Thermal transmission

Tye, R. P., ed. (1977). Thermal Transmission Measurements of Insualtion (STP 660). West Conchohocken, PA American Society for Testing and Materials. 

This is the rate of heat transfer from a surface to the surrounding air (or fluid) due to conduction convection and radiation. It is generally used only in still-air conditions and when the temperature difference between surface and ambient is of the order of 30 K. It is obtained by dividing the thermal transmission per unit area in watts per square meter by the temperature difference between the surface and the surrounding air. It is expressed as W/nf K. 

ISO 8990, 1994. Thermal insulation - Determination of steady state thermal transmission properties - Calibrated and guarded hot box. 

ASTM Cl77, 2004, Standard test method for steady state heat flux measurements and thermal transmission properties by means of the guarded hot plate apparatus. 

ASTM C177-85, Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guaxded-Hot-Plate Apparatus , Annual Book of ASTM Standards, American Society for Testing and Materials, Philadelphia, PA. 

Thermal transmission methods are relatively new techniques for adhesive inspection. Heat flow is determined by monitoring the surface temperature of a test piece a short time immediately after external heating or cooling has been applied. Subsurface anomalies alter the heat flow pattern and, thereby, affect the surface temperature. The surface temperature difference can be detected by thermometers, thermocouples, or heat-sensitive coatings. Liquid crystals applied to the joint can make voids visible if the substrate is heated. 

Thermal transmission testing is an excellent way of detecting various types of anomalies such as surface corrosion under paint before the corrosion becomes visually evident. Thin, single-layer structures, such as aircraft skin panels, can be inspected for surface and subsurface discontinuities. This test is simple and inexpensive, although materials with poor heat-transfer properties are difficult to test, and the joint must be accessible from both sides. For nonmetallic materials, the defect diameter must be on the order of 4 times its depth below the surface to obtain a reliable thermal indication. For metals, the defect diameter must be approximately 8 times its depth. Some bright surfaces such as bare copper and aluminum do not emit sufficient infrared radiation and may require a darkening coating on their surface. 

Heat Transfer. Values observed for heat transfer by conduction and the overall coefficient of heat transmission of flexible plastic sheets reinforced with plain woven fabrics demonstrated that a mathematical model was useful for prediction of the conductivity of the plastic sheets however, bulk density of the fabric had no effect on the conductivity. It was also found that the overall coefficient of thermal transmission of the plastic sheets insulated by woven fabrics depended largely on the conductivity of the insulating fabrics (ll). 

In addition to wool, other hygroscopic textile materials such as cotton and linen underwent a threefold increase in their specific heat at constant vapor pressure. The relatively high specific heats derived from equations in the study, which are considered to represent those incurred in actual use of the hygroscopic textiles, explain the well-known buffering action of these fabrics toward sudden changes in indoor or outdoor temperatures (2l). A compilation of the specific heat of a variety of textile fibers at 20-200°C indicates that considerable variation in the values of this thermophysical property occurs with different fibers (e.g., a value of 0.157 for glass and 0.1 9 cal/g.°C for Nylon 66 are reported), and that additional research is needed to establish the extent to which specific heat affects the characteristics of thermal transmission in textiles (22). 

The thermal transmission apparatus (togmeter) described by Clulow and Rees (27.) uses a heated plate and standardized conducting disks in series with the specimen to compute the heat flow through the textile. Thermal resistance can be measured by using one or two plates, thus simulating various modes of fabric end use,... 

Dix and Lavan conducted studies to measure the effectiveness of draperies and other indoor shading devices on the coefficient of thermal transmission, U, under winter and summer conditions (89)- With the aid of two environmentally controlled chambers and thermistors located near a test window, they determined that a medium-colored drapery with a white plastic backing reduced conductive heat loss in the winter by 6-1%, and conductive and radiant (solar) heat gains in the slimmer by 33%. Because shades inside casements were more effective than draperies in reducing heat loss under winter conditions, they concluded that the ability of the material to block air flow was more important than other properties of the material used (8 ). 

Dechow, F. I. Epstein, K. A. ASTM STP660, Thermal Transmission Measurements of Insulation, American Society for Testing and Materials Philadelphia, PA, 1978, p. 234. 

AH 103 EPS 303 Guidelines for Reporting Thermal Transmission Properties of Polystyrene Foam Insulating Materials, Thermal Conductance, and Transmittance of Built-Up Construction Systems. 

Thermal-break materials are solid or cellular materials, or combinations of materials, of low thermal transmission placed between components of high thermal transmission in order to reduce the heat flow across the assembly. 

Thermal conductivity test JIS A 1412 (Test Method for Thermal Transmission Properties of Thermal Insulations). 

Apparent density of rigid cellular plastics, kg/m3 Thermal transmission resistant (R-value) hr.ft2 °F/Btu.in... 

C 518 Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus... 

Method 7A is based on the absolute guarded hot plate method described in BS 874. 1986 for determining the steady state thermal transmission properties of insulating materials in the temperature range -20 C to 100 C. 

ASTM Cl044 (1985). Practice for using the guarded-hot-plate apparatus in the one-sided mode to measure steady-.state heat flux and thermal transmission properties. 

Steady State Heat Flux Measurement and Thermal Transmission Properties by Measure of Heat Flow Meter Apparatus, ASTM C518, Am. Soc. Testing and Materials. Goodrich B, Toyama B. Liquid encapsulation considerations. Dexter Technology Paper, Dec. 1994. 

Basic physics of thermal transmission general comments... 

The above-mentioned mechanisms of thermal transmission and their interaction towards building elements and their surface are summarized in Figs 5.14-5.17. 

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