A device for separating droplets, which comprises - an outer casing, which has a substantially horizontal cylindrical shell and substantially vertical ends; - an inlet connection for a substance to be vapourised for leading a substance to be vapourised into the outer casing; - a plate pack functioning as an evaporator, which is 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 the plate pack and out of it; - a gravitational droplet separator, which is arranged inside the outer casing, above the plate pack; - an outlet connection for vapourised substance for leading the vapourised substance out of the outer casing from its upper part; characterised in that - the plate pack is a substantially circular cylindrical plate pack, whereby the outer diameter of the plate pack is 30 - 70 % of the inner diameter of the cylindrical shell, wherein the plate pack is in the longitudinal direction of the cylindrical shell, and - longitudinal filler unit (12a, 12b) has been arranged inside the outer casing on both sides of the plate pack, between the outer casing and the plate pack, which filler units (12a, 12b) are arranged to decrease the liquid volume of the substance to be vapourised inside the outer casing, wherein the inner surface of the cylindrical shell and the outer surface of the filler unit (12a, 12b) directed towards it are mutually substantially identical in shape and the outer surface of the plate pack and the outer surface of the filler unit (12a, 12b) directed towards it are mutually substantially identical in shape and in that a flow channel (19a, 19b) for the substance to be vapourised and for the generated vapour has been arranged between the filler unit (12a, 12b) and the cylindrical shell. - an outer casing, which has a substantially horizontal cylindrical shell and substantially vertical ends; - an inlet connection for a substance to be vapourised for leading a substance to be vapourised into the outer casing; - a plate pack functioning as an evaporator, which is 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 the plate pack and out of it; - a gravitational droplet separator, which is arranged inside the outer casing, above the plate pack; - an outlet connection for vapourised substance for leading the vapourised substance out of the outer casing from its upper part; - the plate pack is a substantially circular cylindrical plate pack, whereby the outer diameter of the plate pack is 30 - 70 % of the inner diameter of the cylindrical shell, wherein the plate pack is in the longitudinal direction of the cylindrical shell, and - longitudinal filler unit (12a, 12b) has been arranged inside the outer casing on both sides of the plate pack, between the outer casing and the plate pack, which filler units (12a, 12b) are arranged to decrease the liquid volume of the substance to be vapourised inside the outer casing, wherein the inner surface of the cylindrical shell and the outer surface of the filler unit (12a, 12b) directed towards it are mutually substantially identical in shape and the outer surface of the plate pack and the outer surface of the filler unit (12a, 12b) directed towards it are mutually substantially identical in shape and in that a flow channel (19a, 19b) for the substance to be vapourised and for the generated vapour has been arranged between the filler unit (12a, 12b) and the cylindrical shell.