Modern Machine-Shop Practice Part 227

[Ill.u.s.tration: Fig. 3260.]

Fig. 3260 represents an internally fired flue boiler, known as the Cornish or Lancas.h.i.+re boiler. The furnace is at one end of the flues, the fire pa.s.sing through them to the chimney. There is here obviously more heating surface than in the plain cylinder boiler, but somewhat less facility for cleaning.

The Galloway boiler is of this cla.s.s, but has vertical water tubes placed at intervals in the flues. These water tubes are wider at the top than at the bottom. They serve to break up the body of heat that pa.s.ses through the flues, and increase the heating surface while extracting more of the heat and promoting the circulation of the water in the boiler.

A water tube is one in which the water is inside and the fire outside, as distinguished from a fire tube, in which the fire pa.s.ses through the tube and the water is outside. A water tube is stronger than a fire tube, because the former is subject to bursting pressure and the latter to collapsing pressure.

Vertical boilers are internally fired, and in the ordinary forms have no return tubes or flues, examples of those used for small stationary engines being given as follows.

[Ill.u.s.tration: Fig. 3261.]

Fig. 3261 represents an ordinary form with vertical tubes. The upper ends of the tubes here pa.s.s through the steam s.p.a.ce--a condition that under the moderate pressures and firing that this cla.s.s of boiler is subjected to is of less importance than it is in boilers having higher chimneys and therefore a more rapid draught, and using higher pressures of steam. Furthermore, the small diameters and lengths or heights in which these boilers are made give them ample strength with sh.e.l.ls and tubes of less thickness, while the condition of tube ends with steam on one side and fire on the other is permissible without the injurious effects that ensue under rapid combustion and high pressures.

[Ill.u.s.tration: Fig. 3262.]

The crown sheet of the fire boxes or furnaces of this cla.s.s of boiler is very effective heating surface, first, because of the great depth (and therefore weight) of water resting upon it insuring constant contact between the water and the plate, while there is no danger of the crown sheet burning from shortness of water.

A similar boiler, but with the upper ends of the tubes below the water level, is shown in Fig. 3262.

From the small diameters of these boilers, the flat surfaces are not stayed except to the extent that the holding power of the tubes serves that end.

[Ill.u.s.tration: Fig. 3263.]

[Ill.u.s.tration: Fig. 3264.]

A return flue vertical boiler is shown in Figs. 3263 and 3264. The whole of the surfaces having contact with the fire also have contact with the water, and the height of the crown sheet removes it from the intense heat of the fire. It is stayed to the top of the boiler. The fire box or combustion chamber being taper increases the effectiveness of its sides as heating surface, since the heat in its vertical pa.s.sage impinges against it.

The products of combustion pa.s.s from the top of the combustion chamber through short horizontal flues, which enter an annular s.p.a.ce surrounding the lower section of the boiler, and from this s.p.a.ce vertical flues pa.s.s to a corresponding s.p.a.ce at the bottom of the boiler.

The pa.s.sage of the steam generated at the sides of the combustion chamber is facilitated by the taper of the chamber, which gives increased room for the steam as it gathers in ascending.

Vertical boilers for high pressures, as from 60 to 120 lbs. per inch, are represented in the figures from 3265 to 3269.

In boilers of this cla.s.s, a majority contain water tubes, which, when properly arranged, promote rapid evaporation and circulation.

A boiler with _Field_ tubes is shown in Fig. 3265. It consists of an outer sh.e.l.l and a cylindrical fire box, from the crown sheet of which a number of Field tubes are suspended in the fire box or combustion chamber.

Fig. 3266 is a sectional view of a Field tube, the construction being as follows:

The outer tube, which is expanded into the tube plate, is enclosed at its lower end, and has at its upper end in the water s.p.a.ce of the boiler a perforated mouth piece, from which is suspended an inner tube that extends nearly to the bottom of the outer tube.

As the outer tube is bathed in the fire, steam is generated very rapidly, and a thorough and rapid circulation is kept up, the water pa.s.sing down the inner and up the outer tubes, as denoted by the arrows.

The outer tube is spread out at the upper end to a slight cone, so that it cannot be forced out of the tube sheet by the pressure, and as it hangs free, there is no liability for it to loosen or get leaky from expansion and contraction.

From the great amount of heating surface obtained with these tubes, the fire box may be kept at a minimum diameter for the duty, while still leaving a wide s.p.a.ce for the water leg, which facilitates the circulation.

[Ill.u.s.tration: Fig. 3265.]

[Ill.u.s.tration: Fig. 3266.]

The damper, which is suspended in the uptake, spreads the fire sideways.

[Ill.u.s.tration: Fig. 3267.]

Fig. 3267 represents the arrangement of Field tubes in a boiler.

A boiler of this form may for a given capacity be made lighter and smaller than in any other of the ordinary forms, while the rapid circulation acts to keep the tubes clean.

The inner tubes may be thin, because they are under pressure both inside and out, while the outer tubes may be thin, because they are under a bursting strain, whereas a fire tube is under collapsing pressure.

[Ill.u.s.tration: Fig. 3268.]

A design of high rate boilers, in which the uptake does not come into contact with the water, and water tubes are employed, is shown in Fig.

3268. In the fire box is an inclined tube which promotes the circulation, and is very effective heating surface, and in the combustion chamber are a number of vertical water tubes.

Two manholes give access for cleaning purposes.

The efficiency of the heating surface in this cla.s.s of boiler is increased from the fact that, as the heat does not pa.s.s direct through the boiler, it impinges against the surface. In Fig. 3269, for example, the exit from the spherical fire box is on one side of the boiler, and the uptake on the other, the heat pa.s.sing from the fire box into a combustion chamber, and thence through the horizontal fire tubes to the uptake.

The crown sheet is here stayed by gusset stays, but if made spherical, as in Fig. 3270, the stays may be omitted.

[Ill.u.s.tration: _VOL. II._ =BOILER FOR STATIONARY ENGINES.= _PLATE XXVIII._

Fig. 3271.]

Figs. 3271, 3272, and 3273 ill.u.s.trate a 60-inch horizontal return tubular boiler constructed by the Hartford Steam Boiler Inspection and Insurance Company. This cla.s.s of boiler has found much favor in the United States. It is an externally fired, return tube boiler, the fire pa.s.sing beneath the boiler and returning through the tubes to the front end of the boiler, whence it pa.s.ses through the drum to the chimney.

[Ill.u.s.tration: Fig. 3269.]

[Ill.u.s.tration: Fig. 3270.]

[Ill.u.s.tration: Fig. 3272.]

The boiler is supported on the brackets B, B', the front one, B, resting on an iron plate imbedded in the brickwork, and the back ones on rollers which rest on the plates P' imbedded in the brickwork. This allows the boiler to expand and contract endways under variations of temperature without racking the brickwork.

[Ill.u.s.tration: Fig. 3273.]

A, A, etc., are for holding the brickwork together. The blow-off pipe C is for emptying or blowing down the boiler. The feed-pipe F enters the front end of the boiler, pa.s.ses along it, and then crosses over. A pipe H from the steam s.p.a.ce of the boiler supplies steam to the steam gauge G, and to the upper end of the gauge gla.s.s, which is on the casting K.

The lower end of the gauge gla.s.s receives water from a pipe which pa.s.ses into the water s.p.a.ce of the boiler; at J are the three gauge c.o.c.ks for testing the height of the water in the boiler.

The manhole affords ingress into the boiler for inspecting and for scaling or cleaning it, the nozzles being for a safety valve. At E is a hand-hole for was.h.i.+ng out and cleaning the boiler. P is a damper in the fire door for admitting air above the fire bars, and R is a damper for regulating the draught.

In the brick walls that support the boiler there are air s.p.a.ces to prevent the conduction of the heat through and prevent cracking of the brickwork. The tubes are arranged in vertical and horizontal rows and are equally s.p.a.ced throughout.

Fig. 3274 represents the front end, and Fig. 3275 a longitudinal sectional view of the front end of a boiler of this cla.s.s. In this case, however, the pipes for the water gauge pa.s.s direct into the boiler.

In some practice the tubes are arranged as in Fig. 3276, being wider pitched or s.p.a.ced in the middle of the boiler to increase the circulation of the water in the boiler.