The early stoves of this type were of large diameter in comparison with their height, and contained four passes, each passage of the gas vertically through the stove being called a pass. Following the original Whitwell patent a vast number of modifications sprang up and the shape of the stove has undergone an alteration which has now given it a height of four or five times its diameter.

The number of passes has been the subject of experiment, as has almost everything else connected with the stove. It was gradually realized that the fourth pass frequently contributed very little to the heat of the blast, and as every additional pass meant giving up valuable space in the stove for partition walls, with added complexities of brickwork at the top and bottom to form safe and durable passageways from one pass to another, the number of passes was reduced to three, to two, and finally to one.

Stoves of this latter type were brought out by Mr. Hugh Kennedy some thirty years ago, it has a single pass straight through from the bottom to short chimneys on the top.

This did away with the necessity of building a chimney for the stoves, and simplified the brickwork to the last degree, but the results in practice were unsatisfactory as the stoves did not give the heat desired. They have, therefore, practically passed out of use, and as a result of this and of the gradual disappearance of the four-pass type almost all the stoves built in recent years have been either of the two-pass or the three-pass variety.

The Three-Pass Central Combustion Chamber Stove

The reason for three-pass stoves is that an odd number of passes brings the gas to be discharged at the top of the stove and by adding a relatively short stack immediately on its top we can obtain the draft necessary without the inconvenience of returning the gas back to the ground level and providing a separate stack for it.

The combustion chamber is central for the reason that, as the average temperature of the brickwork is higher around the combustion chamber than elsewhere, it is likely to expand to a greater extent there than anywhere else. When the combustion chamber is in the center this excess expansion takes place along the central axis of the stove with complete symmetry in all directions, while the fact that the annular wall which constitutes the outside of the combustion chamber and presumably attains the highest temperature, does not support anything at the top, makes it perfectly free to expand and contract without the destruction of any part of the stove. It is claimed on behalf of the central combustion chamber stove that the side combustion chamber locates this unequal expansion unsymmetrically in the stove with a tendency to greater stresses on one side than on the other.

Moreover, the last pass is very much lower in average temperature than the combustion chamber, and in the McClure stove this relatively cool pass extends all around the outside of the stove and tends to cut down the radiation losses below what they are when the combustion chamber is just within the stove wall on one side.