Critical orifice diameter has long been held as a standard lab technique to characterise powder flow. Mass flow rate measurement during hopper discharge has also been used as a flowability measure for the design of powder handling and dosing operations. An implicit feature of these methods is that discharge occurs in open atmosphere.
Research into die fill using linear and rotary powder feeders have identified the importance of atmospheric pressure conditions at the point of discharge. A critical orifice measurement apparatus was developed to study hopper discharge into a low pressure enclosure. It was shown that a very small amount of vacuum has decreased the critical orifice diameter significantly. Similarly, flow rate measurements showed identified similarly significant effects.
Two dimensional models were developed to obtain: 1) a relationship between outlet diameter and vacuum level required to initiate flow and 2) a relationship between the flow rate and the applied vacuum. The potential impact of the experimental results and the proposed dimensional model is significant for the design of die fill operations on tablet presses and other powder dosing processes.