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Heat Transfer (continued)
the fluids move through the heat exchangers, unless there is a
phase change, the fluids are continuously changing in tem-
perature, and the temperature gradient from one stream to the
other may be continuously varying. To determine the amount
of heat exchange surface needed for a given process, the de-
signer must evaluate the effective temperature gradient for the
particular conditions and particular heat exchanger design.
Often the heating or cooling of an object is desired. In this case,
the object does not remain at a constant temperature, and such
a process is an unsteady, transient, or time-varying system. The
heating or cooling of food in ovens and refrigerators, respec-
tively; the heating of steel billets in metallurgical furnaces; the
heating of bricks in a kiln; and the calcination of gypsum are all
examples of transient processes.
