When heat is removed cooling the process will take us

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When heat is removed (cooling), the process will take us back from State 5 to State 1 The amount of heat released during cooling is the same as the heat added during heating Exercise Describe the cooling process from superheated steam to subcooled liquid of water in a piston cylinder device. 13
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Saturation Temperature and Pressure When does water boil? At 1 atm pressure, water boils at 100° C At lower pressures, water boils at lower temperatures and vice versa Boiling temperature depends on pressure Saturation Temperature (T sat ) temperature at which a pure substance changes phase at a given pressure Saturation Pressure (P sat ) pressure at which a pure substance changes phase at a given temperature Example: at P atm (P = 101.325 kPa), T sat = 100° C at T = 100° C, P sat = 101.325 kPa T (°C) P sat (kPa) -10 0.26 -5 0.40 0 0.61 5 0.87 10 1.23 15 1.71 20 2.34 25 3.17 30 4.25 40 7.38 50 12.35 100 101.33 150 475.8 200 1554 250 3973 300 8581 14
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Latent Heat During phase change, a substance absorbs or releases energy Latent Heat the amount of energy absorbed or released during phase change Sensible Heat the amount of energy absorbed or released when temperature changes Latent heat depend on temperature or pressure of phase change process Example: at 101.33 kPa (1 atm), the latent heat released during freezing of water is 333.7 kJ/kg at 101.33 kPa, the latent heat absorbed during vaporization of water is 2257.1 kJ/kg Saturated mixture T [°C] v 300 100 20 1 2 3 4 5 Temperature vs specific volume ( T-v ) diagram for heating process of water at constant pressure 15
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During phase change, pressure and temperature are dependent T sat = f (P sat ) T sat increases with P sat Water boils at higher temperature when pressure is high The boiling temperature of water decreases at high altitudes where pressure is low Liquid-vapor saturation curve for water T sat [°C] P sat (kPa) 600 400 200 0 50 100 150 Elevation [m] Patm [kPa] T boiling [°C] 0 101.33 100 1000 89.55 96.3 2000 79.50 93.2 5000 54.05 83.0 10000 26.50 66.2 20000 5.53 34.5 Exercise We can actually control boiling temperature by controlling pressure. Can you give examples of this found in our daily use? 0 20 40 60 80 100 120 0 20 40 60 80 100 120 0 5000 10000 15000 20000 Patm [kPa] Elevation [m] Patm [kPa] Tboiling [°C] 16
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Property Diagrams Property diagrams used in thermodynamics to study the variations of properties during phase-change processes of pure substances The following diagrams are common: Temperature vs Specific volume (T- v ) Pressure vs Specific volume (P- v ) Pressure vs Temperature (P-T) Temperature vs Entropy (T-s) In this topic, we will look at T- v , P- v , P-T 17
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Temperature vs Specific Volume (T- v ) Diagram Recall our discussion about phases of water using the heated piston-cylinder example Let’s say we apply pressure (P > P atm ) to the cylinder by pressing down on the piston and maintain this new pressure Then we plot the temperature of the substance for all phase change stages (stage 1 to stage 5, i.e. compressed liquid to superheated vapor) At P > 1atm, water boils at higher temperature Specific volume of saturated liquid is larger Specific volume of saturated vapor is smaller The latent heat of vaporization decreases as pressure
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