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WorksheetsTHERMAL ENGINEERING II MCQ (MODEL EXAM - 1)
Total questions: 45
Worksheet time: 45mins
At the given inlet condition, a nozzle is said to be choked when mass flow rate is
maximum
is discharging into atmosphere
zero
it is not possible to increase the flow by increasing inlet pressure
The ratio of useful heat drop to the isentropic enthalpy drop is defined as
Total enthalpy drop
Nozzle efficiency
Local enthalpy drop
Rate of heat change
The velocity of fluid at stagnation point is
Zero
Infinite
Very high positive number
Very high Negative
Unity Mach number is observed in this part of a converging-diverging nozzle, in the chocked condition
Convergent portion
Throat
Divergent portion
None of the above
A steam nozzle converts heat energy of steam into
Potential energy
Kinetic energy
Another form of heat energy
None of the above
In a nozzle, friction ___________ the dryness fraction of steam at the exit
Decreases
Same
Increases
None of the above
Steam is expanded in a set of nozzles from 5 bar 300 deg. Celcius to 2 bar. If initial velocity is neglected and isentropic expansion is assumed, then find the minimum area (sq.cm) of the nozzles to flow 1.5kg/s of steam. Consider the isentropic index k=1.3.
22.97
12.32
07.03
09.15
Air having a flow rate of 3.6 kg/s at 7 bar and 170 deg.Celcius expands through a convergent-divergent nozzle into a space at 1.03 bar. Calculate the throat area (sq.mm) of the nozzle. Assume isentropic flow and neglect inlet velocity
1463
3645
2680
2185
The throat and exit areas of a convergent-divergent nozzle are 1280 sq.mm and 1600 sq.mm respectively. Dry saturated steam at 5 bar enters the nozzle at a velocity of 80 m/s. The exit pressure is 1.5 bar. Estimate the mass flow rate in kg/s. Assume isentropic flow up to the throat and take critical pressure ratio to be 0.58
0.689
0.459
0.958
0.632
Which type of steam separator is used for low pressure (<20 bar) boiler?
Cyclone Separator
Baffle Separator
Gravity Separator
Choice of Separator doesn’t depend on boiler pressure
What is the function of air preheater?
To increase air temperature by flue gases
To increase water temperature by air
To increase steam temperature by air
To increase air temperature by steam
In a gas power cycle, hot combustion gas having specific heat 0.98 kJ/Kg-K enters the turbine at 25 bar, 1300 K and exits at 1 bar. The ratio of specific heat of the working fluid is 1.3. The isentropic efficiency of the turbine is 92%. The work developed (kJ/kg) by the turbine is
614.44
725.95
667.87
576.82
A certain chemical plant requires heat from process steam at 125° C at the rate of 6 MJ/s and power at the rate of 1000 kW from the generator terminal. Both the heat and power requirements are met by a back pressure turbine with 85% brake (mechanical) and 80% internal (isentropic) efficiency, which exhausts steam at 125° C in dry saturated state. All the latent heat release during the condensation is utilized in the process heater. The thermodynamic states of water and saturated vapour are given in the table. Here, h is specific enthalpy, s is specific entropy and v the specific volume; subscripts f and g denote saturated liquid state and saturated vapour state respectively.
The enthalpy (kJ/kg) of the steam at the inlet of the turbine is (a)
A binary-vapour cycle operates on mercury and steam. Saturated mercury at 4.5 bar is supplied to the mercury turbine, from which it exhausts at 0.04 bar. The mercury condenser generates saturated steam at 15 bar which is expanded in a steam turbine to 0.04 bar. The thermodynamic states of saturated mercury and saturated vapour are given in the table. Here, h is specific enthalpy, s is specific entropy and v the specific volume; subscripts f and g denote saturated liquid state and saturated vapour state respectively.
The amount of mercury (kg) circulated for 1 kg of steam in the bottoming cycle is (a)
What is the pH value of water permissible for boiler?
0
5.5
6
8.5
The evaporation of 15.653 kg of water per hour from and at 100°C is called
evaporative capacity
factor of evaporation
equivalent evaporation
one boiler h.p.
The air pressure at the fuel bed is reduced below that of atmosphere by means of a fan placed at or near the bottom of the chimney to produce a draught. Such a draught is called
Natural draught
Induced draught
Forced draught
Balanced draught
The ideal combustion process during which methane is burned completely. The air fuel (AF) ratio for this combustion on a mass basis is
13.76 kg air / kg fuel
17.19 kg air / kg fuel
21.17 kg air / kg fuel
23.12 kg air / kg fuel
Steam enters an impulse turbine at a velocity of 500 m/s and having nozzle angle 20 deg. The symmetrical blades have a mean peripheral velocity of 300 m/s. Consider a mass flow rate of 0.75 kg/s. Find the blade angle in degree, assuming the steam enters the blade without shock.
15.1
45.2
25.5
10.4
Steam enters an impulse turbine at a velocity of 500 m/s and having nozzle angle 20 deg. The symmetrical blades have a mean peripheral velocity of 300 m/s. Consider a mass flow rate of 0.75 kg/s. Calculate the tangential forces on the blades (N) for the turbine . Neglect friction effects on blades.
546.36
498.65
660.54
254.77
Steam enters an impulse turbine at a velocity of 500 m/s and having nozzle angle 20 deg. The symmetrical blades have a mean peripheral velocity of 300 m/s. Consider a mass flow rate of 0.75 kg/s. Estimate the axial thrust (N), assuming the steam enters the blade without shock.
0
0.01
0.15
1.52
Steam enters an impulse turbine at a velocity of 700 m/s which has nozzle angle 20 deg. The symmetrical blades have a mean peripheral velocity of 300 m/s. Consider a mass flow rate of 0.75 kg/s. Estimate the axial thrust (N), if the relative velocity is reduced to 80% of that at inlet due to friction
65.38
25.45
35.91
41.04
Steam enters an impulse turbine at a velocity of 700 m/s which has nozzle angle 20 deg. The symmetrical blades have a mean peripheral velocity of 300 m/s. Consider a mass flow rate of 0.75 kg/s. Estimate the diagram power (kW) if the relative velocity is reduced to 80% of that at inlet due to friction
144.84
543.55
232.36
182.90
Steam enters an impulse turbine at a velocity of 700 m/s which has nozzle angle 20 deg. The symmetrical blades have a mean peripheral velocity of 300 m/s. Consider a mass flow rate of 0.75 kg/s. Estimate the diagram efficiency (in %), if the relative velocity is reduced to 80% of that at inlet due to friction
76.21
79.54
57.65
78.82
A 50% reaction turbine is rotating at 2600 rpm with a mean blade speed of 120 m/s. The exit angle of blades is 20 deg. and the velocity ratio is 0.56. Calculate the mass flow of steam through the turbine in kg/h. The mean specific volume of steam is 0.65 cub.m/kg and the mean height of blade is 25 mm. Neglect effect of blade thickness on the annulus area.
28044
25365
35236
20365
A 50% reaction turbine is rotating at 2600 rpm with a mean blade speed of 120 m/s. The exit angle of blades is 20 deg. and the velocity ratio is 0.56. Calculate the diagram power (kW) if there are five pair of blades in the group.. The mean specific volume of steam is 0.65 cub.m/kg and the mean height of blade is 25 mm. Neglect effect of blade thickness on the annulus area.
1324.3
1635.8
1864.2
1565.3
A steam turbine is to operate between 150 bar, 600 deg.Celcius and 0.1 bar. The bucket velocity is limited to 250 m/s and the average nozzle efficiency is expected to be 95% for impulse and 90% for reaction turbine. All stages operate close to the speed corresponding to maximum efficiency. Nozzle angles can be assumed as 20 deg. for impulse and reaction stages. Estimate the number of stages required if all the stages are simple impulse stage.
15
10
5
20
Which of the following statements is not correct
A topping cycle plant generates electricity or mechanical power first
A bottoming cycle plant generates heat first
Both a and b
A bottoming cycle plant generates electricity or mechanical power first
Which of the following statements is not correct
A topping cycle plant generates electricity or mechanical power first
A bottoming cycle plant generates heat first
Both a and b
A bottoming cycle plant generates electricity or mechanical power first
What is a typical efficiency of a cogeneration system?
58%
85%
68%
95%
In a combined cycle power plant consisting of gas turbine and waste heat boiler, the exhaust gas temperature is ____.
around 150 °C
around 500 °C
around 300 °C
around 400 °C
Which of the following depends on physical properties of fluids as well as geometry of the heat exchanger?
Fouling coefficient
Overall heat transfer coefficient
LMTD (Log Mean Temperature Difference)
Effectiveness
A waste heat recovery system (Shell and Tube heat exchanger) receives hot fluid at 200°C and leaves at 70°C, cold medium enters at 30°C and leaves at 90°C, the type of flow involved in this is
cross-flow
co- current flow
counter-current flow
none of the above
Radiation recuperators are used when the furnace flue gas temperature is more than
200ºC
400ºC
500ºC
800ºC
The major limitation of metallic recuperator is -------
limitation of handling COx, NOx etc.
limitation of reduced life for handling temperature more than 1000°C
manufacturing difficulty of the required design
none of the above
Combined cycle is a combination of
conventional boiler first followed by cogeneration system
Rankine cycle first followed by a Brayton cycle
Brayton cycle first followed by a standard Rankine cycle
extraction in the first portion and then a condensing cycle
Which of the following cycles uses ‘air’ as the refrigerant
Ericson
Bell Coleman
Stirling
Carnot
The temperature limits of a heat pump cycle are +27°C and -23°C, then the Carnot COP will be
0.2
1.2
5
6
A cold storage is to be maintained at – 5°C while the surroundings are at 35°C. The heat leakage from the surroundings into the cold storage is estimated to be 29 kW. The actual C.O.P. of the refrigeration plant used is one third that of an ideal plant working between the same temperatures. Find the power required to drive the plant
11.58 kJ/s
12.98 kJ/s
12.90 kJ/s
10.98 kJ/s
The compression, condensation and expansion of refrigerant in a vapour compression refrigeration system is
Isenthalpic, isentropic and isothermal
Isentropic, Isochoric and isothermal
Isothermal, Isentropic and Isochoric
Isentropic, Isobaric and Isenthalpic
In a vapor compression refrigeration plant, the refrigerant leaves the evaporator at 195 KJ/kg and the condenser at 65 KJ/kg. For every kg of refrigerant, the plant can supply per second, a cooling load of
70 KW
130 KW
160 KW
100 KW
If the specific heats of dry air and water vapour are 1.00 kJ/kg-K and 1.88 kJ/kg-K respectively and the humidity ratio is 0.011, then the specific heat of moist air at 25°C and 50% relative humidity will be
1.869 kJ/kg-K
1.891 kJ/kg-K
0.9793 kJ/kg-K
1.0206 kJ/kg-K
If air at dry-bulb temperature of 35° C and dew point temperature of 20° C passes through a cooling coil which is maintained at 25° C, then the process would be
Sensible cooling
Cooling and dehumidification
Cooling and humidification
Cooling at constant wet bulb temperature
Sensible heat factor is given by where S.H. = Sensible heat, and L.H.= Latent heat
It is desired to condition the outside air from 70% relative humidity and 45°C dry bulb temperature to 50% relative humidity and 25% dry bulb temperature (room condition). The practical arrangement will be
Dehumidification
Cooling and humidification
Cooling and dehumidification
Dehumidification and pure sensible cooling
