Efficiency of multi effect evaporator

　　The purpose of using multi effect evaporation is to utilize heat energy, that is, to reduce the consumption of raw steam through the reuse of secondary steam in the evaporation process, so as to improve the economy of the evaporation device. The actual steam consumption includes the heat loss of the heat generating device.

　　With the increase of efficiency, the more steam is saved, but the more efficiency, the better. The efficiency of multi effect evaporation is limited by economic and technical factors.

　　The economic restriction means that it is not reasonable economically when the efficiency exceeds a certain value. 2. In the multi effect evaporator, with the increase of efficiency, the steam tray required for the same total evaporation capacity decreases, which reduces the operation cost. However, the more efficiency, the more equipment cost. And with the increase of efficiency, the less steam is saved. Therefore, we can't increase the efficiency without limitation, and the appropriate efficiency should make the sum of equipment cost and operation cost smaller.

　　The technical limitation is that the number of efficiency is too much, so the evaporation operation will be difficult to carry out. In general industrial production, the pressure of heating steam and the vacuum degree of condenser are limited to a certain extent. Therefore, under certain operating conditions, the theoretical total temperature difference of evaporator is a certain value. When the number of effects increases, the total effective temperature difference decreases due to the increase of the sum of the temperature difference losses of each effect, and the effective temperature difference allocated to each effect will be too small to make each effect boiling normally, so the evaporation operation will be difficult.

　　In order to ensure the normal heat transfer in evaporation operation, according to experience, the temperature difference of each effect should not be less than 5 ~ 7 ℃. Generally, 2-3 effect is used for electrolyte solution, and 4-6 effect is used for non electrolyte solution such as organic solvent.