521,548. Straight - tube heat - exchangers. SPANNER, E. F. Oct. 18, 1938, No. 30085. [Class 64 (iii)] A straight tube with inwardly pressed spiral grooves has at its inlet end a plain length of tube of from 4 to 6 times the internal diameter of the tube and its outlet end a plain length of from 2 to 3¢ times the internal diameter. A tube A has two or more spiral grooves G, which may increase in depth, or decrease in pitch, or which may be in short lengths or their direction of rotation may change towards the outlet end of the tube. A plain or twisted strip of metal may be situated lengthwise in the tube. When the fluid flowing in the tubes is a gas, the number and size of the tubes are chosen so that the gas velocity at entrance is not less than 60 feet per second.