Long tube vertical evaporator consists of a simple, single-pass vertical shell and tube heat exchanger. The tubes are similar diameter and fewer in number than the short tube version. Their length may be 6 m or more. Boiling occurs within the tubes and the vapor-liquid mixture may discharge into an integral or separate separator vessel equipped with a downcomer to recirculate the solution. Due to the high-pressure drop along the longer evaporator tubes, it is usual to arrange these units with a pump in the downcomer to provide forced circulation.