Heat Transfer
HEAT TRANSFER TERMINOLOGY
Heat Flux
The rate at which heat is transferred is represented by the symbol
. Common units for heat
Q
transfer rate is Btu/hr. Sometimes it is important to determine the heat transfer rate per unit area,
or heat flux, which has the symbol
. Units for heat flux are Btu/hr-ft2. The heat flux can be
Q
determined by dividing the heat transfer rate by the area through which the heat is being
transferred.
(2-1)
Q
Q
A
where:
= heat flux (Btu/hr-ft2)
Q
= heat transfer rate (Btu/hr)
Q
A
= area (ft2)
Thermal Conductivity
The heat transfer characteristics of a solid material are measured by a property called the thermal
conductivity (k) measured in Btu/hr-ft-oF. It is a measure of a substance’s ability to transfer heat
through a solid by conduction. The thermal conductivity of most liquids and solids varies with
temperature. For vapors, it depends upon pressure.
Log Mean Temperature Difference
In heat exchanger applications, the inlet and outlet temperatures are commonly specified based
on the fluid in the tubes. The temperature change that takes place across the heat exchanger from
the entrance to the exit is not linear. A precise temperature change between two fluids across
the heat exchanger is best represented by the log mean temperature difference (LMTD or DTlm),
defined in Equation 2-2.
(2-2)
DT1m
(DT2
DT1)
ln(DT2/DT1)
where:
DT2 =
the larger temperature difference between the two fluid streams at either
the entrance or the exit to the heat exchanger
DT1 =
the smaller temperature difference between the two fluid streams at either
the entrance or the exit to the heat exchanger
Rev. 0
Page 3
HT-02
