流体力学与 及传热课件evaporation.pptVIP

Chapter 5evaporation ;Evaporation differs from distillation in that the vapor usually is a single component, and even when the vapor is a mixture ,no attempt is made in the evaporation step to separate the vapor into fractions .;Evaporation differs from crystallization in that emphasis is placed on concentrating a solution rather than forming and building crystals.;Liquid characteristics;Concentration ;Temperature sensitivity;scale;Single- and multiple-effect operation;When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded, the method is called single-effect evaporation.;Additional effects can be added in the same manner. ;Types of evaporators;Once-through and circulation evaporators; In circulation evaporators a pool of liquid is held within the equipment. Incoming feed mixes with the liquid from the pool, and mixture passes through the tubes. Unevaporated liquid discharged from the tubes returns to the pool;Short-tube evaporators;The stream condenses outside the tubes. The tube bundle contains a large central downcomer, the cross-sectional area of which is 25 to 40 per cent of the total cross-sectional area of the tubes;In this evaporator the driving force for flow of liquid through the tubes is the difference in density between the liquid in the downcomer and the mixture of liquid and vapor in the tubes.;Long-tube evaporators with upward flow;Forced-circulation evaporators;Here a centrifugal pump forces liquid through the tubes at an entering velocity of 2 to 5.5m/s;Film evaporators;There are two types of film evaporators:;This is done by inserts the metal plate in the tube ends to cause the liquid to flow evenly into each tube, or by spraying the feed on the inside surface of each tube with the “spider” distributors;For good heat transfer the Reynolds number of the falling film should be greater than 2000 at all point in the tube.;Falling-film evaporators can be used in concentrating sensitive products.