HEAT TRANSFER TERMINOLOGY
Heat Transfer
Convective Heat Transfer Coefficient
The convective heat transfer coefficient (h), defines, in part, the heat transfer due to convection.
The convective heat transfer coefficient is sometimes referred to as a film coefficient and
represents the thermal resistance of a relatively stagnant layer of fluid between a heat transfer
surface and the fluid medium. Common units used to measure the convective heat transfer
coefficient are Btu/hr - ft2 -oF.
Overall Heat Transfer Coefficient
In the case of combined heat transfer, it is common practice to relate the total rate of heat
transfer ( ), the overall cross-sectional area for heat transfer (Ao), and the overall temperature
Q
difference (DTo) using the overall heat transfer coefficient (Uo). The overall heat transfer
coefficient combines the heat transfer coefficient of the two heat exchanger fluids and the thermal
conductivity of the heat exchanger tubes. Uo is specific to the heat exchanger and the fluids that
are used in the heat exchanger.
(2-3)
Q
UoAoDT0
where:
=
the rate heat of transfer (Btu/hr)
Q
Uo
=
the overall heat transfer coefficient (Btu/hr - ft2 -oF)
Ao
=
the overall cross-sectional area for heat transfer (ft2)
DTo
=
the overall temperature difference (oF)
Bulk Temperature
The fluid temperature (Tb), referred to as the bulk temperature, varies according to the details of
the situation. For flow adjacent to a hot or cold surface, Tb is the temperature of the fluid that
is "far" from the surface, for instance, the center of the flow channel.
For boiling or
condensation, Tb is equal to the saturation temperature.
HT-02
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