This dust catcher has given very good satisfaction in use. The principal point to be guarded against is any fluctuation of the water level which will permit the end of the discharge nozzles to become wet, because if they do so the dust rapidly builds upon them and in time they become completely obstructed. A fact not generally recognized is that flue dust has a very cement-like quality and that when dampened it sets almost as hard as a good cement. This fact has proven the destruction of the number of wet dust catchers whose designers did not know how to provide against this difficulty.

Through the courtesy of the representative of the Mullen washer I have obtained a copy of the results of a test made on one of these washers by the superintendent of one of the largest blast-furnace plants in the country, with a view to finding out if the apparatus were suitable for the conditions at his plant. The results of this test showed that the gas leaving the dust catcher contained 2.28 grains per cubic foot of dust and after leaving the gas washer it contained 0.75. The representative of the gas washer claims that it is capable of doing considerably better work than this; even this reduced to a third the total amount of dust carried in the stoves and boilers, and as a considerable proportion of the total is carried through by the stack gases without being deposited, the proportion which lodges in the stoves and boilers to their detriment is reduced in a considerably greater proportion. At the original plant where this washer was installed at Leetonia, Ohio, experience proved that they could run the stoves for the whole blast without taking them off the furnace for cleaning, which would be quite impossible without some means for reducing the dust in the gas far below the percentage which it contains on leaving the dry dust catcher.

Many other types of dust catchers have been built and tried out, but these described give the broad outlines of the art as it existed before the introduction of the gas engine. This forced a further step and a very long one in the direction of cleaning the gas, and as has so often happened, knowledge of the subject gained in learning to clean gas for gas engines has proven of great advantage in directions not contemplated when that work was undertaken. There seem to be definite limits to the degree to which the gas can be cleaned by any type of dust catcher which depends upon gravity, centrifugal force, or impact upon the water surface or combination of any of these. The degree of cleanness so obtainable is barely sufficient for moderately good practice with the stoves, and as has been previously explained, great economies were effected by cleaning the gas a step beyond this point for use with stoves.

Most of the processes used for the next step in cleaning have involved scrubbing the gas with water and this introduced another element in the situation. The gas as scrubbed picks up water vapor enough to saturate itself at the temperature at which it leaves the scrubber and this has a somewhat complicated effect. The gas as it comes from the furnace at a temperature of 300° to 500° carries a very considerable quantity of water vapor picked up from the stock which in a large proportion of modern practice is heavily sprayed with water just before being charged into the furnace. This water vapor is somewhat of a detriment to the gas since it acts as a dead weight during combustion and prevents attainment of as high a temperature as would be reached with the same gas at the same temperature, dry. By scrubbing the gas with water this temperature is reduced and its saturation point is so much lowered that water may actually be removed from it instead of being imparted by the scrubbing operation.

It is obvious that we have here two conflicting effects. The reduction of the temperature of the gas itself is bad because the sensible heat is an appreciable percentage of the total and if this is removed by any means, before the gas reaches the burners such removal represents an absolute loss. On the other hand, hot gas can carry an enormous amount of water vapor and this moisture going through the system acts as a damper on the combustion and prevents the temperature from rising as high as it otherwise would. Therefore, to remove the moisture is in itself a benefit.

There has been a certain amount of misapprehension on this subject. There have been in recent months two papers on the subject of gas cleaning, of which jointly the value probably exceeds anything published in English previous to that time. These are the papers of Mr. W. A. Forbes, "The Cleaning of the Blast-Furnace Gas," before the October, 1913, meeting of the American Institute of Mining Engineers, and that of Mr. A. N. Diehl at the February, 1914, meeting of the same Institute, entitled "Data Pertaining to the Gas Cleaning at the Duquesne Blast-Furnaces".

From the former of these I shall presently quote extensively and from the latter also to a considerable extent, but both of these in my judgment give an erroneous idea as to the relative importance of removing moisture and of cooling the gas. Table 2 of Mr. Diehl's paper is reproduced here as Table 1 (see opposite page).

It will be seen that he gives calculations showing the amount of heat obtainable per cubic foot of gas under three conditions.

First

Washed and cooled to 70° Fahr. and saturated at that temperature with water vapor.

Second

Washed and cooled to 125° Fahr. and saturated at that temperature.