流体力学与传热 ：Capacity and economy of multiple-effect

HUA GONG YUAN LI
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.
HUA GONG YUAN LI
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.
qT UAt
HUA GONG YUAN LI
This is exactly the same equation as that for the multiple-effect evaporator
The boiling-point elevation tends to make the capacity of the multiple-effect evaporator less than that of the corresponding single effect.
HUA GONG YUAN LI
Effect of liquid head and boiling-point elevation
The liquid head and the boiling-point elevation influence the capacity of a multiple-effect evaporator even more than they do that of a single effect
q1 U1A1t1 q2 U2 A2t2 q3 U3 A3t3 【16-13】
The total capacity is proportional to the total rate of heart transfer qT
HUA GONG YUAN LI
qT U1A1t1 U2 A2t2 U3 A3t3 【16-14】 Assume that the surface area is A in each effect and that the overall coefficient U is also the same in each effect. then
HUA GONG YUAN LI
Capacity and economy of multipleeffect evaporators
The increase in economy through the use of multiple-effect evaporation is obtained at the cost of reduced capacity.
Offsetting this are the changes in overall coefficients in a multiple-effect evaporator.
HUA GONG YUAN LI
The average coefficient for the multiple-effect evaporator would be greater than that for the single-effect.
Suppose now that a single-effect evaporator with a surface area A is operating with the same total temperature drop.
If the overall coefficient is the same as in each effect of the triple-effect evaporator. For the single effect
This means that the boiling-point elevation in any effect is lost from the total available temperature drop.
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HUA GONG YUAN LI
It may be thought that by providing several times as much heating surface the evaporating would be increased, but it is not the case.
qT UA(t1 t2 t3) UAt 【16-15】
Δt is the total temperature drop between the steam in the first effect and the vapor in the last effect.
HUA GONG YUAN LI
HUA GONG YUAN LI
The reduction in capacity caused by the liquid head, as before, cannot be estimated quantitatively.
The liquid head reduces the temperature drop available in each effect of a multiple-effect of a multiple-effect evaporator.
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.