Rotary Valve (pneumatically actuated)
Rotary valves should not be confused with Airlock Rotary valves. If the former are electrically driven, rotate 360 degrees, and have tight metal / metal clearances to ensure minimum gas is leaking through it, the rotary valves are actually pneumatically driven, rotate 180 degrees back and forth and are tight only in stand-by position.
These design are particularly popular among italian manufacturer like CORA or STERI.
Figure 1 : Dosing principle of a rotary valve
Rotary valves are used for dosing solids. This dosing can be rough or quite fine.
Rotary valves are made of the following equipment :
- Pneumatic drive
- Rotor with alveoles
- Gasket fit to the housing
Some specific design for fine dosing have actually a second rotor placed in the main one. When running a coarse dosing, the main rotor is activated, then when running fine dosing, the small one is activated. It is a volumetric device, the throughput that can be achieved depends on the size of the valves (pockets) and the frequency at which it is turned. The filling coefficient is usually assumed to be 100% since the system is rotating quite slowly compared to a star valve and because there is no pressure differences between the inlet and outlet of the valve.
The volume dosed for 1 cycle (180 degree in one direction + 180 degree on the other) is given below - this may change slightly from 1 supplier to another.
Table 1 : Dosing volume of rotary valves per complete cycle (order of magnitude)
|Valve diameter mm||Volume 1 cycle in liters|
The sizing can be controlled according to the number of cycles / min that are needed to achieve a given throughput. The calculation should not exceed 60 cycle / min.
Equation 1 : Estimation of rotary valve cycle frequency for a given throughput
With V the volume of 1 complete cycle of the valve in liters
The valve is controlled by playing on the time span between 2 rotation of the valves. It should however not be too close in order to avoid mechanical issues with the actuator of the gasket.
Rotary valves can be used to replace dosing equipment like screw conveyor, vibrating tube or even star valve, if the dosing expected is rough or fine, not too high in flowrate and it does not require an air sealing between 2 parts of the process. The layout should allow such equipment also - next process step located right below the valve.
Key process use for rotary valve :
- Rough control of flow towards a vibrating sifter
- Rough control of the flow towards a pneumatic vacuum transport line
- Fine dosing to a weighing hopper
- Rough control of the flow towards a filling machine hopper
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