Abstract: The heat exchanger is constructed with a plurality of fittings which are disposed in the flow passage. Each fitting is constructed of at least two groups of webs with the webs of each group disposed in spaced parallel relation and in angular relation to the axis of the flow passage. Also, each group of webs is disposed in crossing relation to the webs of the other group. The ratio of web width (b) to diameter (d) of the flow passage is in the range of from 0.08 to 0.5 while the ratio of web spacing (m) to the diameter (d) is in the range of from 0.38 to 0.9. The fittings permit improved heat transfer with reduced pressure losses and a relatively small total area.