Читать книгу Maxims and Instructions for the Boiler Room - N. Hawkins - Страница 30

Table of Contents

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.