Steam and Gas Turbines Multiple Choice Questions on ” Carnot, Rankine and Reheat Cycle-2″.
1. The cycle of evaporation and rain in nature can be assumed to be a kind of Carnot cycle. If ocean water instead of evaporation are boiled by the heat of Sun at a constant temperature of 100 deg C and the condensation of steam into water droplets within the clouds occur at constant temperature of -20 deg C. Find the thermal efficiency of Carnot cycle of nature?
a) 0.2777
b) 0.3030
c) 0.3333
d) 0.50
Answer: b
Clarification: Carnot thermal efficiency = 1-(Mean temperature of heat addition)/( Mean temperature of heat rejection).
2. How is it advantageous to expand steam at 250 deg C in a turbine as compared to expanding saturated steam in similar turbine?
a) Enhances the thermal efficiency of the cycle
b) Reduces the mechanical damage caused to turbine blades due to wet steam
c) Increases the mean temperature of heat addition
d) All of the mentioned
Answer: d
Clarification: Superheating steam makes expansion within turbine to take place in dry region to a greater extent.
3. In a steam turbine power plant, the boiler feedwater temperature is marked as 30 deg C, ocean as sink with temperature 20 deg C and desired thermal efficiency of the plant is 0.46. Find the temperature of steam entering the turbine?
a) 500degC
b) 505.55degC
c) 507.62degC
d) 509.18degC
Answer: d
Clarification: Mean temperature of heat addition = (Temperature of feedwater + Temperature of steam entering turbine)/2.
4. In a newly developed innovative idea brought up by CSIR in India, gas turbine exhaust gas are being utilized to convert feedwater into steam. Certain data of the experiment are as follows: mass flow rate of exhaust gas = 40 kg/hr, enthalpy of exhaust gas at the time of their exit from gas turbine is 600 kJ/kg, steam turbine output as measured by brake dynamometer = 60 hp-sec, thermal efficiency of steam power plant is 0.25, efficiency of exhaust gas in converting feedwater into steam is taken as 50%. Find the enthalpy of exhaust gas remaining after they generate steam from feedwater?
a) 350.0KJ/Kg
b) 241.2KJ/Kg
c) 112.45KJ/Kg
d) 512.56KJ/Kg
Answer: b
Clarification: Heat added = difference of enthalpies * efficiency of exhaust gas in converting feedwater into steam and then apply thermal efficiency formula for Carnot efficiency.
5. Why does Carnot cycle has maximum efficiency?
a) Since all the processes in Carnot cycle are completely reversible
b) Since only process of expansion and compression are reversible
c) Since only the process of heat addition and heat rejection are reversible
d) Since all processes involved are irreversible
Answer: a
Clarification: All the processes in Carnot cycle are completely reversible.
6. A researcher during an experiment found that mean temperature of heat addition and heat rejection for a steam power plant operating on a Carnot cycle is 127 deg C and 27 deg C respectively. Later, he calculated the thermal efficiency of the steam cycle. Can you predict what was it?
a) 0.20
b) 0.25
c) 0.30
d) 0.35
Answer: b
Clarification: Thermal efficiency of a Carnot cycle is given by (Mean temperature of heat addition-Mean temperature of heat rejection)/(Mean temperature of heat addition).
7. If Carnot cycle has maximum possible thermal efficiency, then why don’t the steam turbine power plants run on Carnot cycle?
a) Because the process of heat addition is at constant temperature
b) As mean temperature of heat addition is quite low in case of Carnot cycle
c) Since work consumed by compressor is approximately equal to work produced by turbine
d) Because boiler can’t be maintained at constant pressure in Carnot cycle
Answer: c
Clarification: As compressor and turbine both work on wet steam, therefore they don’t have significant difference in their work.
8. The mass of coal(lignite) used in boiler of a steam power plant is 20 ton per hour and calorific value of lignite is 13600 KJ/kg. Also, if the entropy of steam entering turbine is 750 J/K and that of feedwater entering the boiler is 550J/K. Find the mean temperature of heat addition?
a) 100 deg C
b) 115 deg C
c) 110 deg C
d) 105 deg C
Answer: d
Clarification: Tmean = Q/(change in entropy).
9. In a solar operated steam power plant, water is converted into steam via solar heat. If the effective area of solar plate is 200m2, value of solar constant is 1.362KW per m2, efficiency of solar components is 17% and overall thermal efficiency of steam cycle desired is 12%. Find the turbine power output?
a) 45KW
b) 22.46KW
c) 5.55KW
d) 16.54KW
Answer: c
Clarification: Efficiency of steam cycle = (Turbine power output/(solar constant*area*solar efficiency)).
10. A boiler feed pump is used to raise ___________
a) Static pressure of feedwater
b) Dynamic pressure of feedwater
c) Static & Dynamic pressure of feedwater
d) None of the mentioned
Answer: a
Clarification: Since water needs to be brought to boiler pressure and is not in motion.
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