One is to design the feeder for less frequent larger refills, while the other is to design for more frequent smaller refills. Both have their advantages; however, I would be hesitant to design a TIO2 feeder with a large refill as a large volume of TIO2 coming into the feeder could easily whip up (aerate) the remaining material in the feeder and flush everything out of the discharge end. Therefore, the answer is – it depends on the material(s), but here is a good place to start. A head or heal of material needs to be kept on the feed screw so that the incoming material does not flush out during a refill. As a starting point I recommend using 20-30% of the total hopper volume. Next, the materials natural angle of repose needs to be accounted for. This is the descent angle of the material when piled on a flat surface. Due to the material angle of repose typically the feeder hopper is never fully filled to the top. My preference is to use 80-90% of the total hopper volume to account for the materials natural angle of repose. Let’s say for example, we have a feeder with a total hopper volume of 10 ft³, 20% of 10 ft³ equals 2 ft³ and 80% of 10 ft³ equals 8 ft³. So, the total refill amount then becomes 6 ft³.
At a rate of 1,200 lbs./hr., with the material weighing 20 lbs./ft³, our flow rate would be 1,200/20 = 60 ft³/hr. A refill amount of 6 ft³ would require a refill approximately 10 times per hour or about every 6 minutes.
Before going further, we need to understand the consequences of putting material back into a continuous gravimetric feeder while it is feeding. The controller literally thinks that there is no material coming out of the feeder. This results in trying to speed up to compensate for this. Therefore, the controller will need to ignore the material going into and out of the feeder. This is what is known as the controller’s refill mode. Each manufacturer will have a buzzword or name for what their controller does during a refill. The important thing to know is that the controller cannot control while it is in refill. My opinion and that of many others in the industry is the quicker you refill the feeder and get the controller to come out of refill mode, the more accurate your system is going to be.
Let’s revisit the 6 ft³ of material that is needed for refilling the feeder. If using a best-case scenario where the refill system can supply exactly 6ft³ of material each time the feeder needs it, I would use a rule of thumb that states operate in refill mode no more than 10% of the hour or 6 minutes total for all refills. This would provide one minute to accomplish a refill in the example, but before confirming a one-minute refill time you must take settle time into consideration. Settle time allows a scale (or load cells) time to stabilize after material has been either added or taken away from the feeder. Typically, 5 seconds will be enough to allow the scale to settle. So that leaves 55 seconds to refill the feeder with 6 ft³ of material.
When doing the math that means in 55 seconds the refill system will need to fill the feeder with 120 lbs. of material (6ft³ x 20 lbs./ft³) at a rate of 7,848 lbs./hr. (120 lbs. in 55 seconds = 2.18 lbs./second = 130.8 lbs./min = 7,848 lbs./hr.
When it comes to refilling a gravimetric feeder don’t hesitate to ask an Applications Engineer for advice or to answer any questions.
