PURPOSE: To simply perform the replacement with an existing mono-barrel type natural recirculation system in low cost, by providing a partition plate for partitioning the barrel of a heating boiler basket and the barrel of an evaporation boiler of a mono-barrel type evaporative concn. apparatus in a freely detachable manner.

CONSTITUTION: A partition plate is provided between the heating boiler basket barrel and the evaporation boiler in a mono-barrel type evaporation boiler in a freely detachable manner and an external recirculation liquid is sucked from the section above the partition plate by a recirculation pump to be sent to the evaporation boiler from the part below the partition plate. When the partition plate is detached, the external circulation is not performed, and the titled apparatus can be used as a natural recirculation system.