A dry stream of air is injected at low
pressure on the permeate side to
increase the driving force for water
permeation at the dry end of the
membrane dryer. This dry stream
(fed to the permeate side through the
inlet purge port) is called the external
purge, dry sweep, or inlet purge.
The gas that permeates across the
membrane can be considered internal
purge. The external and internal
purge gas streams together make up
the total purge that exits from the
purge outlet port. Some of the dry
air exiting the membrane dryer can
be used as the external purge or a
secondary source can be used.