The multiple-effect configuration combines two or more evaporator bodies to conserve steam, which is condensed in the first-effect heat exchange only. Water evaporated in the first-effect vapor body is condensed in the second-effect heat exchange, which provides energy for evaporation in the second-effect vapor body (and so on for additional effects). Vapor from the last effect flows to a condenser.
The last-effect vapor body is maintained at a high vacuum with a steam-jet air ejector or mechanical vacuum pump. The driving force is the pressure drop from the first to the last effect.For the same overall pressure differential and process conditions, the evaporator with more effects will require less steam. The evaporation in each effect will be approximately 0.7 to 0.9 pounds for each pound of steam condensed in the first-effect heat exchange.