This chapter concerns the control of a thermosyphon
type of evaporator. The issues are much
the same for calandria and other single effect types
of evaporator.
Figure 31.1 depicts the evaporator in which an
aqueous stream containing suspended particulates
(or dissolved salts) is concentrated by evaporating
off water. The dilute feed stream mixes with
concentrated liquor circulating at its boiling point.
This is partially evaporated on the tube-side of
the reboiler by condensing steam on its shell-side.
The tube-side exit stream consists of water vapour
with entrained droplets of liquor. The separator
provides the space for the droplets to disengage
from the vapour. The droplets fall into the liquor
which is recirculated, a fraction being drawn off as
concentrated product. The reboiler is operated under
vacuum provided by a pump which sucks the
vapour out of the separator. The vapour condenses
on the shell side of a water cooled condenser, the
condensate being discharged via a barometric leg
and pumped away by a centrifugal pump.