Evaporators
offer a multiplicity of choices - Short
residence time for temperature sensitive materials, re-circulation for
materials with a tendency towards scaling, falling film for large
flow rates at high efficiency, rising films for high concentration
differential.
Unlike commonly believed, the choice of the number of effects need not be
a straight trade-off between operating cost against investment cost.
For temperature sensitive materials, for instance, it makes sense to
operate at low steam pressure, and use the high pressure steam for
re-compressing the vent vapors. Again, where low pressure steam is
required by the process, it makes sense to put a single effect operating
with high pressure steam, and recover the low pressure steam to avoid
multiple effects altogether.
