流体力学与传热英文课件：5-2 Capacity and economy of multiple-effect evaporators

1、HUA GONG YUAN LI华南理工大学 化工学院 伍钦apacity and economy of multiple-effect evaporatorsThe increase in economy through the use ofmultiple-effect evaporation is obtained at thecost of reduced capacity.HUA GONG YUAN LI华南理工大学 化工学院 伍钦t may be thought that by providing severaltimes a

2、s much heating surface the evaporatingwould be increased, but it is not the case.The total capacity of a multiple-effect evaporator is usually not greater than that of a single-effect evaporator having a heating surface equal to one of the effects and operating under the same terminal condition.HUA

3、GONG YUAN LI华南理工大学 化工学院 伍钦f the heating load and the heat of dilution areneglected, the capacity of an evaporator isdirectly proportional to the rate of heat transfer.333322221111tAUqtAUqtAUq【16-13】The total capacity is proportional to the total rate of heart transfer qTHUA GONG YUAN L

4、I华南理工大学 化工学院 伍钦ssume that the surface area is A in each effect and that the overall coefficient U is alsothe same in each effect. thentUAtttUAqT)(321【16-14】333222111tAUtAUtAUqT【16-15】t is the total temperature drop between the steam in the first effect and the vapor in the last effect.

5、HUA GONG YUAN LI华南理工大学 化工学院 伍钦uppose now that a single-effect evaporatorwith a surface area A is operating with the sametotal temperature drop.tUAqTIf the overall coefficient is the same as in each effect of the triple-effect evaporator.For the single effectHUA GONG YUAN LI华南理工大学 化工学院

6、伍钦his is exactly the same equation as that for the multiple-effect evaporatorThe boiling-point elevation tends to make the capacity of the multiple-effect evaporator less than that of the corresponding single effect.Offsetting this are the changes in overall coefficients in a multiple-

7、effect evaporator.HUA GONG YUAN LI华南理工大学 化工学院 伍钦he average coefficient for the multiple-effect evaporator would be greater than that for thesingle-effect.HUA GONG YUAN LI华南理工大学 化工学院 伍钦ffect of liquid head and boiling-point elevationThe liquid head and the boiling-point el

8、evationinfluence the capacity of a multiple-effectevaporator even more than they do that of asingle effectHUA GONG YUAN LI华南理工大学 化工学院 伍钦he reduction in capacity caused by the liquid head, as before, cannot be estimated quantitatively. The liquid head reduces the temperature drop availa

9、ble in each effect of a multiple-effect of a multiple-effect evaporator.HUA GONG YUAN LI华南理工大学 化工学院 伍钦he temperature drop in any effect is calculatedfrom the temperature of saturated steam at thepressure of the steam chest, and not from thetemperature of the boiling liquid in the previ

10、ouseffect.This means that the boiling-point elevation in any effect is lost from the total available temperature drop.HUA GONG YUAN LI华南理工大学 化工学院 伍钦his loss occurs in every effect of a multiple-effect evaporator, and the resulting loss of capacity.HUA GONG YUAN LI华南理工大学 化工学院 伍钦 020-871

11、14377Consider the single-effect evaporator. Of the total temperature drop of 181, the shaded part represents the loss in temperature drop 105The actual driving force for heat transfer is represented by the unshaded part.105temperature100176281HUA GONG YUAN LI华南理工大学 化工学院 伍钦he diagram fo

12、r the double-effect evaporator shows two shaded portions becausethere is a boiling-pointelevation in the two effect.The residual unshaded part, 85, is smaller than in the diagram for the single effect.105temperature1001762815010028117635226246HUA GONG YUAN LI华南理工大学 化工学院 伍钦n the triple-

13、effectevaporator there are shaded portionssince there is a losstemperature drop ineach of three effects,and the total netavailable temperaturedrop ,79105temperature1001762815010028117635226246HUA GONG YUAN LI华南理工大学 化工学院 伍钦he economy of a multiple-effect evaporatoris not influenced by b

14、oiling-point elevationsif minor factors, such as the temperature ofthe feed and changes in heats of evaporization,are neglected. Then by Eqs. 【16-16】Substitution from Eq. 【16-2】into Eq. 【16-8】gives vvfpffWttcmD)(【16-16】HUA GONG YUAN LI华南理工大学 化工学院 伍钦 kilogram of steam condensing in the

15、firsteffect generates about a kilogram of vapor, which condenses in the second effect,generating another kilogram there, and so on.HUA GONG YUAN LI华南理工大学 化工学院 伍钦The economy of a multiple-effect, evaporator depending on heat-balance considerations and not on the rate heat transfer.lThe

16、capacity, is reduced by the boiling- point elevationHUA GONG YUAN LI华南理工大学 化工学院 伍钦ptimum number of effectsThe cost of each effect of an evaporator per square meter of surface is a function of itstotal area.The investment required for an N-effect evaporator is about N times that for a sing