Blast-gates
The apparatus for opening and closing pipes supplying blast to cupolas, furnaces, etc.; for use also in exhaust-pipe systems where shavings, dust, smoke, and the like are to be removed, or for regulating the distribution of heated air.
The lever style of blast-gate can be readily manipulated by cords, and is very convenient in cases where it cannot be reached otherwise. The slide style of blast-gate is perhaps as common as any. These should always be made of metal and kept clean; otherwise they become troublesome and inefficacious. It is very important to know that the use of blast-gates to close pipes, when not in use, insures a great saving of power, as a blower requires far less power to drive it with closed connections than with open ones. Blast-gates are furnished by the manufacturers in sizes from 1 1/2 inches to 30 inches, small sizes being made in composition and the larger ones in iron, and these are always to be preferred to such as are usually provided by the foundry tinker. See CUPOLA; BLAST-PIPES; BLOWERS.
Blast-gauge is an apparatus to be attached to the wind-box of a cupola for indicating the pressure in blast-pipes. They are of simple construction and may be purchased from the makers at prices varying from $10 to $15, according to size and degree of finish. Ordinarily, the blast-gauge consists of a siphon-tube with equal legs, half-filled with mercury, one end entering the wind-box, the other being open to the atmosphere. A stop-cock may be provided between this gauge and the wind-box, so that it may be shut off at pleasure. When the stop-cock is open, the blast pressure acting on the mercury in one leg of the gauge presses it down, and the mercury in the other leg rises. The difference between the two columns is the height of mercury, which corresponds to the excess of the pressure of blast in the wind-box above the pressure of the atmosphere; or, in other words, to the effective pressure of blast in the blast-pipes. If 16 ounces be allowed for every 2 inches of the length of this column, or 1 ounce for every 1/8 inch, the effective pressure of blast, in ounces per square inch, is thus obtained. See CUPOLA; BLAST-PIPES; BLOWERS. Blast-pipes are conducting-pipes from the blower to the cupola. These should always be made of iron and perfectly air-tight, and sufficiently large to convey the air without undue loss by friction. When the pipes are too small, a greatly increased velocity is required to discharge a given amount of air, with a larger proportional increase of fractional surfaces. All turns or elbows in conducting-pipes are objectionable in the extreme, and should as much as possible be avoided, as from this cause the direction of the current is changed and the friction greatly increased. Air moving through blast-pipes expends a portion of its force in the friction of its particles along the sides of the pipes, with a consequent reduction in the pressure. In many cases the blower may be condemned as inefficient when the pipe itself is the real cause of the trouble by reason of its too small diameter, its great length, or the number of bends or elbows it contains. The diameter of the blast-pipe should always be increased in proportion as the length is increased.
The main blast-pipe for cupolas 24 to 29 inches diameter should be not less than 10 inches diameter, 30 to 33 inches, 12 inches diameter; 34 to 39 inches, 14 inches diameter; 40 to 45 inches, 16 inches diameter; 46 to 51 inches, 18 inches diameter; 52 to 57 inches, 20 inches diameter; 58 to 70 inches, 22 inches diameter, and 24 inches for cupola's over 70 inches diameter. See CUPOLA; BLAST; BLOWER; COMBUSTION.