A disposable cassette (1) is disclosed having an inlet port (4), an outlet port (5), and a heat exchanger element (3) with a fluid flow channel (9, 10, 15, 29) defined therein, the element being configured for conductive heat transfer between an exterior source and liquid in the fluid flow channel through at least one wall member of the heat exchanger element. The average cross sectional area of the outlet part of the fluid flow channel is substantially smaller than the average cross sectional area of the inlet part of the fluid flow channel. Furthermore is disclosed an assembly for connecting a source of intravenous fluid to a patient, the assembly comprising a sealed sterile packaging enclosing the disposable cassette, a first tube for connecting said source of intravenous fluid to the inlet port of the disposable cassette, and a second tube for connecting the outlet port of the disposable cassette to an intravenous catheter.
