One aspect of the present invention relates to a method for increasing the temperature of a substance which is initially in an at least partly solidified state in a container, where at least one heat exchanger is arranged in the container. One object is to obtain that the temperature of a substance may be changed relatively fast. This is obtained by having pumping means for displacing the substance, exchanging heat between a heat exchanger and the substance, displacing substance with the pumping means for increased heat exchange between the heat exchanger and the substance, as well as stirring the substance with the pumping means by displacing the substance inside the container. When the substance is displaced, then not only stagnant substance is in contact with the heat exchanger for heat exchange. The amount of substance in contact with the heat exchanger is thereby greatly increased, and the heat transfer is less dependent on thermal conductivity of the substance.