ISO 12241-1998 Thermal insulation for building equipment.pdf

STD-IS0 1229l-ENGL 1998 9 Li851703 0739390 5 7 1 9

INTERNATIONAL STANDARD

IS0 12241 First edition 1998-03-01

Thermal insulation for building equipment and industrial installations - Calculation rules Isolation thermique des équipements du bâtiment et des installations industrielles - Méthodes de calcul

This niaterial is reproduced from I S 0 documenu under International Organization for Standardization (ISO) Copyright License Number IHS/ICC/I 996. Not for resale. No part of these I S 0 documents may be reproduced in any forni, electronic retrieval system or otherwise, except as allowed in the copyright law of the country of use, or with the prior written consent of I S 0 (Case postale 56, 12 I I Geneva 20, Switzerland, Fax + 4 1 2 2 734 IO 79), IHS or the I S 0 Licensor's menibers.

Reference number

I S 0 12241:1998(E)

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STD.ISO 12241-ENGL 1998

4 8 5 3 7 0 3 0739391 408

I S 0 12241:1998(E)

Contents Page

1

2 ~

3

...................................................................................................................... Normative references ............................................................................................. Definitions. symbols and abbreviations ...............................................................

Scope

Calculation methods for heat transfer ..................................................................

3

Fundamental equations for heat transfer............................................................... Surface temperature ............................................................................................. Prevention of surface condensation ......................................................................

3 16 19

Calculation of the temperature change in pipes. vessels and containers .........

19

Longitudinal temperature change in a pipe ............................................................ Temperature change and cooling times in pipes. vessels and containers .............

19 20

5.1 5.2

6

1 2 3

4.1 4.2 4.3

5

1

Physical quantities. symbols and units .................................................................. Subscripts .............................................................................................................

3.1 3.2

4

1

Calculation of cooling and freezing times of stationary liquids ......................... 6.1

21

Calculation of the cooling time for a given thickness of insulation to prevent the freezing of water in a pipe .................................................................. Calculation of the freezing time of water in a pipe .................................................

21 22

7

Thermal bridges ......................................................................................................

22

8

Underground pipelines...........................................................................................

23

6.2

Calculation of heat loss (single line) ......................................................................

23

Tables and Diagrams..............................................................................................

26

8.1

9

O I S 0 1998 All rights reserved. Unless otherwise specified. no part of this publication may be reproduced or utilized in any form or by any means. electronic or mechanical. including photocopying and microfilm. without permission in writing from the publisher. InternationalOrganization for Standardization Case postale 56 CH-1211 Genève 20 Switzerland Internet [email protected] X.400 c=ch; a=400net; p=iso; o=isocs; s=central Printed in Switzerland

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STDSISO 122'4l-ENGL 1998 W qB5l703 0739392 3 4 4 IS0 12241:1998(E)

o IS0

Annex A: (informative) Comments on thermal conductivity

30

Annex 6:

......................................... (informative) Examples ....................................................................................

32

Calculation of the necessary insulation thicknesses for a double layered wall of a firebox ................................................................................................................................. Heat flow rate and surface temperature of an insulated pipe............................................... Temperature drop in a pipe ................................................................................................. Temperature drop in a container ......................................................................................... Cooling and freezing times in a pipe ................................................................................... Underground pipeline .......................................................................................................... Required insulation thickness to prevent surface condensation ..........................................

32 33 34 35 36 37 38

B.l

B.2 6.3 B.4 8.5 6.6 B.7

Annex C:

(informative) Bibliography

..............................................................................

39

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Foreword I S 0 (the International Organization for Standardization) is a worldwide federation of national standards bodies (IS0 member bodies). The work of preparing International Standards is normally carried out through I S 0 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. IS0 collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

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. International Standard IS0 12241 was prepared by Technical Committee ISOTTC 163, Thermal insulation, Subcommittee SC 2 , Calculation methods. Annexes A to C of this International Standard are for information only.

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IS0 12241:1998(E)

Introduction Methods relating to conduction are direct mathematical derivations from Fourier’s Law of Heat Conduction, so international consensus is purely a matter of mathematical verification. No significant difference in the equations used in the member countries exists. For convection and radiation, however, there are no methods in practical use which 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, these methods can be applied to most types of industrial thermal insulation heat transfer problems. These methods do not take into account the permeation of air or the transmittance of thermal radiation through transparent media. 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 will depend on the accuracy of the input variables. This tnternational Standard contains no guidelines for accurate measurement of any of the variables. However, it does contain guides which have proven satisfactory for estimating some of the variables for many industrial thermal systems. It should be noted that the steady-state calculations are dependent on boundary conditions. Often a solution at one set of boundary conditions is not sufficient to characterize a thermal system which will operate in a changing thermal environment (process equipment operating year-round, outdoors, for example). In such cases local weather data based on yearly averages or yearly extremes of the weather variables (depending on the nature of the particular calculation) should be used for the calculations in this International Standard. 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. This condition suggests the need for numerous calculations to properly model thermal characteristics of any one surface. Furthermore, the heat flow through a surface at any point is a function of several variables which 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 (often including a great variety of interest). For calculation of dew formation, variability of the local humidity is an important factor. 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.

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Clauses 4 and 5 of this International Standard give the methods used for industrial thermal insulation calculations not covered by more specific standards. In applications where precise values of heat energy conservation or (insulated) surface temperature need not be assured, or where critical temperatures for dew formation are either not approached or not a factor, these methods can be used to calculate heat flow rates. 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.

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INTERNATIONAL STANDARD

0 IS0

IS0 12241:1998(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, assuming onedimensional heat flow only.

2

Normative references

The following standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standards are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and I S 0 maintain registers of currently valid International Standards. I S 0 7345:1987, Thermal insulation - Physical quantities and definitions

IS0 9346:1987, Thermal insulation - Mass transfer - Physical quantities and definitions NOTE - For further publications, see annex C.

3

Definitions, symbols and abbreviations

For the purposes of this International Standard, the definitions given in IS0 7345 and I S 0 9346 apply.

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3.1

Physical quantities, symbols and units

Physical quantities

Symbol

Unit

heat flow rate

W

density of heat flow rate linear density of heat flow rate thermodynamic temperature

W/m2 Wlm K

Celsius temperature

"C

temperature difference

K

thermal conductivity

W/(m-K)

design thermal conductivity

W/( m. K)

surface coefficient of heat transfer thermal resistance linear thermal resistance linear thermal surface resistance surface resistance of heat transfer thermal resistance for hollow sphere thermal transmittance for hollow sphere thermal transmittance linear thermal transmittance specific heat capacity at constant pressure thickness diameter temperature factor radiation coefficient

W/(m2-K) m2.W m-WW m - W m2.W KNV W/K W/( m2-K) W/(m.K) kJ/(kg-K) m m K3 W/( m2.K4)

emissivity Stefan Boltzmann constant (see reference [9]) height length

W/(m2.K4) m m

thickness parameter (see 4.2)

m

perimeter area volume velocity time mass

m m2 m3 m/s S

mass flow rate

kg kgfh

density

kgfm3

specific enthalpy; latent heat of freezing

kJfkg

relative humidity

%

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S T D - I S 0 12241-ENGL 1998 0

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3.2

I S 0 12241 :1998(E)

Subscripts

ambient average cooling convection design, duct, dewpoint exterior,external effective final medium freezing interior, internal initial medium laboratory linear Pipe radiation reference surface exterior surface interior surface spherical soil total vessel water wall

a av C

cv d e ef fm fr I

im lab

I P r ref S

se si SPh E T V W

w

4

Calculation methods for heat transfer

4.1

Fundamental equations for heat transfer

The formulae given in this clause apply only to the case of heat transfer in the steady-state, ¡.e. to the case where temperatures remain constant in time at any point of the medium considered. Generally the thermal conductivity design value is temperature dependent (see figure 1, dashed line). For further purposes of this International Standard, the design value for the mean temperature for each layer shall be used. NOTE -This may imply iterative calculation.

4.1.1

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.

The density of heat flow rate g for a plane wall in the x-direction is given by:

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STD-IS0 12241-ENGL 1778 I S 0 12241:1998(E)

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For a single layer

n (esi-ese)

9 = -. d

w/m?

or

where

il d

eSi O, R

is the thermal conductivity of the material, in W/(m'K); is the thickness of the plane wall, in m; is the temperature of the internal surface, in OC; is the temperature of the external surface, in OC; is the thermal resistance of the wail in (rn2.K)NV.

4

4

d

NOTE -The

straight curve shows the negligible, the dashed one the strong temperature dependence of

Figure 1: Temperature distribution in a single layer wall

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A.

IS0

For multi-layer insulation g=

Os¡ - e s ,

W/m2

R’

(3)

where R‘ is the thermal resistance of the multi-layer wall

di . A’ J=1 I

R’=x-

rn2. K/W

(4)

NOTE - The prime denotes a multi-layer quantity.

c

Figure 2: Temperature distribution in a multi-layer wall

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The linear density of heat flow rate 91 of a single layer hollow cylinder:

where ßl is the linear thermal resistance of a single layer hollow cylinder:

Deis the exterior diameter of the layer, in m; Di is the interior diameter of the layer, in m.

Figure 3: Temperature distribution in a single layer hollow cylinder

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STD-ISO LZZWENGL @

L

m

rn

4a53703

cmLioz n u m I S 0 12241:1998( E)

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For multi-layer holtow cylinder:

where

with DO= Di and D,

I

De

'-2 n-1

Figure 4: Temperature distribution in a multi-layer hollow cylinder

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STD.IS0 122VL-ENGL 1998

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I S 0 12241:1998(E)

The heat flow rate of a single layer hollow sphere is

where ßsph is the thermal resistance of a single layer hollow sphere in W .

0,is the outer diameter of the layer, in m; Di is the inner diameter of the layer, in m.

Figure 5: Temperature distribution in a single layer hollow sphere

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STD-ISO 122'41-ENGL 1 9 9 8 W 4851703 07374114 77b W 0

I S 0 12241:1998(E)

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The heat flow rate of a multi layer hollow sphere is

where

with Do = Q and 0, = De

Figure 6: Temperature distribution in a multi-layer hollow sphere

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The heat flow rate through the wall of a duct with rectangular cross section is given by

The linear thermal resistance of the wall of such a duct can be approximately calculated by

where

Pi is the inner perimeter of the duct, in m; P, is the external perimeter of the duct, in m; d fe=

is the thickness of the insulating layer, in m.

P, + ( 8 - 4

I

Figure 7: Temperature distribution in a wall of a duct with rectangular cross section

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4.1.2

Surface coefficient of heat transfer

In general the surface coefficient of heat transfer is given by:

where

h, hr

is the radiative part of the surface coefficient of heat transfer; is dependent on the temperature and the degree of emissivity of the surface.

NOTE - The emissivity is defined as the ratio between the radiation coefficient of the surface and the black body radiation constant (see IS0 9288).

h,, h,,

is the convective part of the surface coefficient of heat transfer. is in general dependent on a variety of factors such as air movement, temperature, the relative orientation of the surface, the material of the surface and other factors.

4.1.2.1 Radiative part of surface coefficient hr hr

is given by:

A2 1

hr= a,- C, W m .K

a, is the temperature factor. It is given by:

and can be approximated up to a temperature difference of 200 K by ar = 4.(Ta,)3

K3

(17 4

where

Ta, is 0,5 X ( surface temperature + ambient or surface temperature of a radiating surface in the neighbourhood), in K; C, is the radiation coefficient, in W/(m2.K4). Cr is given by

c,=I . o o = 5,67 .

W/(mz.K4)

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IS0 12241 :1998( E)

4.1.2.2 Convective part of surface coefficient h,, For convection a distinction has to be made between surface coefficient inside buildings and in open air. For pipes and containers there is a difference as well between internal surface coefficient hi and the external surface coefficient hse.

a) Inside buildings: In the interior of buildings hcvcan be calculated for plane vertical walls and vertical pipes for laminar free convection ( H 3 . A 6 5 10 m3.K) by h,,

E l(m2.K)

=1,32.4-W

where

A8=18,, -Bal, in K;

e,,

is the surface temperature of the wall, in O C ;

6,

is the temperature of the ambient air inside the building, in "C;

H

is height of the wall or diameter of a pipe, in m.

For vertical plane walls, vertical pipes and in approximation for large spheres inside buildings the convective part hcvforturbulent free convection ( H 3 . A 6> 1O m3.K) is given by:

For horizontal pipes inside buildings hcyis given by

- laminar airflow (L),3.de 5 10 m3.K)

- turbulent airflow (L)e3.de > 10 m3-K) h,, = 1 , 2 1 . m

W/(m'.K)

b) Outside buildings: For vertical plane walls outside of buildings and in approximation for large spheres the convective part h,, of the surface coefficient is given by: laminar airflow ( v H I8 m2/s):

dH 1

h, = 3,96. -W (m2*K)

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IS0 12241:1998(E)

- turbulent air flow (v.H > 8 m2/s):

iH

h,, = 5,76. -W/(m2 . K )

(18f)

For horizontal and vertical pipes which are outside buildings the following equation applies:

- laminar airflow

(V.

-3

2

De I 8 , 5 5 x 10 m /s) :

8,l x 1O-’

hC” =

De

- turbulent airflow ( v hcv =8,9.-

”

a

De > 8,55 x 10” m‘/s) :

03

Dey

’

W/( m2-K)

where 0, is the external insulation diameter, in m;

v

is the wind velocity, in m/s.

NOTE - For calculation of surface temperature, formulas (18a) to (18d) should be used for wall and pipe instead of formulas (18e) to (18h) when the presence of wind is not established.

Table 1 gives a selection of appropriate equations to be used for calculation of h,, Table 1 -Selection of h,,

All the equations for the convective part of the outer thermal surface coefficient inside buildings apply for the heat transfer between surfaces and air at temperature differences A T < 100 K.

4.1.2.3 Approximation for the calculation of hse For approximate calculations the following equations for the outer surface coefficient h,, can be used inside buildings. For horizontal pipes

h,, = CA+ 0,05 . AO W/(m2.K)

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For vertical pipes and walls hse =

C,

+ 0,09 . AO

W/(m2.K)

(20)

using the coefficients in table 2. Equations 19 and 20 can be used for horizontal pipes in range of 0, = 0,25 m to 1,O m and for vertical pipes for all diameters. Table 2 - Coefficients CAand C, for approximate calculation of total exterior thermal surface coefficient

I

I Surface

aluminium, bright rolled aluminium, oxidized galvanized sheet metal, blank galvanized sheet metal, dusty austenitic steel aluminium - zinc sheet nonmetallic surfaces

E

0,05

0,13 0,26 0,44 0,15

0,18 0,94

c, x

W/( m2.K4) 0,28 0,74 1,47 2,49 0,85 1,o2 5,33

For cylindrical ducts with a diameter less than 0,25 m the convective part of the external surface coefficient can be calculated in good approximation by equation (18 c). For larger diameters ¡.e. 0e>0,25 m the equation for plane walls (18 a) can be applied. The respective accuracy is 5 o/o for diameters De>0,4m and 10% for diameters 0,25