Dust catchers were not considered a necessary part of a plant until within the last twenty or twenty-five years. The first ones were crudely designed, simply with the idea of providing an enlargement in the gas main in which the velocity of the gas would be reduced and its dust contents dropped to the bottom by this reduction in velocity. Ordinarily very inadequate means were provided to prevent the gas current from again picking up the dust it had once deposited, and the efficiency of the apparatus was in some cases so low that the quantity of dust recovered was inappreciable, and after several years trial the dust catcher was thown out and replaced by a plain section of pipe. Eventually the idea took shape of combining centrifugal force with the reduction in velocity to throw out the dust.

Fig. 179. Typical form of early dust catcher.

The sketch, Fig. 179, illustrates diagrammatically the construction which came to be standard for the first type. Fig. 180 shows an early type sometimes used to secure the centrifugal effect. The tangential inlet into the dust catcher body constitutes the principal difference in the two types, beyond the much steeper slope of the downcomer in Fig. 180.

The difficulty of preventing the dust from being picked up by the gas current after being once deposited was a serious one and its solution was sought by several furnacemen and several designs were brought out to overcome it by having the dust impinge upon the surface of water to which it would adhere and from which the gas could not pick it up again.

Fig. 180. Typical form at centrifugal dust catcher.

One of the earliest designs to embody this principle was that of Mr. F. E. Bachman, then manager of the Northern Iron Company, at Port Henry, N.Y. Mr. Bachman built a dust catcher in general appearance not differing greatly from the ordinary type provided with a tangential inlet as shown in Fig. 180. The shell of this instead of being lined with firebrick, as in ordinary constructions, was sprayed inside with a great number of small jets of water which produced a water film running down over this whole interior surface. Centrifugal force threw the gas against this surface to which the dust adhered and was carried to the bottom by the flow of the water film. The gas only being exposed to the surface of the water for a short time did not absorb enough water vapor to affect its thermal value materially. This dust catcher was in operation for a number of years (as far as known to me it still is) and gave excellent results. The stoves remained clean and serviceable, although the furnace was charged exclusively with magnetic concentrates, which were quite fine.

Fig. 181. Steese and Ford gas washer.

Another design to secure this result was that of Mr. R. C. Steece, whose dust catcher is shown by Fig. 181. The body of the dust catcher is horizontal and filled with water to about one-third of its height. The gas enters at one end so as to impinge on the surface of the water and is deflected therefrom into a semi-circular passage which brings it back to impinge upon the water a second time. A second semi-circular passage repeats the operation once more and the gas is finally discharged at the far end of the dust catcher.

This design avoids the maintenance of the sprays required by the Bachman apparatus, but, on the other hand, the surface of the water is not constantly and rapidly renewed as it is in the Bachman design, and this is a point of much importance because when the surface of the water becomes fouled by a heavy layer of dust, other dust impinging upon this is not held but liable to be picked up again and carried on by the gas.

Fig. 182. Mullen gas washer.

Another dust catcher which uses the principle of having the gas impinge upon the surface of water is the invention of Mr. B. F. Mullen, of eetonia, Ohio. This dust catcher has had the widest application of any apparatus of its type. The design is shown in Fig. 182.

The external shape of the apparatus is not unlike that of the standard dust catcher. The gas enters at the top and passes into the central chamber shown. This chamber is suspended from the dome of the dust catcher, its shell is lined with firebrick and its bottom constitutes a tube plate supported by the suspension rods L. Through the tube plate project a great number of short tubes through which the gas is forced to pass at a considerable velocity. The bottom cone of the apparatus is filled with water to a level from 4 in. to 6 in. below the bottom of the tubes. This level is maintained by overflow through the ports H around the circumference of the dust catcher body.

Fresh water is introduced into the center through the pipe shown and flows regularly outward to the outlet ports H through which it discharges. This keeps the surface of the water constantly in motion and prevents its becoming too foul to hold the dirt thrown upon it.

After the gas leaves the tubes and impinges upon the water surface it passes into the annular space around the central chamber and so to the outlet at the side. The top of the water outlet trough just beneath the ports H is sealed by a conical plate the bottom of which is riveted to the bottom of the trough, while its top is riveted to the shell of the dust catcher proper.

The light dust which lodges on the surface of the water is carried away by the flow of the water, while the heavy dust which penetrates the surface is flushed off from time to time through the bell valve at the bottom of the cone. A later design is modified so that the outlet trough for the waste water is inside the main dust catcher structure instead of. outside of it, but the principle remains the same.