Patent ID: 7240723

Claim:
A heat exchanger comprising a plurality of tubes extending in parallel relation to one another and defining a tube axis, each of said tubes comprising: a pair of open ends, a tube wall extending between the ends and defining a hollow interior, a portion having an enlarged cross-sectional area and a portion having a relatively smaller cross-sectional area, both the enlarged portion and the smaller portion extending parallel to the tube axis; the enlarged portion of each of the tubes having a cross-sectional shape comprising a plurality of corners and a plurality of side surfaces extending between the corners, the side surfaces being generally parallel to the tube axis; the tubes being arranged as a tube bundle in which a first plurality of said tubes comprise inner tubes and a second plurality of said tubes comprise outer tubes, the outer tubes being located on a periphery of the tube bundle, wherein the enlarged portion of each of the tubes abuts the enlarged portion of at least one other tube, said enlarged portions being in abutment with one another along their side surfaces, with sealed connections being provided between abutting pairs of said side surfaces to prevent axial flow of a fluid between the abutting side surfaces, and with interstitial spaces being formed between the smaller portions of adjacent tubes; the enlarged portion of each of the inner tubes abutting the enlarged portions of adjacent tubes along all of its side surfaces; at least one side surface of the enlarged portion of each outer tube facing generally radially outwardly and not being connected to the side surface of the enlarged portion of an adjacent tube, said radially outwardly facing surfaces defining said periphery of the tube bundle; wherein the enlarged portions of at least some of the tubes are provided with indentations, the indentations forming voids between the abutting enlarged portions of adjacent tubes.