2.  What are the trade-offs in the design choice between a single-effect evaporator and a double- or triple-effect evaporator?  Answer:  Geankoplis shows on page 504 that both the single-effect (with area, A, and overall-heat-transfer coefficient, U) and a multi-effect evaporator (of three effects, where each effect has area A and overall-heat-transfer coefficient, U) will have the same overall capacity, q=UA(T1-T2), where T1-T2 is the total temperature drop (see page 504).  For the multi-effect system, each individual evaporator will have a smaller temperature drop, but the overall temperature drop will be the same for the 3-effect and for the single-effect system.  Therefore, the multi-effect system will cost more in capital costs, since you have to buy three evaporators, versus one for the single-effect system.

Where the multi-effect system is better, however, is in operating costs.  In the single-effect system, you need to buy steam to produce the large overall temperature difference.  For the multi-effect system, the only steam you pay for is the steam to  produce the small temperature difference associated with the first effect.  The second and third effects are heated for free by the steam produced in the effect before it.