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PROB SOLVING PBE > HEAT TRA > Flashcards

HEAT TRA Flashcards

1
Q

Crystal size in a continuous crystalliser depends upon the
-rate of heat transfer.
-degree of turbulence.
-degree of supersaturation.
-all (a), (b) and (c).

A

all (a), (b) and (c).

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2
Q

Heat waves
-can not pass through vacuum.
-travel in straight line.
-can be reflected by a mirror.
-both (b) and (c).

A

-both (b) and (c).

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3
Q

Air is best heated with steam in a heat exchanger of
-plate type.
-double pipe type with fin on steam side.
-double pipe type with fin on air side.
-shell and tube type.

A

-double pipe type with fin on air side.

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4
Q

Which of the following is generally considered as opaque surface towards radiations?
-Gases
-Solids
-Liquids
-Both (b) and (c)

A

Both (b) and (c)

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5
Q

Black liquor generated during paper manufacture is concentrated in a
-single effect evaporator.
-single effect evaporator followed by a crystalliser.
-multiple effect evaporator.
-multiple effect evaporators followed by a crystalliser.

A

-multiple effect evaporator.

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6
Q

When vaporisation takes place directly at the heating surface, it is called
-film boiling
-nucleate boiling
-vapour binding
-none of these

A

-nucleate boiling

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7
Q

Steam consumption in kg/hr in case of an evaporator is given by (where, C & E are capacity the economy of the evaporator respectively)
-C/E
-E/C
-CE
-1/CE

A

C/E

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8
Q

Reynold’s analogy states that
Nst α f
Nst α NRe
NNu α f
NRe α f

A

Nst α f

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9
Q

With increase in temperature, the thermal conductivity of non-metallic amorphous solids
-decreases
-increases
-remains constant
-first decreases upto certain temperature and then increases

A

increases

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10
Q

Fourier’s law applies to the heat transfer by
-convection
-radiation
-conduction
-all (a), (b) & (c)

A

conduction

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11
Q

Shell side pressure drop in a shell and tube heat exchanger does not depend upon the
-baffle spacing & shell diameter.
-tube diameter & pitch.
-viscosity, density & mass velocity of shell side fluid.
-none of these.

A

-none of these.

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12
Q

In the equation Q = UAΔt; Δt is
-geometric mean temperature difference.
-arithmetic mean temperature difference.
-logarithmic mean temperature difference.
-the difference of average bulk temperatures of hot and cold fluids.

A

-logarithmic mean temperature difference.

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13
Q

With increase in porosity, the thermal conductivity of a solid substance
-increases
-decreases
-remains unchanged
-may increase or decrease; depends on the solid

A

-decreases

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14
Q

For an ideal black body
absorptivity = 1
reflectivity = 1
emissivity = 0
transmissivity = 1

A

absorptivity = 1

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15
Q

In Joule’s experiment, an insulated container contains 20 kg of water initially at 25°C. It is stirred by an agitator, which is made to turn by a slowly falling body weighing 40 kg through a height of 4 m. The process is repeated 500 times. The acceleration due to gravity is 9.8 ms-2. Neglecting the heat capacity of agitator, the temperature of water (in °C) is
40.5
34.4
26.8
25

A

34.4

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16
Q

Fouling factor
-is a dimensionless quantity.
-does not provide a safety factor for design.
-accounts for additional resistances to heat flow.
-none of these.

A

accounts for additional resistances to heat flow.

17
Q

In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient
-increases
-decreases
-is not affected
-may increase or decrease; depends on the feed

A

decreases

18
Q

The steam ejector is used to
-remove condensate from the steam pipelines.
-create vacuum.
-superheat the steam.
-none of these.

A

-create vacuum.

19
Q

For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area
-increases
-decreases
-remains constant
-passes through a maxima

A

-passes through a maxima

20
Q

The thermal efficiency of a reversible heat engine operating between two given thermal reservoirs is 0.4. The device is used either as a refrigerator or as a heat pump between the same reservoirs. Then the coefficient of performance as a refrigerator (COP)R and the co-efficient of performance as a heat pump (COP)HP are
-(COP)R = (COP)HP = 0.6
-(COP)R = 2.5; (COP)HP = 1.5
-(COP)R = 1.5; (COP)HP = 2.5
-(COP)R = (COP)HP = 2.5

A

-(COP)R = 1.5; (COP)HP = 2.5

21
Q

Hot water (0.01 m3 /min) enters the tube side of a counter current shell and tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m3/min) of density 800 kg/m3 and specific heat of 2 kJ/kg.K enters at 20°C. The log mean temperature difference in °C is approximately
32
37
45
50

A

32

22
Q

In a heat exchanger, floating head is provided to
-facilitate cleaning of the exchanger.
-increase the heat transfer area.
-relieve stresses caused by thermal expansion.
-increase log mean temperature gradient.

A

-relieve stresses caused by thermal expansion.

23
Q

Leidenfrost point is a term concerned with the
-condensation of the saturated vapor on a cold surface.
-concentration of a corrosive solution by evaporation.
-heat transfer between two highly viscous liquids.
-boiling of a liquid on a hot surface.

A

-boiling of a liquid on a hot surface.

24
Q

A process stream of dilute aqueous solution flowing at the rate of10 Kg.s-1 is to be heated. Steam condensate at 95°C is available for heating purpose, also at a rate of 10 Kg.s-1. A 1 - 1 shell and tube heat exchanger is available. The best arrangement is
-counter flow with process stream on shell side.
-counter flow with process stream on tube side.
-parallel flow with process stream on shell side.
-parallel flow with process stream on tube side.

A

-counter flow with process stream on shell side.

25
Q

Heat produced when a steady state current, I passes through an electrical conductor having resistance, ‘R’ is
IR
I2R
IR2
I2R2

A

I2R

26
Q

Pick out the wrong statement.
-Orifice baffles are never used in a shell and tube heat exchanger.
-Pressure drop on the shell side of a heat exchanger depends upon tube pitch also.
-In a horizontal tube evaporator, surface blanketing by air is avoided.
-Split ring type and pull through type floating heads are two commonly used floating heads is heat exchangers.

A

-Orifice baffles are never used in a shell and tube heat exchanger.

27
Q

Convective heat transfer co-efficient in case of fluid flowing in tubes is not affected by the tube length/diameter ratio, if the flow is in the __________ zone.
-laminar
-transition
-both ‘a’ & ‘b’
-highly turbulent

A

highly turbulent

28
Q

The actual temperature drop across the heating surface in an evaporator depends on the
-feed.
-depth of liquid over heating surface.
-pressure difference between steam chest and vapour space.
-all (a), (b) and (c).

A

all (a), (b) and (c).

29
Q

Which of the following is not used as a medium for high temperature heating ?
-Dowtherm
-mercury
-liquid metal (e.g. molten sodium)
-fused salts (e.g., an eutectic mixture of 53% KNO3, 40% NaNO2and 7% NaNO3)

A

mercury

30
Q

Presence of a non-condensing gas in a condensing vapour
-increases the rate of condensation.
-decreases thermal resistance.
-is desirable to increase the film co-efficient.
-none of these.

A

none of these.

31
Q

Kirchoff s law applies to __________ radiation.
-total
-monochromatic
-both (a) & (b)
-neither (a) nor (b)

A

-both (a) & (b)

32
Q

With the increase of temperature, the Col-burn jH factor
-increases.
-decreases.
-remains unchanged.
-may increase or decrease ; depending on temperature.

A

increases.

33
Q

In case of evaporators, liquid entrainment results primarily due to
-high vacuum in the evaporator.
-high evaporation rate.
-foaming of the solution.
-high heat transfer rate.

A

-foaming of the solution.

34
Q

Baffles in the shell side of a shell and tube heat exchanger
-increase the cross-section of the shell side liquid.
-force the liquid to flow parallel to the bank.
-increase the shell side heat transfer co-efficient.
-decrease the shell side heat transfer co-efficient

A

-increase the shell side heat transfer co-efficient.

35
Q

Air is to be heated by condensing steam. Two heat exchangers are available (i) a shell and tube heat exchanger and (ii) a finned tube heat exchanger. Tube side heat transfer area are equal in both the cases. The recommended arrangement is
-finned tube heat exchanger with air inside and steam outside.
-finned tube heat exchanger with air outside and steam inside.
-shell and tube heat exchanger with air inside tubes and steam on shell side.
-shell and tube heat exchanger with air on shell side and steam inside tubes.

A

-finned tube heat exchanger with air outside and steam inside.

PROB SOLVING PBE (11 decks)

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