Many stoves have been built with arched roofs, but these are a great inconvenience when U-pipes have to be taken out and new ones put in, as unfortunately happens not infrequently with this type of stove; and to tear out a portion of this firebrick arch every time a pipe has to be changed would add considerably to the expense and delay caused by the operation.

The Longdale stoves were, therefore, covered by long cast-iron T-beams whose lower surfaces were of jaw-shaped or dove-tailed section. These were lined with tiles or shaped firebricks, the upper part of which fitted into this jaw-shaped section, while the lower part projected out a little beyond the edges of the T-beam. These "shapes" were slid in from the end of the groove and laid in fireclay to make a tight joint and to hold them in place.

In small stoves one end of this T-beam rests on one wall of the oven and the other on the other, but in a large stove, such as that shown here, the span would be too long for necessary strength, for convenience in handling, and for cheapness in casting the beams, therefore a steel I-beam runs down the center of the oven above the T-beams, and these were cast in two pieces, with the joint in the center, the fend at the joint having a hook which rests on the lower flange of the I-beam. This excellent construction gives strength and complete protection to the iron work of the roof from the flame, together with convenience and accessibility. Moreover, this construction makes a level top for the stove, convenient to work upon.

This latter is important because when changing pipes shear poles of relatively short and convenient length are set up on top of the ovens resting upon the T-beams, and by this means can be placed exactly where desired, which could not possibly be done with an arched roof. By the use of this construction, where a U-pipe requires to be replaced the half-length T-beams immediately above it can be removed to make room for the operation and then restored to place after it is completed with a minimum of labor and time and without requiring work by masons.

The Shape Of The Pipes

The elliptical shape of the pipes is due to a desire to obtain maximum surface for a given area, which may seem at first sight unnecessary, because it might be thought that the area was unobjectionable and that it would be simpler to make the pipes round and not bother about the increased area.

This, however, is open to two objections. First, the length of the bed pipe required for a given number of U-pipes would be greatly increased, which would be objectionable from every point of view, and second, velocity is a matter of much importance in securing proper transfer of heat, much greater transfer being effected at high velocities than at low. Therefore, by constricting the area the velocity of the air passing through the pipes is increased and the temperature obtainable with a given amount of heating surface thereby increased also.

Many stoves have been built with round pipes, and at the end next to the combustion chamber this is probably wise, because the relatively narrow cross section of the elliptical pipes makes them bend about much worse under the influence of the flame than do circular pipes whose diameter is equal to the major diameter of the elliptical pipe. But in the rows which are further from the combustion chamber this does not hold to anything like the same extent, and there is probably ample justification for the use of the elliptical section in those rows.