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

GRATE BARS.

Fig. 81.

The Grate Bars are a very important part of the furnace appliances. These consist of a number of cast iron bars supported on iron bearers placed at and across the front and back of the furnace. Innumerable forms of grate bars have been contrived to meet the cases of special kinds of fuel. The type in common use is represented in [Fig. 82.]

Fig. 82.

These cuts show a side view and a section of a single bar, and a plan of three bars in position. Each bar is in fact a small girder, the top surface of which is wider than the bottom. On each bar are cast lugs, the width of which determines the size of the opening for the passage of air. This opening varies in width according to the character of the fuel; for anthracite ³⁄₄ inch is a maximum, while the soft coals ⁵⁄₈ to ³⁄₄ inch is often used; for pea and nut coal still smaller openings than either of those are used, i.e. ¹⁄₄ and ³⁄₈ inches. For wood the opening should be a full inch in width.

For long furnaces the bars are usually made into two lengths, with a bearer in the middle of the grate, as shown in [Fig. 83]. As a rule long grates are set with a considerable slope towards the bridge in order to facilitate the distribution of the fuel; an inch to a foot is the rule commonly approved.

Fig. 83.

Fig. 84.

Rocking and shaking grates are now very extensively used; these combine a dumping arrangement, and very largely lessen the great labor of the fireman, and by allowing the use of slack and other cheap forms of fuel are very economical. Several patents are issued upon this form of grate bars all working on essentially the same principle. [Fig. 84] exhibits an efficient form of a shaking grate. As shown in the cut, the grates are arranged to dump the ashes and clinkers. By the reverse motion the flat surface of the grates are restored.

Trouble with grate bars comes from warping or twisting caused by excessive heat, and burning out, produced by the same cause—this explains the peculiar shape in which grates are made—very narrow and very deep. A free introduction of air not only causes perfect combustion but tends towards the preservation of the bars.

Grate bars are usually placed so as to incline towards the rear, the inclination being from one to two inches; this facilitates somewhat the throwing of the coal into the furnace.

The proportion between grate and heating surface should be determined by the kind of fuel to be used. The greatest economy will be attained when the grate is of a size to cause the fire to be forced, and have the gases enter the chimney only a few degrees hotter than the water in the boiler.

If the grate is too large to admit of forcing the fire, the combustion is naturally slower, and consequently the temperature in the furnace is lower, and the loss from the escaping gases is greater.

It must be borne in mind that the only heat which can be utilized is that due to the difference in temperature between the fire and the water in the boiler. For example, if the temperature in the furnace be 975°, and the water in the boiler have a temperature due to 80 pounds of steam, viz.: 325°, it is evident that the heat which can be utilized is the difference between them, or ²⁄₃ of the total heat. Now if the fire be forced, and the furnace temperature raised to 2600°, ⁷⁄₈ of the total heat can be utilized; so it can be readily seen that the grate should be of such a size as to have the fire burn rapidly.

The actual ratio of grate to heating surface should not in any case be less than 1 to 40, and may with advantage, in many cases, be 1 to 50. This proportion will admit of very sharp fires, and still insure the greater portion of the heat being transmitted to the water in the boiler.

The water grate bars, invented in 1824, and since frequently applied to locomotives and marine boilers, do not seem to grow in popular favor, and are scarcely known in stationary boilers.

The objections urged against them are the expense of maintenance, their fittings and attachments, and the possibility of serious consequences should they rupture or burn out.