One of the earliest forms of chimney valves is of the same form as the cold-blast valve, but very much larger and water-cooled to withstand the action of the hot chimney gases; its seat is also water-cooled. Such a valve designed by Frank C. Roberts & Co. is shown by Figs. 153 and 154. Its construction is so clearly shown by the cuts as not to need much description.

Like the hot-blast valve this valve is counterweighted and operated by a hand-wheel and drum around which the suspension rope passes. The side furthest removed from the stove is turned to a joint on the face and the pressure of the blast when the stove is on blast holds the valve to its seat. When the stove is blown off it then swings comparatively freely and can be raised by the operator without much difficulty.

A mushroom valve of the same type as that used for hot blast has also been largely used in the past. A type of this valve designed by Frank C. Roberts & Co. is shown by Figs. 155 and 156 (page 219). This also is water-cooled and operated exactly like the hot-blast valve.

Valves of these two types, however, have proven very large and unwieldy and rather troublesome in operation, and there has been a strong tendency in the last few years to develop a type of valve which was more accessible, could be more easily operated, and which did not require water-cooling. One of the first and most successful of these was the Kennedy-Morrison valve shown by Fig. 157 (page 220). This valve is exactly on the principle of the Spearman burner except that it makes as close a joint as possible against the reinforcing ring around the chimney opening in the stove, so as to prevent the ingress of any outside air to the flue. When the elbow which carries the burnt gas from the stove to the underground flue is withdrawn its slide seals the flue opening exactly as in the Spearman burner, and the opening in the stove is sealed by a cap held to place by one or more screws mounted in swinging yokes, these screws forcing in the ends of a crossbar or "spider" pivoted to the center of the cap. (See Fig. 158 on page 221).

Figs. 153,154. Roberts slide chimney valve.

Figs. 155, 156. Roberts mushroom chimney valve.

The Inland Steel Company has in use a modification of this valve designed by its chief engineer, Mr. Ole Stenbakken, in which the gooseneck instead of sliding back and forth rests on a circular seat, the center of the circle being set off from the central plane of the gooseneck proper so that when the gooseneck is revolved around this base it is withdrawn from the stove, the opening is then closed by a cap as in the Kennedy-Morrison burner. The construction is shown by Figs. 158 and 159 (page 221). As these goosenecks are very large and heavy it is desirable to reduce the amount of movement and effort required to operate them to a minimum and this is admirably accomplished by the rotation of the gooseneck around a vertical axis; its opening, when removed from the stove, is closed by a light steel door.

The chimney valves which are used on the three-pass type of stoves have been shown in conjunction with those stoves and do not need further comment here.

Fig. 157. Kennedy-Morrison chimney valve.