“For grains, flour and agricultural dust, which are only mildly abrasive, an open rotor will be the best option. Besides being cheaper, it prevents the buildup of dirt between the ends and the housing. It does have some wear problems, especially in the seal that protects the bearings of the shaft, but basic regular maintenance (checking and replacing) will prevent any problems. It would be much worse to have the airlock completely disassembled every week just for cleaning.”
Industrial Mechanical Engineer
Licensed Grain Inspector
MBA Project Management
SOSA – Engineering Consultants
In most grain handling and milling facilities, pneumatic conveying and dust control gets very little attention. Except in countries with stringent regulation, the only requirement is to have SOME dust control system, for which there aren’t any specifications of performance. Of course, flour milling is very dependent on pneumatic conveying, as it is the most efficient way to transport flour from one point to the other of the facility, but even there, operation tends to be deficient because of the lack of technical preparation of the workers.
In the general universe of pneumatic conveying, cyclones are treated like the bum brother nobody talks about. Several times I have had people come to my company and ask for an off the shelf cyclone (which shouldn’t exist), and when I asked about the air flow required and the head loss in the piping, they had no idea. Particle size distribution? They act like it is a bad word. To be clear, air flow determines the size of the cyclone. Both air flow and head loss determine the specific fan you need. And particle size distribution helps to choose the general design of the cyclone (among the different design methods) and the airlock valve. One can design some cyclone without the information on particle size, but not without the other two.
Given this situation, it is understandable that the rotary airlock valve gets minimum if any attention. The fan and the airlock valve are the only moving pieces in the cyclone, and the airlock valve is the only one (hopefully) that gets in contact with the product.
For the proper functioning of the cyclone, the air should enter only through the tangential inlet, and should enter only through the main inlet. The exit of the air will always only through the roof because the fan is sucking the air from the cyclone. The content of the cyclone must be emptied regularly so the vortex of the air inside it isn’t impeded. This is achieved with a rotary airlock valve. What this little piece of equipment does is to empty the cyclone continually, while keeping the flow of air entering through the bottom at a minimum. This means the clearance between the blades and the housing must be minimum too.
Again, this is not a simple question, and any manufacturer has to invest a lot of money and time trying with different products, flows, and pressures to achieve an optimal solution.
The gap between blades and housing can’t be 0. That would get the valve stuck just because of changes in temperature, either work or ambient. Too little a gap would get it stuck as soon as there is some dirt in the housing. To wide, and it lets too much air go through. The ideal gap is in a range for different designs of blades and housings, depending on the variables mentioned before.
For grains, flour and agricultural dust, which are only mildly abrasive, an open rotor will be the best option. Besides being cheaper, it prevents the buildup of dirt between the ends and the housing. It does have some wear problems, especially in the seal that protects the bearings of the shaft, but basic regular maintenance (checking and replacing) will prevent any problems. It would be much worse to have the airlock completely disassembled every week just for cleaning.
That leads me to the basic rule of the airlock valve. CLEAN IT.
It is easy to overlook the cleaning of the airlock if you don’t care about performance. Just make the gap big enough, with a fan big enough to compensate, and you are done. But you are wasting energy, and energy means money.
In a classic airlock with flat tips, the gap is in the order of 0,1 mm. If the tip has a bevel, the gap may be around 0,05 mm, even 0,01 mm. My personal preference is blades with rubber tips (wiper seals) that allow a clearance of down to 0,08 mm, but give some way when the dirt builds up inside the housing. Not all “rubber blades” are equal. Some blades only have a wiper seal at the tip, while others are almost entirely made of rubber. I have seen different manufacturers making different choices, and it looks like the rubber part gets longer when the equipment is to be operated in a low income rural area, where there is a lack of trained technicians. In those places you need something that may not be so good, but keeps working where others fail. It is the difference between a burnt motor and some air leaking.
In any case, the basic preventive maintenance is to completely empty the cyclone at the end of the shift (just keep the airlock moving a little bit) and clean it with compressed air. Remember to NOT stick your fingers inside it.
If it is in the outdoors, you may consider placing it inside a housing that protects it from the rain. Organic dust and moisture don’t make a pretty combination.
If you have any questions or suggestions about this article, please, feel free to write to me.
We also suggest you to read our previous article titled "Pneumatic conveying and food safety".