Commonly two-pass stoves have shared the preeminence with three-pass stoves in the best modern practice. In the case of the two-pass stove the valves are all at the ground level, so that any blast leakage has at least a fair chance of being detected by the sound it makes, and the interior construction of the stoves is the simplest possible.

Only a single brick dome is required, and therefore the gas in passing from the combustion chamber to the checkers has only to pass over the top of the combustion chamber wall and down. At the bottom the arrangement of flues for gathering burnt gases from the checker work and leading them to the chimney valve, and distributing the blast from the cold-blast valve to the checkers is also extremely simple. Moreover, no space is given up to any intermediate wall of any kind.

Two-Pass Side Combustion Chamber Stoves

Some operators claim that, while the central combustion chamber is theoretically correct, in practice the side combustion chamber type gives little or no trouble if properly constructed, and, owing to the fact that the side combustion chamber is immediately adjacent to the gas and clean-out openings, it is easier to keep clean than is the central combustion chamber because of the distance of the latter from the operator, and the consequent necessity of removing the accumulation of dirt and slag in the center of the stove at a distance of 8 to 12 ft. from the closest point which the operator can reach. Moreover, the tendency of the blast to wander in the checker work of the two-pass central combustion type of stove seems to be established beyond question, which is a serious drawback to its efficiency.

The cross section of the checkers for a stove of this type of a given size is almost twice as great as that of the checkers of the second and third passes in the McClure stove, and this excess area causes a very low velocity through the checkers, so that a very small difference in draft on one side or another will cause a difference in the flow on that side.

The chimney is necessarily at one side in this type of stove and the gas passing through the checkers nearest it has the shortest path to the stack, and for that reason the greatest flow will generally take place on that side, with corresponding inactivity in some other region, whose usefulness is thereby greatly reduced. This is for a reason which must never be forgotten in matters of heat conductivity to or from a fluid, particularly a gas, that high velocity of the fluid absorbing or emitting the heat is of the same order of importance as temperature-difference and surface.

Efforts have been made to overcome this tendency by equipping each stove with two or even three chimney valves, but while this is undoubtedly good practice, and while two such valves are becoming recognized as correct practice, even for the side combustion stove, it does not, in the experience of some skilled operators, overcome the difficulty completely. Some recent work done in Germany makes it appear possible that we have neglected this question of velocity in the passes far more than its importance warrants and that our stove practice may be materially improved by paying more attention to it.

Fig. 139. Section through combustion chamber.

Fig. 140. Section through arches at base of stove.

Figs. 139, 140. M'Kee Nelson stove.

For these reasons some operators and engineers prefer the two-pass side combustion chamber stove.