Exhaust air valves previously known for use in ventilation systems generally have a valve casing which is provided with a seat resembling a truncated cone. A similarly designed valve body cooperates with the seat, but has smaller dimensions so that the air passage through the seat more or less assumes the shape of a slot with a large crescent-like cross section. In a commonly used valve, the valve body is mounted onto the seat by means of a self-tapping screw. Adjustment is afforded by providing a slot in the seat through which the screw passes so that when the screw is loose the valve body may be adjusted along the slot and then tightened in the desired position before the valve casing is mounted in the exhaust passage. In another valve which is commonly used, the valve body is rotatably mounted in registry with the seat and is held in adjusted position by friction. Other arrangements for anchoring and installing valve bodies are also known.
Conventional valve structures of this type are advantageous in many respects, but in view of the considerable period of time which has elapsed since they were first conceived, various new requirements have been imposed relating to the various functions and characteristics of an exhaust air valve. For example there is currently a high requirement for low generation of noise in ventilating systems at normal operating positions and under all normal operating conditions, and in certain cases, the ventilating system must be noiseless at a certain special operating position or under a certain special operating condition.
Capability of variable adjustment is another requisite, and in many cases full adjustment between maximum opening and complete closing of the ventilation passages is required. Preferably the adjustability should be achieved through simple and secure fastening of the valve body. Desirably the adjustment may be accomplished manually and the adjusted position should be secured by simple frictional resistance. The valve operator may comprise a cord or the like for manual adjustment and may also embody an automatic anti-fire tripping mechanism which automatically closes the valve at higher temperatures.
It is desirable that the valve casing and/or the valve body are readily adapted to different environments and should be compact. The valve should have long operating life and be resistant to jamming or other frictional problems under all conditions and yet be lightweight and simple, and inexpensive to manufacture, install and adjust. Adaptation of the valve to special purposes should be easily accomplished in a simple manner. Last, but not least, the valve should be attractive in appearance since it is frequently completely exposed to view in an apartment or a home.