For some manufacturers, there is an inherent issue with “pressure loop” conveying. These manufacturers may or may not be aware of this issue. When you convey this specific way in a pressure pneumatic conveying system, it is a normal occurrence for material to go back into a source hopper or silo. This means that every time you convey to a scale and the scale reaches its set-point, that which is in the pipe travels back to the top of your hopper or silo.
While there are a few industries in which these concerns wouldn’t detract from essentially handling the same product twice over, in specific industries (like food-related industries) it is critical to avoid this occurrence.
The destination point for a dilute phase vacuum pneumatic conveying system
Take the difference between a PVC manufacturer and a baked goods manufacturer, for instance. The resin that the PVC manufacturer conveys out of their silo and into their mixer won’t make a difference if it is used again in the process. Still, for fresh-baked products and virtually any material where it is dire to avoid contamination (and even mold), vacuum scaling presents some major benefits to your pneumatic conveying system. From more consistent metering to advantages in dust control, here are the three major ways scaling with a vacuum pneumatic conveying system will fit these specific processes.
“Double-handling” of product means you lose “first-in/first-out” capability
In a loop-style conveying process where you first convey material from the silo to the scale, material has to go back into the silo after reaching a set-point. Essentially, the material bypasses the receiver and loops back into the silo. This could mean pipe-loads of older material, as well as undesirable dust-layering, returning on top of newer and fresher ingredients or material located in the silo.
In this scenario, every time you run the system, you return some percentage of material to your silo (where the newer product is located). The standard “first-in/first-out” practice is not achievable for your product when you utilize a pressure loop system. If cross-contamination is a concern, this kind of loop conveying can be extremely counter-productive. In the worst cases, this can lead to product recalls (which are, in turn, made less easy by the very nature of this loop style conveying).
The pickup point for a dilute phase vacuum pneumatic conveying system
More consistent metering is accomplished with vacuum scaling
If you have a feeder in your operation that is dosing into a pneumatic conveying line under negative pressure, you’re able to fill the pockets or the displacement of this feeder more consistently. Since air is trying to force its way up through the feeder in the opposite direction of product flow in a positive pressure system, erratic product flow can occur. This can cause inconsistent batch amounts.
Using negative pressure, or vacuum flow, moves the product in the same direction as the air within the feeder. As a result, more consistent metering is achieved in the conveying line.
Vacuum scaling poses benefits in regards to dust control
One additional benefit to vacuum scaling that is used with vacuum conveying is the overall inherent dust-free system. Connection issues happen often in bulk material handling. There are many opportunities for leaks in systems to occur. The difference between a leak happening in a pressure and a vacuum pneumatic conveying system is that a vacuum sucks air into the convey line. A pressure system blows air and material/dust out into the facility.
Proper dust mitigation is a topic we have covered before in the AZO blog. Having a dust-tight conveying system is just one benefit that contributes to a general peace of mind for those seeking to attain critical adherence to standards set by the NFPA.
In fact, we at AZO have published an infographic to explain exactly how NFPA 652 and the new 2020 edition of NFPA 61 relate to the ingredient handling world. The sleek graphic covers specifically what has changed in the 2020 edition of NFPA 61, key facts to keep in mind for a dust hazards analysis (DHA) and even steps to perform this critical evaluation. Altogether we have more than seven decades of experience in handling raw materials and shaping ingredient automation along the way. Feel free to contact the AZO sales team for any questions on how to help your plant and processes run smoothly.