Calculation of Multi-Effect Evaporator by Applying Enthalpy-Concentration Diagram

Abstract

Calculation of a multi-effect evaporator by applying enthalpy-concentration diagram, had been reported by several researchers, but their methods were so complicated due to many assumptions on trial, that we felt the necessity of simplifying them.Our calculation which was based on Matz's theory7) consisted of three kinds of balance equation… heat balance, material balance and heat transfer balance…and the enthalpy concentration diagram. The only assumption needed at the start of this trial was the first effect pressure p1, which might easily be derived from Fig. 5 by one or two trials. We then proceeded on with our calculation by using p1 and the next given condition: concentration, temperature and feed rate of liquid to the first effect, the concentration of product liquor, the operating conditions (pressure and temperature of heating steam, number of effect, last effect pressure) and the over-all heat transfer coefficient of each effect. Last of all we derived on trial the necessary values (pressure, temperature and concentration) for each effect, by satisfying the terminal condition that the last effect temperature coincides with the given one. Further more, we easily came to know the heating surface and the liquor temperature of each effect, as well as the steam consumption and the amount of vapor leaving the last effect and going to the condenser.It must be admitted, however, that the present method is also one of the approximate calculations, because it comprizes errors on drawing and the assumption that the heat requirement in the first effect of the n effects evaporator is 1/n of that of the single effect evaporator, having the same amount of product liquor as that of the n effects.