Abstract

ROUILLARD, E. E. A.. Boiling viscous liquids in natural circulation vacuum evaporators. R&D j. (Matieland, Online) [online]. 1990, vol.6, pp.13-19. ISSN 2309-8988.

Vacuum evaporative crystallizers are used by the South African sugar industry to produce more than 2 000 000 tons of sugar annually. The process fluid is very viscous and laminar conditions prevail in the apparatus. To date the equipment has been designed by rule of thumb. In order to develop a procedure for optimizing the design and operation of this equipment, research was undertaken to determine how the equations proposed for heat transfer and vapour hold-up, should be modified for boiling in laminar flow under vacuum. It was found that the heat transfer coefficient can be estimated by the superposition method of Rohsenow and Griffith, but that different exponents and constants are required. A new equation is proposed for calculating the void fraction in the highly subcooled region, and the equation of Bowring for subcooling at the point of bubble departure must also be modified.

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