BLOWING, an operation by which a continued stream of air is projected with great force and veloci ty into a furnace, for the purpose of increasing its combustion.
When large bellows are employed for this purpose, or when the air is -pumped from a cylinder directly into the furnace, it is introduced in irregular puffs, are completely insufficient for the intended purpose. In order to equalise and continue the blast, three different contrivances have been adopted. The first method is -by a regulating cylinder, which is fit ted with a piston heavily loaded, having at least three pounds on the square inch. The air which is pump ed from the blowing cylinder, passes into the regu lating cylinder, and as this cannot escape, it will, of 'course, raise the loaded piston. The twere, or pipe, •which conveys the air out of this cylinder into the furnace, is -connected with this cylinder, so that when the mouth of •the mere is open, the air will rush from the regulating cylinder into the furnace, and the weight of the loaded piston will force the air through the twere with a constant blast during the intervals between each stroke of the piston of the blowing cy linder. This method was originally •adopted in blast furnaces ; but though the quality of the air is subject to little alteration from any change in the atmo sphere, yet the regulating cylinder has several disad vantages. -Owing to the small capacity of this cy linder, the blast is not altogether free from irregula rities. A considerable quantity of dense air, likewise, escapes by the sides of the piston ; and if this is re medied by fitting the piston closer to the cylinder, the friction is so much increased, that the piston does not follow the air fast enough down, and the blast weakens a little at the end of every stroke ; while at the beginning of the succeeding stroke; the air must overcome this friction before the piston will rise, and hence another puff accompanies the commencement of each stroke.
The second method of equalizing the blast, is by discharging the air from the blowing cylinder into an airtight apartment, or air vault, the air being pre vented from returning into the cylinder by a valve. Let us suppose that the air vault is of such a size as to contain 100 fulls of the blowing cylinder, and that the nose pipe, which discharges the air into the fur nace, is stopped. After the engine has made 25
strokes, and forced into the air vault 25 cylinders of air, the air vault will then contain 125 cylinders of air in a state of condensation, and having a force 'of three pounds upon the square inch. Let the nose pipe be now opened, and let it be of such a size as to discharge onefull of the blowing cylinder during one stroke of -the engine. The blowing cylinder will then supply the air vault as fast as the air is carried off by the twere, and the blast will be very equal ; the end of each blast .being only abolit two parts weaker than the beginning -of the blast. It is ob vious, that the elasticity of the condensed air will .keep up the regularity of the blast during the inter vals between each stroke of the engine. The air vault.at the Devon iron works is excavated out of the solid rock. It is 72 feet long, 14. feet wide, and 13 feet high, and contains about 13,000 cubic feet of air. Mr Mushet is of opinion, that the air from the air vault is of a very bad quality for the purpose for which it is required. " This immense magazine of -ompressed air," he observes, " generates a consider able portion of heat, which greedily seizes the damps which are unavoidable in under ground excavations, and conveys them to the furnace. In the summer months, the air becomes so debased,_as to affect the quality of the iron, and change it from grey to white. Every change in the temperature of the atmosphere, during this period, is indicated -by various changes in the furnace." The third method of equalizing the blast is, by the water vault, or water regulator, which is described in the. following article, and represented in Plate LXI II. The water regulator has the advantage of a steady and cold blast. No air is lost, as in the case of the regulating cylinder, and no irregularity arises from friction..• The air, however, always contains a con siderable portion of moisture from its being in con tact with water; and the blast is so cold, that the temperature of the discharged air seldom exceeds 38°, when the temperature of the atmosphere is 60°— 65° and 70°.