Tube metal is usually:
Low carbon steel
Low alloy steel
Stainless steel
Copper
Admiralty
Cupronickel
Inconel
Aluminum (in the form of alloys),
or titanium.
Tube diameters (8–15 mm) are preferred for greater area/volume density but are limited, for purposes of in-tube cleaning larger tube diameters are often required for condensers and boilers. The tubes may be either bare or have low fins on the outside. Low fin tubes are used when the fluid on the outside of the tubes has a substantially lower heat transfer coefficient than the fluid on the inside of the tubes.
shell-diameter-to-tube-length ratio is typically between 1/5 to 1/15 [ Ds/L =1/5 to 1/15]