A heat exchange element of flexible plastic film, a heat exchanger including such elements, and apparatus for manufacturing the elements. The element includes a pair of opposite film sheets bonded by welds to form an expandable bag with inside and outside heat exchange surfaces, an inlet opening for supplying a pressurized heat exchange fluid to the bag, an outlet opening for discharging the fluid from the bag after heat exchange, and an array of welds defining routes for fluid flow inside the bag. There are spot welds in a first zone of the element, parallel extended welds in a second zone of the element, defining fluid flow channels through the second zone, and oblique welds in a third zone of the element, defining channels for fluid flow towards the outlet opening. The heat exchanger includes adjacent elements for heat exchange between a pressurized first fluid.