There are several basic objectives when discharging a bulk
bag. First, to be able to hang the bag safely and securely
in a stable structure above the point of discharge. Second,
to obtain good flow of the bag contents from the bag. Third,
to empty the contents completely, with as little dust to the
operator environment as possible. If a liner is used, clear
monitoring of the liner position should be facilitated, and
the liner should be properly secured to prevent it from tearing,
or discharging with the ingredients or tearing. Completely
sealed discharge of dusty or sensitive ingredients should
be offered.
There
are eleven basic functions to look for in a discharging
machine. If the discharging machine you select offers these
features, the above objectives will be met, even with the
most difficult of ingredients.
1.
An external device (rigging frame) to secure the top of
the bag for lifting and leaving the bottom unobstructed.
2. A
means of lifting the bulk bag into place over the discharger.
3. A
protective (safe) support for the bottom of the bag.
4. A
support for the top of the bag (or rigging frame).
5. A
means to untie the bag outlet.
6. Dust
control:
•
at bag sides
•
at outlet spout
7. A
means to handle the liner:
•
top retraction
•
bottom seal
8. A
means to empty entire contents of bag:
•
flow inducement
9. Downstream
flow control ranging from free flowing, conveying, feeding,
batching.
10.
Removal of the empty bag dribble/dust control.
11.
Operator controls:
•
vibration
•
discharge rate
A bulk
bag typically has four loops to properly suspend it from
hanger arms on the discharging machine. The suspended bag
will be centered and supported by a discharger hopper. This
is important because it ensures that the bag will not shift
or tip during discharge.
The
slope of the discharger hopper, using the same design principles
as silos, should be steep enough to break the ingredient
bridge and promote flow. The discharger hopper must offer
safe access to the bag outspout for tying and untying functions.
Unlined
bags come in a variety of styles and require different design
to discharge them. A bag without an outlet spout, and a
flat bottom needs a knifed discharger to pierce the bottom
of the bag for the contents to be released. A bag with an
outlet spout is discharged when the outlet spout is untied
and flow initiated. Some ingredients require a steep hopper
to promote flow, and more ingredients need the supplement
of vibration to ensure complete discharge.
Downstream
Device
Lined
bags require careful design to discharge without tearing,
or contaminating the downstream process. A liner tensioner
keeps the liner in place and centered in the outspout. As
the liner empties and takes on a thin tubular shape, it
essentially becomes longer and can slip out of the downspout
into the process below. The rack and pinion liner tensioner
pulls the liner up and winds it onto a spool, for up to
42", and thus prevents liner slippage.
