Thermodynamics

590 | ThermodynamicsPROBLEMS*Carnot Vapor Cycle10–1C Why is excessive moisture in steam undesirable insteam turbines? What is the highest moisture contentallowed?10–2C Why is the Carnot cycle not a realistic model forsteam power plants?10–3E Water enters the boiler of a steady-flow Carnotengine as a saturated liquid at 180 psia and leaves with aquality of 0.90. Steam leaves the turbine at a pressure of 14.7psia. Show the cycle on a T-s diagram relative to the saturationlines, and determine (a) the thermal efficiency, (b) thequality at the end of the isothermal heat-rejection process,and (c) the net work output. Answers: (a) 19.3 percent,(b) 0.153, (c) 148 Btu/lbm10–4 A steady-flow Carnot cycle uses water as the workingfluid. Water changes from saturated liquid to saturated vaporas heat is transferred to it from a source at 250°C. Heat rejectiontakes place at a pressure of 20 kPa. Show the cycle ona T-s diagram relative to the saturation lines, and determine(a) the thermal efficiency, (b) the amount of heat rejected, inkJ/kg, and (c) the net work output.10–5 Repeat Prob. 10–4 for a heat rejection pressure of10 kPa.10–6 Consider a steady-flow Carnot cycle with water as theworking fluid. The maximum and minimum temperatures inthe cycle are 350 and 60°C. The quality of water is 0.891 atthe beginning of the heat-rejection process and 0.1 at the end.Show the cycle on a T-s diagram relative to the saturationlines, and determine (a) the thermal efficiency, (b) the pressureat the turbine inlet, and (c) the net work output.Answers: (a) 0.465, (b) 1.40 MPa, (c) 1623 kJ/kgThe Simple Rankine Cycle10–7C What four processes make up the simple ideal Rankinecycle?10–8C Consider a simple ideal Rankine cycle with fixedturbine inlet conditions. What is the effect of lowering thecondenser pressure on*Problems designated by a “C” are concept questions, and studentsare encouraged to answer them all. Problems designated by an “E”are in English units, and the SI users can ignore them. Problemswith a CD-EES icon are solved using EES, and complete solutionstogether with parametric studies are included on the enclosed DVD.Problems with a computer-EES icon are comprehensive in nature,and are intended to be solved with a computer, preferably using theEES software that accompanies this text.Pump work input: (a) increases, (b) decreases,(c) remains the sameTurbine work (a) increases, (b) decreases,output: (c) remains the sameHeat supplied: (a) increases, (b) decreases,(c) remains the sameHeat rejected: (a) increases, (b) decreases,(c) remains the sameCycle efficiency: (a) increases, (b) decreases,(c) remains the sameMoisture content (a) increases, (b) decreases,at turbine exit: (c) remains the same10–9C Consider a simple ideal Rankine cycle with fixedturbine inlet temperature and condenser pressure. What is theeffect of increasing the boiler pressure onPump work input: (a) increases, (b) decreases,(c) remains the sameTurbine work (a) increases, (b) decreases,output: (c) remains the sameHeat supplied: (a) increases, (b) decreases,(c) remains the sameHeat rejected: (a) increases, (b) decreases,(c) remains the sameCycle efficiency: (a) increases, (b) decreases,(c) remains the sameMoisture content (a) increases, (b) decreases,at turbine exit: (c) remains the same10–10C Consider a simple ideal Rankine cycle with fixedboiler and condenser pressures. What is the effect of superheatingthe steam to a higher temperature onPump work input: (a) increases, (b) decreases,(c) remains the sameTurbine work (a) increases, (b) decreases,output: (c) remains the sameHeat supplied: (a) increases, (b) decreases,(c) remains the sameHeat rejected: (a) increases, (b) decreases,(c) remains the sameCycle efficiency: (a) increases, (b) decreases,(c) remains the sameMoisture content (a) increases, (b) decreases,at turbine exit: (c) remains the same10–11C How do actual vapor power cycles differ from idealizedones?