Roxul Industrial Insulation Process - Manual (Page 125)

2.3 Bases for thermal calculations
2.3.1 Heat Transfer – ASTM C168 and C680 (North American basis and terms)
Surface Coefficient
Surface Coefficient (surface transfer conductance)
is the ratio of the steady-state heat exchange rate
(time rate of heat flow per unit area of a particular
surface by the combined effects of radiation,
conduction and convection) between a surface and
its external surroundings (air or other fluid and
other visible surface) to the temperature difference
between the surfaces and its surroundings. The
surface coefficient includes the combined effects of
radiant, convective and conductive heat transfer, it
is defined by:
h = h
r
+ h
c
where h
r
is the component due to radiation (Radiant
Heat Transfer Conductance) and h
c
is the
component dues to convection and conduction
(Convective Heat Transfer Conductance). Units are
BTU/hr.ft
2 .°F (W/m 2 K).
Radiant Heat Transfer Conductance
The radiant component of surface transfer
conductance, defined by:
h
r
=
σε(T
s
4 — T
o
4 )
T
s
— T
o
Convective Heat Transfer Conductance (Film
Conductance)
The time rate of heat flow from a unit area of a
surface to its surroundings, induced by a unit
temperature difference between the surface and
the environment where the environment is a fluid
(liquid or gases). Convective heat transfer
conductance is the convective (and conductive)
component of surface transfer conductance, h
c
; it
does not include radiative component. Film
conductance depends on the nature of fluid motion
past the surface (laminar or turbulent). Units are
BTU/hr.ft
2 .°F (W/m 2 K).
In general, indoor or outdoor heat transfer
coefficient (or surface film coefficient) are denoted
by h
i
and h
o
respectively for the interior and exterior
surface.
Emittance, ε
The ratio of the radiant flux emitted by a specimen
to that emitted by a black body at the same
temperature under the same conditions.
Resistance, R
The thermal resistance, R, is the reciprocal of the
thermal conductance. The units of thermal
resistance are °F.ft
2 .hr/BTU (m 2 K/W).
R =
1
C
Overall resistance to thermal transfer is sum of the
thermal resistance of individual layers, it is the
reciprocal of the overall heat transfer coefficient ,
U.
R
total
=
R
total
= R
i
+ R
1
+ R
2
+ ... + R
n
+ R
o
=
1
U
=
1
U
+
1
U
i
+
1
U
1
+
1
U
n
1
U
o
+ . . . +
1
U
2
1
Heat Transfer
Resistance
inside
+
Heat Transfer
Resistance
material
+
Heat Transfer
Resistance
outside
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