Plain tubes
The most common arrangement is for one fluid to flow within straight or curved tubes of circular cross section, while a second fluid washes the outside of these tubes in
a longitudinal, perpendicular, or oblique direction.
The interface is thus represented by a tube wall, and contact is indirect. The tubes may, of course, be noncircular in cross section; often they are bent into hairpin shapes and sometimes they take the form of helical coils.
Finned tubes
Sometimes, when heat transfer is affected more easily on the inside of the tubes than on the outside, the latter surface is extended by the provision of fins, these excrescences may be integral with the tube wall, or they may be soldered, brazed, or welded to it; they may comprise annular disks, helical tapes, or plane sheets aligned with the tube axis.
The presence of the fins entails that, per unit volume of exchanger, there is more interface area between metal and fluid for one stream than for the other. Occasionally it is the inside of the tube that is provided with such fins; this is appropriate when it is the internal heat transfer coefficient that is the lower