Thermodynamics

The Reheat Rankine Cycle10–29C How do the following quantities change when asimple ideal Rankine cycle is modified with reheating?Assume the mass flow rate is maintained the same.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 sameMoisture content (a) increases, (b) decreases,at turbine exit: (c) remains the same10–30C Show the ideal Rankine cycle with three stages ofreheating on a T-s diagram. Assume the turbine inlet temperatureis the same for all stages. How does the cycle efficiencyvary with the number of reheat stages?10–31C Consider a simple Rankine cycle and an idealRankine cycle with three reheat stages. Both cycles operatebetween the same pressure limits. The maximum temperatureis 700°C in the simple cycle and 450°C in the reheat cycle.Which cycle do you think will have a higher thermalefficiency?10–32 A steam power plant operates on the idealreheat Rankine cycle. Steam enters the highpressureturbine at 8 MPa and 500°C and leaves at 3 MPa.Steam is then reheated at constant pressure to 500°C before itexpands to 20 kPa in the low-pressure turbine. Determine theturbine work output, in kJ/kg, and the thermal efficiency ofthe cycle. Also, show the cycle on a T-s diagram with respectto saturation lines.10–33 Reconsider Prob. 10–32. Using EES (or other)software, solve this problem by the diagramwindow data entry feature of EES. Include the effects of theturbine and pump efficiencies and also show the effects ofreheat on the steam quality at the low-pressure turbine exit.Plot the cycle on a T-s diagram with respect to the saturationlines. Discuss the results of your parametric studies.10–34 Consider a steam power plant that operates on areheat Rankine cycle and has a net power output of 80 MW.Steam enters the high-pressure turbine at 10 MPa and 500°Cand the low-pressure turbine at 1 MPa and 500°C. Steamleaves the condenser as a saturated liquid at a pressure of10 kPa. The isentropic efficiency of the turbine is 80 percent,and that of the pump is 95 percent. Show the cycle on a T-sdiagram with respect to saturation lines, and determine(a) the quality (or temperature, if superheated) of the steam atthe turbine exit, (b) the thermal efficiency of the cycle, and(c) the mass flow rate of the steam. Answers: (a) 88.1°C,(b) 34.1 percent, (c) 62.7 kg/sChapter 10 | 59310–35 Repeat Prob. 10–34 assuming both the pump and theturbine are isentropic. Answers: (a) 0.949, (b) 41.3 percent,(c) 50.0 kg/s10–36E Steam enters the high-pressure turbine of a steampower plant that operates on the ideal reheat Rankine cycle at800 psia and 900°F and leaves as saturated vapor. Steam isthen reheated to 800°F before it expands to a pressure of1 psia. Heat is transferred to the steam in the boiler at a rate of6 10 4 Btu/s. Steam is cooled in the condenser by the coolingwater from a nearby river, which enters the condenser at 45°F.Show the cycle on a T-s diagram with respect to saturationlines, and determine (a) the pressure at which reheating takesplace, (b) the net power output and thermal efficiency, and(c) the minimum mass flow rate of the cooling water required.10–37 A steam power plant operates on an ideal reheat Rankinecycle between the pressure limits of 15 MPa and 10 kPa.The mass flow rate of steam through the cycle is 12 kg/s. Steamenters both stages of the turbine at 500°C. If the moisture contentof the steam at the exit of the low-pressure turbine is not toexceed 10 percent, determine (a) the pressure at which reheatingtakes place, (b) the total rate of heat input in the boiler, and(c) the thermal efficiency of the cycle. Also, show the cycle ona T-s diagram with respect to saturation lines.10–38 A steam power plant operates on the reheat Rankinecycle. Steam enters the high-pressure turbine at 12.5 MPaand 550°C at a rate of 7.7 kg/s and leaves at 2 MPa. Steam isthen reheated at constant pressure to 450°C before it expandsin the low-pressure turbine. The isentropic efficiencies of theturbine and the pump are 85 percent and 90 percent, respectively.Steam leaves the condenser as a saturated liquid. If themoisture content of the steam at the exit of the turbine is notto exceed 5 percent, determine (a) the condenser pressure,(b) the net power output, and (c) the thermal efficiency.Answers: (a) 9.73 kPa, (b) 10.2 MW, (c) 36.9 percent2Boiler45Pump3FIGURE P10–38Turbine16Condenser