Roxul Industrial Insulation Process - Manual (Page 126)

2.3.2 Heat transfer
(European basis and terms)
Terms
Heat quantity Q
The heat quantity is the thermal energy that is
supplied to or dissipates from a body. The unit
used to designate the heat quantity is J.
Heat flow Q`
The heat flow Q` is the heat quantity flowing in a
body or being transferred between two bodies per
time unit. The unit used to designate the heat flow
is W (1W = 1J/s).
Heat flow density q
The heat flow density q is the heat flow being
applied to the unit of the surface that the heat is
flowing through. The unit is expressed in W/m² for
surfaces or in W/m for pipes, for example. In the
field of insulation technology, the heat flow density
refers to the surface of the insulation system.
Thermal conductivity λ
The heat-insulating effect of insulation materials
is described in terms of the thermal conductivity
λ. λ is specified in the physical unit of W(m K). It
indicates the quantity of heat “Q” that, in “t”
amount of time and at a temperature difference of
“∆T”, flows across the thickness “s” through the
surface.
λ =
⋅
⋅ ⋅
=
⋅
⋅ ⋅
=
⋅ ⋅
=
⋅
⎡
⎣
⎢
⎤
⎦
⎥
Q l
A t T
J m
m s K
J
m s K
W
m K Δ 2
The unit of the thermal conductivity is expressed
in terms of J/(m s K) or W/(m K).
Thermal conductance Λ
The coefficient of thermal conductance “Λ”
indicates, for a given layer, the heat flow density
flowing vertically between the surfaces over an
area of 1 m² at a temperature difference of 1 K.
The unit used to express the coefficients of
thermal conductance is W/(m² K).
Λ = =
s
λ Thermal conductivity
Applied insulation thickness
W
m K 2 ⋅
( )
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
Thermal resistance R
The thermal resistance “R” is the reciprocal of the
coefficients of thermal conductance. The unit used
to express the thermal resistance is (m² K)/W.
R
s
λ
Applied insulation thickness
Thermal conductivity
m K
W
2 ⋅
( )
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
R
d
d
Pipe
a
i =

 

⋅ ⋅
ln
2 π λ
m K
W
⋅
( )
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
Surface coefficient of heat transfer α
The surface coefficient of heat transfer “α” gives
the heat flow density circulating at the surface of a
body in a medium or vice versa, when the
temperature difference between the body and
the liquid or gaseous medium amounts to 1 K.
The unit used to express surface coefficients of
heat transfer is W/(m²K).
for walls
for pipe insulation
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