86

CYLINDER BOILER,

a

robbed of almost all its heat before it can make its escape." The boiler, as originally constructed by Watt and Bolton, was slightly concave at its bottom surface, with perpendicular sides, flat ends, and a semi-cylindrical top. The form of the sides and ends was, certainly, not conducive to strength; indeed, the steam intended to be generated in this boiler was of a pressure not exceeding that of the atmosphere by more than from 3 to 5 lbs. on the square inch. An improvement was afterwards introduced in this form of boiler, by hollowing in the sides, as represented in the transverse section of the improved waggon boiler in the annexed figure. The heated air from the fire below the boiler, after passing under its entire length, is brought back, previously to its escape to the chimney, by flues o o, in order that the sides of the boiler may receive all the effect of the heated air. The modes by which the boiler is supplied with water, will be more particularly noticed hereafter. One mode may be here described. By means of the apparatus represented in the figure, the water is supplied in proper quantity to the boiler, and maintained at a uniform level from a reservoir by means of a tube g, which passes through a box c into the boiler. The box contains a valve v, which is made to regulate the supply of water: when the water is at the required level in the boiler, the valve v is kept in its seat by pressure from the lever a b, which is loaded at a. But to the other branch be of the lever, an iron rod is attached, which descends into the boiler, and carries a float of wood f; when the level of the water sinks, the float sinks with it, the point e is brought down, the valve is raised, and water admitted into the boiler.

Fig. 29.

64. Cylinder Boiler.-Another form of the boiler is the cylindrical. A simple cylinder with convex ends appears to be the best form of boiler for the production of highpressure steam. Of this form, two modifications occur: that

CYLINDER BOILER.

Fig. 30.

87

in which the furnace is within, and that in which it is outside, the boiler. 1. The former of these, or the tubular-flued boiler, is represented, in transverse section, in fig. 30. It consists of two cylinders, the one placed within the other, and the space between them being occupied to a certain extent by the water. The outer cylinder may be six feet in diameter, and is frequently fifty or sixty feet in length. The heated air from the fire, after traversing the inner cylinder, is conveyed under the boiler by the flues o o, by which means it imparts the last effects of its heat to the boiler, before it is carried to the chimney. The cylinder boiler, as figured above, is preferred as the most economical, for the great steam engines at the mines in Cornwall. The boiler employed by Smeaton was of a spherical form, but, like the one above, it was enclosed entirely within the boiler. Tredgold says, that on the large scale, Smeaton's boilers were admirably adapted for generating steam, and were little inferior to any which have since been contrived. 2. The following figures illustrate a simple cylinder boiler, in which the furnace is on the outside; the smaller figure represents a transverse, the larger a vertical and horizontal, section, of this apparatus. This kind of

boiler has a diameter of four feet, and is about thirty feet in length; its ends are flattened segments of spheres; it is

88

TUBULAR BOILERS.

placed in the horizontal position, as in the figure. The furnace is situated below it, and encased in brickwork. By this position, the fire communicates its greatest heat to the bottom surface of the boiler; the flame and hot air then traverse the flues, the direction of which is denoted by the white channels, in a backward direction, so as to expend all their heat on the several parts of the boiler before they are conveyed to the chimney. A boiler of this kind evidently requires a large consumption of fuel, and has the further disadvantage of being very bulky. It is much used in the American engines.

65. Tubular Boilers.-Various other forms of boiler have been suggested, with a view to economise fuel, bulk, and weight. In 1774, Blakey had proposed to use cylindrical tubes for boilers, in order to obtain a larger heating surface in a smaller space; for it is found that the effective generation of steam depends more on the extent of surface exposed to the direct action of the furnace, than on the general bulk of the boiler. Several varieties of tubular boilers were contrived by Count Rumford and Arthur Woolf; these were, however, of complicated construction, and were necessarily weak from being made of cast iron. The following figures exhibit a form of boiler, which may be considered as a transition form from the tubular-flued already described; the smaller figure presents a transverse, the larger a horizontal and vertical section, of this apparatus. The

Fig. 33.

Fig. 34.

flame and heated air pass into the large tube, which has half its circumference in contact with the water, while two

GENERAL REMARKS ON BOILERS.

89

vertical portions of the boiler, called 'water legs,' receive the heat on each side, as seen in the smaller figure. In this apparatus the heat is economised, being conveyed to the boiler without any expenditure of it upon the brickwork of the furnace. A further economy of heat has been effected by substituting, in room of one large tube, a great many smaller tubes of about six inches in diameter, as in the following figures:

Fig. 35.

Fig. 36.

In this form of boiler, the flame and hot air, after traversing the tubes, circulate in the space beneath the boiler previously to their being conveyed to the chimney.

66. General Remarks on Boilers.-The following observations on the construction and economy of boilers, taken from Mr. Scott Russell's Treatise on the Steam Engine, published in the Encyclopedia Britannica, serve to show the difficulty of adjusting the several parts of this important piece of apparatus. "The conclusion to be drawn from all that has been attempted or achieved in boilers, is, we believe, the following:-That there exist certain limits prescribed by the constitution of fuel, the nature of metals, and the properties of water and steam, which cannot be exceeded without incurring evils that greatly overbalance the partial gain. The best boilers that have ever existed have been those in which a large number of principles have been applied, and so adjusted in relation to each other as to gain the

90

GENERAL REMARKS ON BOILERS.

maximum, not of any one property, but of all the valuable properties, each in the degree of its individual importance. The first cost of the boiler must not be rendered too great, or that will neutralize the economy of using it; the space to which it is confined may be as small as possible; but, if that be produced by intricacy of construction, the loss may surpass the advantage. Then, again, if complex and confined, it may be impossible to cleanse or to repair the boiler; and therefore it must be remembered, that unless easy access can be gained to every part of a boiler, and of its flues, that boiler will soon become totally useless. Then, it is further demanded of a good practical boiler, that, if one part should be damaged or give way, the whole should be so constructed that the damage done to that part must not endanger the rest. An extensive heating surface is to be obtained for economy's sake; but that large surface must at the same time remain unimpaired to resist burning; a property to a certain extent inconsistent with extensive surface. The surface which is thus spread as widely as possible, so as to apply the fire to the water through every part of its mass minutely and in great subdivisions, if extended beyond a certain degree, will not have over it a body of water capable of conducting heat from it with the rapidity adequate to the rapid generation of steam, and to the preservation of the intensely heated metal from the destructive action of the fire. Then, again, it is desirable to have long and tortuous flues, to extricate as much heat as possible from the fuel and the products of combustion; but these, by their very length, may interfere with the draught of the chimney, so as to diminish the efficacy and vigour of the combustion of the fuel, and produce loss instead of gain. Thus it happens that the whole question of boilers is an exact and judicious combination and adjustment of parts, so as to obtain each of these many points in that degree which is most advantageous for every one of the other qualities, and of all of them together. The question is a practical one of no common difficulty." These are important remarks on a most important subject.