print barcode labels using vb.net The Choice of Refrigerant in Software

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93 The Choice of Refrigerant
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Table 91 Thermodynamic Properties of Saturated Tetrafluoroethanej
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Temperature "C K Saturation pressure MPa Liquid density kg m-3 Specific volume of vapor m3 kg-' Enthalpy Entropy
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kJ kg-'
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k kg-' K-' J
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normal boiling point +Reproducedwith permission from ASHRAE Handbook: Fundamentals, p 1729, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc, Atlanta, 1993
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CHAPTER 9 Refrigeration and Liquefaction
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independent of the refrigerant However, the irreversibilities inherent in the vapor-compression cycle cause the coefficient of performance of practical refrigerators to depend to some extent on the refrigerant Nevertheless, such characteristics as its toxicity, flammability, cost, corrosion properties, and vapor pressure in relation to temperature are of greater importance in the choice of refrigerant So that air cannot leak into the refrigeration system, the vapor pressure of the refrigerant at the evaporator temperature should be greater than atmospheric pressure On the other hand, the vapor pressure at the condenser temperature should not be unduly high, because of the initial cost and operating expense of high-pressure equipment These two requirements limit the choice of refrigerant to relatively few fluids The final selection then depends on the other characteristics mentioned Ammonia, methyl chloride, carbon dioxide, propane and other hydrocarbons can serve as refrigerants Halogenated hydrocarbons came into common use as refrigerants in the 1930s Most common were the fully halogenated chlorofluorocarbons,CC13F(trichlorofluoromethane or CFC-1 I ) and CC12F2(dichlorodifluoromethane or CFC-12) These stable molecules per~ sist in the atmosphere for hundreds of years, causing severe ozone depletion Their production has mostly ended Replacements are certain hydrochlorofluorocarbons, less than fully halogenated hydrocarbons which cause relatively little ozone depletion, and hydrofluorocarbons, which contain no chlorine and cause no ozone depletion Examples are CHC12CF3(dichlorotrifluoroethane or HCFC-123), CF3CH2F(tetrafluoroethane or HFC-134a), and CHF2CF3 (pentafluoroethane or HFC-125) A pressurelenthalpy diagram for tetrafluoroethane (HFC134a) is shown in Fig G2; Table 91 provides saturation data for the same refrigerant Tables and diagrams for a variety of other refrigerants are readily a ~ a i l a b l e ~ Limits placed on the operating pressures of the evaporatorand condenser of a refrigeration system also limit the temperature difference TH - TC over which a simple vapor-compression cycle can operate With TH fixed by the temperature of the surroundings, a lower limit is placed on the temperature level of refrigeration This can be overcome by the operation of two or more refrigeration cycles employing different refrigerants in a cascade A two-stage cascade is shown in Fig 93 Here, the two cycles operate so that the heat absorbed in the interchanger by the refrigerant of the higher-temperature cycle 2 serves to condense the refrigerant in the lower-temperature cycle 1 The two refrigerants are so chosen that at the required temperature levels each cycle operates at reasonable pressures For example, assume the following operating temperatures (Fig 93):
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If tetrafluoroethane(HFC-134a) is the refrigerant in cycle 2, then the intake and discharge pressures for the compressor are about 145 bar and 772 bar, and the pressure ratio is about 532 If propylene is the refrigerant in cycle 1, these pressures are about 11 bar and 4 bar, and the pressure ratio is about 364 These are all reasonable values On the other hand, for a single cycle operating between 227 K and 303 K with HFC-134a as refrigerant, the intake
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3 ~ h abbreviated designation is nomenclature of the American Society of Heating, Refrigerating, and Aire Conditioning Engineers 4 ~ ~ Handbook: A ~ ~ R Fundamentals, Chap 17, 1989; R H Peny and D Green, Perry's Chemical Engineers' Handbook, 7th ed, Sec 2, 1997 Extensive data for ammonia are given by L Haar and J S Gallagher, J Phys Chem Re$ Data, vol 7, pp 635-792, 1978
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94 Absorption Refrigeration
Figure 93 A two-stage cascade refrigeration system
pressure to the condenser is about 04 bar, well below atmospheric pressure Moreover, for a discharge pressure of about 772 bar the pressure ratio is 193, too high a value for a single-stage compressor
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