Boiler Accessories Natural and Forced Drafts

BOILER ACCESSORIES: NATURAL AND FORCED DRAFTS The draft in a chimney is caused by the difference in weight between the volume of heated gases inside and the outside air. This being so, it is apparent that the taller the chimney, the greater this difference will be. The force or intensity of a draft is increased, and additional draft is induced, by the force of the wind as it whistles by the chimney top. The intensity may at any time be measured by a draft-gauge. The most satisfactory instrument of this sort is the "Eames Differential" draft-gauge, shown in Fig. 20. The tube is filled with a special non-drying, non-evaporating oil of known specific gravity. The incline and diameter of the tube are so proportioned that the readings are equivalent to inches of water, in which terms the draft is invariably measured.

Other things being equal, the rate of combustion depends upon the height of the chimney. A chimney 20 to 25 feet in height will cause a draft sufficient to burn about 8 lbs. of coal per square foot of grate area per hour. If the height is increased to about 100 feet, the rate of combustion will be increased to approximately 15 lbs. per square foot; and to burn 25 lbs., the chimney should be about 175 feet high. This is measured above the grate of the boiler. For good bituminous or anthracite coal, the chimney must be higher than for wood, if the same rate of combustion is desired. If the boiler has small or winding passages, the chimney must be higher to produce the same effective draft. High chimneys are costly; and it is frequently the practice to build two or three small chimneys in place of the big one, and to supplement them with some form of forced draft.

By means of forced draft, the rate of fuel combustion can be increased under favorable conditions to 100 lbs. of coal per square foot of grate surface per hour. This, of course, greatly increases the power of the plant, but is likely to injure the boiler, and is uneconomical under most conditions. There are three systems of forced draft in common use: 1. The closed stoke-hold, as used in marine work; 2. The closed ash-pit; 3. The induced draft.

Closed Stoke-Hold. One of the most common forms of forced draft, especially as used on warships, is obtained by closing the stokeholds and blowing a fresh supply of air into the fire-room. This gives an exceedingly good ventilation and keeps the fire-room in good condition; but its chief objection is that when the furnace doors are opened there is a tremendous indraft of cold air, which tends to lower the efficiency of the boiler. If this system is employed, the bulk

heads adjacent to the boiler-room must be provided with double doors, forming an air-lock between. By opening only one door at a time, the pressure in the fire-room is not lost. This system seems to possess but one distinct advantage, and that is coolness and therefore comfort for the firemen; but the disadvantage of the inrush of air to the furnaces- when firing, is sufficient, in some cases, to make the system questionable.

Closed Ash-Pit. The essential features of forced draft by this method consist merely in closing the ash-pit tight, and blowing the air directly under the grate. When the fires are cleaned, the draft, of course, must be shut off; otherwise the flames will be blown out into the fire-room. The fire-room, under this system, is likely to be hotter than by the other method; but this system would seem to be the better from a mechanical point of view.

There are several patented devices in connection with the forced draft, of which the "Howden" and the "Ellis and Eaves" systems may be specially mentioned. It may be worth while to note that if fueloil is burned, any one of these systems of forced draft will work better than with coal, for the fire can be tended without opening the firedoors.

Induced Draft. Perhaps the most common example of induced draft is to be found in the locomotive, where the exhaust steam is turned into the smokestack. The rush of this steam up the stack, by carrying a large volume of air with it, induces a tremendous draft. Induced draft may also be obtained in stationary and marine plants by placing a blower in the chimney or stack. In marine work, of course, induced draft by exhaust steam is out of the question. When a blower is placed in the smokestack, an economizer should be used, so that the gases may be cooled before they reach the blower. The draft obtained on locomotives is frequently equivalent to a column of five or six inches of water; while a forced draft of two inches is usually considered large, except for torpedo-boats, which may have as strong a draft as a locomotive has.