Multiple-effect evaporators Heat recovery systems

If a multiple-effect evaporator system produces 10 pounds of fresh water per pound of saturated steam at 35 p.s.i.a. (t = 259° F.) and t0 = 70° F., the work equivalent per 1000 gallons of fresh water is 60 kw.-hr. and the energy efficiency using the differential process with 50% recovery as the standard, is 6.9%. This calculation assumes that the available heat is simply the latent heat of condensation at the constant temperature of 259° F. 

Disregarding economics, there is a maximum number of effects in a multiple-effect system which is fixed by the boiling point elevation (BPE). The number depends on the over-all temperature range, the initial salt concentration, and the per cent yield. For the special case of a 35,000-p.p.m. NaCl feed, temperature range of 100° to 25° C., and 50% recovery, the maximum is about 107. Economics fixes a much smaller number and present indications are that the optimum number of effects will be somewhere between 10 and 20. This is for the case of boiling on the heat-transfer surface and not for flash evaporation, where the optimum number of stages is probably much greater. 

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