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.
WATER GAUGE COCKS.
It is of the first importance that those in charge of a boiler shall know with certainty the position of the water level within the boiler.
