Heat transfer, also called heat exchange, is a flow of heat energy from one object to another. This flow of energy occurs at all times
when two or bodies at different temperatures are in thermal contact. Three
methods of heat transfer can be distinguished: conduction, convection, and radiation. In conduction,
heat is transferred from molecule to molecule within a body, as in an iron
rod stuck in a fire; in convection, heat is transferred by circulation of
a fluid, as in boiling; in radiation, heat
is transferred in the form of electromagnetic waves, as in sunlight.
|The three ways in which heat moves all take place
when a pan is heated (A) – conduction through the metal walls
of the pan (1), convection by fluid motion (2), and radiation from
the heat source to the pan (3). In theory an insulated good conductor
with ice at one end and boiling water at the other varies in temperature
linearly with distance along the bar (B), as in the straight-line
graph. With poor insulation a curve like the dotted line results.
A vacuum flask (C) has a vacuum (4) to prevent conduction and convection
and silvered walls (5) to minimize heat loss by radiation.
Heat exchangers are essential in many industrial processes
where heat may be extracted from one source for use elsewhere without the
two sources combining. The most simple heat exchangers use conduction to
achieve this. For example, a hot fluid is run through tunes running parallel
to one another (and with a high surface area). This arrangement of tubes
is immersed or built around a container, into, and through, which another,
cold fluid flows. By conduction the heat is transferred from the hot fluid
to the cold.