ASTM C108-46(1998)

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Designation:C 108–46(Reapproved1998)
Standard Symbols for
Heat Transmission
This standard is issued under the fixed designation C 108; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
SPECIALPRACTICES RELATING TO HEAT FLOW
q is in general the time derivative of total quantity of heat
transferred. For steady-state heat flow, q is total quantity of heat
TERMS
divided by time.
Terms Ending in “ivity”—Terms ending in “ivity” designate
k 5 thermal conductivity; heat flow rate, per unit of area, per “degree
characteristics of materials, normally independent of size or
per unit of length”
shape, sometimes called “specific properties.”
dq/dA 5 −k (dt/dL) general expression giving value at each point
Examples: Conductivity and resistivity.
k 5 (q/A)/(Dt/L) for substantially uniform and steady-state heat flow
Terms Ending in “ance”—Terms ending in “ance” designate
through a homogeneous medium of thickness L, with plane parallel
properties of a particular object, depending not only on the faces of area A, and a constant temperature difference Dt applied
to the faces
material, but also upon size and shape, sometimes called
“total quantities.”
1/k 5 thermal resistivity; reciprocal of conductivity
R 5 thermal resistance; degrees, per unit of heat flow rate, for a
Examples: Conductance and transmittance.
particular body or setup (where the area may not be known)
Terms Ending in “ion”—Terms ending in “ion” designate
time rate of the process of transfer; flux, flow rate.
R 5Dt/q for substantially uniform and steady-state heat flow,
generally
Examples: Conduction and transmission.
R 5 L/kA for substantially uniform and steady-state heat flow
Transmission—“Transmission,” “transmissivity,” “transmit-
through a homogeneous medium of thickness L with plane parallel
tance” usually refer to transfer by one or more of the
faces of area A
processes of conduction, convection, and radiation.
1/R 5 thermal conductance; reciprocal of thermal resistance (C is also
Conduction—“Conduction,” “conductivity,” “conductance”
used)
usually refer to transfer within a medium, and without bodily
1/RA 5 thermal conductance per unit of area; heat flow, rate, per unit of
area, per degree
displacementasoccurswithconvection,andwithouttransfer
RA 5 thermal resistance of unit area; degrees, per “unit of heat flow rate
at a distance as occurs with radiation.
per unit of area.” R is used for resistance for a setup with a
particular area (which may not be known) and RA for resistance of
HEAT FLOW SYMBOLS
unit area. In some British texts R is used for thermal resistance of
A 5 area
unit area, here called RA
p 5 density; pounds, kilograms, etc., per unit of volume
h 5 surface coefficient of heat transfer; heat flow rate, per unit of area,
L 5 length of path of heat flow
per degree, across a boundary surface
Q 5 total quantity of heat transferred (with subscripts for particular cases
and to distinguish for Q for volume rate)
dq/dA 5 hDt general expression giving value at each point
Q 5 volume rate; discharge by volume; fluid rate of flow by volume.
h 5 (q/A)/Dt for substantially uniform and steady-state
m m
(There also is used q with a subscript to distinguish from heat flow
heat flow, where h and Dt are constant over the area A or where
m m
rate.)
one (but not both) of them varies but may substantially be
c 5 specific he
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