thermodynamics

thermodynamics File #1 FLORIDA TECH DEPARTMENT OF MECHANICAL AND AEROSPACE ENGINEERING MAE 3191-E2 Engineering Thermodynamics I Chapter 5 Homework due 10/26/15 1. Consider a refrigerator that accepts 15 kW of heat in the evaporator and rejects 19.76 kW of heat from the condenser. The compressor requires an input of 5 kW of power and loses 0.24 kW of heat. Determine the coefficient of performance for this refrigerator. 2. Each set of values below represents the operating conditions for a new refrigerator claimed by an inventor. Determine whether each claim satisfies both the first and second laws of thermodynamics. a) = 7 kW = 7 kW = 0 kW b) = 8 kW = 7 kW = 2 kW c) = 6 kW = 1 kW = 5 kW d) = 10 kW = 4 kW = 12 kW 3. Heat is transferred from a geothermal vent to heat a greenhouse. The temperature inside the greenhouse must be 300 K at all times. If the outside temperature is lower than 300 K, heat is lost through the walls. To keep the greenhouse warm, a heat pump with a coefficient of performance equal to 1.8 has been constructed and requires 12 kW of power to operate. Assuming the outside temperature is 260 K, how much heat is lost through the walls per degree temperature difference? 4. A Carnot engine operates between the temperatures of 550°C and 10°C, producing 115 kJ of work. Calculate the heat input to the engine. 5. A heat engine operating on the Carnot cycle uses solar energy as the source of hightemperature heat input. The solar irradiation, averaged over the day, has a value of 0.51 kW/m2 . This provides energy to the cycle at a uniform temperature of 450 K, and the cycle rejects heat to the environment at a temperature of 300 K. The engine produces 2 MW of power. Determine the minimum area needed to provide this power. 6. A Carnot refrigerator, consuming 3.0 kW, is located in a room where the air temperature is 25°C. The coefficient of performance is found to be 5.0. Determine the amount of heat removed from and the temperature of the refrigerated space. 7. Consider a tank containing 1 kg of water at 20°C and 150 kPa. It is desired to cool the water to 5°C using a Carnot refrigerator. If this refrigerator sits in a room with a temperature of 30°C, and assuming the refrigerated space is at 5°C, what is the amount of work required to operate the refrigerator? 8. A heat pump maintains a temperature of 25°C inside a house. The house leaks 0.8 kW per degree temperature difference between the inside and outside of the house. Assuming that this is a real heat pump having a coefficient of performance equal to only 70% that of a Carnot heat pump, and assuming that the heat pump requires 1.5 kW of power, what is the minimum temperature outside that can exist to allow the heat pump to work? 9. A Carnot heat engine using 1 kg of air has the following conditions: isothermal heat addition beginning at 15 MPa and 1200 K and continuing until the pressure is 10 MPa; adiabatic compression from 5 MPa and 500 K, continuing until the pressure is 15 MPa. Determine a) the heat transfer into the cycle; b) the heat transfer from the cycle; c) the work for each of the processes; d) the cycle thermal efficiency. 10. Reverse the heat pump cycle shown in Problem 4.123 such that it is a heat engine. Assume the net work into the compressor is now the net work out of a turbine. Determine the thermal efficiency for this engine.