Meaning
Gas flow limits occur in extraction piping when the velocity of the gas reaches the speed of sound at a restriction. In devolatilizing extruders, vacuum line choked flow prevents any further increase in the extraction rate of volatile contaminants even if the downstream vacuum pump is upgraded. This limit is reached when pressure drops across the line are too high, which typically happens when the pipe diameter is too small or if the line is partially blocked by condensed volatiles.
Flow Restriction
Pipe diameters, bends and filters create resistance that restricts the movement of extracted gases. When vacuum line choked flow occurs, the mass flow rate through the pipe becomes independent of the downstream pressure. This restriction means that the vacuum level at the extruder dome remains insufficient.
System Pressure
High pressure differences across the system are necessary to drive the volatiles from the melt to the extraction system. If vacuum line choked flow is present, the pressure at the extruder vent dome will rise, reducing the driving force for volatile removal. This rise leads to high amounts of residual monomers in the polymer.
Process Instability
Fluctuation of volatile concentration in the melt stream causes bubble formation and surging at the extruder die. Properly sizing the extraction lines to avoid vacuum line choked flow ensures that the system maintains a stable vacuum during the run. This stability is critical for recycled polymer compounding.
