The vapour from the last effect of the evaporator (temp. 45-50șC) is usually
passed through a tubular heat exchanger, where the vapour is cooled by means of
the cold incoming product, which is reheated. The precooled vapour is passed on
to a condenser, where it is condensed using cooling water either from a cooling
tower or a natural water reservoir.
It is an obvious idea to utilize
the drying air instead of cooling water in a heat exchanger for condensing the
vapour simultaneously preheating the drying air.
In the four-effect
evaporator operating in conjunction with the two-stage spray dryer at a drying
temperature of 230șC, as described on page 145, there is approximately 800 kg/h
vapour with a temperature of about 47șC available. However, by means of 31,500
kg/h air at 10șC it is only possible to condense 400 kg/h.
Therefore,
an additional condenser, operated with water, is necessary. Another
inconvenience of this system is that the additional condenser should have
sufficient capacity to cope with the extra duty in case the ambient temperature
increases. Further, during start-up of the evaporator it is necessary to have
at least the fans of the spray dryer operating to get stable conditions in the
evaporator. Therefore, this way of saving energy in the spray dryer is not used
in practice, but only included, as there is a patent describing it as a
possibility.