Thermal diffusivity of the condensed

The supersaturation in condensers arises for two reasons. First, the condensable vapor is generally of higher molecular weight than the noncondensable gas. This means that the molecular diffusivity of the vapor will be much less than the thermal diffusivity of the gas. Restated, the ratio of NSc/Npr is greater than 1. The result is that a condenser yields more heat-transfer units dTg/(Tg — Tt) than mass-transfer units dYg/(Yg — Yt). Second, both transfer processes derive their driving force from the temperature difference between the gas Tg and the interface Tt. Each incremental decrease in interface temperature... 

Similar to the expression for the condensed phase, the thermal diffusivity in the gas phase is given by g = Xg/pgCg and is assumed to be independent of temperature. The thermal wave thickness in the gas phase 6g is defined according to 6g = Og/Mg. Then, Eq. (3.46a) can be written as ... 

Foamed-in-place polyurethane is prepared by allowing a polyol [po y(ethy ene glycol), polyester alcohols, etc.] to react with a diisocyanate in the presence of an amine catalyst. The gas which creates the foam may be a dissolved material, such as a Freon, which volatilizes during the exothermic polymerization reaction.7 A second method involves the use of water in the reaction mixture this hydrolyzes part of the isocyanate to produce an amine and C02 gas. The Freon-formed material is preferred for the insulation of low-temperature apparatus because the thermal conductivity of the foam is greatly reduced at low temperatures by the condensation of the Freon in the cells. It is probable that the longterm effectiveness of this phenomenon must be maintained by surrounding the foamed plastic with an airtight enclosure which will prevent diffusion of air into and Freon out of the cells. 

The thermal behaviour of the metal phosphates differs from those of the two groups above in that elimination of water results in the (irreversible) condensation of the anionic groups. Rates of many of these dehydrations are controlled by diffusion within the residual solids. 

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