A heat exchanger cap configured to conform to a human head, the cap comprising: a first element for covering one side of the head; a second element for covering the other side of the head; and an intermediate joining element joined to, and which spaces apart, the first element and second element; the three elements (60,62,64) define a single passageway for the passage of fluid through the cap; each of the elements (60,62,64) being provided with a flow interface (70,72) in the region where the elements (60,62,64) are joined; the flow interface (70,72) defining an inlet for the passage of fluid into the passageway of one of the elements (60,62,64), and an outlet for the passage of fluid from the passageway out of the same element (60,62,64); and characterised in that: each of the interface regions (70,72) of the first and second elements are provided in an abutment wall (74,76) which extends along one side of the element (60, 62) and describes an arc extending from a location corresponding to the temple and parietal regions to the nape region of their respective side of the head, the intermediate element having an abutment wall (78,80) on two sides along its length, each of the intermediate element abutment walls (78,80) configured for abutment with one of the first element or second element abutment walls (74,76), and the abutting walls are bonded together along their length; the abutment walls (74,76,78,80) and element (60,62,64) geometry being configured such that, when the abutment walls (74,76,78,80) are bonded together the elements (60,62,64) are biased towards defining a predetermined cap geometry which is resistant to distortion; and the passageway has a serpentine route in each of the elements (60,62,64), and the serpentine route is back and forth in the direction from the nominal front of the cap to the nominal back of the cap. a first element for covering one side of the head; a second element for covering the other side of the head; and an intermediate joining element joined to, and which spaces apart, the first element and second element; the three elements (60,62,64) define a single passageway for the passage of fluid through the cap; each of the elements (60,62,64) being provided with a flow interface (70,72) in the region where the elements (60,62,64) are joined; the flow interface (70,72) defining an inlet for the passage of fluid into the passageway of one of the elements (60,62,64), and an outlet for the passage of fluid from the passageway out of the same element (60,62,64); and characterised in that: each of the interface regions (70,72) of the first and second elements are provided in an abutment wall (74,76) which extends along one side of the element (60, 62) and describes an arc extending from a location corresponding to the temple and parietal regions to the nape region of their respective side of the head, the intermediate element having an abutment wall (78,80) on two sides along its length, each of the intermediate element abutment walls (78,80) configured for abutment with one of the first element or second element abutment walls (74,76), and the abutting walls are bonded together along their length; the abutment walls (74,76,78,80) and element (60,62,64) geometry being configured such that, when the abutment walls (74,76,78,80) are bonded together the elements (60,62,64) are biased towards defining a predetermined cap geometry which is resistant to distortion; and the passageway has a serpentine route in each of the elements (60,62,64), and the serpentine route is back and forth in the direction from the nominal front of the cap to the nominal back of the cap. each of the interface regions (70,72) of the first and second elements are provided in an abutment wall (74,76) which extends along one side of the element (60, 62) and describes an arc extending from a location corresponding to the temple and parietal regions to the nape region of their respective side of the head, the intermediate element having an abutment wall (78,80) on two sides along its length, each of the intermediate element abutment walls (78,80) configured for abutment with one of the first element or second element abutment walls (74,76), and the abutting walls are bonded together along their length; the abutment walls (74,76,78,80) and element (60,62,64) geometry being configured such that, when the abutment walls (74,76,78,80) are bonded together the elements (60,62,64) are biased towards defining a predetermined cap geometry which is resistant to distortion; and the passageway has a serpentine route in each of the elements (60,62,64), and the serpentine route is back and forth in the direction from the nominal front of the cap to the nominal back of the cap.