tailieunhanh - Sổ tay kỹ sư cơ khí P3

Tham khảo tài liệu 'sổ tay kỹ sư cơ khí p3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | SYSTEM COMPONENTS 1897 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 -20 C limits its use to approximately -12 C. The eutectic freezing point of calcium chloride is -53 C but achieving this limit requires such an accuracy of mixture that -40 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 0 C the vapor pressure of R-ll is kPa 299 mm Hg that of a 22 solution of calcium chloride is only kPa 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-ll and other chlorofluorocarbons. This is a refrigerant with a high ozone-depletion potential and halocarbon global warming 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 .