Thermodynamics

422 | Thermodynamicsisentropic efficiency of 80 percent, the power produced bythe turbine is(a) 194 kW (b) 291 kW (c) 484 kW(d) 363 kW(e) 605 kW7–228 Water enters a pump steadily at 100 kPa at a rate of35 L/s and leaves at 800 kPa. The flow velocities at the inletand the exit are the same, but the pump exit where the dischargepressure is measured is 6.1 m above the inlet section.The minimum power input to the pump is(a) 34 kW (b) 22 kW (c) 27 kW(d) 52 kW(e) 44 kW7–229 Air at 15°C is compressed steadily and isothermallyfrom 100 kPa to 700 kPa at a rate of 0.12 kg/s. The minimumpower input to the compressor is(a) 1.0 kW (b) 11.2 kW (c) 25.8 kW(d) 19.3 kW (e) 161 kW7–230 Air is to be compressed steadily and isentropicallyfrom 1 atm to 25 atm by a two-stage compressor. To minimizethe total compression work, the intermediate pressurebetween the two stages must be(a) 3 atm (b) 5 atm (c) 8 atm(d) 10 atm(e) 13 atm7–231 Helium gas enters an adiabatic nozzle steadily at500°C and 600 kPa with a low velocity, and exits at a pressureof 90 kPa. The highest possible velocity of helium gas atthe nozzle exit is(a) 1475 m/s (b) 1662 m/s (c) 1839 m/s(d) 2066 m/s (e) 3040 m/s7–232 Combustion gases with a specific heat ratio of 1.3enter an adiabatic nozzle steadily at 800°C and 800 kPa witha low velocity, and exit at a pressure of 85 kPa. The lowestpossible temperature of combustion gases at the nozzle exit is(a) 43°C (b) 237°C (c) 367°C(d) 477°C (e) 640°C7–233 Steam enters an adiabatic turbine steadily at 400°Cand 3 MPa, and leaves at 50 kPa. The highest possible percentageof mass of steam that condenses at the turbine exitand leaves the turbine as a liquid is(a) 5% (b) 10% (c) 15%(d) 20% (e) 0%7–234 Liquid water enters an adiabatic piping system at15°C at a rate of 8 kg/s. If the water temperature rises by0.2°C during flow due to friction, the rate of entropy generationin the pipe is(a) 23 W/K (b) 55 W/K (c) 68 W/K(d) 220 W/K (e) 443 W/K7–235 Liquid water is to be compressed by a pump whoseisentropic efficiency is 75 percent from 0.2 MPa to 5 MPa at arate of 0.15 m 3 /min. The required power input to this pump is(a) 4.8 kW (b) 6.4 kW (c) 9.0 kW(d) 16.0 kW (e) 12 kW7–236 Steam enters an adiabatic turbine at 8 MPa and500°C at a rate of 18 kg/s, and exits at 0.2 MPa and 300°C.The rate of entropy generation in the turbine is(a) 0 kW/K (b) 7.2 kW/K (c) 21 kW/K(d) 15 kW/K (e) 17 kW/K7–237 Helium gas is compressed steadily from 90 kPa and25°C to 600 kPa at a rate of 2 kg/min by an adiabatic compressor.If the compressor consumes 70 kW of power whileoperating, the isentropic efficiency of this compressor is(a) 56.7% (b) 83.7% (c) 75.4%(d) 92.1% (e) 100.0%Design and Essay Problems7–238 It is well-known that the temperature of a gas riseswhile it is compressed as a result of the energy input in theform of compression work. At high compression ratios, theair temperature may rise above the autoignition temperatureof some hydrocarbons, including some lubricating oil. Therefore,the presence of some lubricating oil vapor in highpressureair raises the possibility of an explosion, creating afire hazard. The concentration of the oil within the compressoris usually too low to create a real danger. However, the oilthat collects on the inner walls of exhaust piping of the compressormay cause an explosion. Such explosions havelargely been eliminated by using the proper lubricating oils,carefully designing the equipment, intercooling betweencompressor stages, and keeping the system clean.A compressor is to be designed for an industrial applicationin Los Angeles. If the compressor exit temperature is notto exceed 250°C for safety consideration, determine the maximumallowable compression ratio that is safe for all possibleweather conditions for that area.7–239 Identify the major sources of entropy generation inyour house and propose ways of reducing them.7–240 Obtain the following information about a powerplant that is closest to your town: the net power output; thetype and amount of fuel; the power consumed by the pumps,fans, and other auxiliary equipment; stack gas losses; temperaturesat several locations; and the rate of heat rejection at thecondenser. Using these and other relevant data, determine therate of entropy generation in that power plant.7–241 Compressors powered by natural gas engines areincreasing in popularity. Several major manufacturing facilitieshave already replaced the electric motors that drive their compressorsby gas driven engines in order to reduce their energybills since the cost of natural gas is much lower than the costof electricity. Consider a facility that has a 130-kW compressorthat runs 4400 h/yr at an average load factor of 0.6. Makingreasonable assumptions and using unit costs for natural gasand electricity at your location, determine the potential costsavings per year by switching to gas driven engines.