A rotary valve is a mechanism that changes the path of liquids or gases by use of a circular, rotating closure. It contrasts with the linear valve, which moves a gate up and down to control flow. Rotary valves typically take up less space than linear valves while performing the same basic function. Some rotary valves have an airlock design which allows them to be used in pressurized conditions.
Any valve that uses a circular motion to control flow is a type of rotary valve. "Rotary valve" is a broad term that covers a wide range of applications. Naturally, valves are used in plumbing and manufacturing, but they are also used in combustion engines, musical instruments and the human body. Even a revolving door is a type of rotary valve.
A ball valve is a common type of rotary valve. It is also called a stopcock valve. The ball valve is round with a hole through its middle, which is turned by a handle. Inserted into a pipe, it can be closed, partially open or completely open, which allows for some degree of flow control.
The butterfly valve is also part of this category. It is more space efficient than the ball valve, because the closing mechanism is a flat disc. The disc is attached to the pipe at the top and bottom and it turns on a vertical axis using a hand wheel. A similar valve is the tilting disc, which is used in prosthetic hearts.
Musical instruments such as the tuba, French horn and valve trombone also use rotary valves. The levers — commonly called keys — operate the valves and allow air to flow into another line of tubing. This produces a different set of notes. The valves serves two purposes: to open a new path for the air and to close the alternative path. If the alternative path is not completely sealed, then the musical sound is compromised.
Some combustion engines use a very sophisticated type of rotary valve. In this case, the valve closures are shaped like a rounded triangle and fitted inside an oblong encasement. The tips of the triangle form a seal with the encasement, and this creates air pockets. As the engine turns, all four strokes of the combustion cycle — intake, compression, combustion and exhaust — are completed in a single movement.
A rotary feeder is a type of airlock rotary valve. The feeder transfers solids from one chamber to the next without changing the pressure of either. Rotary feeders are often used in manufacturing facilities that deal with granulated solids.