameter should be made wherewith to surround it while it is running.

If a little bowl with edges turned over be accurately centred on the wheel, a demonstration of the effects of centrifugal force may be made with water, quicksilver, or shot, which fly up into the rim and disappear as the top attains high speed, and come into sight again when its velocity decreases to a certain figure. A perforated metal globe threaded on the spindle gives the familiar humming sound.

A spectrum disc of the seven primary colours—violet, indigo, blue, green, yellow, orange, red—revolved by the top, will appear more or less white, the purity of which depends on the accuracy of the tints used.

XVIII. MODEL BOILERS.

A chapter devoted to the construction of model boilers may well open with a few cautionary words, as the dangers connected with steam-raisers are very real; and though model-boiler explosions are fortunately rare, if they do occur they may be extremely disastrous.

Therefore the following warnings:—

(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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

CONSTRUCTION.

The centres for the water-tubes, G and H, should be laid out, in accordance with Fig. 88, on the tops of B and C and the lower part of A, along lines scribed in the manner explained on p. 207. Tubes H must be bent to a template to get them all of the same shape and length, and all the tubes be prepared before any are put in place. If the tubes are set 7/8 inch apart, centre to centre, instead of 1-1/4 inches, the heating surface will be greatly increased and the furnace casing better protected.

Assembling.—When all necessary holes have been made and are of the correct size, begin by riveting and silver-soldering in the ends of the drums. Next fix the cross tubes, E, taking care that they and B and C form rectangles. Then slip the F, G, and H tubes half an inch into the main drum, and support A, by means of strips passed between the G and H tubes, in its correct position relatively to B and C. The E tubes can now be pushed into B and C and silver-soldered. The supports may then be removed, and the a and H tubes be got into position and secured. Drum D then demands attention. The connecting tubes, KK, should be silver-soldered in, as the boiler, if properly made, can be worked at pressures up to 100 lbs. per square inch.

The casing is of 1/20-inch sheet iron, and in five parts. The back end must be holed to allow A, B, and C to project 1 inch, and have a furnace-door opening, and an airway at the bottom, 5 inches wide and 1 inch deep, cut in it. The airway may be provided with a flap, to assist in damping down the fire if too much steam is being raised. In the front end make an inspection opening to facilitate cleaning the tubes and removing cinders, etc.

The side plates, m m, are bent as shown in Fig. 86, and bolted to a semicircular top plate, n, bent to a radius of 6 inches. A slot, 1-1/2 inches wide and 11-1/2 inches long, must be cut in the top, n, to allow it to be passed over drum D; and there must also be a 3 or 3-1/2 inch hole for the chimney. A plate, p, covers in D. A little plate, o, is slipped over the slot in n, and asbestos is packed in all round D. The interior of the end, side, and the top plates should be lined with sheet asbestos held on by large tin washers and screw bolts. To protect the asbestos, movable iron sheets may be interposed on the furnace side. These are replaced easily if burnt away. The pieces m m are bent out at the bottom, and screwed down to a base-plate extending the whole length of the boiler.

The fire-bars fill the rectangle formed by the tubes B, El, and E2. A plate extends from the top of E2 to the front plate of the casing, to prevent the furnace draught being “short circuited.”

Boiler Fittings.

Safety Valves.—The best all-round type is that shown in Fig. 89. There is no danger of the setting being accidentally altered, as is very possible with a lever and sliding weight. The valve should be set by the steam gauge. Screw it down, and raise steam to the point at which you wish the safety valve to act, and then slacken off the regulating nuts until steam issues freely. The lock nuts under the cross-bar should then be tightened up. In the case of a boiler with a large heating surface, which makes steam quickly, it is important that the safety-valve should be large enough to master the steam. If the valve is too small, the pressure may rise to a dangerous height, even with the steam coming out as fast as the valve can pass it.

Steam Gauges.—The steam gauge should register pressures considerably higher than that to be used, so that there may be no danger of the boiler being forced unwittingly beyond the limit registered. A siphon piece should be interposed between boiler and gauge (Fig. 90), to protect the latter from the direct action of the steam. Water condenses in the siphon, and does not become very hot.

Water Gauges should have three taps (Fig. 91), two between glass and boiler, to cut off the water if the glass should burst, and one for blowing off through. Very small gauges are a mistake, as the water jumps about in a small tube. When fitting a gauge, put packings between the bushes and the glass-holders, substitute a piece of metal rod for the glass tube, and pack the rod tightly. If the bushes are now sweated into the boiler end while thus directed, the gauge must be in line for the glass. This method is advisable in all cases, and is necessary if the boiler end is not perfectly flat.

Pumps.—Where a pump is used, the supply should enter the boiler below low-water level through a non-return valve fitted with a tap, so that water can be prevented from blowing back through the pump. As regards the construction of pumps, the reader is referred to p. 164 and to