used in a climbing-film arrangement, thereby increasing turbulence in the liquid film
compared with standard plate evaporators. A typical arrangement of an APV “Paravap”
plant is shown in Figure 14.26. Feed liquor from balance tank 1 is pumped 2 through the
feed preheater 3 to the evaporator 4 where it boils. Concentrated product and evaporated
vapour are discharged to the separator 5 from which product is pumped 6. Vapour passes
to the condenser 7 from which condensate is pumped 8, vacuum being maintained by a
liquid ring pump 9. Single-pass operation is used for low concentration ratios between
feed and product, whilst higher ratios require the recirculation of some of the product.
This can be to the balance tank or feed pipework, although in some cases it is necessary
to use in-line devices to achieve satisfactory mixing.
For some products it has been found advantageous to pressurise the plate heat
exchanger, with an orifice or valve preventing boiling until the liquor enters the separator,
in what is known as the APV “Paraflash” system. This is a special case of the forced
circulation evaporator described earlier.
APV “Paravap” and “Paraflash” evaporators are used for products with viscosities up
to 5 Ns/m2 and concentrations in excess of 99 per cent by mass. Evaporation rates are
up to 4 kg/s.
It may be noted that the gasket is a key component in plate heat exchangers, and this
may limit the maximum temperature which can be used and may indeed prevent the use
of this type of equipment with some corrosive fluids.