Different basic flow configurations are observed in different parts of the heat exchangers, for example, the fluid may flow between closely spaced plates that effectively form the configuration of flow through a duct. Although laminar duct flows do not occur as extensively as turbulent duct flows, they do occur in a number of important situations. In the case of plate-fin heat exchangers, the fin spacing is so small and the mean velocity range is such that the flows are often laminar. Conventionally, it is usual to assume that a higher heat transfer rate is achieved with turbulent flow than with laminar flow. However, sometimes a design that involves laminar flow is more efficient from the viewpoint of energy transfer since associated pressure penalty is less.