8.3. OVERALL HEAT-TRANSFER COEFFICIENTS IN EVAPORATORS
The overall heat-transfer coefficient U in an evaporator is composed of the steam-side condensing coefficient, which has a value of about 5700 W/m2 · K (1000 btu/h · ft2 · °F); the metal wall, which has a high thermal conductivity and usually a negligible resistance; the resistance of the scale on the liquid side; and the liquid film coefficient, which is usually inside the tubes.
The steam-side condensing coefficient outside the tubes can be estimated using Eq. (4.8-20) through (4.8-26). The resistance due to scale formation usually cannot be predicted. Increasing the velocity of the liquid in the tubes greatly decreases the rate of scale formation, which is one important advantage of forced-circulation ...
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