SIST EN ISO 12241:2022

SLOVENSKI STANDARD
oSIST prEN ISO 12241:2021
01-maj-2021
Toplotna izolacija za opremo stavb in industrijske inštalacije - Pravila za računanje
(ISO/DIS 12241:2021)
Thermal insulation for building equipment and industrial installations - Calculation rules
(ISO/DIS 12241:2021)
Wärmedämmung an haus- und betriebstechnischen Anlagen - Berechnungsregeln
(ISO/DIS 12241:2021)
Isolation thermique des équipements de bâtiments et des installations industrielles -
Méthodes de calcul (ISO/DIS 12241:2021)
Ta slovenski standard je istoveten z: prEN ISO 12241
ICS:
91.120.10 Toplotna izolacija stavb Thermal insulation of
buildings
91.140.01 Napeljave v stavbah na Installations in buildings in
splošno general
oSIST prEN ISO 12241:2021 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN ISO 12241:2021

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oSIST prEN ISO 12241:2021
DRAFT INTERNATIONAL STANDARD
ISO/DIS 12241
ISO/TC 163/SC 2 Secretariat: SN
Voting begins on: Voting terminates on:
2021-02-23 2021-05-18
Thermal insulation for building equipment and industrial
installations — Calculation rules
Isolation thermique des équipements de bâtiments et des installations industrielles — Règles de calcul
ICS: 91.140.01; 91.120.10
THIS DOCUMENT IS A DRAFT CIRCULATED
This document is circulated as received from the committee secretariat.
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
ISO/CEN PARALLEL PROCESSING
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 12241:2021(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
©
PROVIDE SUPPORTING DOCUMENTATION. ISO 2021

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oSIST prEN ISO 12241:2021
ISO/DIS 12241:2021(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
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ii © ISO 2021 – All rights reserved

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oSIST prEN ISO 12241:2021
ISO/DIS 12241:2021(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions . 1
3.2 Definition of symbols . 1
3.3 Subscripts . 2
4 Calculation methods for heat transfer . 3
4.1 Fundamental equations for heat transfer . 3
4.1.1 General. 3
4.1.2 Thermal conduction . 3
4.1.3 Surface coefficient of heat transfer . 9
4.1.4 External surface resistance .15
4.1.5 Thermal transmittance . .16
4.1.6 Heat flow rate .17
4.1.7 Temperatures of the layer boundaries .18
4.2 Determination of the influence of thermal bridges .19
4.2.1 General.19
4.2.2 Insulation system related thermal bridges .19
4.2.3 Installation related thermal bridges .19
4.3 Determination of total heat flow rate for plane walls, pipes and spheres .20
4.4 Surface temperature .20
4.5 Prevention of surface condensation .21
5 Calculation of the temperature change in pipes, vessels, and containers .22
5.1 General .22
5.2 Longitudinal temperature change in a pipe .23
5.3 Temperature change and cooling times in pipes, vessels, and containers .23
6 Calculation of cooling and freezing times of stationary liquids .24
6.1 Calculation of the cooling time to prevent the freezing of water in a pipe .24
6.2 Calculation of the freezing time of water in a pipe .25
7 Underground pipelines.25
7.1 General .25
7.2 Calculation of heat loss (single line) without channels .26
7.2.1 Uninsulated pipe .26
7.2.2 Insulated pipe .27
7.3 Other cases .27
Annex A (informative) Thermal Bridges .28
Annex B (informative) Examples .40
Bibliography .49
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oSIST prEN ISO 12241:2021
ISO/DIS 12241:2021(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International
Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies
casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 12241 was prepared by Technical Committee ISO/TC 163, Thermal performance and energy use in
the built environment, Subcommittee SC 2, Calculation methods.
This second edition cancels and replaces the first edition (ISO 12241:1998), which has been technically
revised, including methods to determine the correction terms for thermal transmittance and linear
thermal transmittance for pipes that are added to the calculated thermal transmittance to obtain the
total thermal transmittance to calculate the total heat losses for an industrial installation.
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oSIST prEN ISO 12241:2021
ISO/DIS 12241:2021(E)

Introduction
Methods relating to conduction are direct mathematical derivations from Fourier’s law of heat
conduction, so no significant difference in the equations used in the member countries exists. For
convection and radiation, however, there are no methods in practical use that are mathematically
traceable to Newton’s law of cooling or the Stefan-Boltzman law of thermal radiation, without some
empirical element. For convection in particular, many different equations have been developed, based
on laboratory data. Different equations have become popular in different countries, and no exact means
are available to select between these equations.
Within the limitations given below, these methods can be applied to most types of industrial, thermal-
insulation, heat-transfer problems.
1. These methods do not take into account the permeation of air and the transmittance of thermal
radiation through transparent media.
2. The equations in these methods require for their solution that some system variables be known,
given, assumed or measured. In all cases, the accuracy of the results depends on the accuracy of
the input variables. This International Standard contains no guidelines for accurate measurement
of any of the variables. However, it does contain guides that have proven satisfactory for estimating
some of the variables for many industrial thermal systems.
3. When the steady-state calculations are used in a changing thermal environment (process
equipment operating year-round, outdoors, for example), it is necessary to use local weather data
based on yearly averages or yearly extremes of the weather variables (depending on the nature of
the particular calculation) for the calculations in this International Standard.
4. In particular, the user should not infer from the methods of this International Standard that either
insulation quality or avoidance of dew formation can be reliably assured based on minimal, simple
measurements and application of the basic calculation methods given here. For most industrial heat
flow surfaces, there is no isothermal state (no one, homogeneous temperature across the surface),
but rather a varying temperature profile. Furthermore, the heat flow through a surface at any point
is a function of several variables that are not directly related to insulation quality. Among others,
these variables include ambient temperature, movement of the air, roughness and emissivity of the
heat flow surface, and the radiation exchange with the surroundings (which often vary widely). For
calculation of dew formation, variability of the local humidity is an important factor.
5. Except inside buildings, the average temperature of the radiant background seldom corresponds
to the air temperature, and measurement of background temperatures, emissivities and exposure
areas is beyond the scope of this International Standard. For these reasons, neither the surface
temperature nor the temperature difference between the surface and the air can be used as a
reliable indicator of insulation performance or avoidance of dew formation.
Clauses 4 and 5 of this International Standard give the methods used for industrial thermal insulation
calculations not covered by more specific standards.
Clauses 6 and 7 of this International Standard are adaptations of the general equation for specific
applications of calculating heat flow, temperature drop, and freezing times in pipes and other vessels.
Thermal insulation to heating/cooling systems such as a boiler and refrigerator are not dealt with by
this International Standard.
Annexes A and B of this International Standard are for information only.
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oSIST prEN ISO 12241:2021
DRAFT INTERNATIONAL STANDARD ISO/DIS 12241:2021(E)
Thermal insulation for building equipment and industrial
installations — Calculation rules
1 Scope
This International Standard gives rules for the calculation of heat-transfer-related properties of
building equipment and industrial installations, predominantly under steady-state conditions. This
International Standard also gives a simplified approach for the treatment of thermal bridges.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 7345, Thermal performance of buildings and building components — Physical quantities and definitions
ISO 9346, Hygrothermal performance of buildings and building materials — Physical quantities for mass
transfer — Vocabulary
ISO 10211, Thermal bridges in building construction — Heat flows and surface temperatures — Detailed
calculations
ISO 13787, Thermal insulation products for building equipment and industrial installations —
Determination of declared thermal conductivity
ISO 23993, Thermal insulation products for building equipment and industrial installations —
Determination of design thermal conductivity
VDI 4610-2, Energy efficiency of operational installations - Thermal bridges catalogue
VDI 2055-1, Thermal insulation of heated and refrigerated operational installation – Calculation rules
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7345, ISO 9346, ISO 13787
and ISO 23993 apply.
3.2 Definition of symbols
Symbol Definition Unit
2
A area m
A solar absorption coefficient
s
a length of a rectangle m
3
a temperature factor K
r
b width of a rectangle m
C′ thickness parameter (see 4.2.2) m
2 4
C radiation coefficient W/(m ⋅K )
r
c specific heat capacity at constant pressure J/(kg⋅K)
p
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oSIST prEN ISO 12241:2021
ISO/DIS 12241:2021(E)

Symbol Definition Unit
D diameter m
d thickness m
F overall conversion factor for thermal conductivity
Gr Grashof number
H height m
2
h surface coefficient of heat transfer W/(m ⋅K)
2
J solar radiation W/m
s
K thermal bridge coefficient W/K
L length, pipe length m
l characteristic length m
m mass kg

m mass flow rate kg/s
Nu Nusselt number
P perimeter m
p pressure Pa
Pr Prandtl number
2
q density of heat flow rate W/m or W/m
2
R thermal resistance m ⋅K/W or m⋅K/W or K/W
Re Reynolds number
T thermodynamic temperature K
t time s
2
W/(m ⋅K) or W/(m⋅K) or
U thermal transmittance
W/K
w velocity of the air or other fluid m/s
-1
α coefficient of longitudinal temperature drop m
-1
α′ coefficient of cooling time s
Δh specific enthalpy; latent heat J/kg
ε emissivity
Φ heat flow rate W
λ design thermal conductivity W/(m⋅K)
λ declared thermal conductivity W/(m⋅K)
D
θ Celsius temperature °C
3
ρ density kg/m
φ relative humidity %
2 4
σ Stefan-Boltzmann constant (see Reference[8]) W/(m ⋅K )
2
v kinematic viscosity of air or other fluid m /s
Δ difference
2
ΔA equivalent area m
Δl equivalent length m
3.3 Subscripts
A valve Ka insulation box
a ambient, anchore l linear
av average lab laboratory
B thermal bridge MRT mean radiant temperature
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oSIST prEN ISO 12241:2021
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A valve Ka insulation box
c cooling P pump
cv convection p pipe
cs cross section r radiation
d duct, dew point ref reference
E soil s surface
e exterior, external se exterior surface
ef effective si interior surface
en entrance sph spherical
ex exit sq per square
f fluid T total
fa frontal of the fin tw start freezing
fi final V vertical
fl flange v vessel, cooling
fr freezing W wall
H horizontal w water
i interior, internal wp start freezing
in initial
4 Calculation methods for heat transfer
4.1 Fundamental equations for heat transfer
4.1.1 General
The equations given in Clause 4 apply only to the case of heat transfer in steady state, i.e. to the case
where temperatures remain constant in time at any point of the medium considered. The design thermal
conductivity is temperature-dependent; see Figure 1, dashed line. However, in this International
Standard, the design value for the mean temperature for each layer shall be used.
4.1.2 Thermal conduction
Thermal conduction normally describes molecular heat transfer in solids, liquids, and gases under the
effect of a temperature gradient.
It is assumed in the calculation that a temperature gradient exists in one direction only and that the
temperature is constant in planes perpendicular to it.
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oSIST prEN ISO 12241:2021
ISO/DIS 12241:2021(E)

The density of heat flow rate, q, for a plane wall in the x-direction is given by Equation (1):
dθ
q=−λ⋅ (1)
dx
For a single layer, Equations (2), (3) and (4) hold:
λ
q=− ⋅−()θθ (2)
si se
d
or
θθ−
 
si se
q= (3)
 
R
 
Where
d
R= (4)
λ
where
λ is the design thermal conductivity of the insulation product or system, expressed in W/(m·K);
d is the thickness of the plane wall, expressed in m;
θ is the temperature of the internal surface;
si
θ is the temperature of the external surface;
se
2
R is the thermal resistance of the wall, expressed in m ·K/W.

Figure 1 — Left: Temperature distribution in a single-layer wall. Right: Thermal conductivity as
function of the temperature (dashed curve) and non-temperature-depending (solid line).
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oSIST prEN ISO 12241:2021
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NOTE The dashed curve in Figure 1, left, represents the temperature variation in a wall, considering that the
thermal conductivity depends on the temperature, this curve corresponds to the dashed curve on the right. In
case that the thermal conductivity is considered as temperature-independent, the variation of the temperature
inside a wall is represented by the solid line on the left; it corresponds to the solid line in the figure on the right,
which shows no change in the thermal conductivity over the temperature.
For multi-layer wall (see Figure 2), q is calculated according to Equation (3), where R is the thermal
resistance of the multi-layer wall, as given in Equation (5):
n
d
j
R= (5)
∑
λ
j
j=1
Figure 2 — Temperature distribution in a multi-layer wall
The linear density of heat flow rate, q , of a single-layer hollow cylinder (see Figure 3) is given in
l
Equation (6):
θθ−
si se
q = (6)
l
R
l
where R is the linear thermal resistance of a single-layer hollow cylinder, as given in Equation (7):
l
D
e
ln
D
i
R = (7)
l
2⋅⋅π λ
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oSIST prEN ISO 12241:2021
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Figure 3 — Temperature distribution in a single-layer hollow cylinder
For multi-layer hollow cylinder (see Figure 4), the linear density of heat flow rate, q, is given in
l
Equation (6), where R is given by Equation (8)
l
n
D
 
1 1 ej,
R =  ln  (8)
∑
l
 
2⋅π λ D
j ij,
j=1 
where
D = D
i,1 i
D = D
e,n e

Figure 4 — Temperature distribution in a multi-layer hollow cylinder
NOTE For curved surfaces with a diameter larger than 1 200 mm, it is recommended to use formulas for
wall surface.
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oSIST prEN ISO 12241:2021
ISO/DIS 12241:2021(E)

The heat flow rate of a sphere, Φ , of a single-layer hollow sphere (see Figure 5) is given by Equation (9):
sph
θθ−
si se
Φ = (9)
sph
R
sph
where R is the thermal resistance of a single-layer hollow sphere, as given in Equation (10):
sph
 
1 11
R = − (10)
 
sph
2⋅⋅π λ DD
 
ie
where
D is the outer diameter of the layer, expressed in m;
e
D is the inner diameter of the layer, expressed in m.
i

Figure 5 — Temperature distribution in a single-layer hollow sphere
For multi-layer hollow sphere (see Figure 6), the heat flow rate of a sphere, Φ , is given in Equation (9),
sph
where R is given by Equation (11)
sph
n
 
1 1 11
R = ⋅⋅ −  (11)
∑
sph
 
2⋅π λ DD
jj−1 j
j=1  
where:
D = D
0 i
D = D
n e

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oSIST prEN ISO 12241:2021
ISO/DIS 12241:2021(E)

Figure 6 — Temperature distribution in a multi-layer hollow sphere
NOTE For hollow sphere heat flow rate is given, not density of heat flow
The linear density of heat flow rate, q , through the wall of a duct with rectangular cross-section (see
d
Figure 7) is given by Equation (12):
θθ−
si se
q = (12)
d
R
d
The linear thermal resistance, R , of the wall of such a duct can be approximately calculated by
d
Equation (13):
2⋅d
R = (13)
d
λ⋅+()PP
ei
where
d is the thickness of the insulating layer, expressed in m;
P is the inner perimeter of the duct, expressed in m;
i
P is the external perimeter of the duct, expressed in m, as given in Equation (14):
e
PP=+ ()8⋅d (14)
ei
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oSIST prEN ISO 12241:2021
ISO/DIS 12241:2021(E)

Figure 7 — Temperature distribution in a wall of a duct with rectangular cross-section
at temperature-dependent thermal conductivity
4.1.3 Surface coefficient of heat transfer
In general, the radiative and convective heat transfer at the surface are given by Equations (15) and (16):
qh=⋅()θθ− (15)
rr 1 MRT
qh=⋅()θθ− (16)
cv cv 1 a
where
q is the radiative heat flow;
r
q is the convective heat flow;
cv
2
h is the radiative part of the surface coefficient of heat transfer, expressed in W/(m ·K);
r
2
h is the convective part of the surface coefficient of heat transfer, expressed in W/(m ·K);
cv
is the surface temperature of surface 1;
θ1
is the mean radiant temperature of the surrounding;
θMRT
is the ambient air temperature.
θa
NOTE 1 h is dependent on the temperature and the emissivity of the surface. Emissivity is defined as the ratio
r
between the radiation coefficient of the surface and the black body radiation constant (see ISO 9288).
NOTE 2 h is, in general, dependent on a variety of factors, such as air movement, temperature, the relative
cv
orientation of the surface, the material of the surface and other factors.
The combined surface heat transfer can be given by Equation (17):
qq=+qh=⋅()θθ−+h ⋅−()θθ (17)
rcvr 11MRTcva
When the mean radiant temperature is almost equal to the ambient air temperature, the combined heat
transfer at the surface is given by Equations (18):
qh=⋅()θθ−+hh⋅−()θθ =+()hh⋅−()θθ =⋅()θθ− (18)
ra11cv ar cv 1 asesea
where
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oSIST prEN ISO 12241:2021
ISO/DIS 12241:2021(E)

2
h is the external surface coefficient of heat transfer expressed in W/(m ·K)
se
θ is the external surface temperature expressed in Celcius
se
In 4.1.4 and 4.1.5, the coefficient h = h + h is used to calculate the external surface resistance, R
se r cv se
and thermal transmittance, U, hence the approximation, mean radiant temperature equals the ambient
temperature, is considered.
NOTE 3 When a surface receives the solar radiation (e.g outdoor pipes, tank roofs, etc), the total heat flow due
to radiant and convective heat transfer can be calculated using the following equation (19).
 AJ⋅ 
ss
qq=+qh=⋅ θθ−− (19)
 
rcvsesea
h
 
se
where,
2
J is the solar radiation, expressed in W/m ,
s
A is the absorption coefficient of solar radiation.
s
4.1.3.1 Radiative part of surface coefficient, h
r
The surface coefficient between two surfaces at different temperatures, h , is given by Equation (20):
r
ha=⋅C (20)
rr r
where
a is the temperature factor;
r
C is the radiation coefficient, as given by Equation (23).
r
The temperature factor, a , is given by Equation (21):
r
4 4
TT−
1 2
a = (21)
r
TT−
12
where
T is the absolute temperature of surface 1, expressed in K
1
T is the absolute temperature of surface 2, expressed in K
2
Equation (21) can be approximated as follows.
3
TT+
 
12
3
a ≈⋅4 =⋅4T (22)
 
rav
2
 
where T is the arithmetic mean of the temperatures T and T .
av 1 2
NOTE This approximation is only valid up to 200 K temperature difference between the component and the
surroundings.
When a component is surrounded by different surfaces at different temperatures, the temperature T
2
should be the mean radiant temperature of the surroundings.
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oSIST prEN ISO 12241:2021
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The radiation coefficient, C , is given by Equation (23):
r
C =⋅εσ (23)
r
where
−8 2 4
σ is the Stefan-Boltzmann constant [5,67×10 W/(m ·K )];
ε is the effective emissivity consisted of emissivity ε , ε and configuration factor.
1 2

Figure 8 — Radiation exchange between two surfaces: a) open system (free standing surfaces)
and b) closed system (surface 1 inside, surface 2 outside)
When the mean radiant temperature is considered, the surface 2 is assumed as black (ε =1), and εε= .
2
1
Usually, the surrounding surface consists of several surfaces, each of them has generally different
temperature, emissivity, and configuration factor. Here, we assume (approximate) that the surrounding
surface has hypothetical uniform temperature T and emissivity ε .
2 2
Table 1 gives some general values of
...