UNIT NO-03 [8 hrs] Second Law Of Thermodynamics: Introduction (Law of degradation of energy), Thermal energy reservoirs, Kelvin-Plank & Clausius

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1 UNIT NO-03 [8 hrs] Second Law Of Thermodynamics: Introduction (Law of degradation of energy), Thermal energy reservoirs, Kelvin-Plank & Clausius statements, Heat engines, Refrigerator and Heat pump, Perpetual motion machines, Reversible and Irreversible processes, Carnot cycle, Thermodynamic temperature scale. 1 (W97) State and prove. Clausius inequality. 2 (Nov 04, May 03) State Kelvin Planck's and Clausis statement of second law of thermodynamics and prove that violation of one statement implies violation of the second and vice versa. 3 (A-95) State Carnot theorem. 4 (A96) Prove that efficiency of reversible engine depends on temperature of source and sink and is independent of working substance. 5 (W97) Define the term Coefficient of performance as applied to a refrigerator and heat pump. Show that, COP (Heat Pump) = COP (Refrigerator ) (W97) A kitchen provided with a standard refrigerator and a window air-conditioner How it is that the refrigerator heats the kitchen where as the air conditioner cools it? 7 (W97) Explain Principle of increase of entropy. 8 (A-95) A house wife, on a warm summer day, decides to beat heat by closing the windows and doors in the kitchen and opening the refrigerator door. At first she feels cool and refreshed, but after a while the effect begins to wear off. Evaluate the situation as it relates to First law of thermodynamics, considering the room including the refrigerator as the system. 9 (A96) A household refrigerator is in thermal communication solely with atmosphere of constant temperature room. The refrigerator works in a complete cycle, receiving work from an electrical main and exchanging heat only with the atmosphere. Does this violates second law of thermodynamics? Explain. 10 (W00) Which is the most effective way to increase the efficiency of a' Carnot engine to increase T keeping T constant, or to decrease T keeping T constant? 11 (W97) A manufacturer of Carnot refrigerator has a choice of varying, either evaporator temperature. T L = 250K or the condenser temperature TH = 500K ± 25K. No restrictions are

2 placed on the operating temperature. In the best Interest of economy, what would you recounted to this manufacturer? 12 (A-88) A refrigerating machine has COP = 5. The heat engine running this machine has 30% efficiency. Find the heat removed from refrigerating machine for 1MJ of heat input to the engine, If the same machine acts for heat pump find the heat input for 1MJ of heat pumped out. 13 (A01) A Carnot cycle heat engine operates between reservoirs at C and 0 0 C and receives 1000 kj of heat from the high temperature reservoirs (i) Calculate the network and thermal efficiency of this engine. (ii) Calculate change in entropy of the high temperature and low temperature reservoirs. 14 (A-95) A domestic food refrigerator maintains a temperature of C. The ambient air temperature is 35 0 C. If heat leaks into the freezer at a continuous rate of 2 KJ/s, determine the total power necessary to pump this heat out continuously. 15 (W00) A fish freezing plant requires 40 tons of refrigeration. The freezing temperature is C while the ambient temperature is 30 0 C. If the performance of the plant is 20% of the theoretical reversed Carnot cycle working within the same temperature limits, calculate the power required. 16 (A96) A meat cold storage plant requires kj/min of refrigeration. The minimum temperature to be maintained is C. If actual COP of refrigerating plant is 15 of theoretical for the same temperature limits, calculate the power required to run the plant. 17 (W96) A Carnot refrigerator requires 1.3 kw per ton of refrigeration to maintain a region temperature of C. Determine (i) COP of Carnot Refrigerator (ii) Higher temperature of the cycle (iii) COP when this device is used as heat pump. 18 (W-95) A Carnot engine operates between two reservoirs at temperature of Ti & Tz K. The work output of the engine is 0.6 times that of heat rejected. Given that the difference in temperatures between the source and the sink is C. Calculate the source temperature, the sink temperature and the thermal efficiency. 19 (W96) A cold storage plant is required to store 20 tonnes of fish. The temperature of the fish when supplied = 250C; Storage temperature of fish required = -8 0 C. Specific heat of fish above freezing point = 2.93 kj/kg-c; Specific heat of fish below freezing point 1.25 kj/kg-c. Freezing point of fish = -3 0 C. Latent heat of fish = 232kJ/kg. If the cooling is achieved within 8 hours. find :(i) Capacity of the Refrigerating Plant (ii) Carnot cycle COP between this

3 temperature range (iii) If the actual COP is 1/3rd of the Carnot COP, find the power required to run the plant. (3+2+2). 20 (May 02) A Carnot refrigeration system used for air conditioning operates between a low temperature of 50C and a high temperature of 450C. If it is used for maintaining food stuffs in a cold storage operating at a low temperature of 150C and a high temperature of 45 0 C as before. What percentage increase in work input would be required for the food refrigeration unit over the air conditioning unit for the same quantity of Q. 21 (A-95) Verify the Validity or otherwise of the following Systems (a) An engine having the thermal efficiency of 25% receives 1200 KJ of heat at 400K, rejects 500 KJ to one sink at 300K and 400 KJ another sink at 350 K simultaneously. (b) Output of an engine receiving 600 KJ of heat and rejecting 400 KJ of heat is 220 KJ. (c) A refrigerator system operating between 33 0 C and 27 0 C produces 17.6 kw of refrigerating and rejects heat at the rate of 19.8 kw. 22 (A96) An inventor claims to have heat engine which is capable of developing 19 kw, while working between the temperature limits of 30 0 C and 40 0 C. It receives only 1000 kj/min of heat. Discuss the possibility of claim. 23 (W97) A reversible engine is supplied with heat from two constant temperature sources at 900K and 600K and reject energy to a constant temperature sink at 300. Assuming the engine to execute a complete cycle while developing 4800 kj of work per minute and rejecting 3200 kj of heat minute. Calculate the heat supplied by each source and the efficiency of the engine. 24 (Nov 04) A reversible engine receives heat from two constant temperature sources at 870K and 580K. It rejects 3000 kj/min to a sink at 290K. The engine develops 85kW. Determine heat supplied by each source and the efficiency of the engine. 25 Nov 03) A reversible heat pump is required to maintain a temperature of 0 0 C in a refrigerator when it rejects the heat to surrounding at 25 C. If the heat removal rate from the refrigerator is 1440 kj/min, determine the COP of the machine and work required. If the required input to run the pump is developed by a reversible engine which receives heat at C and rejects heat to atmospheric then determine the overall COP of the system. 26 (May03) A reversible heat pump is required to maintain a temperature of 5 0 C in a refrigerator when it rejects the heat to surrounding at 40 0 C. If the heat removal rate from the refrigerator is 1500 kj/min, determine the COP of the machine and work required. If the required input to run

4 the pump is developed by a reversible engine which receives heat at C and rejects heat to atmospheric then determine the overall COP of the system. 27 (Nov 02) A reversible heat engine operates between two reservoirs at temperature of 600 and 40 0 C. The engine drives a reversible refrigerator which operates between reservoirs at temperature of 40 0 C and C. The heat transfer to the engine is 2000 kj and net work done of the combined engine and refrigerator plant is 360 kj then evaluate the heat transfer to the refrigerator and net heat transfer to reservoir at 40 0 C. 28 (A-99) A reversible heat engine operates between two reservoirs at temperatures C and 50 0 C. The engine drives a reversible refrigerator, which operates between reservoirs at temperatures of 50 0 C and 25 0 C. The heat transfer to the engine is 2500 kj and the network output of the combined engine refrigerator plant is 400 kj (i) Determine the heat transfer to the refrigerant and the net heat transfer to the reservoir at 50 0 C. (ii) Reconsider (i) given that the efficiency of the heat engine and the C.O.P. of the refrigerator are each 45% of their maximum possible values. 29 (A-02) A reversible heat engine operates between two reservoirs at temperatures of 6000C and 400C. The engine drives a reversible refrigerator, which operates, between reservoir at temperatures of 400C and 200C. The heat transfer to the heat engine is 2000 kj and the network output of the combined engine refrigerator plant is 360 kj. (a) Evaluate the heat transfer to the refrigerant and the net heat transfer to the reservoir at 400C (b) Reconsider (a) given that the efficiency of the heat engine and COP of the refrigerator are each 40% of their maximum possible values. 30 (W 98) A heat engines uses 5000 kg/hr of water of cool oil from 1500C to 500C, The rate of flow of oil is 2500 kg/hr, The average specific heal of oil is 2.5 kj/kg-k the water enters the heat exchanger at 21 0 C. Determine (i) change in entropy due to heat exchange process. (ii) the amount of work obtained if cooling of oil is done by using the heat to run a Carnot engine with sink temperature of 21 0 C. 31 (W-01, W-96, A00) An Ice plant working on a reversed Carnot cycle heat pump produces 15 m tonnes of Ice per day. The ice is formed from water at 0 0 C and the formed ice is maintained at 0 0 C. The heat is rejected to the atmosphere at 25 0 C. The heat pump used to run the ice plant is coupled to a Carnot engine which absorbs heat from a source maintained at C by burring liquid fuel of kj/kg calorific value and rejects heat to the atmosphere.

5 Determine: (i) Power developed by the engine (ii) Fuel consumed per hour. Take enthalpy of fusion of ice 335kJ/kg. 32 (A- 00) Two Carnot engines work in series between the source and sink temperatures of 550 K and 350 K. If both engines develop equal power determine the intermediate temperature. 33 (W-99) It is desired to produce refrigeration at 3 0 C. A reservoir is available at a temperature of C and the ambient temperature is 30 0 C. Thus work can be done by heat engine operating between the C reservoir and the ambient, and this work can be used to drive the refrigerator. Determine the ratio of the heat transferred from the high temperature reservoir to the heat transferred from the refrigerated space, assuming all processes to be reversible.

Chapter 6 THE SECOND LAW OF THERMODYNAMICS

Thermodynamics: An Engineering Approach, 6 th Edition Yunus A. Cengel, Michael A. Boles McGraw-Hill, 2008 Chapter 6 THE SECOND LAW OF THERMODYNAMICS Copyright The McGraw-Hill Companies, Inc. Permission