When a solution evaporates in an evaporator and the secondary vapor produced is no longer utilized, this evaporation operation is called single effect evaporation. An evaporator that uses a single effect evaporation method is called a single effect evaporator. When the required production capacity is small, steam is cheap, materials are corrosive and require very expensive structural materials, or steam is contaminated and cannot be reused, a single effect evaporator should be used.

Single effect evaporators can be operated intermittently, semi intermittently, continuously in batches, or continuously. Strictly speaking, the intermittent evaporator involves a step-by-step process of feeding, evaporation, and discharge. This operation method is rarely used because it requires a sufficiently large shell to accommodate all the materials fed, and the heating element is placed quite low so that it cannot be exposed when the volume is reduced to the volume of the product. The more common operating method is semi intermittent, where the feed is continuously added to maintain a fixed liquid level until the entire charge reaches its final density. Continuous batch evaporators typically have continuous feed and continuous discharge over at least a portion of the cycle range. One method of operation is to circulate from the storage tank to the evaporator, and then return to the original position until the entire storage tank reaches the desired concentration, thus completing the processing in batches. Continuous evaporators basically feed and discharge continuously, with the concentration of feed and product remaining largely unchanged.