ASTM C335 - C 335M - 23

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Designation: C335/C335M −23
Standard Test Method for
Steady-State Heat Transfer Properties of Pipe Insulation
1
This standard is issued under the ﬁxed designation C335/C335M; 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 (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope
1.1 This test method covers the measurement of the steady-
state heat transfer properties of pipe insulations. Specimen
types include rigid, ﬂexible, and loose ﬁll; homogeneous and
nonhomogeneous; isotropic and nonisotropic; circular or non-
circular cross section. Measurement of metallic reﬂective
insulation and mass insulations with metal jackets or other
elements of high axial conductance is included; however,
additional precautions must be taken and speciﬁed special
procedures must be followed.
1.2 The test apparatus for this purpose is a guarded-end or
calibrated-end pipe apparatus. The guarded-end apparatus is a
primary (or absolute) method. The guarded-end method is
comparable, but not identical to ISO 8497. The ISO method
does not use the calculation procedure in Practice C1045.
1.3 The values stated in either SI units or inch-pound units
are to be regarded separately as standard. The values stated in
each system may not be exact equivalents; therefore, each
system shall be used independently of the other. Combining
values from the two systems may result in non-conformance
with the standard.
1.4 When appropriate, or as required by speciﬁcations or
other test methods, the following thermal transfer properties
for the specimen can be calculated from the measured data (see
3.2):
1.4.1 The pipe insulation lineal thermal resistance and
conductance,
1.4.2 The pipe insulation lineal thermal transference,
1.4.3 The surface areal resistance and heat transfer
coefficient,
1.4.4 The thermal resistivity and conductivity,
1.4.5 The areal thermal resistance and conductance, and
1.4.6 The areal thermal transference.
NOTE 1—In this test method the preferred resistance, conductance, and
transference are the lineal values computed for a unit length of pipe. These
must not be confused with the corresponding areal properties computed on
a unit area basis which are more applicable to ﬂat slab geometry. If these
areal properties are computed, the area used in their computation must be
reported.
NOTE 2—Discussions of the appropriateness of these properties to
particular specimens or materials may be found in Test Method C177, Test
Method C518, and in the literature (1).
2
1.5 This test method allows for operation over a wide range
of temperatures. The upper and lower limit of the pipe surface
temperature is determined by the maximum and minimum
service temperature of the specimen or of the materials used in
constructing the apparatus. In any case, the apparatus must be
operated such that the temperature difference between the
exposed surface and the ambient is sufficiently large enough to
provide the precision of measurement desired. Normally the
apparatus is operated in closely controlled still air ambient
from 15 to 30°C, but other temperatures, other gases, and other
velocities are acceptable. It is also acceptable to control the
outer specimen surface temperature by the use of a heated or
cooled outer sheath or blanket or by the use of an additional
uniform layer of insulation.
1.6 The use any size or shape of test pipe is allowable
provided that it matches the specimens to be tested. Normally
the test method is used with circular pipes; however, its use is
permitted with pipes or ducts of noncircular cross section
(square, rectangular, hexagonal, etc.). One common size used
for interlaboratory comparison is a pipe with a circular cross
section of 88.9-mm diameter (standard nominal 80-mm [3-in.]
pipe size), although several other sizes are reported in the
literature (2-4).
1.7 The test method applies only to test pipes with a
horizontal or vertical axis. For the horizontal axis, the literature
includes using the guarded-end, the calibrated, and the
calibrated-end cap methods. For the vertical axis, no experi-
ence has been found to support the use of the calibrated or
calibrated-end methods. Therefore the method is restricted to
using the guarded-end pipe apparatus for vertical axis mea-
surements.
1.8 This test method covers two distinctly different types of
pipe apparatus, the guarded-end and the calibrated or
1
This test method is under the jurisdiction of ASTM Committee C16 on Thermal
Insulation and is the direct responsibility of Subcommittee C16.30 on Thermal
Measurement.
Current edition approved March 1, 2023. Published April 2023. Originally
approved in 1954. Last previous edition approved in 2017 as C335/C335M – 17.
DOI: 10.1520/C0335_C0335M-23.
2
The boldface numbers in parentheses refer to the references at the end of this
test method.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
1

calculated-end types, which differ in the treatment of axial heat
transfer at the end of the test section.
1.8.1 The guarded-end apparatus utilizes separately heated
guard sections at each end, which are controlled at the same
temperature as the test section to limit axial heat transfer. This
type of apparatus is preferred for all types of specimens within
the scope of this test method and must be used for specimens
incorporating elements of high axial conductance.
1.8.2 The calibrated or calculated-end apparatus utilizes
insulated end caps at each end of the test section to minimize
axial heat transfer. Corrections based either on the calibration
of the end caps under the conditions of test or on calculations
using known material properties, are applied to the measured
test section heat transfer. These apparatuses are not applicable
for tests on specimens with elements of high axial conductance
such as reﬂective insulations or metallic jackets. There is no
known experience on using these apparatuses for measure-
ments using a vertical axis.
1.9 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.10 This international standard was developed in accor-
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
3
C168 Terminology Relating to Thermal Insulation
C177 Test Method for Steady-State Heat Flux Measure-
ments and Thermal Transmission Properties by Means of
the Guarded-Hot-Plate Apparatus
C302 Test Method for Density and Dimensions of Pre-
formed Pipe-Covering-Type Thermal Insulation
C518 Test Method for Steady-State Thermal Transmission
Properties by Means of the Heat Flow Meter Apparatus
C680 Practice for Estimate of the Heat Gain or Loss and the
Surface Temperatures of Insulated Flat, Cylindrical, and
Spherical Systems by Use of Computer Programs
C870 Practice for Conditioning of Thermal Insulating Ma-
terials
C1045 Practice for Calculating Thermal Transmission Prop-
erties Under Steady-State Conditions
C1058 Practice for Selecting Temperatures for Evaluating
and Reporting Thermal Properties of Thermal Insulation
E230 Speciﬁcation for Temperature-Electromotive Force
(emf) Tables for Standardized Thermocouples
2.2 ISO Standards:
ISO 8497 Thermal Insulation-Dermination of Steady State
Thermal Transmission Properties of Thermal Insulation
for Circular Pipes
3. Terminology
3.1 Deﬁnitions—For deﬁnitions of terms used in this test
method, refer to Terminology C168.
3.2 Deﬁnitions of Terms Speciﬁc to This Standard:
3.2.1 areal thermal conductance, C—the steady-state time
rate of heat ﬂow per unit area of a speciﬁed surface (Note 3)
divided by the difference between the average pipe surface
temperature and the average insulation outer surface tempera-
ture. It is the reciprocal of the areal thermal resistance, R.
C5Q
A
~
t
o
2t
2
!
51
R(1)
where the surface of the area, A, must be speciﬁed (usu-
ally the pipe surface or sometimes the insulation outer sur-
face).
NOTE 3—The value of C, the areal thermal conductance, is arbitrary
since it depends upon an arbitrary choice of the area, A. For a homoge-
neous material for which the thermal conductivity is deﬁned as in 3.2.7
(Eq 8), the areal conductance, C,is given as follows:
C52πLλ
p
Aln
~
r
2
/r
o
!
(2)
If the area is specially chosen to be the “log mean area,”
equal to 2πL(r
2
−r
o
)/l n(r
2
/r
o
), then C=λ
p
/(r
2
−r
o
). Since
(r
2
−r
o
) is equal to the insulation thickness measured from
the pipe surface, this is analogous to the relation between
conductance and conductivity for ﬂat slab geometry. Similar
relations exist for the areal thermal resistance deﬁned in
3.2.2. Since these areal coefficients are arbitrary, and since
the area used is often not stated, thus leading to possible
confusion, it is recommended that these areal coefficients not
be used unless speciﬁcally requested.
3.2.2 areal thermal resistance, R—the average temperature
difference between the pipe surface and the insulation outer
surface required to produce a steady-state unit rate of heat ﬂow
per unit area of a speciﬁed surface (Note 3). It is the reciprocal
of the areal thermal conductance, C.
R5A
~
t
o
2t
2
!
Q51
C(3)
where the surface of the area, A, must be speciﬁed (usu-
ally the pipe surface or sometimes the insulation outer sur-
face).
3.2.3 areal thermal transference, T
r
—the time rate of heat
ﬂow per unit surface area of the insulation divided by the
difference between the average pipe surface temperature and
the average air ambient temperature.
T
r
5Q
2πr
2
L
~
t
o
2t
a
!
(4)
3.2.4 pipe insulation lineal thermal conductance, C
L
—the
steady-state time rate of heat ﬂow per unit pipe insulation
length divided by the difference between the average pipe
surface temperature and the average insulation outer surface
3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
C335/C335M − 23
2

temperature. It is the reciprocal of the pipe insulation lineal

thermal resistance, R

(5)

3.2.5 pipe insulation lineal thermal resistance, R

—the

average temperature difference between the pipe surface and

the insulation outer surface required to produce a steady-state

unit time rate of heat ﬂow per unit of pipe insulation length. It

is the reciprocal of the pipe insulation lineal thermal

conductance, C

Q51

(6)

3.2.6 pipe insulation lineal thermal transference, T

—the

steady-state time rate of heat ﬂow per unit pipe insulation

length divided by the difference between the average pipe

surface temperature and the average air ambient temperature. It

is a measure of the heat transferred through the insulation to the

ambient environment.

(7)

3.2.7 pipe insulation thermal conductivity,λ

—of homoge-

neous material, the ratio of the steady-state time rate of heat

ﬂow per unit area to the average temperature gradient (tem-

perature difference per unit distance of heat ﬂow path). It

includes the effect of the ﬁt upon the test pipe and is the

reciprocal of the pipe insulation thermal resistivity, r

. For pipe

insulation of circular cross section, the pipe insulation thermal

conductivity is:

5Q1n

L2π

(8)

3.2.8 pipe insulation thermal resistivity, r

—of homoge-

neous material, the ratio of the average temperature gradient

(temperature difference per unit distance of heat ﬂow path) to

the steady-state time rate of heat ﬂow per unit area. It includes

the effect of the ﬁt upon the test pipe and is the reciprocal of the

pipe insulation thermal conductivity, λ

. For pipe insulation of

circular cross section, the pipe insulation thermal resistivity is

calculated as follows:

52πL

Q1n

(9)

3.2.9 surface areal heat transfer coeffıcient, h

—the ratio of

the steady-state time rate of heat ﬂow per unit surface area to

the average temperature difference between the surface and the

ambient surroundings. The inverse of the surface heat transfer

coefficient is the surface resistance. For circular cross sections:

2πr

(10)

3.3 Symbols: see 1.3:

= pipe insulation lineal thermal conductance, W/m·K

[Btu · in ⁄F · hr · ft

= pipe insulation lineal thermal resistance, K·m/W

[Btu · in ⁄F · hr · ft

= pipe insulation lineal thermal transference, W/m·K

[Btu · in ⁄F · hr · ft

= pipe insulation thermal conductivity, W/m·K

[Btu · in ⁄F · hr · ft

= pipe insulation thermal resistivity, K·m/W [F · hr · ft

= surface areal heat transfer coefficient of insulation

outer surface, W/m

·K [Btu · in ⁄F · hr · ft

C= areal thermal conductance, W/m

·K

[Btu · in ⁄F · hr · ft

R= areal thermal resistance, K·m

/W [F · hr · ft

= areal thermal transference, W/m

·K

[Btu · in ⁄F · hr · ft

Q= time rate of heat ﬂow to the test section of length L, W

[Btu/hr],

= temperature of pipe surface, K [F],

= temperature of insulation inside surface, K [F],

= temperature of insulation outside surface, K [F],

= temperature of ambient air or gas, K [F],

= outer radius of circular pipe, m [ft],

= inner radius of circular insulation, m [ft],

= outer radius of circular insulation, m ft],

L= length of test section (see 8.1.1), m [ft], and

A= area of speciﬁed surface, m

[ft

4. Signiﬁcance and Use

4.1 As determined by this test method, the pipe insulation

lineal thermal resistance or conductance (and, when applicable,

the thermal resistivity or conductivity) are means of comparing

insulations which include the effects of the insulation and its ﬁt

upon the pipe, circumferential and longitudinal jointing, and

variations in construction, but do not include the effects of the

outer surface resistance or heat transfer coefficient. They are

thus appropriate when the insulation outer-surface temperature

and the pipe temperature are known or speciﬁed. However,

since the thermal properties determined by this test method

include the effects of ﬁt and jointing, they are not true material

properties. Therefore, properties determined by this test

method are somewhat different from those obtained on appar-

ently similar material in ﬂat form using the guarded hot plate,

Test Method C177, or the heat ﬂow meter apparatus, Test

Method C518.

4.2 The pipe insulation lineal thermal transference incorpo-

rates both the effect of the insulation and its ﬁt upon the pipe

and also the effect of the surface heat-transfer coefficient. It is

appropriate when the ambient conditions and the pipe tempera-

ture are known or speciﬁed and the thermal effects of the

surface are to be included.

4.3 Because of the test condition requirements prescribed in

this test method, recognize that the thermal transfer properties

obtained will not necessarily be the value pertaining under all

service conditions. As an example, this test method provides

that the thermal properties shall be obtained by tests on dry or

conditioned specimens, while such conditions are not neces-

sarily realized in service. The results obtained are strictly

applicable only for the conditions of test and for the product

construction tested, and must not be applied without proper

adjustment when the material is used at other conditions, such

as mean temperatures that differ appreciably from those of the

test. With these qualiﬁcations in mind, the following apply:

4.3.1 For horizontal or vertical pipes of the same size and

temperature, operating in the same ambient environment,

C335/C335M − 23

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摘要：

ASTM C335 - C 335M - 23 是当前最新版本的标准化测试方法，专门用于测定水平管道绝热材料在稳态条件下的传热性能，即线性热导率。该标准由美国材料与试验协会（ASTM）发布，适用于评估各类管道保温材料的实际隔热效果，确保工业、建筑及能源系统中管道系统的能效与安全。通过规定的测试装置、温度条件与计算步骤，该标准为制造商、工程师和质检机构提供了统一、可重复的试验依据，对于优化管道设计、控制热损失以及满足节能规范具有关键参考价值。

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