Page:The American Cyclopædia (1879) Volume II.djvu/766

 74G BLOWING MACHINES other, by the shutting of the valve through which the air entered, is made to furnish its contents through the side opening to the same main pipe, which connects with the other end. The principle of the machine is thus the same as that of the double-acting force pump for pro- pelling water. By the alternate motion of the piston, a current of air is maintained of con- siderable steadiness, and of quantity and pres- sure according to the size of the cylinder and its valves, the rapidity of the movement, and the power applied. The pressure is equalized by the use of an air receiver of great capacity, into which the air is forced through a larger aperture than that for its exit ; its elasticity is thus made to act as a perfect spring. For pro- pelling the air into blast furnaces, the blowing cylinders are made of great size and strength. They are often set in pairs, upon horizontal frames of cast iron, the piston rods being con- nected with cranks geared to the main shaft of the steam engine. Two such cylinders, of 5 ft. diameter and 6 ft. stroke, afford at a com- mon rate of running (as eight full strokes per minute), sufficient air for a first class furnace. No allowance being made for escape of air, and room occupied by the piston and rod, each movement of the piston should discharge the contents of the cylinder, which are 117'81 ft. A full revolution of the crank discharges it twice, and this being repeated eight times in a minute, the effect of the two cylinders is to drive forward 3,370 cubic feet every minute. Instead of being placed horizontally, a single blowing cylinder is sometimes used of great dimensions, placed upright, and the piston rod attached to one end of the lever beam of the steam engine, the steam cylinder connecting with the other end. Some are also connected by the same piston rod passing through the steam cylinder and blowing cylinder, without the intervention of either beam or gearing. A fan blower is a short cylinder of cast iron, through the axis of which passes a shaft, made to revolve by a pulley attached to it outside of the cylinder. Upon the shaft within the box are placed four or five wings, which when ro- tating pass near to the inner surface of the cyl- inder. The apparatus, drawn in section, is like an undershot water wheel enclosed in a box. Around the axle, openings are left in the sides of the box for the admission of the air. This may for purposes of ventilation be drawn from a distance through air pipes discharging into the box. The motion of the wings carries the air around, and a new supply enters to be taken on by the next wing. The discharge is through a box or pipe placed at a tangent to the cylinder and opening into it. The bottom of this box forms the base upon which the ap- paratus rests; and in some machines, as this lower plate curves around to form the case of the blower, it is made to take a spiral form instead of that of a true cylinder, the radius of the cir- cle lessening as the arc is produced. This is called the eccentric fan ; the other, in which the revolving axis is in the centre of the cylinder, is the concentric fan. The latter is supposed to work to disadvantage by carrying around a por- tion of the compressed air a second time, while the wings of tbe other, revolving above the bottom of the discharge box, afford more room for the escape of the air, and at the same time cut off, as they pass into the upper portion of the box, and close to its inner surface, the en- trance for any air from without. By the high speed at which the fans are made to revolve a large body of air is discharged through the aperture, but with little pressure. It is not unusual to run them at the rate of 1,800 revo- lutions per minute, and for the air at its dis- charge to have a velocity of 3,280 ft. in the same time. According to the statements of Dr. Ure, published in the " Philosophical Trans- actions," the velocity of the discharge is actu- ally about three fourths of that of the extremi- ties of the fan blades. If the effective velocity of these be 70 ft. per second, and the area of the discharge pipe be 3 ft., the quantity of air discharged is 210 ft., or 12,600 ft. per minute. The weight of this amount of air is about 9C9 Ibs. For a heavy body falling to acquire a velocity of 70 ft. per second, the height of the fall must be 76-5 ft. This, multiplied by the number of pounds moved, and divided by 33,000, will give the horse power, which in this case is 2 '24, required to produce this result. The pressure of the blast is rarely more than from one quarter to half a pound upon the square inch ; hence the fan can only be used where no great resistance is offered to the blast. It is admirably adapted for .blowing a large number of open fires, or for cupola furnaces. The trompe is a machine dependent upon a current of water falling from a considerable height. It consists of a large pipe, about 2 ft. square, leading from an upper reservoir of water to a cistern or box, 25 to 30 ft. or more below it. A few feet under the cistern, the pipe is contracted in the shape of a funnel in order to divide the water into many stream- lets in its fall. Below this narrow place are a number of holes through the pipe for the ad- mission of air. This is taken down by the water as it descends, and passes into the mid- dle of the cistern at the bottom, where a block is placed, upon which the water dashes, caus- ing the air to separate from it. The water passes through a hole in the bottom of the cis- tern into a side box, in which is placed a valve for checking the exit of the water, that the air which collects in the upper part of the cistern may be kept at any desired pressure. From the top of the cistern a small air pipe conveys the blast to any required point. This appara- tus is used for furnishing air to cupelling and melting furnaces. The ventilator of the llartz is an apparatus of great simplicity, designed to be connected with any part of the machinery about mines that will give a slow alternating motion, and which is usually kept in action, the object being to furnish a continual supply