流体力学与 及传热课件Capacity and economy of multiple-effect evaporators.pptVIP

Capacity and economy of multiple-effect evaporators;It may be thought that by providing several times as much heating surface the evaporating would be increased, but it is not the case.;If the heating load and the heat of dilution are neglected, the capacity of an evaporator is directly proportional to the rate of heat transfer.;Assume that the surface area is A in each effect and that the overall coefficient U is also the same in each effect. then;Suppose now that a single-effect evaporator with a surface area A is operating with the same total temperature drop.;Effect of liquid head and boiling-point elevation;The reduction in capacity caused by the liquid head, as before, cannot be estimated quantitatively. ;The temperature drop in any effect is calculated from the temperature of saturated steam at the pressure of the steam chest, and not from the temperature of the boiling liquid in the previous effect.;This loss occurs in every effect of a multiple- effect evaporator, and the resulting loss of capacity.;Consider the single-effect evaporator. ;The diagram for the double- effect evaporator shows two shaded portions because there is a boiling-point elevation in the two effect.;In the triple-effect evaporator there are shaded portions since there is a loss temperature drop in each of three effects, and the total net available temperature drop ,79o;The economy of a multiple-effect evaporator is not influenced by boiling-point elevations if minor factors, such as the temperature of the feed and changes in heats of evaporization, are neglected. Then by Eqs. 【16-16】;A kilogram of steam condensing in the first effect generates about a kilogram of vapor, which condenses in the second effect, generating another kilogram there, and so on.;The economy of a multiple-effect, evaporator depending on heat-balance considerations and not on the rate heat transfer.;Optimum number of effects;The optimum number of effects must be found from an economic balance between the savi