In the process design, we should first consider the use of single-effect or multi-effect evaporation. In order to make full use of heat energy, multi-effect evaporation is generally used in production . Since in the multi-effect evaporation, the former secondary steam is used as the latter effect heating steam, the raw steam consumption can be saved. But the more the better, the better, the effectiveness is limited by economic and technical factors.

 

Economic restrictions mean that economic efficiency is not economical when the effectiveness exceeds a certain level. In multi-effect evaporation, as the effect increases, the amount of steam required for the same total evaporation is reduced, which reduces operating costs. However, with the increase of the efficiency, the equipment cost will increase exponentially, and the amount of raw steam saved will be less and less. Therefore, it is meaningless to increase the efficiency without limitation. The most suitable effect should be the equipment cost and operation fee. The sum is the smallest.

 

Technical limitations mean that there are too many efficiencies and evaporation operations will be difficult. In general industrial production, the heating vapor pressure and the operating pressure of the condenser have certain limits, so under certain operating conditions, the theoretical total heat transfer temperature difference of the evaporator is a certain value. When the effect is increased, the total effective heat transfer temperature difference is reduced due to an increase in the sum of the respective effect temperature difference losses.

 

When the effective temperature difference assigned to each effect is too small to ensure that the operation is in a normal boiling state, the evaporation operation will not proceed.   Therefore, based on the above factors, the actual multi-effect evaporation process is not much. Multi-effect evaporation is most commonly used in practice n = 2 ~ 4 , the design uses three- effect evaporation.