The vapor from the first effect becomes the heating source for the second effect. This is possible because the second effect’s boiling temperature has been lowered to 68°C (at 29 kPa), which is less than the saturated temperature of the first effect, 82°C (at 51.8 kPa). The latent heat is reused again as the vapor from the second effect becomes the heat source of the third effect, where the boiling point temperature has been lowered to 57°C (at 17.5 kPa).
Evaporators are generally rated based on their evaporation rate; the amount of water they evaporate per hour (e.g., kg/hr, ton/hr).