Process Considerations and Performance

Process Considerations and Performance The capacity, or performance, of a thin-film evaporator is often reported as a feed mass flow-rate scale factor (unit ma per unit surface area) for a specific process step. Performance is a function of a few process and equipment design variables  

Figura 11-28 Mean residence time In a thin-fil n evaporator equipped with a residence time (RT) control ring at various rotor speeds. Feed rate Is 157 Ib/hr/ft. The ratio b/bo Is the RT ring width relative to the standard RT ring width. 

While mathematical models have been developed for predicting performance of thin-film evaporators, suppliers of thin-film machinery maintain extensive pilot plant testing laboratories for obtaining development and process data with small evaporators. Data obtained from one or two days of testing on several drums of feed material are sufficient for predicting and scaling performance to large evaporator applications. 

Clearance between the rotor blades and the thermal wall is not a variable for most suppliers of thin-film evaporators. While the clearance for fixed-clearance designs should be close to zero, the rotor blades should not touch the wall. Thermal expansion and contraction of the rotor and the body and manufacturing considerations (especially of large rotors) dictate an optimum clearance typically 1.25 millimeters for small rotors, up to 5 millimeters for large rotors. 

Maintenance Because thin-film evaporators are rotating machinery, they are precision machines and preventative and corrective maintenance are important consideration. Bearings and mechanicai seais must be properiy appiied and maintained. Properly maintained, thin-film evaporators provide long, trouble-free service. Premature failure can be caused by  

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