circulate the water while running, the valve n is opened, when the feed-water itself will maintain the circulation.

To

heat and circulate the water before fires are lighted, the valve / is closed, the valves M and n are opened, and steam from the donkey boiler admitted to the apparatus, the steam in the manner previously explained heating and circulating the water, thus bringing all the parts of the boiler to a uniform temperature, and thereby greatly reducing the local strains in the material of the boiler due to starting a fire in the furnaces. The apparatus may be connected by suitable piping to serve for the main feed as well as for the donkey feed.

NATURAL AND FORCED DRAFT.

851. The necessary quantity of air is supplied to a furnace by a draft, either artificial or natural.

852. Forced draft, or artificial draft, is produced mechanically by fans or blowers; natural draft is pro duced by the smokestack, the impelling force being the difference between the weight of the hot gases in the stack and the weight of an equal column of cold air.

The smokestack serves the double purpose of creating a draft and carrying away obnoxious gases. The production. of the draft depends upon the fact that the furnace gases (the products of combustion) passing up the stack have a high temperature, and are, consequently, lighter than an equal volume of outside air at ordinary temperature; that is, the pressure within the smokestack is slightly less than the pressure of the outside air. Consequently, the air will flow from the place of higher pressure to the place of lower pressure; that is, into the smokestack through the furnace.

The area of the smokestack must be such that the gases of combustion may be discharged freely. Experience has shown that an area of 1 square foot of smokestack to every 8 square feet of grate surface is a fair ratio, representing the practice of some of the best builders. The intensity of the draft depends upon the height of the smokestack; hence, the amount of coal that may be burned per square foot of grate surface per hour may be found from the height of the smokestack, provided the area of the stack is of the required

size.

NOTE. The height of a smokestack is the perpendicular distance between the top of the stack and the top of the grate.

853. The maximum rates of combustion attainable under natural draft are given by the following rules, which have been deduced from the experiments of Isherwood, U. S. N.:

Let W

=

weight (in pounds) of coal burned per square foot of grate area per hour;

H height of smokestack in feet.

=

Rule 161.—To find the amount of anthracite coal, that may be burned per square foot of grate surface per hour under the most favorable conditions, subtract 1 from twice the square root of the height of the smokestack.

Rule 162. For anthracite burning under ordinary conditions, subtract one from one and one-half times the square root of the height of the smokestack.

Rule 163.—For best semi-anthracite and bituminous coals, multiply the square root of the height of the smokestack by two and one-quarter.

Rule 164. For ordinary soft coals, multiply the square root of the height of the smokestack by three.

854. The maximum rate of combustion is thus fixed by the height of the smokestack; the minimum rate may be anything less.

EXAMPLE.-What is the maximum coal consumption per hour of a vessel fitted with six boilers with two furnaces each, the length of the grate being 6 feet, the width 3 feet 6 inches, and the height of the smokestack 65 feet? Ordinary soft coal is being used.

SOLUTION. Using rule 164, we get

W = 3465 = 24.187 lb. per sq. ft. The total grate area = 6 × 2 × 6 × 35 imum coal consumption 252 X 24.187:

of grate area per hour.

252 sq. ft. Hence, the max6,095.12 lb. per hour. Ans.

855. As previously explained, natural draft is caused by the upward flow of heated air.

The force of the draft depends a great deal upon various conditions, such as the direction and the force of the wind, the temperature of the air, the height of the smokestack, the thickness of the fire, etc. To make the draft

independent of any of these conditions, various mechanical arrangements are used.

856. Forced draft is practically applied in one of three ways:

1. The air is forced by a blower into an air-tight fire

room.

3.

2. The air is forced by a blower into air-tight ash pits. The blower is placed in the uptake, and by driving out the waste gases creates a partial vacuum, which draws the air in through the grates.

The pressure of the draft is measured in inches of water; thus, in the above cases the air is forced into the fire room