Maxims and Instructions for the Boiler Room / Useful to Engineers, Firemen & Mechanics; Relating to Steam Generators, Pumps, Appliances, Steam Heating, Practical Plumbing, etc.

by N. Hawkins

THE GROWTH OF THE STEAM BOILER.

After the first crude forms, such as that used in connection with the Baranca and Newcomen engine, and numerous others, the steam boiler which came into very general use was the plain cylinder boiler. An illustration is given of this in figures [14] and [15].

It consists of a cylinder A, formed of iron plate with hemispherical ends B. B. set horizontally in brick work C. The lower part of this cylinder contains the water, the upper part the steam. The furnace D is outside the cylinder, being beneath one end; it consists simply of grate bars e e set in the brick work at a convenient distance below the bottom of the boiler.

Fig. 14.

Fig. 15.

The sides and front of the furnace are walls of brick work, which, being continued upwards support the end of the cylinder. The fuel is thrown on the bars through the door which is set in the front brick work. The air enters between the grate bars from below. The portion below the bars is called the ash pit. The flame and hot gases, when formed, first strike on the bottom of the boiler, and are then carried forward by the draft, to the so-called bridge wall o, which is a projecting piece of brick work which contracts the area of the flue n and forces all the products of combustion to keep close to the bottom of the boiler.

Thence the gases pass along the flue n, and return part one side of the cylinder in the flue m ([fig. 15]) and back again by the other side flue m to the far end of the boiler, whence they escape up the chimney. This latter is provided with a door or damper p, which can be closed or opened at will, so as to regulate the draught.

This boiler has been in use for nearly one hundred years, but has two great defects. The first is that the area of heating surface, that is the parts of the boiler in contact with the flames, is too small in proportion to the bulk of the boiler; the second is, that if the water contains solid matter in solution, as nearly all the water does to a greater or less extent, this matter becomes deposited on the bottom of the boiler just where the greatest evaporation takes place. The deposit, being a non-conductor, prevents the heat of the fuel from reaching the water in sufficient quantities, thus rendering the heating surface inefficient; and further, by preventing the heat from escaping to the water, it causes the plates to become unduly heated, and quickly burnt out.

There is another defect belonging to this system of boiler to which many engineers attach great importance, viz.: that the temperature in each of the three flues n, m, m´ is very different, and consequently that the metal of which the shell of the boiler is composed expands very unequally in each of the flues, and cracks are very likely to take place when the effects of the changes of temperature are most felt. It will be noted that the flames and gases in this earliest type of steam boiler make three turns before reaching the chimney, and as these boilers were made frequently as much as 40 feet long it gave the extreme length of 120 feet to the heat products.

The Cornish Boiler is the next form in time and excellence. This is illustrated in figures [16] and [17].

It consists also of a cylindrical shell A, with flat ends as exhibited in cuts. The furnace, however, instead of being situated underneath the front end of the shell, is enclosed in it in a second cylinder B, having usually a diameter a little greater than half that of the boiler shell. The arrangement of the grate and bridge is evident from the diagram. After passing the bridge wall the heat products travel along through the internal cylinder B, till they reach the back end of the boiler; then return to the front again, by the two side flues m, m´, and thence back again to the chimney by the bottom of flue n.

In this form of boiler the heating surface exceeds that of the last described by an amount equal to the area of the internal flues, while the internal capacity is diminished by its cubic contents; hence for boilers of equal external dimensions, the ratio of heating surface to mass of water to be heated, is greatly increased. Boilers of this sort can, however, never be made of as small diameters as the plain cylindrical sort, on account of the necessity of finding room inside, below the water level, for the furnace and flue.

Fig. 16.

Fig. 17.

The disadvantage, too, of the deposits mentioned in the plain cylinder is, to a great extent got over in the Cornish boiler, for the bottom, where the deposit chiefly takes place, is the coolest instead of being the hottest part of the heating surface.

But the disadvantage of unequal expansion also exists in this type of boiler, as the internal flue in the Cornish system is the hottest portion of the boiler, and consequently undergoes a greater lengthways expansion than the flues. The result is to bulge out the ends, and when the boiler is out of use, the flue returns to its regular size, and thus has a tendency to work loose from the ends to which it is riveted and if the ends are too rigid to move, a very serious strain comes on the points of the flue.

Even while in use the flue of a Cornish boiler is liable to undergo great changes in temperature, according to the state of the fire; when this latter is very low, or when fresh fuel has been thrown on, the temperature is a minimum and reaches a maximum again when the fresh fuel commences to burn fiercely. This constant expansion and contraction is found in practice to also so weaken the tube that it frequently collapses or is pressed together, resulting in great disaster.

This led to the production and adoption of the—

Lancashire Boiler, contrived to remedy this inconvenience and also to attain a more perfect combustion, the arrangement of the furnaces of which is shown in [fig. 19 and fig. 20].

It will be observed that there are two internal furnaces instead of one, as in the Cornish type. These furnaces are sometimes each continued as a separate flue to the other end of the boiler as shown in the cuts; but as a rule they emerge into one internal flue. They are supposed to be fired alternately, and the smoke and unburned gases issuing from the fresh fuel are ignited in the flue by the hot air proceeding from the other furnace, the fuel in which is in a state of incandescence. Thus all violent changes in the temperature are avoided, and the waste of fuel due to unburned gases is avoided, if the firing is properly conducted.

LANCASHIRE BOILER—Fig. 18.

The disadvantage of the Lancashire boiler is the difficulty of finding adequate room for the two furnaces without unduly increasing the diameter of the shell. Low furnaces are extremely unfavorable to complete combustion, the comparatively cold crown plates, when they are in contact with the water of the boiler, extinguishing the flames from the fuel, when they are just formed, while the narrow space between the fuel and the crown does not admit the proper quantity of air being supplied above the fuel to complete the combustion of the gases, as they arise.

On the other hand, though this boiler favors the formation of the smoke, it supplies the means of completing the combustion afterwards, as before explained, by means of the hot air from the second furnace.

Fig. 18 (a)

Another disadvantage is the danger of collapsing the internal flue already spoken of; this is remedied by the introduction of what are called the galloway tubes, illustrated in the cut shown on [this page], which is a cross section of the water tubes shown in Figs. [18] and [20].

These tubes not only contribute to strengthen the flues but they add to the heating surface and greatly promote the circulation so important in the water space.