An evaporator for vaporizing a substance into its gaseous form, which comprises at least - an outer casing, which comprises a cylindrical shell, which is longitudinal in the horizontal direction, and vertical first and second end plate (3a, 3b), - an inlet connection for a substance to be vaporized for leading a substance to be vaporized into the outer casing, - a plate pack functioning as an evaporator, which is longitudinal in the longitudinal direction of the cylindrical shell and arranged inside the outer casing, in its lower part, - an inlet connection and an outlet connection for a heating substance for leading a heating substance into and out from the plate pack, - an outlet connection for vaporized substance for leading the vaporized substance out of the outer casing from its upper part, - flow guides or plates (10a, 10b) arranged inside the outer casing, between the cylindrical shell and the plate pack, and which flow guides or plates are arranged tightly against the outer casing and the plate pack, - a circulation pipe arrangement arranged to guide a part of a substance to be vaporized from inside the outer casing back into the outer casing through the inlet connection for a substance to be vaporized, - a droplet separator, which is arranged inside the outer casing, above the plate pack, characterized in that the evaporator further comprises - at least one collection pipe (11, 11a, 11b) having openings arranged above the flow guide or plate (10a, 10b) inside the outer casing, which collection pipe (11, 11a, 11b) extends in a longitudinal direction of the horizontal cylindrical shell from the first end plate towards the second end plate, and the collection pipe (11, 11a, 11b) comprises openings in the whole length of the collection pipe, wherein the collection pipe (11, 11a, 11b) is connected to the circulation pipe arrangement and arranged to suck a substance to be vaporized from the interior of the outer casing, and at least one ejector pipe arranged into the circulation pipe arrangement. - an outer casing, which comprises a cylindrical shell, which is longitudinal in the horizontal direction, and vertical first and second end plate (3a, 3b), - an inlet connection for a substance to be vaporized for leading a substance to be vaporized into the outer casing, - a plate pack functioning as an evaporator, which is longitudinal in the longitudinal direction of the cylindrical shell and arranged inside the outer casing, in its lower part, - an inlet connection and an outlet connection for a heating substance for leading a heating substance into and out from the plate pack, - an outlet connection for vaporized substance for leading the vaporized substance out of the outer casing from its upper part, - flow guides or plates (10a, 10b) arranged inside the outer casing, between the cylindrical shell and the plate pack, and which flow guides or plates are arranged tightly against the outer casing and the plate pack, - a circulation pipe arrangement arranged to guide a part of a substance to be vaporized from inside the outer casing back into the outer casing through the inlet connection for a substance to be vaporized, - a droplet separator, which is arranged inside the outer casing, above the plate pack, - at least one collection pipe (11, 11a, 11b) having openings arranged above the flow guide or plate (10a, 10b) inside the outer casing, which collection pipe (11, 11a, 11b) extends in a longitudinal direction of the horizontal cylindrical shell from the first end plate towards the second end plate, and the collection pipe (11, 11a, 11b) comprises openings in the whole length of the collection pipe,