Thermodynamics

408 | Thermodynamics7–91 Liquid water enters a 25-kW pump at 100-kPa pressureat a rate of 5 kg/s. Determine the highest pressure the liquidwater can have at the exit of the pump. Neglect the kineticand potential energy changes of water, and take the specificvolume of water to be 0.001 m 3 /kg. Answer: 5100 kPaPUMP100 kPaP 2FIGURE P7–9125 kW7–92E Saturated refrigerant-134a vapor at 15 psia is compressedreversibly in an adiabatic compressor to 80 psia.Determine the work input to the compressor. What wouldyour answer be if the refrigerant were first condensed at constantpressure before it was compressed?7–93 Consider a steam power plant that operates betweenthe pressure limits of 10 MPa and 20 kPa. Steam enters thepump as saturated liquid and leaves the turbine as saturatedvapor. Determine the ratio of the work delivered by the turbineto the work consumed by the pump. Assume the entirecycle to be reversible and the heat losses from the pump andthe turbine to be negligible.7–94 Reconsider Prob. 7–93. Using EES (or other)software, investigate the effect of the quality ofthe steam at the turbine exit on the net work output. Vary thequality from 0.5 to 1.0, and plot the net work output as afunction of this quality.7–95 Liquid water at 120 kPa enters a 7-kW pump whereits pressure is raised to 5 MPa. If the elevation differencebetween the exit and the inlet levels is 10 m, determine thehighest mass flow rate of liquid water this pump can handle.Neglect the kinetic energy change of water, and take the specificvolume of water to be 0.001 m 3 /kg.7–96E Helium gas is compressed from 14 psia and 70°F to120 psia at a rate of 5 ft 3 /s. Determine the power input to thecompressor, assuming the compression process to be (a) isentropic,(b) polytropic with n 1.2, (c) isothermal, and (d) idealtwo-stage polytropic with n 1.2.7–97E Reconsider Prob. 7–96E. Using EES (or other)software, evaluate and plot the work of compressionand entropy change of the helium as functions of thepolytropic exponent as it varies from 1 to 1.667. Discuss yourresults.7–98 Nitrogen gas is compressed from 80 kPa and 27°C to480 kPa by a 10-kW compressor. Determine the mass flowrate of nitrogen through the compressor, assuming the com-pression process to be (a) isentropic, (b) polytropic with n 1.3, (c) isothermal, and (d) ideal two-stage polytropic with n 1.3. Answers: (a) 0.048 kg/s, (b) 0.051 kg/s, (c) 0.063 kg/s,(d) 0.056 kg/s7–99 The compression stages in the axial compressor of theindustrial gas turbine are close coupled, making intercoolingvery impractical. To cool the air in such compressors and toreduce the compression power, it is proposed to spray watermist with drop size on the order of 5 microns into the airstream as it is compressed and to cool the air continuously asthe water evaporates. Although the collision of water dropletswith turbine blades is a concern, experience with steam turbinesindicates that they can cope with water droplet concentrationsof up to 14 percent. Assuming air is compressedisentropically at a rate of 2 kg/s from 300 K and 100 kPa to1200 kPa and the water is injected at a temperature of 20°Cat a rate of 0.2 kg/s, determine the reduction in the exit temperatureof the compressed air and the compressor powersaved. Assume the water vaporizes completely before leavingthe compressor, and assume an average mass flow rate of 2.1kg/s throughout the compressor.7–100 Reconsider Prob. 7–99. The water-injected compressoris used in a gas turbine power plant. It is claimed that thepower output of a gas turbine will increase because of theincrease in the mass flow rate of the gas (air water vapor)through the turbine. Do you agree?Isentropic Efficiencies of Steady-Flow Devices7–101C Describe the ideal process for an (a) adiabatic turbine,(b) adiabatic compressor, and (c) adiabatic nozzle, anddefine the isentropic efficiency for each device.7–102C Is the isentropic process a suitable model for compressorsthat are cooled intentionally? Explain.7–103C On a T-s diagram, does the actual exit state (state2) of an adiabatic turbine have to be on the right-hand side ofthe isentropic exit state (state 2s)? Why?7–104 Steam enters an adiabatic turbine at 8 MPa and500°C with a mass flow rate of 3 kg/s and leaves at 30 kPa.The isentropic efficiency of the turbine is 0.90. Neglecting8 MPa500°CSTEAMTURBINEη T = 90%30 kPaFIGURE P7–104