256. Q.—­What is the proper proportion of fire grate per horse power?

A.—­Boulton and Watt allow 0.64 of a square foot area of grate bars per nominal horse power in their marine boilers, and a good effect arises from this proportion; but sometimes so large an area of fire grate cannot be conveniently got, and the proportion of half a square foot per horse power, which is the proportion adopted in the original boiler of the Great Western, seems to answer very well in engines working with a moderate pressure, and with some expansion; and this proportion is now very widely adopted.  With this allowance, there will be 22 to 24 square feet of heating surface per square foot of fire grate; and if the consumption of fuel be taken at 6 lbs. per nominal horse power per hour, there will be about 12 lbs. of coal consumed per hour on each square foot of grate.  The furnaces should not be more than 6 ft. long, as, if much longer than this, it will be impossible to work them properly for any considerable length of time, as they will become choked with clinker at the back ends.

257. Q.—­What quantity of fuel is usually consumed per hour on each square foot of fire grate?

A.—­The quantity of fuel burned on each square foot of fire grate per hour, varies very much in different boilers; in wagon boilers it is from 10 to 13 lbs.; in Cornish boilers from 3-1/2 to 4 lbs.; and in locomotive boilers from 80 to 150 lbs.; but about 1 cwt. per hour is a good proportion in locomotives, as has been already explained.

CALORIMETER AND VENT.

258. Q.—­In what manner are the proper sectional area and the proper capacity of the flue of a boiler determined?

A.—­The proper collective area for the escape of the smoke and flame over the furnace bridges in marine boilers is 19 square inches per nominal horse power, according to Boulton and Watt’s practice, and for the sectional area of the flue they allow 18 square inches per horse power.  The sectional area of the flue in square inches is what is termed the calorimeter of the boiler, and the calorimeter divided by the length of the flue in feet is what is termed the vent.  In marine flue boilers of good construction the vent varies between the limits of 20 and 25, according to the size of the boiler and other circumstances—­the largest boilers having generally the largest vents; and the calorimeter divided by the vent will give the length of the flue in feet.  The flues of all flue boilers diminish in their calorimeter as they approach the chimney, as the smoke contracts in its volume in proportion as it parts with its heat.

259. Q.—­Is the method of determining the dimensions of a boiler flue, by a reference to its vent and calorimeter, the method generally pursued?