Thermodynamics Quiz 2

A tank fully insulated contains 1 m3 of air at 50°C and 2 MPa. The valve is open and air escapes until half the original mass is gone and then the valve is closed. After valve is closed, the pressure inside the tank is

The air with enthalpy of 100 kJ/kg is compressed by an air compressor to a pressure and temperature at which its enthalpy becomes 200 kJ/kg. The loss of heat is 40 kJ/kg from the compressor as the air passes through it. Neglecting kinetic and potential energies, the power required for an air mass flow of 0.5 kg/s is ___________ kW.

Air enters an air turbine at a pressure of 700 kPa, 65°C with a velocity of 90 m/s and leaves the turbine at 140 kPa, 5°C and 60 m/s velocity. There is no heat transfer from the turbine. Assume air to be an ideal gas with constant specific heat. The second law efficiency of the turbine is _________%. (Assume T0 = 25°C, p0 = 101.325 kPa)

For water at 25°C, dps /dTs = 0.189 kPa/K (ps is the saturation pressure in kPa and Ts is the saturation temperature in K) and the specific volume of dry saturated vapour is 43.38 m3/kg. Assume that the specific volume of liquid is negligible in comparison with that of vapour. Using the Clausius-Clapeyron equation, an estimate of the enthalpy of evaporation of water at 25°C is ___________kJ/kg.

A household refrigerator is maintained at a temperature of 2°C. Each time the door opens, warm food is placed inside introducing an average heat of 400 kJ. The refrigerator operates at 20% of the ideal COP. The monthly bill at the rate of Rs. 3 per kWhr is Rs. 200. If the atmosphere is at 27°C, the number of times in a day the door is opened is