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There are several different evaporator designs used to concentrate black liquor, and they all contain two major parts. The first is the heating element, usually a vertically-arranged heat exchanger where the vapor or steam condenses and transfers its heat to the black liquor. The second part is the vapor body, also referred to as the vapor head or dome, which provides space for the concentrated liquor and water vapor to separate. The heating element is designed to provide as much surface area as possible for the vapor to condense and transfer heat to the liquor. The more heating surface that is available, the more the solution can be heated. Sets of tubes or parallel plates are commonly used. Black liquor and vapor or steam are both continuously fed into the evaporator, and concentrated liquor, water vapor, and condensate are all continuously discharged.