 # 2.4. Compressed air with initial properties: p1 = 0.35 MPa, T1 =30 ...

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2.4. Compressed air with initial properties: p1 = 0.35 MPa, T1 =30 °C is adiabatically and irreversibly expanded in a pneumatic engine. The air leaving the engine has pressure p2 = 0.1MPaand temperature T1 = -35 C. The mass rate of air flowing through the engine is equal to il = 1.0-10-3 kmols/s . Find exergy loss of the air per unit time the if the environmental temperature and pressure are pe = 0.1MPa, Te = 10 °C. 2.7. A refrigerator runs according to the Carnot cycle. The working medium absorbs heat in the evaporator having the constant temperature T1 = 255K and gives it over in the condenser having the constant temperature T11 = 300K. Temperature of the working medium in the evaporator is 10K lower than the evaporator temperature while temperature of the working medium in the condenser is higher by 10K than the condenser temperature. Find the second law efficiency of the refrigerator. 2.9. The air having inlet properties: P1 = 0.5MPa, T1 = Te = 20 °C flows through a long, horizontal, well thermally insulated tube. The tube diameter is equal to d = 40 mm. The outlet temperature and pressure of the air are, p2 = 0.2MPa, T2 = 30 °C respectively. Find the exergy loss per unit mass of the air. The mass rate of the air is m = 0.2 kg/s and the particular gas constant of the air R = 287 J/(kg.K). Estimate the loss of exergy in the case when the air in the tube would be well cooled and its temperature would not vary along the duct.

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