and a turbulator arranged generally helically around the cylinder and having edgewise adjacent fins in the form of separate tongues in which each fin is at an angle of about 100°-110° from the inner cylinder in the direction of flow of the liquid flowing through the tube, each said fin being generally planar but formed with a radial channel intermediate the radial edges of the fin, adjacent edges of adjacent fins describing a divergent V from the cylinder toward the tube thereby describing channels between adjacent fins for directing the liquid outwardly toward the tube, longitudinally adjacent fins in successive reaches of said helix being spaced apart a distance equal to 0.8-1.2 times the inner diameter of the tube, the outer tip edge of each fin being spaced from the inner surface of the tube a distance equal to about 5 to 8% of the inner diameter of the tube.