In evaporation, solids may come out of solution and form a deposit or scale on the heat
transfer surfaces. This causes a gradual increase in the resistance to heat transfer and, if
the same temperature difference is maintained, the rate of evaporation decreases with time
and it is necessary to shut down the unit for cleaning at periodic intervals. The longer
the boiling time, the lower is the number of shutdowns which are required in a given
period although the rate of evaporation would fall to very low levels and the cost per unit
mass of material handled would become very high. A far better approach is to make a
balance which gives a minimum number of shutdowns whilst maintaining an acceptable
throughput.
It has long been established(18) that, with scale formation, the overall coefficient of heat
transfer may be expressed as a function of the boiling time by an equation of the form:
1/U2 = atb +b (14.11)