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Thermodynamics Tutorial: Second law of Thermodynamics
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1
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What is the minimum amount of work input necessary to a refrigerator which will convert 1 kg of water at 20 oC into ice at -5oC? (Ans: 40 kJ)
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2
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A heat engine receives heat from a solar collector plate. Solar radiation of the amount 1880 kJ/m2 falls on the solar collector and raises its temperature to 95 oC. The engine rejects heat to the atmosphere having 20oC. Calculate the minimum solar collector area required for a plant producing 1 kW of useful shaft power. (Ans: 9.394 square meters)
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3
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During the summer to cool water for drinking purpose 1 kg of ice at -2 oC is mixed with 5 kg of water at 27 oC in an insulated container that is subjected to the atmospheric pressure of 1 bar. Calculate the temperature of the mixture and the change in entropy for the spontaneous process. Assume specific heat of ice as 2.093 kJ/kg K and latent heat of ice as 335 kJ/kg. (Ans: 9 oC, 0.085 kJ/K)
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4
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A Carnot engine works between two thermal reservoirs A having a constant temperature of 600 oC and B having a constant temperature of 250 oC. Half of the power developed by the Carnot engine is used to drive a generator to produce electricity and the other half is used to drive the heat pump which receives heat from thermal reservoir B and rejects heat to a thermal reservoir C having 400 oC temperature. Calculate the heat rejected to thermal reservoir C by the heat pump as a percentage of heat from thermal reservoir A to the Carnot engine. Also, calculate the heat rejected per hour to thermal reservoir C if 480 kW are generated by the generated assuming 100 % generator efficiency. (Ans: 4.486, 89.92%, 2153.13 kW)
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5
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What is the more effective way to increase the efficiency of a Carnot engine? Comment on the results.
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Thermodynamics Tutorial: First law of Thermodynamics
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6
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In a vessel,10 kg of O2 is heated in a reversible, nonflow and constant volume process so that the pressure of oxygen is increased two times that of the initial value. The initial temperature is 20 oC. Calculate (a) the final temperature (b) the change in internal energy, (c) the change in enthalpy (d) the heat transfer. Take R = 0.259 kJ/kgK and Cv=0.652 kJ/kgK for oxygen. (Ans: 586 K, 1910.36 kJ, 2669.2 kJ, 1910.36 kJ)
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7
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A steam turbine operates under steady-flow conditions. It receives 7200 kg/h of steam from the boiler. The steam enters the turbine at enthalpy of 2800 kJ/kg, a velocity of 4000 m/min and elevation of 4 m. The steam leaves at enthalpy of 2000 kJ/kg, the velocity of 8000 m/min and an elevation of 1 m. Due to radiation, heat losses from the turbine to the surrounding area are 1580 kJ per hour. Calculate the output of the turbine. (Ans: 1586.72 kJ/s)
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8
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An axial-flow compressor of a gas turbine plant receives air from the atmosphere at a pressure of 1 bar, a temperature of 300 K, and a velocity of 60 m/s. At the discharge of the compressor, the pressure is 5 bar and the velocity is 100 m/s. The mass flow rate through the compressor is 20 kg/s. Assuming the isentropic compression, calculate the power required to drive the compressor and inlet and outlet pipe diameters. (Ans: 3584.2 kW, 0.604 m)
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Thermodynamics Tutorial: Properties of Pure Substance
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9
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A vessel of volume 0.05 m3 contains a mixture of saturated water and saturated steam at a temperature of 250 oC. The mass of the liquid present is 10 kg. Calculate the pressure, the mass, the specific volume, the enthalpy, and entropy. (Ans: 39.73 bar, 10.7478 kg, 0.0695, 0.0046482 m3/kg, 1204.63 kJ/kg, 3.02 kJ/kgK)
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10
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5000 kg/hr of steam enters a surface condenser at a pressure of 0.07 bar. 3 lacks liter of cooling water are passed through the tubes per hour and its temperature rises from 21 oC to 32 oC. The condensed steam leaves the condenser at a temperature of 30 oC. Calculate the condition of the steam entering the condenser. (Ans: 0.919)
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Thermodynamics Tutorial: Heat and Work interactions
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11
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The pressure of the gas in a pipeline is measured with a mercury manometer having one limb open to the atmosphere. If the difference in height of mercury in the two limbs is 562 mm, calculate the gas pressure. The barometer reads 761 mm of Hg. The density of mercury is 13640 kg/m3(Specific gravity of mercury: 13.64) (Answer: 1.77 bar)
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12
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A piston and cylinder machine containing a fluid system has a stirring device in the cylinder. The piston is frictionless and it is held against the fluid due to the atmospheric pressure of 101.325 kPa. The stirring device is turned 10000 revolutions with an average torque against the fluid of 1.275 m.N. Meanwhile, the piston having 0.6 m diameter moves out 0.8 m. Find the network transfer for the system. (Answer: -57.1 kJ)
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13
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An electric heater has a resistance of 50 ohm. It is connected across a power supply of 240 V for a period of 2 hour. Determine the work done by the power supply on the heater. How many ‘units’ of electricity are consumed?
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