Heat transfer fluids thermal degradation

Synthetic fluids are safe, noncorrosive, essentially nontoxic, and thermally stable when operated under conditions recommended by the manufacturers. Generally, these fluids are more expensive than petroleum oils, but the synthetics can usually be reprocessed to remove degradation products. There are several classes of chemicals offered permitting a wide temperature range of appHcation. Any heat-transfer fluid in use should be examined periodically to monitor degradation or contamination. 

Ucon HTF-500. Union Carbide Corp. manufactures Ucon HTE-500, a polyalkylene glycol suitable for Hquid-phase heat transfer. The fluid exhibits good thermal stabHity in the recommended temperature range and is inhibited against oxidation. The products of decomposition are soluble and viscosity increases as decomposition proceeds. The vapor pressure of the fluid is negligible and it is not feasible to recover the used fluid by distiHation. Also, because the degradation products are soluble in the fluid, it is not possible to remove them by filtration any spent fluid usuaHy must be burned as fuel or discarded. The fluid is soluble in water. 

Heat transfer may also be required to maintain isothermal or adiabatic conditions in the presence of endothermic and/or exothermic reactions, as the result of mixing product components, surfactant neutralization, and other chemical reactions. In these cases, heat transfer requirements may be severe to minimize exposure of the bulk fluid to high temperatures for extended time periods, resulting in irreversible thermal degradation. 

Prandtl number is analogous to Schmitt number but focuses on heat transfer. It is the ratio of kinematic viscosity to thermal diffusivity. Typical Prandtl numbers are about 1 for gases and about 10 for supercritical fluids. The implieation thermal inhomogeneity in gases degrade rapidly, liquids need stirring, and supercritical fluids mix much more rapidly than liquids. 

These results make LDHs excellent candidates as fillers for polymer matrices, to be used as degradant additives, which permits a precise control of the lifetime of the plastic product, decreasing its negative impact on the environment. The improvement in the thermal properties of PE/LDH nanocomposites is usually explained assuming that the LDH layers act as a sort of barriers. Yue et al. [56] claim formation of nanostructures from the dispersed LDH particles, which decrease heat transfer, thus stabilizing the polymer chains. Dispersion of the LDH particles thus improves the barrier properties of the polymer nanocompounds because of their easy exfoliation. Fluid diffusion, especially gases, is strongly affected by this barrier effect. 

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