The upstream production process feeds a preweigh bulk bagging system capable of filling 40 bulk bag per hour. Also, the system that conveys product to the bagging location, is at the same rate – product in must equal product out.
This is accomplished in two ways:
Often, the production linel continuously conveys product to a large silo (50+ tons capacity). The preweigh bulk bagging system is underneath the silo. So the bulk bagging occurs continuously, if production keeps up with the bulk bagging rate. Or it occurs periodically when production fills the silo to an appropriate level.
A less common arrangement is where the production line continuously matches the bagging rate. In this case product continuously conveys to the bagging line where it continuously fills into bulk bags.
The C1-2 gain-in-weight bulk bag filler has a flat plate base that removes bags on pallets.
The weighing sub-system design is the most challenging aspect of implementing a preweigh bulk bagging system.
As was shown in the previous article, a 40 bag per hour system has a total cycle time of 90 seconds. Of that, it allocates 15 seconds to emptying the preweigh hopper (the vessel wherein the product shot is weighed) into the bulk bag. From the preweigh hopper’s perspective, the remainder of the cycle time – 75 seconds – weighs the next shot.
Assuming a typical dribble feed duration of 15 seconds, the fast fill portion of the weighing cycle is 60 seconds.
Typically, 50 – 100 lb of the total shot weight dribble fills. Therefore, in the case of a 2,000 lb bag, 1,950 lb of product meters into the preweigh hopper in 60 seconds. Hence this equates to a very high instantaneous flow rate of approximately 117,000 lb/hr.
Two key factors maintain accurate and consistent weights:
- Flow control. The device that meters the product into the preweigh hopper is properly selected.
- Flow rate. The rate at which the product flows into the preweigh hopper is consistent from bag to bag.
Also there are a number of ways to design such a weighing system according to the desired weighing accuracy and capital budget.
Certainly note that preweigh systems consistently maintain accuracy of +/- 2 lb.
The faster bulk bags fill, the less time there is for densification. Further, bag stability becomes more of a concern as the bulk bag filling rate increases.
We must carefully assess applications with bagging rates over 20 per hour. They must determine densification required to produce safe and stable packages.
As the bagging rate approaches 25 bulk bags per hour and beyond, stability and densification is a major concern with almost any product. Even more, this is particularly true if stacking the bulk bags is a requirement.
Finally, in a preweigh system capable of filling 40 bulk bags per hour, the filling machine is carefully designed. We ensure the filled bags are stable and safe.