Abstract: A method and apparatus for cleaning a heat exchanger during operation thereof. The heat exchanger includes a plurality of tubes through which a first fluid is conducted from an inlet end to an outlet end in indirect heat transfer relationship with a second fluid disposed on the outside of the tubes intermediate the inlet and outlet ends of the tubes. Further, the heat exchanger includes an inlet chamber for the first fluid communicating with the inlet ends of the tubes, and a tube sheet for supporting the inlet ends of the tubes and isolating the inlet chamber from the second fluid, the inlet ends of the tubes extending into the inlet chamber beyond the tube sheet.