Abstract: In a tube heat exchanger, each tube in a set has a non-round cross-section and two or more ridges extending helically around the tube's center line for keeping the center lines of the tubes at defined distances from each other, some of the tubes (14) being arranged so that the distance between the center lines of two adjacent tubes is essentially the same as or less than the sum of the radii of the circles circumscribing these tubes. Flattened cross-sections of the tubes may form a pattern of squares; or the center lines of the tubes may form a dividing pattern in the form of equilateral triangles, two adjacent tubes resting against each other at supporting points where the flattened cross-section of one tube forms a right angle with the flattened cross-section of the adjacent tube on the same level.