It has been the practice in the past to make the checkers about 9" square since smaller ones could not be maintained in operation, but this practice is now beginning to undergo a change which is likely to be very rapid.

Until quite recently gas cleaning was not practiced to any considerable extent in this country, while the blast-furnace gas produced here probably carries more dirt and fine ore than that of any other country, because of the predominant portion of fine sandy ore used. This dust is carried into the stove by the gas and deposits on the brickwork which, of course, is composed largely of silica; under the intense heat to which this brickwork is exposed the iron ore constituting the larger portion of the flue-dirt is fused to the brickwork, and the wall of the combustion chamber soon becomes glazed over and finally is slagged away by this action, so much so that a false wall is always built in the combustion chamber, so that this wall may be renewed when it is burnt out without affecting the stove as a whole. In one plant, formerly under my management, we never counted that these walls would last over eight or nine months without repair, and this was due almost entirely to the slagging action of the iron ore carried by the gas.

In the checkers the conditions are not quite so bad, because the temperatures are much lower, but vast quantities of dirt settle on these in light flocculent masses, about the best non-conductor of heat which could be found. These flocculent masses greatly reduce the flow of gas through the stove and seriously impede the passage of blast through it also. Therefore, it is now necessary at intervals to take each stove off the furnace for several days, cool it down so that it can be entered, put men in the upper dome and clean these checkers by dropping through them weights suspended from a rope or chain. This is a highly laborious performance, and from the nature of the case more or less ineffective. In order to carry out this operation at all, it is necessary to lose the use of the stove for from one to two weeks under the best conditions, and during that time, unless the furnace is more than usually well provided with stove capacity, the blast is improperly heated, and the fuel economy of the furnace suffers.

In recent years it has been found that with the exercise of a certain amount of care the gas can be scrubbed practically free of these impurities at comparatively small expense. This eliminates the necessity of allowing for a coating of flue-dust from one to three inches thick on all sides of the checkers. When this is done it becomes a simple matter to use checkers six inches square which are not only as good as, but much better than checkers nine inches square when dirty gas is used, even smaller checkers are now being used.

It is obvious that the amount of heating surface which can be put into a given stove with six-inch checkers is far greater than that which can be put into it with nine-inch checkers and this means that the stove capacity needed to supply a given amount of heat to the blast may be more cheaply obtained, not only in operating cost but also in first cost, by putting in a gas-washing plant to supply clean gas, and using somewhat smaller stoves with smaller checkers. The smaller stoves so designed contain more heating surface and maintain a higher efficiency than the larger stoves under the old conditions with dirty gas.