In commercial installations high
values of overall transfer coefficients have been achieved
14.7.5. Film-type units
In all the units so far discussed, the liquor remains for some considerable time in the
evaporator, up to several hours with batch operation, and this may be undesirable as
many liquors decompose if kept at temperatures at or near their boiling points for any
length of time. The temperature can be reduced by operating under a vacuum, as discussed
previously, though there are many liquors which are very heat-sensitive, such as orange
juice, blood plasma, liver extracts and vitamins. If a unit is designed so that the residence
time is only a few seconds, then these dangers are very much reduced. This is the principle
of the Kestner long tube evaporator, introduced in 1909, which is fitted with tubes of 38 to
50 mm diameter, mounted in a simple vertical steam chest. The liquor enters at the bottom,
and a mixture of vapour and entrained liquor leaves at the top and enters a separator,
usually of the tangential type. The vapour passes out from the top and the liquid from
the bottom of the separator. In the early models the thick liquid was recirculated through
the unit, although the once-through system is now normally used.