Blowing Machines. Besides the common bellows (see Bellows), a variety of other machines have been devised for the purpose of propelling air in large volume, or with great pressure and volume together. The most efficient of these machines are the blowing cylinders, which are used to supply air to blast furnaces, and by their great size and strength are made to furnish immense bodies of air under great pressures. Fan blowers are used for supplying large volumes of air, but for purposes in which a high pressure is unimportant. The water blowing machine, for which we have neither name nor use in this country, but which is well known in the mining regions of central and southern Europe by the name of trompe, is so ingenious, and may in some situations prove so valuable a contrivance, that it cannot be passed over without notice. There is also, in the same countries, a very simple blowing apparatus, used for ventilating mines, also too little known in this country, called the ventilator of the Hartz, which is well worthy of notice. - Blowing cylinders of the best construction are made of cast iron, the inner surface turned perfectly true, fitted with airtight iron heads, each of which is furnished with a large valve, corresponding to the clapper of the bellows, opening inward.
Through the centre of the heads the smooth iron piston rod moves in close packing, carrying a piston which is fitted accurately to the cylinder. As the piston moves in one direction, the air enters through the valve in the head behind it, while that in front is forced through an aperture on one side, which is furnished with a valve opening outward, and connects with a pipe leading to any desired point. By reversing the motion the end exhausted of air is refilled, while the 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 propelling water. By the alternate motion of the piston, a current of air is maintained of considerable steadiness, and of quantity and pressure 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 propelling 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 connected with cranks geared to the main shaft of the steam engine. Two such cylinders, of 5 ft. diameter and 6 ft. stroke, atford at a common 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 rotating pass near to the inner surface of the cylinder. The apparatus, drawn in section, is like an undershot water wheel enclosed in a box. Around the axle, openings are left in thesides 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 apparatus 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 circle 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 portion of the compressed air a second time, while the wings of the 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 entrance 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 revolutions per minute, and for the air at its discharge to have a velocity of 3,280 ft. in the same time. According to the statements of Dr. Ure, published in the "Philosophical Transactions," the velocity of the discharge is actually about three fourths of that of the extremities 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 969 lbs.
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 streamlets in its fall. Below this narrow place are a number of holes through the pipe for the admission of air. This is taken down by the water as it descends, and passes into the middle of the cistern at the bottom, where a block is placed, upon which the water dashes, causing the air to separate from it.
The water passes through a hole in the bottom of the cistern 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 apparatus is used for furnishing air to cupelling and melting furnaces. - The ventilator of the Hartz 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 of air to mines. Two cylindrical-shaped vessels, such as long casks, are selected, of such sizes that one, when inverted, may easily move up and down within the other. The outer one is nearly filled with water, and is furnished with an air pipe, which leads from its upper part through the water, and through its bottom down into the mine. Upon the upper end of this pipe is a valve opening downward. The inner inverted cask surrounds this pipe. It has upon its upper end a large valve opening within.
Being suspended by a chain to the end of a lever beam, or to the arm of a bob, air passes within as it is lifted up, and is propelled as it descends through the pipe. By this alternating motion a continual current of air is supplied with little cost of power or attention. A more perfect arrangement of this machine is in making it double, by attaching one to each end of the lever beam. For blowing furnaces these machines have the common disadvantage of all water blasts, that they cause the air to take up more or less moisture, which is discharged into the furnace, and must to some extent diminish the effect of the blast.