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that in case of an accident to one of them, necessitating repairs, five of them may still be in proper working condition. Four of these boilers will be needed to run the engine, which leaves a margin of 100 horse-power with which to supply the necessary steam for other purposes about the plant.

Under ordinary conditions a sixth boiler may be added, giving 200 horsepower for these purposes. By this arrangement there is the additional advantage that the boilers may be cleaned one at a time without shutting down the power.

The styles and types of boilers in the market are many and various, and most of them have good and practical claims to consideration in one way or another. But it is somewhat doubtful if any type will be devised that will become as popular for general use — or in the long run any more efficient and economical for hard, every-day service — as the return tubular type.

Ofttimes the space in which the boilers must be located will determine the type, whether they shall be upright or horizontal; and the method of firing them, as well as the kind of fuel to be used, must also be taken into consideration.

Mechanical stoking is used with success in some instances, but as yet has not come into general use. Both oil and natural gas as a fuel are much used in such localities as render them more economical than coal.

Our boilers will therefore be of the return tubular type, fired by hand, with the usual kind of soft or bituminous coal. They will be 66 inches diameter and 16 feet long, exclusive of the curtain sheet under the space occupied by the “up-take," or smoke connection.

The arrangement for setting the boilers is shown in vertical, longitudinal section in Fig. 75; in a half vertical cross-section, and half front elevation in Fig. 76, and in a horizontal section above the grate line in Fig. 77.

There are several matters in connection with the setting of boilers which should be strictly attended to. Among these are the following: Two courses of bricks should be laid above the floor line of the boiler room for the boiler fronts to rest upon; the top course at least should be headers, and carefully leveled up. They should be so located that at least two inches will project in front of the boiler fronts. The ashpits should be cemented so as to allow of the introduction of a few inches of water.

The front supporting brackets should rest fairly upon iron plates in the side walls, while the rear brackets rest on rollers, which in turn rest on the iron plates set in the walls, by which arrangement all expansion of the boiler is toward the rear. The brickwork around the brackets should be entirely clear of them so as to leave the boilers opportunity to expand and contract without injury to the walls.

The grates should incline from front to back from 1 to 1 inch per foot.

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The bridge wall should come up to within 12 or 14 inches of the bottom of the boiler, and be curved to suit its form, although this is not absolutely necessary. The width of grate surface should be equal to the diameter of the boiler. The side walls of the furnace are to incline outwards, so as to be two

inches from the sides of the boiler, at a point one course of bricks below the bottom of the brackets. The fire bricks should be laid with a header course every five courses, so that burned-out bricks may be conveniently replaced.

At each side of the fire doors cast iron “cheek-pieces” should be put in, the cast iron arch plate over the door resting on them. These “cheek-pieces" should be about 11 inches thick, of the form shown in Fig. 77, and have as many half-inch holes cored in them as possible, the holes spaced two inches from center

to center, for an air supply to prevent Fig. 76. — Vertical Half Cross Section and Elevation of Boiler Setting.

them from burning out. Their height is

equal to the height of the fire door at the side. They will be found to be very durable and to save much expense in fire brick repairs.

They may be removed and replaced whenever the furnace is cool, by jacking up the arch plate a trifle and letting it down on the new "cheek-piece"

introduced. Their inclined form renders the cleaning of the fire much more convenient and the extreme front corner which they cut off is of little benefit in making steam.

Both outside and division walls should have a two-inch air space, as shown. The top of the boiler should be covered with asbestos, or with a brick arch. If the latter, there should be a two-inch air space left between it and the boiler. The boilers must rest only on the supporting brackets and in no case on the boiler fronts.

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The fronts are held in place by anchor bolts { inch in diameter and 4 feet long, with the inner ends bent to a right angle 10 or 12 inches long. Their front ends are threaded for nuts coming outside of the boiler fronts, so that a defective or cracked portion of the front may be readily removed and replaced without disturbing the brickwork.

The smoke connections from the boilers to the stack may be the same width all the way through, in which case its height is to be increased from the first to the sixth boiler to include the additional area necessary for each boiler as it progresses toward the stack. Thus it may be 36 inches wide and 20 inches high at the first boiler, and increasing to 78 inches high at the sixth boiler. It will perhaps be more convenient to increase also the width, in order that the area at the large end may equal that of the stack without increasing the height to such an extent.

By this method we may make the larger end 48 inches wide and 60 inches high. There should be a cleaning door in the end of the smoke connection at the first boiler, and a pivoted damper properly balanced between the sixth boiler and the stack. It will be convenient also to have dampers in the "uptake” from each boiler to the main smoke connection, so as to shut these off whenever a boiler is laid off for cleaning or repairs.

The steam connections from the tops of the boilers are to be so arranged

that any one or more of the boilers may be "cut out” and the use of all the others continued. All steam pipes of three inches or over should be covered with an efficient and lasting non-conducting material. That containing a large portion of asbestos will probably be found the best.

The foundations for the boiler settings should be prepared in the same manner as for a machine foundation, as described in Chapter X, due consideration being given to the weight to be supported, say about 1,200 pounds per square foot.

The plan of boiler settings shown, that is, supporting the boiler on brackets attached to it, is the ordinary method. The more modern method, however, is to erect iron columns at each side of the boiler and upon these to lay I-beams, from which the boiler is suspended by iron rods, entirely clear of the brickwork, and with no part resting upon it. While this plan is no doubt correct in theory and practice, it is considerably more expensive than the method shown herewith and for that reason it may not receive the favor it deserves.

The general plan and arrangement of the boiler room with the boiler settings, the smoke connections with the stack, the coal-delivering tram track, scales, etc., and the engine room, with the location of the engine and its connection with the main shaft, is shown in Fig. 78, and in so far as it relates to the boilers and settings it is substantially the system adopted by the Bigelow Company, New Haven, Conn.

As to the engine, it seems fairly well conceded that for economy of steam and general efficiency in furnishing the power for machine shop work the horizontal, cross compound condensing engine will be the best. This type of engine is made by a number of well-known engine builders, and while all of them have certain convenient features and peculiarities of design and construction which commend them to different purchasers, it is probable that there is no very great difference in their efficiency or economy in the general results.

The subject of gas engines has not been considered in connection with our plans as they do not seem suitable where a large amount of power is required, whatever may be their advantages in small or isolated plants, although gas engines have been built as large as a thousand horse-power that have been fairly successful.

Prominent among the builders of steam engines of the type referred to above are the Allis-Chalmers Company, of Milwaukee, Wis., and the William A. Harris Engine Company, of Providence, R. I., and it is this type of engine which is illustrated and described in this article, and commended for machine shop use.

The size selected may be 16 and 32 x 48 inches, or 18 and 36 x 42 inches, with a balance wheel 18 feet in diameter with a 36-inch face, and capable of

generating 400 horse-power at 80 revolutions per minute, and steam at 125 pounds pressure.

It will be proper to consider whether to drive direct from the engine to the machines by means of shafting and belts, or whether the engine shall drive dynamos, from which the electric current may be transmitted by proper

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conductors to motors, which in turn may drive main lines of shafting, or to a number of motors located in different parts of the works, driving short lines of shafting operating groups of machines, or again, whether we shall place a small motor upon each individual machine to drive it.

All of these methods have their peculiar advantages and necessarily their

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