In heat exchanger design, the objective is to have maximum overall heat transfer coefficient for the given allowable pressure drop value and space constraints; low lifecycle cost is also the criterion in most of the case.

The value of the overall heat transfer coefficient is mainly controlled by the lower of the two heat transfer coefficients (also termed as film coefficient by some references) on the two sides of the heat transferring surface. If one of the sides of the heat transferring surface is subjected to boiling or condensing fluid, which gives an extremely high heat transfer coefficient, and other side is in contact with a single phase flow, then the heat transfer coefficient at the surface in contact with the single phase fluid will control the value of the overall heat transfer coefficient. So, the design attention must concentrate on increasing this single phase flow side heat transfer coefficient by changing the flow configuration, increasing the flow velocity, and/or using some heat transfer enhancement technique within the constraint of the allowable pressure drop.

2. Fins are never used on the surface in contact with a fluid undergoing phase change.