Effect of rapid cooling

The question of the effect of rapid cooling on the crystal is often raised, mainly with the concern that phase transitions or thermal stress may destroy the crystal. From experience with hundreds of compounds it can be stated with confidence that damage from cooling is rare. Perhaps about % of the crystals will undergo a phase transition under the conditions described here. Most of these cases can be handled by raising the temperature to a few degrees above the transition temperature. In practically no case is slow cooling of value more frequently it will cause problems. 

Figure 4. X-ray patterns showing the effect of rapid cooling on the amount and ordering of oxygen.

Chihib, N.-E., Tholozan, J.-L. (1999). Effect of rapid cooling and acidic pH on cellular homeostasis Pectinatus frisingenis, a strictly anaerobic beer-spoilage bacterium. International Journal of Pood Microbiology, 48, 191-202. 

The mould heating oven may be heated by direct gas flame, hot air, infra red, molten salts or fuel oil. The moulds are usually cooled by water immersion or sprays, and occasionally by cold air. Cooling is not critical but one must be aware of the effect of rapid cooling on the relative crystallinity of polymers. 

Bachman, K.C., Wilcox, C.J. 1990a. Effect of time of onset of rapid cooling on bovine milk fat hydrolysis. J. Dairy Sci. 73, 617-620. 

Often, it is preferable to avoid production of high-molar-mass polymer, and control can be effected by rapidly cooling the reaction at the appropriate stage or by adding calculated quantities of monofunctional materials as in the preparation of nylon-6,6 from the salt. 

This intricate mode of crystallization requires more time to accomplish than, say, the entry of small ions into growing salt crystals. This, coupled with low chain mobility due to viscous effects, makes the rate of crystallization slow and accounts in part for the fact that with rapid cooling-called quenching-the temperature drops below T without crystallization. 

Equipment for food freezing is designed to maximize the rate at which foods are cooled to —18° C to ensure as brief a time as possible in the temperature zone of maximum ice crystal formation (12,13). This rapid cooling favors the formation of small ice crystals which minimize the dismption of ceUs and may reduce the effects of solute concentration damage. Rapid freezing requires equipment that can deHver large temperature differences and/or high heat-transfer rates. 

Evaporation Retardants. Small molecule solvents that make up the most effective paint removers also have high vapor pressure and evaporate easily, sometimes before the remover has time to penetrate the finish. Low vapor pressure cosolvents are added to help reduce evaporation. The best approach has been to add a low melting point paraffin wax (mp = 46-57° C) to the paint remover formulation. When evaporation occurs the solvent is chilled and the wax is shocked-out forming a film on the surface of the remover that acts as a barrier to evaporation (5,6). The addition of certain esters enhances the effectiveness of the wax film. It is important not to break the wax film with excessive bmshing or scraping until the remover has penetrated and lifted the finish from the substrate. Likewise, it is important that the remover be used at warm temperatures, since at cool temperatures the wax film may not form, or if it does it will be brittle and fracture. Rapid evaporation occurs when the wax film is absent or broken. 

Natural-draft cooling towers are extremely sensitive to air-inlet conditions owing to the effects on draft. It can rapidly be estabUshed from these approximate equations that as the air-inlet temperature approaches the water-inlet temperature, the allowable heat load decreases rapidly. For this reason, natural-draft towers are unsuitable in many regions of the United States. Figure 10 shows the effect of air-inlet temperature on the allowable heat load of a natural-draft tower for some arbitrary numerical values and inlet rh of 50%. The trend is typical. 

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