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**Significance and Use**

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 apparently similar material in flat form using the guarded hot plate, Test Method C177, or the heat flow meter apparatus, Test Method C518.

The pipe insulation lineal thermal transference incorporates 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 temperature are known or specified and the thermal effects of the surface are to be included.

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 necessarily 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:

For horizontal or vertical pipes of the same size and temperature, operating in the same ambient environment, values obtained by this test method can be used for the direct comparison of several specimens, for comparison to specification values, and for engineering data for estimating heat loss of actual applications of specimens identical to those tested (including any jackets or surface treatments). When appropriate, correct for the effect of end joints and other recurring irregularities (4.4).

When applying the results to insulation sizes different from those used in the test, an appropriate mathematical analysis is required. For homogeneous materials, this consists of the use of the thermal conductivity or resistivity values (corrected for any changes in mean temperature) plus the use of the surface heat transfer coefficient when the ambient temperature is considered (for example, see Practice C680). For nonhomogeneous and reflective insulation materials, a more detailed mathematical model is required which properly accounts for the individual modes of heat transfer (conduction, convection, radiation) and the variation of each mode with changing pipe size, insulation thickness, and temperature.

It is difficult to measure the thermal performance of reflective insulation that incorporate air cavities, since the geometry and orientation of the air cavities can affect convective heat transfer. While it is always desirable to test full-length pipe sections, this is not always possible due to size limitations of existing pipe insulation testers. If insulation sections are tested less than full length, internal convective heat transfer are usually altered, which would affect the measured performance. Therefore, it must be recognized that the measured thermal performance of less than full-length insulation sections is not necessarily representative of full-length sections.

**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.

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.

**2. Referenced Documents** *(purchase separately)* The documents listed below are referenced within the subject standard but are not provided as part of the standard.

**ASTM Standards**

C168 Terminology Relating to Thermal Insulation

C177 Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus

C302 Test Method for Density and Dimensions of Preformed 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 Materials

C1045 Practice for Calculating Thermal Transmission Properties Under Steady-State Conditions

C1058 Practice for Selecting Temperatures for Evaluating and Reporting Thermal Properties of Thermal Insulation

E230 Specification and Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples

**ISO Standards**

**ICS Code**

ICS Number Code 91.100.60 (Thermal and sound insulating materials)

**UNSPSC Code**

UNSPSC Code 30141500(Thermal insulation); 40171500(Commercial pipe and piping)

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**DOI:** 10.1520/C0335_C0335M-10E01

**Citation Format**

ASTM C335 / C335M-10e1, Standard Test Method for Steady-State Heat Transfer Properties of Pipe Insulation, ASTM International, West Conshohocken, PA, 2010, www.astm.org

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