Abstract: A heat exchanger (1) has a plurality of metallic fluid flow tubes (5) secured adjacent their ends by tube sheets (6, 7). A non-corrodible, erosion resistance, non-metallic elongated sleeve-like insert (30) is telescopingly mounted within a tube. The insert length corresponds closely with the length of the finite tube end section (21) subject to damage from fluid turbulence, which has been found to be in the range of about 5 to 12 inches depending upon the tube I.D. as well as fluid velocity and pressures. To prevent undesirable penetration of corrosive materials and gases into any void that may exist between the plastic insert and metallic heat exchanger tube, a filler (33) of adhesive epoxy or the like may be applied to fill the void. The insert (30) provides only a single element reduction in passage diameter. When a tube cleaning brush (25) and basket (26) system is also utilized, brush diameter reduction relative to the tube I.D. is minimized.