means of variously directed jets or drafts of air or else to deposit a stream of coal in a moving stream of air. In some cases a specially constructed injection nozzle discharges a stream of coal in an annular stream of air, or it may be that an annular stream of coal is discharged around an inner jet of air. Any method of breaking up or eroding a body of coal by a stream of air flowing over the surface, will require both time and a considerable amount of excess air. Again, where the coal is deposited in a moving stream of air, either this air must move at comparatively high velocity, in order to carry the comparatively dense body of dust until broken up, or else a volume of air in excess of combustion requirements must be used. Either horn of high velocity or excess air is objectionable.

Like Breaking a Grenade

It appears safe to state that none of the "streamline" methods of feeding are capable of completely diffusing the coal dust uniformly throughout the entire volume of combustion air, or of accurately proportioning the amount of air to coal in a strict combustion mixture. Such a complete mixture can be had in an apparatus, designed especially for the purpose, wherein the volumes of coal and air are successively divided into various smaller volumes, and so handled as to create a large number of eddying currents of air within the apparatus, sufficient in number and strength of action to bring about complete diffusion of the dust particles within the air. All problems calling for rapid and wide diffusion, whether it be a bursting hand grenade, or the mere breaking up of a dust body, call for action from the inside out. On this principle an apparatus can be designed to diffuse a sufficient volume of dust to meet the large capacity and exacting requirements of the largest furnaces, calling for low discharge velocity and accurate control of combustion conditions. Our ideal condition, then, will be to conduct a certain amount of air within a volume of coal dust, and by there turning it loose, burst the particles apart in all directions. Then by subjecting this rough mixture to further treatment in an apparatus, creating a large number of eddying currents, we insure uniformity of dust diffusion throughout the entire volume of combustion air.

The first phase of this ideal process can be only approximated in practice, but the action of the coal control, the upper part of the apparatus seen in Fig. 1, approximates the desired condition sufficiently for all practical purposes. Here the coal dust as thrown off by the feed screw into the mixing drum of the coal control, is immediately attacked by a heavy blast of air, A, directed at right angles across its path and finding

a ready entrance into the body of coal, owing to its disturbed condition as showered from the screw end. The coal and air must then pass through the holes of the perforated screen in the drum, and in so doing there is a kind of kneading or wire-drawing effect, tending to equalize the dust diffusion in the air. The jets issuing from the screen holes, B, flare, and on coming in contact with the outer drum, burst into reverse

flow lines somewhat on the order of a mushroom head. The distance between shell and drum being small there is opportunity for a rebound of the current against the outside of the perforated shell, if indeed the flow lines have not at this time been lost in the mass of eddying currents, all of which are ultimately drawn together at the one large outlet, C, there to receive a general kneading to make diffusion more uniform. The action here may be taken as rather complete, but as we wish to work on accurately proportioned mixtures of air and coal, the diffusion must be still further carried out in the discharge section of the apparatus, which is the mixing chamber proper.

Detail of Burner

The partially mixed material coming from the coal control enters the burner at C, Fig. 1, which shows what is known as a quadruplex burner, so called on account of its having four sets of mixing shells concentrically arranged. This burner, which is 30 inches in diameter and about 10 feet long over all, is of the size and type generally used for the air melting furnace. The design is capable of being used with any number of the mixing shells and in the smaller sizes one or two sets are usually employed. On the average malleable annealing oven, for example, two sets of shells will ordinarily be employed and the outside diameter of the burner will be from 6 to 10 inches and the overall length from 3 to 4 feet. Fig. 2 shows a general assembly of the apparatus in front of a melting furnace.

Fig. 3 is a general view of the front end of this furnace, showing the coal hopper to the bottom of which is attached. the coal control, containing the feed screw which delivers the coal to the control mixing drum seen at the left behind the post. Below is the burner proper with its discharge nozzle, which is 6 inches high and 5 feet wide. The blast fan is mounted immediately at the right of the short outlet connection which connects it with the 24-inch valve and hence to the burner. It is here shown driven by belt, though direct-connection will later be installed, being considered preferable for this drive. The general assembly of this apparatus is most clearly

shown, with some of the principal dimensions in the vertical elevations, Fig. 2, where the position of the valves and speed reduction set for the direct-connected drive of the feed screw are also shown. This view is on the side opposite from that of Fig. 3. Fig. 4, which is another view taken farther to the right, clearly shows the motor and gear reduction set of the feed screw drive. This view also clearly shows the arrangement of the fan and its motor and the controllers which are immediately to the rear of this motor beside the fan are more clearly seen in Fig. 5, while Fig. 9 gives a good idea of the arrangement for top blast as provided from a direct motor-driven, positive-pressure blower. This unit has considerably more capacity than is actually required, but was installed to avoid delay in erection on account of being on hand. At the right of Fig. 9, it will be noted that provision is made to remove a bung of narrow width in order to check the stack draft as required, the stack provided being of rather more liberal proportions than necessary with this method of firing.

Referring again to Fig. 1 in which an attempt is made to indicate the general tendency to formation of air currents, it will be noted that the partially mixed air and coal on coming from the coal control and entering the burner at the point C is divided by means of the successive communicating tubes. E. This mixture enters each of the four coal and air (C-andA) strata, while at the same time additional air in an amount regulated to give a perfectly balanced mixture, enters the open end of the burner through a 24-inch valve. Some of this air passes into the center pipe, while the remainder goes into the air strata which are concentric and just inside their respective C-and-A strata. The outer shells of all air strata and the inner pipe contain a number of holes arranged in successive staggered rows, a total of 1516 holes being used in all the shells of the 30-inch burner. It will be seen that these 1516 jets of air give a tremendous mixing effect in the comparatively thin strata of partially mixed coal and air streams. At this point it will be discovered that, aside from the space economy in having the shells arranged concentrically, there is a purpose in having the radial distance between the shells comparatively short, in

order to insure a good jet-piercing effect into the resilient Cand-A strata of material. The analogy here is that of cutting

One's first instinct is to With usual fan blast, the

a thick piece of rubber with a knife. place the rubber on a hard surface. air jet will pierce little more than 2 inches, so at this distance a solid backing is provided.

Eddy Currents are Useful

Consideration will show that the structure illustrated will cause a large number of complicated, cross-firing and eddying

FIG. 5-OPERATING SIDE OF BURNER SHOWING LOCATION CONTROLS

air currents, thus affording every practical assurance of the thorough and complete mixing-diffusion-of the coal dust in the entire volume of air. In the first place it is noted that the partially mixed material, on entering the various strata of the burner, is considerably expanded, by reason of the fact