← MES Jr. Grade
Past Questions — 2012
In a regenerative feed heating cycle, the optimum value of the fraction of steam extracted for feed heating
A)
decreases with increase in Rankine cycle efficiency
B)
increases with increase in Rankine cycle efficiency
C)
is unaffected by increase in Rankine cycle efficiency
D)
none of the above
In a regenerative feed heating cycle, the fraction of steam extracted for feed heating is chosen to maximize the overall cycle performance. As the Rankine cycle efficiency increases, the need for large extraction fractions reduces because the cycle is already operating more effectively. Therefore, the optimum extracted steam fraction decreases with increase in Rankine cycle efficiency. Hence, option (a) is correct.
If a refrigerant is having low specific heat, then coefficient of performance (COP) will be
A)
lower
B)
higher
C)
same
D)
none of these
A refrigerant with low specific heat absorbs less sensible heat in the liquid state before evaporation, so the refrigeration effect per unit mass increases relative to the compressor work in many standard cycle comparisons. Hence the coefficient of performance (COP) tends to be higher. Therefore, the correct option is (b) higher.
The compression ratio of Petrol engine is in the range of
A)
2 to 3
B)
7 to 10
C)
16 to 20
D)
above 20
In a petrol (spark-ignition) engine, the compression ratio is generally kept moderate to avoid knocking. Typical compression ratios are in the range of about 7:1 to 10:1. Option (a) is too low for normal petrol engines, while (c) and (d) are characteristic of diesel engines, which use much higher compression ratios.
A fluid in which resistance of deformation is independent of the shear stress is known as
A)
Bingham plastic fluid
B)
Pseudo plastic fluid
C)
Dilatent fluid
D)
Newtonian fluid
A Newtonian fluid is one in which the viscosity (resistance to deformation) is constant and independent of the applied shear stress. In such fluids, shear stress is directly proportional to rate of shear strain. Bingham plastic fluids require a yield stress before they start flowing, while pseudo plastic and dilatent fluids are non-Newtonian and their viscosity changes with shear rate.
In a free vortex motion, tangential velocity of the water particles is proportional to
A)
R
B)
R2
C)
1/R
D)
1/R2
In a free vortex, there is no external torque acting on the fluid, so angular momentum is conserved. Hence, the tangential velocity varies inversely with radius: v ∝ 1/R. Therefore, the correct option is (c). Options (a), (b), and (d) do not satisfy the free-vortex relation.
In a vapour compression refrigeration, dry compression means
A)
entire compression of vapour should be in wet region
B)
entire compression of vapour should be superheated region
C)
vapour enters the compression in dry saturated and leaves in a wet state
D)
none of these
In vapour compression refrigeration, dry compression means the refrigerant vapour remains dry throughout the compression process, i.e., it does not contain any moisture and is compressed in the superheated region. This avoids liquid droplets entering the compressor. Option (a) is wrong because wet region compression is not dry compression. Option (c) is wrong because entering dry saturated and leaving wet is the opposite of dry compression. Hence, the correct answer is (b).
Psychometric process is
A)
sensible heating
B)
humidification
C)
dehumidification
D)
all of these
A psychrometric process is any process involving a change in the state of moist air on the psychrometric chart. This includes sensible heating, humidification, dehumidification, cooling, and combinations of these. Since all the listed processes are psychrometric processes, the correct answer is 'all of these'.
Temperature of air recorded by an ordinary thermometer is known as
A)
wet bulb temperature
B)
dry bulb temperature
C)
dew point temperature
D)
saturation temperature
The temperature of air measured by an ordinary thermometer exposed to the air is called the dry bulb temperature. A wet bulb thermometer gives wet bulb temperature, dew point is the temperature at which moisture starts condensing, and saturation temperature refers to the temperature at which air becomes saturated with water vapour.
The refrigerant used in air craft is
A)
Ammonia
B)
CO2
C)
air
D)
Freon -12
In aircraft refrigeration systems, air is commonly used as the refrigerant because it is safe, light, readily available, and suitable for air-cycle refrigeration. Ammonia is toxic, CO2 is not the standard choice for aircraft cooling, and Freon-12 is used in vapour compression systems but not typically for aircraft refrigeration in this context.
The Zeroth law deals with thermal equilibrium and establishes a concept of following
A)
Thermal energy
B)
Temperature
C)
Heat
D)
Temperature
The Zeroth law of thermodynamics states that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This law establishes the concept of temperature as the property that determines thermal equilibrium. Hence, it deals with and defines the concept of temperature. Thermal energy and heat are different concepts, and option (d) is a duplicate of (b).
In a four stroke cycle engine, the four operations namely suction, compression, expansion and exhaust are completed in the number of revolutions of crank shaft equal to
A)
four
B)
three
C)
two
D)
one
In a four-stroke engine, one complete cycle requires 4 strokes of the piston: suction, compression, expansion (power), and exhaust. Since the crankshaft completes 2 revolutions for these 4 strokes, the correct answer is two revolutions. Hence, option (c) is correct.
Specific heat is the amount of heat needed to raise the temperature
A)
by unit degree of a substance
B)
by unit degree of unit mass
C)
of a unit mass by ten
D)
None of the above
Specific heat (specific heat capacity) is defined as the amount of heat required to raise the temperature of unit mass of a substance by 1 degree. Option (b) matches this definition. Option (a) is incomplete because it does not specify unit mass, and option (c) is incorrect.
The second law of thermodynamics throws the light on the concept of increase in
A)
Entropy
B)
Temperature
C)
Heat
D)
Pressure
The second law of thermodynamics states that for an isolated system, entropy tends to increase in spontaneous processes. It introduces the concept of entropy as the measure of irreversibility or disorder. Temperature, heat, and pressure are not the central quantity highlighted by the second law in this way.
A refrigerator works on
A)
Bell-Coleman cycle
B)
reversed Carnot cycle
C)
Carnot cycle
D)
both (a) and (b)
A refrigerator is a heat pump operating on a reversed cycle. In theory, the ideal refrigerator works on the reversed Carnot cycle, which gives the maximum possible COP. In practical air refrigeration systems, the Bell-Coleman cycle is used. Since both are associated with refrigerator operation, the correct option is both (a) and (b).
In a refrigeration cycle, the heat is absorbed by a refrigerant in a
A)
evaporator
B)
condenser
C)
compressor
D)
expansion valve
In a refrigeration cycle, the refrigerant absorbs heat from the space to be cooled in the evaporator. Here the low-pressure refrigerant evaporates by taking in latent heat. The condenser rejects heat, the compressor raises pressure and temperature, and the expansion valve reduces pressure.
Which of the following is an extensive property of a thermodynamic system?
A)
Pressure
B)
Volume
C)
Temperature
D)
Density
An extensive property depends on the size or mass of the system. Volume increases when the amount of substance increases, so it is extensive. Pressure, temperature, and density do not depend on system size, so they are intensive properties.
Which one of the following components is not boiler mounting?
A)
Safety valve
B)
Pressure gauge
C)
Feed pump
D)
Stop valve
Boiler mountings are the essential fittings attached to a boiler for safe operation. Safety valve, pressure gauge, and stop valve are all boiler mountings. A feed pump is not a mounting; it is a boiler accessory used to supply water to the boiler.
Temperature of air recorded by a thermometer when the moisture present in it begins to condense, is known as
A)
wet bulb temperature
B)
dry bulb temperature
C)
dew point
D)
saturation temperature
The temperature at which the moisture (water vapour) in air starts condensing is called the dew point. It is the point where air becomes saturated and any further cooling causes condensation. Wet bulb temperature is measured by a wetted thermometer, dry bulb temperature is the normal air temperature, and saturation temperature is a broader term related to phase change at saturation, not specifically the onset of condensation in air.
The sequence of process that eventually returns the working substance to its original state, is known as
A)
event
B)
thermodynamic cycle
C)
thermodynamic property
D)
none of the above
A sequence of thermodynamic processes that brings the working substance back to its initial state is called a thermodynamic cycle. In a cycle, the system ends where it started, so all state properties return to their original values. Option (a) event is unrelated, and option (c) thermodynamic property refers to characteristics like pressure, temperature, and volume, not a process sequence.
The ratio of specific heats of a gas at constant pressure and at constant volume
A)
varies with temperature
B)
varies with pressure
C)
is always constant
D)
none of the above
The ratio of specific heats is denoted by γ = Cp/Cv. For a given gas in the ideal-gas approximation, Cp and Cv are constants for a fixed composition, so their ratio is also constant. Hence the correct option is (c). Options (a) and (b) are not generally true for the standard thermodynamics treatment used in such exams.
An optical pyrometer can measure temperatures up to a limit of
A)
1000ºC
B)
2000ºC
C)
3000ºC
D)
4000ºC
An optical pyrometer is used for very high temperatures, typically in the range where contact thermometers fail. In standard electrical engineering instrumentation texts, its upper measuring limit is about 3000ºC. Hence option (c) is correct. 1000ºC is too low for an optical pyrometer, while 4000ºC is beyond the usual practical limit.
The bank of tubes at the back of domestic refrigerator are
A)
condenser tubes
B)
evaporated tubes
C)
refrigerant cooling tubes
D)
capillary tubes
In a domestic refrigerator, the coil/tube arrangement at the back is the condenser. It rejects heat from the hot, high-pressure refrigerant to the surrounding air, so the refrigerant condenses there. The evaporator is inside the cabinet, capillary tubes are small metering tubes, and 'refrigerant cooling tubes' is not the standard term.
The vacuum recorded in the condenser of a steam power plant is 740mm of mercury. If the barometer reading is 760mm of mercury then the absolute pressure in the condenser is
A)
2668 Pa
B)
6682 Pa
C)
2668 KPa
D)
6826 Pa
Vacuum reading means the pressure in the condenser is below atmospheric pressure by 740 mm of Hg. So, absolute pressure = barometric pressure - vacuum = 760 - 740 = 20 mm of Hg. Converting to pascal: 1 mm Hg ≈ 133.3 Pa, so 20 × 133.3 ≈ 2666 Pa. However, among the given options, the intended answer appears to be 6682 Pa, which corresponds to a likely misprint in the question/options. Since the standard calculation gives about 2668 Pa, the correct physical result is 2668 Pa, matching option (a).
The thermal efficiency of the engine with condenser compared to one without condenser, for a given pressure and temperature of steam is
A)
higher
B)
lower
C)
unaffected
D)
same so long temperature remains constant
A steam engine with a condenser operates at a lower exhaust pressure (vacuum at the outlet). This increases the pressure drop across the engine, so the steam does more work during expansion. For the same steam pressure and temperature at admission, the cycle efficiency therefore increases. Hence, compared to an engine without a condenser, the thermal efficiency is higher.
Required bandwidth of wideband amplifier to reproduce a pulse faithfully, depends upon .............. of the pulse
A)
width
B)
rise time
C)
fall time
D)
frequency
A pulse is reproduced faithfully only if the amplifier can pass the high-frequency components that determine the sharpness of its leading edge. The required bandwidth of a wideband amplifier is therefore mainly related to the pulse's rise time: shorter rise time means higher bandwidth is needed. Width, fall time, and frequency are not the primary determining factor here.