30079_62b - Since a secondary coolant cannot be used below...

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Since a secondary coolant cannot be used below its freezing point, certain ones are not applicable at the lower temperatures. Sodium chloride's eutectic freezing point of -2O 0 C limits its use to ap- proximately -12 0 C. The eutectic freezing point of calcium chloride is -53 0 C, but achieving this limit requires such an accuracy of mixture that -4O 0 C is a practical low limit of usage. Water solubility in any open or semi-open system can be important. The dilution of a salt or glycol brine, or of alcohol by entering moisture, merely necessitates strengthening of the brine. But for a brine that is not water-soluble, such as trichloroethylene or methylene chloride, precautions must be taken to prevent free water from freezing on the surfaces of the heat exchanger. This may require provision for dehydration or periodic mechanical removal of ice, perhaps accompanied by replacement with fresh brine. Vapor pressure is an important consideration for coolants that will be used in open systems, especially where it may be allowed to warm to room temperature between periods of operation. It may be necessary to pressurize such systems during periods of moderate temperature operation. For example, at O 0 C the vapor pressure of R-Il is 39.9 kPa (299 mm Hg); that of a 22% solution of calcium chloride is only 0.49 kPa (3.7 mm Hg). The cost of vapor losses, the toxicity of the escaping vapors, and their flammability should be carefully considered in the design of the semiclosed or open system. Environmental effects are important in the consideration of trichlorofluoromethane (R-Il) and other chlorofluorocarbons. This is a refrigerant with a high ozone-depletion potential and halocarbon global wanning potential. The environmental effect of each of the coolants should be reviewed before the use of it in a system is seriously considered. Energy requirements of brine systems may be greater because of the power required to circulate the brine and because of the extra heat-transfer process, which necessitates the maintenance of a lower evaporator temperature. 62.7.1 Use of Ice Where water is not harmful to a product or process, ice may be used to provide refrigeration. Direct application of ice or of ice and water is a rapid way to control a chemical reaction or remove heat from a process. The rapid melting of ice furnishes large amounts of refrigeration in a short time and allows leveling out of the refrigeration capacity required for batch processes. This stored refrigeration also is desirable in some processes where cooling is critical from the standpoint of safety or serious product spoilage. Large ice plants, such as the block-ice plants built during the 1930s, are not being built today. However, ice still is used extensively, and equipment to make flake or cube ice at the point of use
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This note was uploaded on 05/02/2010 for the course ME 100 taught by Professor Any during the Spring '10 term at Purdue University-West Lafayette.

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30079_62b - Since a secondary coolant cannot be used below...

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