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适用范围：<p>Manufacturers of thermal insulation express the performance of their products in charts and tables showing heat gain or loss per unit surface area or unit length of pipe. This data is presented for typical insulation thicknesses, operating temperatures, surface orientations (facing up, down, horizontal, vertical), and in the case of pipes, different pipe sizes. The exterior surface temperature of the insulation is often shown to provide information on personnel protection or surface condensation. However, additional information on effects of wind velocity, jacket emittance, ambient conditions and other influential parameters may also be required to properly select an insulation system. Due to the large number of combinations of size, temperature, humidity, thickness, jacket properties, surface emittance, orientation, and ambient conditions, it is not practical to publish data for each possible case, Refs <span class="bold">(31,32)</span>.</p> <p>Users of thermal insulation faced with the problem of designing large thermal insulation systems encounter substantial engineering cost to obtain the required information. This cost can be substantially reduced by the use of accurate engineering data tables, or available computer analysis tools, or both. The use of this practice by both manufacturers and users of thermal insulation will provide standardized engineering data of sufficient accuracy for predicting thermal insulation system performance. However, it is important to note that the accuracy of results is extremely dependent on the accuracy of the input data. Certain applications may need specific data to produce meaningful results.</p> <p>The use of analysis procedures described in this practice can also apply to designed or existing systems. In the rectangular coordinate system, Practice C 680 can be applied to heat flows normal to flat, horizontal or vertical surfaces for all types of enclosures, such as boilers, furnaces, refrigerated chambers and building envelopes. In the cylindrical coordinate system, Practice C 680 can be applied to radial heat flows for all types of piping circuits. In the spherical coordinate system, Practice C 680 can be applied to radial heat flows to or from stored fluids such as liquefied natural gas (LNG).</p> <p>Practice C 680 is referenced for use with Guide C 1055<astmref rid="a00006"> and Practice C 1057<astmref rid="a00007"> for burn hazard evaluation for heated surfaces. Infrared inspection, in-situ heat flux measurements, or both are often used in conjunction with Practice C 680 to evaluate insulation system performance and durability of operating systems. This type of analysis is often made prior to system upgrades or replacements.</p> <p>All porous and non-porous solids of natural or man-made origin have temperature dependent thermal conductivities. The change in thermal conductivity with temperature is different for different materials, and for operation at a relatively small temperature difference, an average thermal conductivity may suffice. Thermal insulating materials (<span class="italic">k</span> <span class=''unicode''>x003C;</span> 0.85 {Btu<span class=''unicode''>x00B7;</span>in}/{h<span class=''unicode''>x00B7;</span>ft²<span class=''unicode''>x00B7;</span><span class=''unicode''>x00B0;</span>F}) are porous solids where the heat transfer modes include conduction in series and parallel flow through the matrix of solid and gaseous portions, radiant heat exchange between the surfaces of the pores or interstices, as well as transmission through non-opaque surfaces, and to a lesser extent, convection within and between the gaseous portions. With the existence of radiation and convection modes of heat transfer, the measured value should be called apparent thermal conductivity as described in Terminology C 168<astmref rid="a00001">. The main reason for this is that the premise for pure heat conduction is no longer valid, because the ot...........