RESIDENCE TIME OF LIQUID IN A THIN-LAYER EVAPORATOR EQUIPPED WITH A SPIRAL ELEMENT

A new apparatus for conservative evaporation of thermally sensitive li quids is thoroughly examined. The application of a spiral element in a static type thin layer evaporator (STLE) enables controlled extension of the residence time of liquid inside the evaporator, and consequent ly a controlled degree of evaporation. The residence time distribution of a tracer injected into a falling-water film on the outside of a cy lindrical tube is measured by the impulse response method. A total of 221 measurements are performed using an STLE and a TLE equipped with s piral elements of two different pitches. On the basis of experimental data, detailed correlations are elaborated which express relationships between the mean residence time (t) over bar and dimensional moduli o f Reynolds and Froude numbers.