Y W W

where hb and h are for convective heat transfer with and without boiling, respectively.

In falling-film evaporation three different flow regimes normally exist, depending on the Reynolds number (Re) of the film: laminar, wavy laminar, and turbulent. The truly laminar regime takes place for Re < 20-30, the wavy laminar regime starts at Re = 30-50, and at 1,000-3,000 a fully turbulent flow is established. The Reynolds number for a film is calculated according to the following equation (8):

where G is the mass flow rate per unit width of the wall.

For the laminar region, the local heat-transfer coefficient may be written as (8):

r JOY033

Table 1 gives formulas for local heat-transfer coefficients according to different investigators.

Experimental product-side heat-transfer coefficients hp were determined (9) for thin-film wiped-surface evaporators, which are extensively used for concentrating high-viscosity or heat-sensitive products. Product degradation is minimized by vigorous agitation and continuous removal of liquid films on the heat-transfer wall. The correlation obtained for the transition-flow regime (Re = 100-1,000)

Table 1. Local Heat-Transfer Coefficients

Flow regime

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