[Illustration: 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.
[Illustration: 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.
[Illustration: Fig. 83.]
[Illustration: 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.