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.