Transport Processes and Separation Process Principles (Includes Unit Operations) Fourth Edition

8.5. CALCULATION METHODS FOR MULTIPLE-EFFECT EVAPORATORS

8.5A. Introduction

In evaporation of solutions in a single-effect evaporator, a major cost is the cost of the steam used to evaporate the water. A single-effect evaporator is wasteful in terms of steam costs, since the latent heat of the vapor leaving the evaporator is usually not used. To reduce this cost, multiple-effect evaporators are used, which recover the latent heat of the vapor leaving and reuse it.

A three-effect evaporator, discussed briefly in Section 8.2B, is shown in Fig. 8.2-3. In this system each effect in itself acts as a single-effect evaporator. Raw steam is used as the heating medium to the first effect, which is boiling at temperature T₁ and pressure P₁. The vapor removed ...