Stoves of this general type are divided into two classes, known respectively as the "Glendon" and "Durham," from the former well-known works in the Lehigh Valley, which respectively used and advocated the two types.

There is no essential difference in the construction of these two types, the only difference being in the directions in which the flame and air pass through them. The Glendon stove, as illustrated by the drawings, Figs. 126-129, is on the counter-current principle. The cold air enters at the draft stack end of the stove and progresses through increasing temperatures to the combustion chamber end, where it is discharged, while in the Durham stove the opposite procedure is followed. This latter at first sight is amazing and contrary to reason, but it appears to be an established fact that higher blast heats can be obtained from stoves of this kind than can be obtained from the Glendon type. The latter type is definitely limited, if it is to give long life and low repair cost, to 850° F., while from 900° to 1000° may be obtained in the Durham type of stove. This is for the reason that when the counter-current principle is used the pipes next to the combustion chamber carry air already heated up almost to the limit of temperature for the stove, and are exposed on the outside to the flame directly from the combustion chamber. They are, therefore, not cooled off or protected on either side, and if very moderate temperatures be exceeded they will fail in one way or another under this combined assault from within and from without.

In the Durham stove, on the other hand, the coldest air passes immediately into the hottest pipes and cools them down to such a degree that they can withstand the external heat. Of course, in this type of stove the waste gases must pass off at a temperature at least 200° or 300° higher than that of the hot blast they are discharging, otherwise the last rows of pipes would be worthless and would absorb no heat, while in the Glendon type of stove the hot gas may be discharged at a temperature several hundred degrees lower than this and still impart heat to the cold blast just entering.

I do not know of a single case in which any investigation has ever been made of the thermal efficiency of iron-pipe stoves, but I am convinced that that of the Glendon type is considerably higher than that of the Durham. On the other hand, if there be no use for the gas saved by this superior stove economy, as was generally the case at plants where these stoves were in service, then it is much better to burn this gas even though the economy of the operation be low, and convert its heat into increased blast temperature which exercises a beneficial effect upon the fuel economy of the plant as a whole.