Thermal conductivity, building design

The other principal thermal properties of plastics which are relevant to design are thermal conductivity and coefficient of thermal expansion. Compared with most materials, plastics offer very low values of thermal conductivity, particularly if they are foamed. Fig. 1.10 shows comparisons between the thermal conductivity of a selection of metals, plastics and building materials. In contrast to their low conductivity, plastics have high coefficients of expansion when compared with metals. This is illustrated in Fig. 1.11 and Table 1.8 gives fuller information on the thermal properties of pl tics and metals. 

HDI Heat buildup An abbreviation for hexamethylene diisocyanate. The temperature rise within an elastomer due to hysteresis. In many end-use applications, an elastomer can be subjected to repeated cycles of deformation-relaxation. As this occurs, friction between the elastomer molecules generates heat. As elastomers have relatively poor thermal conductivity, the heat generated builds up over time, progressively increasing the internal temperature of the elastomer. If the temperature increases above 70°C, the elastomer physical properties can begin to reduce. Design of the elastomer part can play an important role in minimizing the effects of heat buildup. 

The sample container rests on a boundary layer at the top of the shelf surface. Such a layer is a region where the flow of heat transfer fluid is minimal or zero (i.e. the fluid is stationary). As a result, the sublimation step, which involves the transfer of heat from the fluid to the shelf surface, creates a temperature gradient across the boundary layer that depends on the thermal load exerted by the sublimation process and on the nature (viscosity, thermal conductivity and flow across the shelves) of the heat transfer fluid. The temperature gradient also depends on the number of shelves, their design and build, and on the pumping capacity of the circulation pump. These variables in turn depend on the size and particular manufacturer of the freeze-dryer, so the software used should include an input of data for the materials used, and for the dryer s design and build. 

The effect of the detrimental exothermic reaction is assisted by the low thermal conductivity of the adhesive/resin itself and the FRP adherends. If the dimensions cannot be changed, the heat build-up should be limited, for example, by adding fillers into the adhesive/resin and thus reducing the mass of thermoset polymer in the adhesive/resin. Depending on the type of filler, the thermal conductivity may also be improved. In any case, the adverse impact of the above points should be reduced by the design or other appropriate precautions. 

Other commercially available units, mainly designed for determination of the thermal conductivity of building materials in the form of plates, like the Linseis L91 unit, suffer from the shortcoming that in order to minimize heat losses, the sample diameter is 80 to 120mm. This large diameter makes it difficult to obtain good contact between the heaters and the catalyst sample. Furthermore, with electric heaters it is difficult to realize a constant temperature across the entire cross section of the probe. Finally the sample size demands a rather large amount of catalyst. 

The thermal conductivity (the ability to conduct heat) varies between materials and is a key element in building design and construction, which will be considered in Chapter 9. 

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