. "The surface area of any pipe, tube, channel, conduit, passage or other like structure for irrigant and/or wound exudate flow in contact with the heater or any pipe, tube, channel, conduit, passage or other like structure for a heat exchanger fluid (including those in a heated metal sinter) may suitably be no less than 100 mm2, such as 625 to 2500 mm2, e.g. up to 6400 mm2." . . . . . .