Application of Rotary Valve

Rotary airlock feeders/ rotary airlock valves are used in pneumatic conveying systems, dust control equipment, and as volumetric feed-controls.

Application wise feeder can be classified into two categories:
1)    Rotary feeder and
2)    Rotary airlock valve

Although the actual rotary feeder is the same in both cases, the efficiency of each application is different, making of the valve and the drive speed different. 

The rotary valve controls the flow of (metering) material. There is no pressure differential across the feeder; however, a rotary feeder can be used under a head of material, if

a)    It is located immediately below an Air slide bottom bin,
b)    It is located below a small batch hopper having aeration pads,
c)    If the head of material above the rotary feeder is held to a minimum by the use of a vented expansion hopper or
d)    If located below a bag dump hopper where the head is limited.

Rotary feeder is used where the flow control of the material is required and pressure difference or vacuum differential is not essential. 

The rotary airlock application operates with either a pressure or vacuum differential, but does not necessarily controls the flow of material.  Rotary airlock valves are normally used either as a charging device for low-pressure pneumatic conveying systems, or as a suction lock for dust collectors.

It can be mounted below the bag filter, which can make the pressure difference between atmosphere & inside the bag filter, which in turn makes effective dust collection system.  It can also be used in dilute phase and continuous dense phase.

Open and Closed Ended Rotor in Rotary Airlock Valve

The rotor can also classified based on the open and closed ended construction of rotor or rotor assembly. Majority of the manufacturers offer both types. In case of open end rotor construction / assembly, the pockets between the vanes is contained on both sides by the rotor assembly of the end plates of the housing of the rotary valve. This is the most common type of style of construction of the rotary valve. The bulk solids material passing through such rotor assembly comes in contact with the end plates and the bore of the valve housing. Due to the open end construction the bulk solids material when getting discharged through the rotary valve gets migrated towards the seals. There is no obstruction or any arrangement to contain this migration. Hence this causes wear of the seal. This is a major drawback of the open ended construction of the rotor assembly. The ability of the valve to contain the material leakage depends solely on the quality and type of seal.

Rotary Valve

Contrary to the open ended design, the closed ended design has two plates to seal the pockets from the side, this prevents the material from migrating to the seal when the material is getting discharged from the inlet to the outlet. In case the rotor assembly is fabricated the plates are welded to each end of the blades or vanes. While when the rotor is manufactured out of casting or molding, the plates are cast as part of the rotor. The end plates are also known as shrouds. The wear and tear in this case is greatly reduced.

In general closed ended rotor assembly has better performance capability compared to the open ended construction wherever wear and tear is a problem. It is also a better option in case of better sealing requirement for pressure difference. 

Points to Consider While Selection of Rotary Feeder

Rotary feeder is also called as rotary valve or rotary airlock valve. The application of this versatile valve is volumetric feeding / dosing of dry bulk solids material. The key factors that determine the capacity of the valve include:

1.       Bulk density of the material

2.       RPM

3.       Size of the rotor

To fine tune the throughput capacity, speed adjustment can be provided through variable frequency drives for special feeding applications.  The selection of rotary valve is important. In today’s competitive business environment companies focus on energy efficiency like never before. As for example in cement industry, the production process consumes vast amounts of energy. Hence energy efficiency of all equipment is important. Rotary valve is extensively used in cement industries, energy efficient rotary feeders help in reducing the energy costs. 

Where wet and sticky materials need to be handled via rotary airlock valve, care needs to be taken in selection of the motor hp. A positive mechanical scrapper can be used to avoid choking of the valve. However such scrapper mechanism increases the size of the valve considerably. Certain materials change their property when they catch moisture from the atmosphere. They may become cohesive and thereby trip the motor due to the resulting sluggishness due to moisture. It is important to understand such characteristics of materials before selection of the feeding equipment. In case of handling abrasive materials, the material of construction of rotary feeders need to be wear-resistant or in stainless steel. For materials with adhesive properties a coating of non-stick needs to be applied for effective operation of the equipment.

Cast iron or cast stainless steel is used as material of construction for rotary valves. Abrasion resistant liners like Teflon can be used at the tip of the rotor vanes for wear resistance. Attachable gear motors provide a compact manufactured size which is easy to install. Outbound bearings ensure that the dust do not ingress into the bearings thereby causing undue problems in the operation of the valve.

The material passes through the inlet to the discharge outlet through the rotor. The rotor is housed in the body and rotates. Pockets are formed in the rotor through vanes which can be 6 or 8 in number depending on the capacity required, material to be handled and the application.