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A heat
exchanger is a device built for efficient heat
transfer from one medium to another, whether the
media are separated by a solid wall so that they
never mix, or the media are in direct contact. They
are widely used in space heating, refrigeration, air
conditioning, power plants, chemical plants,
petrochemical plants, petroleum refineries, and
natural gas processing. One common example of a heat
exchanger is the radiator in a car, in which a hot
engine-cooling fluid, like antifreeze, transfers
heat to air flowing through the radiator.
Heat exchangers may
be classified according to their flow arrangement.
In parallel-flow heat exchangers, the two fluids
enter the exchanger at the same end, and travel in
parallel to one another to the other side. In
counter-flow heat exchangers the fluids enter the
exchanger from opposite ends. The counter current
design is most efficient, in that it can transfer
the most heat. See countercurrent exchange. In a
cross-flow heat exchanger, the fluids travel roughly
perpendicular to one another through the exchanger.
For efficiency, heat exchangers are designed to
maximize the surface area of the wall between the
two fluids, while minimizing resistance to fluid
flow through the exchanger. The exchanger's
performance can also be affected by the addition of
fins or corrugations in one or both directions,
which increase surface area and may channel fluid
flow or induce turbulence.
The driving temperature across the heat transfer
surface varies with position, but an appropriate
mean temperature can be defined. In most simple
systems this is the log mean temperature difference
(LMTD). Sometimes direct knowledge of the LMTD is
not available and the NTU method is used. |
