An expansion evaporator includes a heat exchanger and a replaceable freezer cylinder detachably received within a receiving channel of the heat exchanger, wherein the replaceable freezer cylinder has a feeding channel for raw material feeding therethrough to thermally communicate with the heat exchanger so as to form a frozen product within the feeding channel. The replaceable freezer cylinder is detached from the heat exchanger for cleaning purpose as the raw material is not contacted with the heat exchanger.