A feed containing 15% by weight of solids is to be concentrated to 40 % solids using a double effect backward feed evaporator. The first effect is heated by steam at 413 K. Vapour from the second effect is condensed at a pressure corresponding to 325 K. The heat transfer area is 10 m2 and the overall heat transfer coefficients are 3 and 2 kW/m2 in the first and second effects respectively. The boiling point rise is 3 K in the first effect and zero in the second effect. If the feed enters at 310 K what is the maximum feed rate that can be accommodated?