Heat always moves from a warmer to a colder medium. Heat exchangers make use of this basic physical principle. They transfer thermal energy from flowing media such as water, air or gas to other media, without the media coming into contact with one another or being mixed. Depending on requirements and the application scenario, air or water can be cooled or heated in this way. In a countercurrent heat exchanger, hot and cold fluids flow past each other in opposite directions along the dividing panel.