Description of Boiler.Grate
Area
sq. ft.		Heating
Surface
sq. ft.		Duration
of Trial
Hours.		Coal
burned
Per sq. ft.
of Grate
per Hour.		Air
Pressure
in Stoke-
hold—
Inches of
Water.		Chimney
Draught—
Inches of
Water		Water Evaporated
per ℔ of Coal.		Water
Evapor-
ated per
sq. ft. of
Heating
Surface.		Thermal
Units per
℔ of
coal.		Effic-
iency
of
Boiler
%.
ActualFrom and
at
212° F.
℔ ℔ ℔
Ordinary cylindrical single-
 ended; 3 furnaces; 155 ℔
 working pressure; closed
 stokehold system.*		81 2308 25 14.2 Nil 0.36 8.56 10.26 4.26 14,267 69.7
” ” 24 13.9 ” 0.50 8.84 10.33 4.32 14,697 68.0
” ”  9 30.3 0.81 0.39 7.93 9.27 8.46 14,686 61.4
” ”  8½ 29.1 0.65 0.32 8.84 10.34 9.05 14,612 68.4
Ordinary cylindrical single-
 ended; 3 furnaces; 210 ℔
 working pressure; closed
 ashpit, Howden system.**		63.2 2876 in
boiler,
766 in
air heaters		 13 20.6 In Ash-
pit
1.53		 0.58 11.30 12.33 5.14 14,475 82.3
Niclausse water-tube; 160
 ℔ working pressure.		46 1322  8 12.8 Nil 0.20 8.41 10.15 3.75 14,680 66.9
” ”  8 21.9 ” 0.20 8.01 9.40 6.11 14,760 62.1
” ” 37 20.2 ” 0.29 7.62 9.00 5.44 14,600 60.5
Niclausse water-tube;
 250 ℔ working pressure.		34 990  9 14.0 0.10 0.23 8.77 10.50 4.17 14,640 69.8
” ”  9 22.0 0.27 0.23 7.68 9.06 5.74 14,640 60.4
” ” 90 15.4 Nil Not asce-
rtained		 7.61 9.08 4.00 14,630 59.9
Babcock water-tube; 33⁄16
 in. tubes; 260 ℔ working
 pressure.		36 1010  9 13.0 ” 0.26 9.31 11.02 4.30 14,590 73.2
” ”  9 20.0 0.18 0.20 8.58 10.11 6.13 14,590 67.0
” ” 90 14.5 Nil Not asce-
rtained		 8.09 9.53 4.18 · · 63.1
Babcock water-tube; 113⁄16
 in. tubes; 270 ℔ working
 pressure.***		62 2167 28 18.4 ” 0.45 8.94 10.61 4.61 14,520 70.7
” ” 24 19.2 ” 0.47 8.93 10.59 4.82 14,390 71.1
” ” 12 20.5 ” 0.42 9.42 11.04 5.41 14,080 75.8
” ”  7 28.9 0.50 Not asce-
rtained		 8.54 9.88 6.91 14,390 66.3
” ” 30 19.9 Nil 0.38 10.11 12.00 6.01 14,530 79.9
” ” 29 27.1 0.66 0.23 9.96 11.67 8.05 14,630 77.1
Belleville water-tube with
 economizers; 320 ℔
 working pressure.		44 910 in boiler; 24½ 15.8 Nil 0.36 9.65 11.46 4.94 14,697 77.2
” 447 in econo- 24 17.4 ” 0.39 9.33 11.00 5.30 14,805 71.8
” mizer; 11 19.8 ” 0.43 9.39 11.03 6.38 14,578 73.3
” 1357 total.  8 27.2 ” 0.39 8.28 9.79 7.78 14,611 65.0
Yarrow water tube; 1¾ in.
 tubes; 250 ℔ working
 pressure.		56 2896 26 16.9 Nil 0.31 9.57 11.45 3.12 14,750 75.0
” ” 26 18.2 ” 0.31 9.37 11.33 3.30 14,500 75.7
” ” 25 21.3 ” 0.31 8.83 10.45 3.63 13,500 75.2
” ” 30 35.4 0.53 0.26 8.82 10.59 6.04 14,430 70.9
” ”  8 41.9 0.86 0.31 8.24 9.94 6.69 14,500 66.3
” ”  8 33.7 0.31 0.30 8.39 9.93 5.47 14,680 65.4
” ”  8 39.8 0.82 0.24 8.85 10.43 6.81 14,530 69.5
Dürr water-tube; 250 ℔
 working pressure.		 712671 in boiler,
140 in super-
heater;
2811 total.		26 16.1 Nil 0.39 7.95 9.50 3.24 14,500 63.8
” 26 17.7 ” 0.30 7.06 9.28 3.43 14,620 61.7
” 25 21.1 ” 0.31 7.62 9.08 4.05 14,650 60.3
”  7 33.8 0.70 0.36 7.72 9.29 6.59 14,570 62.7
”  8 26.7 0.33 0.35 7.86 9.26 5.30 14,320 63.1
”  8 34.6 1.11 0.20 8.02 9.53 7.02 14,230 64.8
” 22 34.8 0.73 0.16 6.84 8.06 6.02 14,430 54.0
” 24 29.9 0.35 0.12 7.62 9.00 5.75 14,240 61.2
” 20 19.9 Nil 0.21 7.30 8.33 3.66 14,240 8.6
*  In the first three trials no retarders were used in the tubes.In the last trial retarders were used.
**  In this trial retarders were used in the tubes.
*** The first four trials were made with horizontal baffles above the tubes;the last two trials with the baffling described in the text.

Boiler Making

The practice of the boiler, bridge and girder shops may here be conveniently treated together, because similar materials and methods are employed in each, notwithstanding that many points of divergence in practice generally relegate them to separate departments. The materials used are chiefly iron and steel. The methods mostly adopted are those involved in the working of plates and rolled sections, which vastly predominate over the bars and rods used chiefly in the smithy. But there are numerous differences in methods of construction. Flanging occupies a large place in boilermaking, for end-plates, tube-plates, furnace flues, &c., but is scarcely represented in bridge and girder work. Plates are bent to cylindrical shapes in boilermaking, for shells and furnaces, but not in girder work. Welding is much more common in the first than in the second, furnace flues being always welded and stand pipes frequently. In boiler work holes are generally drilled through the seams of adjacent plates. In bridge work each plate or bar is usually drilled or punched apart from its fellows. Boilers, again, being subject to high temperatures and pressures, must be constructed with provisions to ensure some elasticity and freedom of movement under varying temperatures to prevent fractures or grooving, and must be made of materials that combine high ductility with strength when heated to furnace temperatures. Flanging of certain parts, judicious staying, limitation of the length of the tubes, the forms of which are inherently weak, provide for the first; the selection of steel or iron of high percentage elongation, and the imposition of temper, or bending tests, both hot and cold, provide for the second.

The following are the leading features of present-day methods.

It might be hastily supposed that, because plates, angles, tees, channels and joist sections are rolled ready for use, little work could be left for the plater and boilermaker. But actually so much is involved that subdivisions of tasks are numerous; the operations of templet-making, rolling, planing, punching and shearing, bending, welding and forging, flanging, drilling, riveting, caulking, and tubing require the labours of several groups of machine attendants, and of gangs of unskilled labourers or helpers. Some operations also have to be done at a red or white heat, others cold. To the first belong flanging and welding, to the latter generally all the other operations. Heating is necessary for the rolling of tubes of small diameter; bending is done cold or hot according to circumstances.

Fig. 20.—Thornycroft-Schulz Water-tube Boiler.

The fact that some kinds of treatment, as shearing and punching, flanging and bending, are of a very violent character explains why practice has changed radically in regard to the method of performing these operations in cases where safety is a cardinal matter. Shearing and punching are both severely detrusive operations performed on cold metal; both leave jagged edges and, as experience has proved, very minute cracks, the tendency of which is to extend under subsequent stress, with liability to produce fracture. But it has been found that, when a shorn edge is planed and a punched hole enlarged by reamering, no harm results, provided not less than about 1⁄16 in. is removed. A great advance was therefore made when specifications first insisted on the removal of the rough edges before the parts were united.

In the work of riveting another evil long existed. When holes are punched it is practically impossible to ensure the exact coincidence of holes in different plates which have to be brought together for the purpose of riveting. From this followed the use of the drift,—a tapered rod driven forcibly by hammer blows through corresponding holes in adjacent plates, by which violent treatment the holes were forcibly drawn into alignment. This drifting stressed the plates, setting up permanent strains and enlarging incipient cracks, and many boiler explosions have been clearly traceable to the abuse of this tool. Then, next, specifications insisted that all holes should be enlarged by reamering after the plates were in place. But even that did not prove a safeguard, because it often happened that the metal reamered was nearly all removed from one side of a hole, so leaving the other side just as the punch had torn it. Ultimately came the era of drilling rivet-holes, to which there is no exception now in high-class boiler work. For average girder and bridge work the practice of punching and reamering is still in use, because the conditions of service are not so severe as are those in steam boilers.

Flanging signifies the turning or bending over of the edges of a plate to afford a means of union to other plates. Examples occur in the back end-plates of Lancashire and Cornish boilers, the front and back plates of marine boilers, the fire-boxes of locomotive boilers, the crowns of vertical boilers, the ends of conical cross-tubes, and the Adamson seams of furnace flues. This practice has superseded the older system of effecting union by means of rings forming two sides of a rectangular section (angle iron rings). These were a fruitful source of grooving and explosions in steam boilers, because their sharp angular form lacked elasticity; hence the reason for the substitution of a flange turned with a large radius, which afforded the elasticity necessary to counteract the effects of changes in temperature. In girder work where such conditions do not exist, the method of union with angles is of course retained. In the early days of flanging the process was performed in detail by a skilled workman (the angle ironsmith), and it is still so done in small establishments. A length of edge of about 10 in. or a foot is heated, and bent by hammering around the edge of a block of iron of suitable shape. Then another “heat” is taken and flanged, and another, until the work is complete. But in modern boiler shops little hand work is ever done; instead, plates 4 ft., 6 ft., or 8 ft. in diameter, and fire-box plates for locomotive boilers, have their entire flanges bent at a single squeeze between massive dies in a hydraulic press. In the case of the ends of marine boilers which are too large for such treatment, a special form of press bends the edges over in successive heats. The flanges of Adamson seams are rolled over in a special machine. A length of flue is rotated on a table, while the flange is turned over within a minute between revolving rollers. There is another advantage in the adoption of machine-flanging, besides the enormous saving of time, namely, that the material suffers far less injury than it does in hand-flanging.

These differences in practice would not have assumed such magnitude but for the introduction of mild steel in place of malleable iron. Iron suffers less from overheating and irregular heating than does steel. Steel possesses higher ductility, but it is also more liable to develop cracks if subjected to improper treatment. All this and much more is writ large in the early testing of steel, and is reflected in present-day practice.