A comprehensive discussion of the main aspects of the wiped-film evaporator technique
covering thin-film technology in general, the equipment, and its economics and process
applications is given by MUTZENBURG(36)
, PARKER(37)
, FISCHER(38)
, and RYLEY(39)
. An
additional advantage of wiped-film evaporators, especially those producing a scraped
surface, is the reduction or complete suppression of scale formation though, in processes
where the throughput is very high, this type of unit obviously becomes uneconomic and
the traditional way of avoiding scale formation, by operating a flash evaporation process,
is more suitable.
14.7.7. Plate-type units
A plate evaporator consists of a series of gasketted plates mounted within a support
frame. Film-type plate evaporators can be climbing-film, falling-film or a combination of
these. Figure 14.25 shows the flow and plate arrangement of an APV falling-film plate
evaporator. Each unit comprises a product plate and a steam plate, and this arrangement
is repeated to provide the required heat transfer area. Product flow down each side of the
plate may be in series where this is advantageous in terms of wetting rates.
Both the vapour evaporated from the boiling film and the concentrated product are
discharged from the evaporator to a vapour–liquid separator from which the product
is pumped, the vapour passing to the next effect of the evaporator, or the condenser.
Compared with tubular evaporators, plate evaporators can offer important advantages in
terms of headroom, floorspace, accessibility and flexibility.