Superheated vapor this table shows properties in

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Superheated Vapor This table shows properties in single phase only (vapor) Properties are listed for different temperatures at selected pressures The saturated temperatures (T sat ) are shown beside the selected pressures Property table for Superheated Steam 38
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To check if the substance is in superheated vapor phase: For a given pressure, check T sat (in the Saturated Water table) If the existing temperature is higher than T sat , then the substance has to be in superheated phase For a given temperature, check P sat (in the Saturated Water table) If existing pressure is lower than P sat , then the substance has to be in superheated phase Then use the Superheated Vapor table to look up the values Example Determine the internal energy of water at 200 kPa and 200 °C Example Determine the temperature of water at the following state: P = 0.5 Mpa, h = 2890 kJ/kg 39
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Reference State The values of u , h and s are changes in properties and are based on reference states Reference state for water is the saturated liquid at 0.01°C At this reference state for water: P sat = 0.6113 Mpa), u f = 0.0 kJ/kg, h f = 0.01 kJ/kg and s f = 0.000 kJ/kg.K Reference state for refrigerant R-134a is saturated liquid at -40°C At this reference state for R-134a: P sat = 0.05164 MPa, u f = 0.04 kJ/kg, h f = 0.00 kJ/kg and s f = 0.0000 kJ/kg.K 40
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Example Complete the following table for properties of water T [°C] P [kPa] u [kJ/kg] x Phase (a) 200 0.6 (b) 125 1600 (c) 1000 2950 (d) 75 500 (e) 850 0.0 41
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Ideal Gas Equation of State Equation of State relates pressure , temperature and specific volume of a substance Ideal Gas Equation of State provides simplest form of P-v-T relation Pressure of gas is inversely proportional to volume At low pressures, volume is proportional to temperature Ideal Gas Equation of State (Ideal Gas Relation) is given as: 𝑃 = 𝑅 𝑇 ? 𝑃? = 𝑅? where R = gas constant (different for each gas) [kJ/kg.K] P = absolute pressure T = absolute temperature 42
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Gas Constant The individual gas constant is given by: 𝑅 = 𝑅 ? 𝑀 ?? ??.? or ?𝑃𝑎.? 3 ??.? where, R u = universal gas constant M = molar mass (molecular weight) of gas Gas Constants R air = 0.2870 kJ/kg.K R O2 = 0.2598 kJ/kg.K R H2 = 4.1240 kJ/kg.K R CO2 = 0.1889 kJ/kg.K R CO = 0.2968 kJ/kg.K R He = 2.0769 kJ/kg.K R Ar = 0.2081 kJ/kg.K R N2 = 0.2968 kJ/kg.K R CH4 = 0.5182 kJ/kg.K R H2O = 0.4615 kJ/kg.K 43
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Universal Gas Constant R u R u value is the same for all gases 𝑅 ? = 8.314 ?? ????.? or ?𝑃𝑎.? 3 ????.? Mass of gas The molar mass (M) is defined as the mass of one mole of gas (in g or kg) Example: The mass of 1 kmol of nitrogen is 28 kg. Therefore, the molar mass of nitrogen is 28 The mass of a substance is given as 𝑚 = ?? 𝑘𝑔 Where, M = molar mass N = number of moles 44
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Example Determine the mass of air in a 4 m x 5 m x 6 m room at P = 100 kPa and T = 25 °C Solution Assume air as ideal gas Volume, V = 4 x 5 x 6 = 120 m 3 From table: R air = 0.2870 kJ/kg.K From ideal gas relations, Pv = RT and v = V/m m = PV/RT = (100)(120)/(0.287)(25 + 273) = 140.3 kg 45
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Validity of Ideal Gas Assumption At low pressures, most gases behave close to ideal gas The compressibility effects are negligible at low pressures (relative to critical pressure)
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