Heat Transfer Flashcards by Pauline Yarah Morales

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

All

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

double pipe type with fin on air side.

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

multiple effect evaporator.

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When vaporisation takes place directly at the heating surface, it is called
a. film boiling
b. nucleate boiling
c. vapour binding
d. none of these

nucleate boiling

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

C/E

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Reynold’s analogy states that
a. Nst α f
b. Nst α NRe
c. NNu α f
d. NRe α f

Nst α f

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

increases

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

conduction

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

none of these.

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

logarithmic mean temperature difference.

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

decreases

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For an ideal black body
a. absorptivity = 1
b. reflectivity = 1
c. emissivity = 0
d. transmissivity = 1

absorptivity = 1

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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
a. 40.5
b. 34.4
c. 26.8
d. 25

34.4

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

accounts for additional resistances to heat flow

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

decreases

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The steam ejector is used to
a. remove condensate from the steam pipelines.
b. create vacuum.
c. superheat the steam.
d. none of these.

create vacuum.

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

passes through a maxima

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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
a. (COP)R = (COP)HP = 0.6
b. (COP)R = 2.5; (COP)HP = 1.5
c. (COP)R = 1.5; (COP)HP = 2.5
d. (COP)R = (COP)HP = 2.5

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

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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
a. 32
b. 37
c. 45
d. 50

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

relieve stresses caused by thermal expansion.

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

boiling of a liquid on a hot surface.

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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
a. counter flow with process stream on shell side.
b. counter flow with process stream on tube side.
c. parallel flow with process stream on shell side.
d. parallel flow with process stream on tube side.

counter flow with process stream on shell side.

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

I2R

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

all

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

mercury

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

none of these.

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

increases

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

foaming of the solution

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

increase the shell side heat transfer co-efficient

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 a. finned tube heat exchanger with air inside and steam outside. b. finned tube heat exchanger with air outside and steam inside. c. shell and tube heat exchanger with air inside tubes and steam on shell side. d. shell and tube heat exchanger with air on shell side and steam inside tubes.

finned tube heat exchanger with air outside and steam inside.

The unit of heat transfer co-efficient in SI unit is a. J/M2°K b. W/m2°K c. W/m°K d. J/m°K

W/m2°K

The number of kg vaporised per kg of steam fed to the evaporator is defined as a. capacity b. rate of evaporation c. economy d. rate of vaporisation

economy

Pick out the wrong statement. a. In case of heat transfer by purely forced convection, GR/Re 2 ≤ 1. b. The equivalent diameter of heat transfer for a duct of square cross-section (having each side as 'x') is equal to 4x. c. Distillation process is not the same as evaporation. d. The effectiveness of nucleate boiling depends basically on the ease with which the bubbles are formed and detached from the heating surface.

The equivalent diameter of heat transfer for a duct of square cross-section (having each side as 'x') is equal to 4x.

Economy of a multiple effect evaporator is not influenced much by the a. boiling point elevations b. temperature of the feed c. rate of heat transfer d. ratio of the weight of the thin liquor to thick liquor

boiling point elevations

Increasing the liquor level in the evaporator results in the a. decreased capacity. b. increase in liquor film co-efficient. c. decreased effect of hydrostatic head. d. increased true temperature drop.

decreased capacity.

The equivalent diameter for pressure drop is __________ that for heat transfer. a. smaller than b. greater than c. equal to d. not related with

smaller than

Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer. a. no b. the least c. the largest d. lower

the largest

The heat flux (from outside to inside) across an insulating wall with thermal conductivity, K= 0.04 W/m.°K and thickness 0.16m is 10 W/m2. The temperature of the inside wall is - 5°C. The outside wall temperature is a. 25°C b. 30°C c. 35°C d. 40°C

35°C

Pick out the wrong statement. a. The controlling resistance in case of heating of air by condensing steam is in the air film. b. The log mean temperature difference (LMTD) for counter flow and parallel flow can be theoretically same when any one of the fluids (hot or cold fluid) passes through the heat exchanger at constant temperature. c. In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs. d. Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice-versa occurs at saturation temperature.

In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.

The average heat transfer co-efficient for laminar film condensation on vertical surface is inversely proportional to (where, ΔT = Temperature drop across condensate film ) a. (ΔT)^2 b. ΔT c. (ΔT)^1/4 d. (ΔT)^3/2

(ΔT)^1/4

Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is a. in the central core of the fluid. b. uniformly distributed throughout the fluid. c. mainly confined to a thin film of fluid near the surface. d. none of these.

mainly confined to a thin film of fluid near the surface.

Which of the following is concerned with both heat and mass transfer ? a. Lewis relationship b. Nusselt number c. Kutateladze number d. Froude number

Lewis relationship

A 10 cm dia steam pipe, carrying steam at 180°C, is covered with an insulation (conductivity = 0.6 W/m.°C). It losses heat to the surroundings at 30°C. Assume a heat transfer co-efficient of 0.8 W/m2.°C for heat transfer from surface to the surroundings. Neglect wall resistance of the pipe and film resistance of steam. If the insulation thickness is 2 cms, the rate of heat loss from this insulated pipe will be a. greater than that for uninsulated steam pipe. b. less than that of the uninsulated steam pipe. c. equal to that of the uninsulated steam pipe. d. less than the steam pipe with 5 cms insulation.

less than that of the uninsulated steam pipe.

Which area is used in case of heat flow by conduction through a cylinder ? a. Logarithmic mean area b. Arithmetic mean area c. Geometric mean area d. None of these

Logarithmic mean area

The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is a. Nusselt number. b. Sherwood number. c. Schmidt number. d. Stanton number.

Schmidt number

Finned tube heat exchangers a. give larger area per tube. b. use metal fins of low thermal conductivity. c. facilitate very large temperature drop through tube wall. d. are used for smaller heat load.

give larger area per tube.

The absorptivity of a grey body at a given temperature __________ with increasing wavelength of radiation. a. increases b. decreases c. remains constant d. may increase or decrease ; depends on the material

may increase or decrease ; depends on the material

Which one gives the monochromatic emissive power for black body radiation ? a. Planck's law b. Kirchhoffs law c. Wien's law d. Stefan-Boltzman law

Planck's law

For what value of Prandtl number, St = f/2? a. 1.5 b. 1 c. > 1 d. < 1

In a shell and tube heat exchanger, a. the temperature drops in the two fluids and the wall are proportional to individual resistances. b. the temperature drop is inversely proportional to the resistance across which the drop occurs. c. there is no relationship between temperature drop and resistance. d. the relationship is not generalised.

the temperature drops in the two fluids and the wall are proportional to individual resistances.

Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid a. for fluids in laminar flow. b. for fluids in tubulent flow. c. when Grashhoff number is very important. d. for liquid metals.

for fluids in tubulent flow.

For large heat transfer area requirement, shell and tube heat exchanger is preferred, because it a. occupies smaller space. b. is more economical. c. is easy to operate and maintain. d. all (a), (b) and (c).

all

Heat flux increases with temperature drop beyond the Leiden frost point in the plot of heat flux vs. temperature drop for a boiling liquid, because a. convection becomes important b. conduction becomes important c. radiation becomes important d. sub-cooled boiling occurs

radiation becomes important

Resistance to heat flow by conduction is proportional to (where, t & ρ are thickness & density of the material respectively and A = area normal to the direction of heat flow. ) a. t b. 1/ρ c. 1/A d. all (a), (b) & (c)

all

Temperature profile in steady state heat transfer is a. asymptotic b. hyperbolic c. parabolic d. linear

linear

For the same heat load and mass flow rate in the tube side of a shell and tube heat exchanger, one may use multipass on the tube side, because it a. decreases the pressure drop. b. decreases the outlet temperature of cooling medium. c. increases the overall heat transfer coefficient. d. none of these.

increases the overall heat transfer coefficient.

When vaporisation takes place through a blanketting film of gas, the phenomenon is termed as __________ boiling. a. pool b. nucleate c. transition d. film

film

Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter) a. 0.2 D b. 0.5 D c. 0.66 D d. 0.80 D

0.2 D

Stefan-Boltzman law which describes the radiation heat transfer states that, it is proportional to (where, t = temperature in °C T = absolute temperature in ° K ) a. t4 b. T4 c. 1/t4 d. 1/T4

What is the unit of thermal conductivity ? a. Kcal/hr. m2 °C b. Kcal/hr.m.°C c. Kcal/hr.m d. Kcal/hr. °C

Kcal/hr.m.°C

The thermal radiation emitted by a body is proportional to Tn, where T is its absolute temperature. The value of 'n' is exactly 4 for a. black painted bodies only b. all bodies c. polished bodies only d. a black body

all bodies

It is not preferable to use superheated steam in evaporators, because of its very a. high temperature b. high pressure c. low film co-efficient d. none of these

low film co-efficient

It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of solution) to a 30% salt solution in an evaporator. Consider a feed of 300 kg/min at 30°C. The boiling point of the solution is 110°C, the latent heat of vaporization is 2100 kJ/kg and the specific heat of the solution is 4 kJ/kg.K. The rate at which the heat has to be supplied in (kJ min) to the evaporator is a. 3.06 x l05 b. 6.12 x 105 c. 7.24 x 105 d. 9.08 x 105

3.06 x l05

LMTD for counterflow and prallel flow heat exchanger will be the same, when the a. cold fluid is heated to a certain temperature by condensing steam (isothermal fluid). b. outlent temperature of both the hot and cold fluid are same. c. outlet temperature of hot fluid is less than the outlet temperature of the cold fluid. d. none of these.

cold fluid is heated to a certain temperature by condensing steam (isothermal fluid).

For flow over a flat plate, the ratio of thermal boundary layer thickness, 'xt' and hydrodynamic boundary layer thickness 'x' is equal to (where, NPr = Prandtl number) a. NPr b. NPr1/3 c. NPr-1 d. NPr-1/3

NPr1/3

Use of transverse baffles in a shell and tube heat exchanger is done to increase the a. rate of heat transfer. b. flow velocity. c. turbulence of shell side fluid. d. all (a), (b) and (c).

all

The actual temperature drop across the heating surface of an evaporator depends on the a. liquid depth over the heating surface. b. solution being evaporated. c. pressure difference between the steam chest and the vapor space above the boiling liquid. d. all (a), (b) and (c)

all

The Graetz number is concerned with the a. mass transfer between a gas and a liquid. b. absorption with chemical reaction. c. heat transfer in turbulent flow. d. heat transfer in laminar flow.

heat transfer in laminar flow

Solid angle subtended by the finite surface at the radiating element is a. called the view factor. b. called the angle of vision. c. proportional to the square of the distance between surfaces. d. expressed in terms of radians.

called the angle of vision.

Pick out the wrong statement. a. The emissivity of a surface decreases, if it gets corroded by atmospheric environment. b. The emissivity of a surface increases with increase in surface roughness. c. The emissivity of a polished surface is quite low. d. The emissivity of a non-metallic surface decreases with increase in the temperature.

The emissivity of a surface decreases, if it gets corroded by atmospheric environment.

In forced convection, the heat transfer depends on a. Re, Pr b. Re, Gr c. mainly Gr d. Re only

Re, Pr

With increase in temperature, the thermal conductivity of most liquids a. increases b. decreases c. remains same d. first increases upto a certain temperature and then becomes constant

decreases

Heat transfer by __________ may not necessarily require the presence of a medium. a. conduction b. natural convection c. forced convection d. radiation

radiation

Pick out the correct statement. a. Higher is the temperature of the radiating body, higher is the wavelength of radiation. b. Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder. c. The wavelength corresponding to maximum mono-chromatic emissive power increases with rise in temperature. d. Solid angle subtended by the finite surface at the radiating element is called the angle of incidence.

Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder.

In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used a. in low range of temperature differences. b. in high range of temperature differences. c. because of its low cost. d. to prevent corrosion of the tube bundles.

in high range of temperature differences.

Stefan-Boltzman law applies to __________ body. a. black b. white c. grey d. any colour

black

The film co-efficient is decreased due to the presence of non-condensing gases in the vapors. The film co-efficient of superheated vapor as compared to that of saturated vapor is a. more b. less c. some d. either more or less ; depends on the nature of vapor

some

In case of parallel flow heat exchanger, the lowest temperature theoretically attainable by the hot fluid is __________ the outlet temperature of the cold fluid. a. equal to b. more than c. less than d. either more or less than (depending upon the fluid)

equal to

An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a a. boiler b. heat exchanger c. recuperator d. regenerator

heat exchanger

Which of the following is directly concerned with the heat transfer ? a. Strouhal number b. Sherwood number c. Euler number d. Grashoff number

Grashoff number

Pick out the wrong statement: a. With change in temperature, the radiant energy emitted by a black body remains unchanged. b. Absorptivity of a body approaches unity in case of diffuse reflection. c. Absorptivity of a perfectly black body is unity. d. Value of Stefan-Boltzman constant is 4.876 x 10-8 KCal/m2.hr.°K4.

With change in temperature, the radiant energy emitted by a black body remains unchanged.

Planck's distribution law is valid for __________ bodies. a. black b. white c. coloured d. all (a), (b) & (c)

coloured

For hot feed, forward feeding as compared to backward feeding results in __________ economy. a. increased b. decreased c. no effect on d. none of these

increased

Radiation energy is emitted by all the substances, which are above a. 0°K b. 0°C c. 100°C d. room temperature

0°K

What is the steam economy in case of a single effect evaporator system ? a. 1 b. >1 c. < 1 d. 0.1

< 1

If average heat transfer co-efficient is ha and the local coefficient at the end of the plate is hl then in case of heat transfer to a fluid flowing over a flat plate, heated over its entire length a. ha= hl b. ha = 2hl c. ha = 0.5 hl d. ha = 0.75 hl

ha = 2hl

The Fourier number (defined as a.t/L2) is used in the analysis of problem involving heat transfer by a. forced convection b. natural convection c. transient conduction d. steady state conduction

transient conduction

The energy radiated from a surface Q at absolute temperature T is related as a. Q ∝ T2 b. Q ∝ T4 c. Q ∝ T3 d. none of these

Q ∝ T4

Baffles are provided in heat exchangers to increase the a. fouling factor b. heat transfer area c. heat transfer co-efficient d. heat transfer rate

heat transfer co-efficient

200 kg of solids (on dry basis) is subjected to a drying process for a period of 5000 seconds. The drying occurs in the constant rate period with the drying rate as, Nc = 0.5 x 10-3 kg/m2.s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid. The interfacial area available for drying is 4 m2/1000 kg of dry solid. The moisture content at the end of the drying period is (in kg moisture/kg dry solid) a. 0.5 b. 0.05 c. 0.1 d. 0.15

0.1

The ratio of total radiating power to the absorptivity of the body depends upon the __________ as per Kirchoffs law. a. wavelength of radiation b. nature of the body c. temperature of the body d. none of these

temperature of the body

For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally a. decreases b. increases c. remains unchanged d. increases exponentially

decreases

A 2-4 heat exchanger involves a. only counter-flow of fluids. b. only parallel-flow of fluids. c. both counter and parallel-flow of the fluids. d. smaller pressure drop compared to 1-2 exchanger.

both counter and parallel-flow of the fluids.

In a liquid-liquid heat exchanger, for the same process temperature, the ratio of the LMTD in parallel flow to the LMTD in counter flow is always a. < 1 b. >1 c. 1 d. ∞

< 1

With increase in temperature, the thermal conductivity of steel a. increases b. decreases c. remains unchanged d. increases exponentially

decreases

Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids? a. Dittus-Boeltier equation b. Sieder-Tate equation c. Nusselt equation a. None of these

Sieder-Tate equation

The main function of baffles provided in a shell and tube heat exchanger is to a. facilitate the cleaning of outer tube surface. b. enhance turbulence. c. hold the tubes in position. d. all 'a', 'b' & 'c'.

enhance turbulence.

In a single effect evaporator, the economy is a. 1 b. < 1 c. >1 d. none of these

< 1

LMTD correction factor which is to be applied for a cross-flow heat exchanger increases with increase in the number of shell passes. Its value for a single pass cross flow heat exchanger is a. 0 b. 1 c. >1 d. <1

Which of the following is not concerned with the heat transfer? a. Brinkman number b. Stanton number c. Schmidt number d. Peclet number

Schmidt number

Latent heat absorbed by 1 lb of water at 212°F, when it is changed to steam at 212°F, may be around __________ BTU. a. 180 b. 970 c. 3.97 d. none of these

180

The thermal radiative flux from a surface of emissivity = 0.4 is 22.68 kW/m2. The approximate surface temperature (K) is (Stefan-Boltzman constant = 5.67xl0-8 W/m2.K4) a. 1000 b. 727 c. 800 d. 1200

1000

Crystallisation of solids from a homogeneous solution is a/an __________ process. a. exothermic b. mildly endothermic c. highly endothermic d. none of these

exothermic

A multiple effect evaporator has a capacity to process 4000 kg of solid caustic soda per day, when it is concentrating from 10% to 25% solids. The water evaporated in kg per day is a. 6000 b. 24000 c. 60000 d. 48000

24000

Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by a. eddies current b. conduction c. convection d. none of these

conduction

For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is a. forward b. backward c. parallel d. none of these

backward

The sum of reflectivity and absorptivity for an opaque body is equal to a. 0.5 b. 1 c. 0 d. 2

The film thickness for laminar film condensation on vertical surface __________ from top to bottom. a. cummulatively increases b. cummulatively decreases c. remains constant d. and the surface conductance increase

cummulatively increases

In case of a multipass shell and tube heat exchanger, providing a baffle on the shellside __________ the heat transfer rate. a. increases b. decreases c. does not affect d. may increase or decrease, depends on the type of baffle

increases

The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness) a. ∝ t b. ∝ 1/t c. ∝ t d. independent of 't'

∝ 1/t

Heat transfer co-efficient (h1) for liquids increases with a. increasing temperature. b. decreasing temperature. c. decreasing Reynolds number. d. none of these.

increasing temperature.

A diathermaneous substance __________ the thermal radiation completely. a. absorbs b. reflects c. transmits d. none of these

transmits

A hot body will radiate heat most rapidly, if its surface is a. white & rough b. black & rough c. white & polished d. black & polished

black & rough

A __________ surface has the maximum thermal emissivity out of the following. a. black & smooth b. black & rough c. white & smooth d. white & rough

black & rough

Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.) a. (d)^0.5 b. d c. d2 d. d1.5

Mechanical recompression evaporation is used in the production of a. alcohol b. distilled water c. salt d. fruits jam

distilled water

A measure of the extent to which viscous heating is important relative to the heat flow resulting from the impressed temperature difference is represented by the __________ number. a. Condensation b. Grashoff c. Stantan d. Brinkman

Brinkman

The purpose of providing a 'catchall' in the vapor line of an evaporator is to a. create vacuum b. regulate the vapor flow c. vent the non-condensible gases d. arrest the entrained liquid

arrest the entrained liquid

What is the thermal conductivity of a perfect heat insulator ? a. zero b. one c. ∞ d. between 0 and ∞

zero

'Fouling factor' used in the design of a multipass shell and tube heat exchanger is a a. non-dimensional factor. b. factor of safety. c. conversion factor for individual film heat transfer co-efficient to overall heat transfer co-efficient. d. none of these.

factor of safety.

A fluid is flowing inside the inner tube of a double pipe heat exchanger with diameter 'd'. For a fixed mass flow rate, the tube side heat transfer co-efficient for turbulent flow conditions is proportional to a. d0.8 b. d-0.2 c. d-1 d. d-1.8

d-0.2

Prandtl and Reynolds analogy are same, when Prandtl number is a. 0.5 b. 1 c. >2 d. 1.5

Heat transfer occurs by natural convection because change in temperature causes difference in a. viscosity b. density c. thermal conductivity d. heat capacity

density

The ratio of kinematic viscosity to thermal diffusivity is called the __________ number. a. Peclet b. Prandtl c. Stanton d. Nusselt

Prandtl

The critical radius 'r' of insulation on a pipe is given by a. r = 2k/h b. r = k/h c. r = k/2h d. r = h/k

r = k/h

If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid a. remains unchanged. b. increases. c. increases or decreases depending on number of shell passes. d. decreases.

decreases

The heat transfer by radiation from a mild steel surface is to be reduced by reducing the emissivity of the surface. This can be best achieved by a. painting the surface black. b. painting the surface white (with aluminium paint). c. giving the surface a mirror finish. d. roughening the surface.

painting the surface white (with aluminium paint)

_________ paint has the maximum absorption coefficient. a. Black b. Yellow c. White d. Grey

Black

Steam traps are provided in steam carrying pipelines to a. condense steam. b. release excess steam pressure by bleeding steam. c. remove condensate and inert gases. d. none of these.

remove condensate and inert gases.

A hot liquid is kept in a big room. The logorithm of the numerical value of the temperature difference between the liquid and the room is plotted against time. The plot will be very nearly a/an a. ellipse b. straight line c. parabola d. circular arc

straight line

Radiation heat transfer rates does not depend upon the a. type of absorbing surface. b. distance between the heat source and the object receiving the heat. c. surface area and temperature of the heat source. d. none of these.

none of these.

Which of the following is correct? a. Rate = Driving force x Resistance b. Driving force = Rate x Resistance c. Resistance = Driving force x Rate d. Rate = Resistance/Driving force

Driving force = Rate x Resistance

Convective heat transfer, in which heat is transferred by movement of warmed matter is described by a. Fourier's law b. Newton's law of cooling c. Fick's law d. none of these

Newton's law of cooling

As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient a. continues to increase. b. continues to decrease. c. goes through a minimum. d. goes through a maximum.

goes through a minimum.

Heat transfer rate per unit area is called a. thermal conductivity b. heat flux c. heat transfer co-efficient d. thermal diffusivity

heat flux

Log mean temperature difference (LMTD) cannot be used, if a. heat transfer co-efficient over the entire heat exchanger is not constant. b. there exists an unsteady state. c. the heat capacity is not constant and there is a phase change. d. none of these.

none of these.

In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head. a. twice b. four times c. square root of d. square of

four times

For a given ambient air temperature with increase in the thickness of insulation of a hot cylinderical pipe, the rate of heat loss from the surface would a. decrease b. increase c. first decrease and then increase d. first increase and then decrease

first increase and then decrease

In a parallel flow heat exchanger, if the outlet temperature of hot and cold fluids are the same, then the log mean temperature difference (LMTD) is a. minimum b. maximum c. zero d. infinity

zero

In a forward feed multiple effect, the pressure build up will be a. least at the inlet of the first effect. b. least at the outlet of the last effect. c. highest at the inlet of the last effect. d. highest at the outlet of the last effect.

least at the outlet of the last effect.

Film boiling is usually not desired in commercial equipments, because a. the heat transfer rate is low in view of the large temperature drop. b. it is difficult to maintain. c. it is not economic. d. none of these.

the heat transfer rate is low in view of the large temperature drop.

The variation of thermal conductivity of a metal with temperature is often correlated using an expression of the form K = K0 + at. where, K is the thermal conductivity and T is the temperature (in °K). The units of 'a' in SI system will be a. W/m.k b. W/m c. W/m.k2 d. none, 'a' is just a number

W/m.k2

Film condensation is promoted on a/an __________msurface. a. oily b. coated c. clean & smooth d. dirty

clean & smooth

Dietus-Boelter equation cannot be used for molten metals mainly due to its very low a. Prandtl number b. Grashoff number c. thermal conductivity d. viscosity

Prandtl number

If Prandtl number is greater than the Schmidt number, then the a. thermal boundary layer lies inside the concentration boundary layer. b. concentration boundary layer lies inside the thermal boundary layer. c. thermal & concentration boundary layers are of equal thickness. d. hydrodynamic(i.e., momentum)boundary layer is thicker than the other two.

thermal boundary layer lies inside the concentration boundary layer.

Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid ? a. Bleed point b. Vent c. Catchall d. Baffle

Catchall

Rate of heat transfer by vaporisation from pools of water is affected by the a. nature of heating surface and distribution of bubbles. b. surface tension of water. c. viscosity of water. d. all (a), (b) and (c).

all

Multiple effect evaporators are commonly used in the manufacture of P. Paper Q. Superphosphate R. Sugar S. Fats a. P and Q b. P and R c. P and S d. R and S

P and S

Multiple effect evaporators are commonly used in the manufacture of P. Paper Q. Superphosphate R. Sugar S. Fats a. P and Q b. P and R c. P and S d. R and S

P and S

Which of the following has the highest thermal conductivity? a. Brick b. Air c. Water d. Silver

Silver

Which of the following is unimportant in forced convection? a. Reynolds number b. Prandtl number c. Grashhoff number d. none of these

Grashhoff number

Steam is to be condensed in a shell and tube heat exchanger, 5 m long with a shell diameter of 1 m. Cooling water is to be used for removing the heat. Heat transfer co-efficient for the cooling water, whether on shell side or tube side is the same. The best arrangement is a. vertical heat exchanger with steam on tube side. b. vertical heat exchanger with steam on shell side. c. horizontal heat exchanger with steam on tube side. d. horizontal heat exchanger with steam on shell side.

vertical heat exchanger with steam on shell side.

In a multiple effect evaporator, the effect of boiling point elevation is to a. reduce the capacity b. reduce the economy c. increase the economy d. none of these

reduce the capacity

Dropwise condensation occurs on __________ surfaces. a. clean and dirt free b. smooth clean c. contaminated cooling d. polished

contaminated cooling

For specified tube outside diameter, higher BWG means higher a. tube thickness b. cross-sectional area c. weight per unit length d. none of these

cross-sectional area

Pick out the wrong statement. a. Swenson-Walker crystalliser is a batch crystalliser. b. Super saturation of the solution is the driving potential for a crystal growth. c. The liquor left after the removal of crystals from a solution is called mother liquor. d. The first stage of crystal formation is called nucleation.

Swenson-Walker crystalliser is a batch crystalliser.

Heat flux is the time rate of heat transfer per unit a. length b. area c. volume d. none of these

area

Film boiling occurs at __________ pressure. a. atmospheric b. sub-atmospheric c. negative d. very high

very high

Heat flux through several resistances in series in analogous to the current flowing through several a. resistances in parallel. b. capacitors in series. c. resistances in series. d. none of these.

resistances in series.

Extended heat transfer surface like fins are used to increase the heat transfer rate. Fin efficiency is defined as the ratio of heat transferred across the fin surface to the theoretical heat transfer across an equal area held at the a. surrounding temperature. b. average temperature of the fin. c. temperature of the fin end. d. constant temperature equal to that of the base

constant temperature equal to that of the base

Vacuum is generally maintained in the vapour space of an evaporator mainly to a. get economical temperature difference by using moderate pressure steam. b. facilitate forward feeding in multiple effect evaporation. c. concentrate heat sensitive materials. d. achieve very high concentration of the final product.

get economical temperature difference by using moderate pressure steam.

Double pipe heat exchangers are used a. when heat transfer area required is very high. b. when heat transfer area required is very low, i.e (100-200 ft2). c. because it occupies less floor area. d. because it is less costly.

when heat transfer area required is very low, i.e (100-200 ft2).

Pick out the wrong statement. a. Heat transfer by radiation can not occur across an absolute volume. b. In case of a shell and tube heat exchanger, the pressure drop through the shell is,proportional to the number of times the fluid crosses the bundle between baffles. c. Propagation velocity for travel of heat radiation through vacuum is equal to the velocity of the light. d. The amount of heat involved in the condensation or vaporisation of 1 kg of a fluid is the same.

Heat transfer by radiation can not occur across an absolute volume.

Which type of heat exchanger is preferred for heavy heat loads? a. Double pipe b. Plate fine c. Series and parallel set of shell and tube d. None of these

Series and parallel set of shell and tube

Pick out the wrong statement. a. Reciprocal of the resistance to heat flow is called thermal conductance. b. Unit of thermal conductance is W/°K. c. Thermal conductance of a wall of thickness 'L', thermal conductivity 'k' and heat flow area 'A' is kL/A. d. none of these.

Thermal conductance of a wall of thickness 'L', thermal conductivity 'k' and heat flow area 'A' is kL/A.

While the total emissivity of a perfect black body is unity, the same for a real body is a. 0 b. 1 c. >1 d. between 0 and 1

between 0 and 1

The Grashoff number is defined as the ratio of the a. buoyancy to inertial forces. b. buoyancy to viscous forces. c. inertial to viscous forces. d. buoyancy to surface tension forces.

buoyancy to viscous forces.

According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor) a. 2f b. f c. f/2 d. f/4

f/2

The absorptivity of a body is equal to its emissivity a. at a particular temperature. b. for circular bodies. c. under thermal equilibrium. d. none of these.

under thermal equilibrium.

Absorptivity and refletivity of a perfect black body are respectively a. 1 and 0 b. 0 and 1 c. 1 and ∞ d. 0 and 0.5

1 and 0

In thermal radiation for a black body (where, ε is emissivity and α is absorptivity) a. α = 1; ε ≠ 1 b. α ≠ 1, ε = 1 c. α ≠ 1, ε ≠ 1 d. α = 1; ε = 1

α = 1; ε = 1

Pick out the wrong statement. a. Superheated steam is preferably not used for process heating because of its low heat transfer film co-efficient. b. In a shell and tube heat exchanger, the shell pressure drop is maximum for orifice baffles. c. S.I. unit of fouling factor is Watt/m2.°K. d. Longitudinal fins are used in extended surface heat exchangers, when the direction of fluid flow is parallel to the axis of the tube.

S.I. unit of fouling factor is Watt/m2.°K.

A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300°C and allowed to cool in natural air. Which one will cool the slowest? a. Cube b. Plate c. Sphere d. All will cool at the same rate

Cube

Vent pipes are provided in a condenser to a. remove non-condensable gases. b. purge the condenser. c. facilitate easy cleaning of tubes. d. none of these.

remove non-condensable gases.

Heat transfer by natural convection is enhanced in system with a. high viscosity. b. high co-efficient of thermal expansion. c. low temperature gradients. d. low density change with temperature.

high co-efficient of thermal expansion.

In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid. a. nucleate b. pool c. low pressure d. none of these

pool

A body cools down from 75°C to 70°C in 10 minutes. It will cool down from 70° C to 65° C in __________ minutes. a. 10 b. >10 c. <10 d. either (b) or (c), depends on the mass of the body

>10

In forced convection, the Nusselt number is a function of a. Re and Pr b. Re and Gr c. Pr and Gr d. Re and Sc

Re and Pr

A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because a. total heat transfer area of all the effects is -less than that in a single effect evaporator system. b. total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect. c. boiling point elevation in a single effect system is much higher than that in any effect in a multieffect system. d. heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system.

total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.

Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by a. keeping the heat transfer surface clean. b. enhancing the fluid pumping rate. c. increasing the tube length. d. none of these.

keeping the heat transfer surface clean.

Trap is used to remove __________ from steam pipe lines. a. steam b. condensate c. non-condensables d. none of these

condensate

A composite flat wall of a furnace is made of two materials 'A' and 'B'. The thermal conductivity of 'A' is twice of that of material 'B', while the thickness of layer of 'A' is half that of B. If the temperature at the two sides of the wall are 400 and 1200°K, then the temperature drop (in °K) across the layer of material 'A' is a. 125 b. 133 c. 150 d. 160

160

Tube pitch is the __________ of tube diameters and the clearances. a. sum b. difference c. ratio d. none of these

sum

With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer a. decreases b. increases c. increases exponentially d. remains unaffected

decreases

The equivalent diameter for the annulus of a double pipe heat exchanger, whose inner pipe has fins on the outside is __________ compared to the same size pipes without fins. a. more b. less c. same d. unpredictable

less

Natural convection is characterised by a. Grashhoff number b. Peclet number c. Reynolds number d. Prandtl number

Grashhoff number

Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is a. 1 b. < 1 c. >1 d. >5

< 1

The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe ) a. K/h0 b. 2K/h0 c. hi/K d. 2hi/K

K/h0

The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature) a. (ΔT)2 b. (ΔT)4 c. (ΔT)3 d. (ΔT)^0.5

(ΔT)3

In a 1-1 cocurrent heat exchanger, if the tube side fluid outlet temperature is equal to the shell side fluid outlet temperature, then the LMTD is a. ∞ b. 0 c. equal to the difference between hot and cold fluids inlet temperature. d. equal to the difference between hot fluid inlet temperature and cold fluid outlet temperature.

A body is called grey if the monochromatic emissivity of the body is a. zero b. unity c. same for all wavelengths d. different for all wavelengths

same for all wavelengths

In an extended surface heat exchanger, fluid having lower co-efficient a. flows through the tube. b. flows outside the tubes. c. can flow either inside or outside the tubes. d. should not be used as it gives very high pressure drop.

flows outside the tubes.

In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is a. infinity. b. zero. c. same as average heat transfer co-efficient for tube side. d. none of these.

infinity

Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force. a. viscous b. elastic c. inertial d. none of these

none of these

Pick out the wrong statement. a. By increasing the number of shell passes, the temperature cross in a shell and tube heat exchanger can be prevented. b. An underdesigned steam trap will back up the condensate instead of discharging it out. c. Steam condensate is corrosive because of the presence of dissolved oxygen in it. d. Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop

By increasing the number of shell passes, the temperature cross in a shell and tube heat exchanger can be prevented.

For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in a. increased economy b. decreased economy c. lower capacity d. no effect on economy

increased economy

If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows __________ law. a. hyperbolic b. parabolic c. linear d. logarithmic

linear

In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred a. through a metallic wall. b. by direct contact of hot flue gas with air. c. by heating an intermediate material (like chequor bricks) and then heating the air from this hot material. d. none of these.

by heating an intermediate material (like chequor bricks) and then heating the air from this hot material.

__________ equation relates the thermal conductivity of a solid to its temperature. a. Antonie b. Kopp's c. Lee's d. Kistyakowski

Lee's

For small temperature difference, the heat transfer rate as per Newton's law of cooling is proportional to (where, Δt = excess temperature) a. Δt b. Δt2 c. Δt3 d. Δt^0.5

Δt

Pick out the wrong statement. a. The condensing film co-efficient is about 3 times lower for vertical condenser as compared to the equivalent horizontal condenser for identical situation. b. Film co-efficient for vaporisation decreases as a result of vapor binding. c. In industrial practice, sub-cooling of condensate is required, when the condensate is a volatile liquid and is to be transferred for storage. d. Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.

Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.

Viscous & heat sensitive liquids are concentrated in __________ evaporators. a. open pan b. long tube c. agitated film d. none of these

agitated film

Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on a. T1 - T2 b. T1 - Ta c. T2 - Ta d. none of these

none of these

The ratio of momentum diffusivity to thermal diffusivity is the __________ number. a. Prandtl b. Nusselt c. Stanton d. Grashoff

Prandtl

For a perfectly transparent surface (like gases), the a. absorptivity = 0 b. transmissivity = 1 c. reflectivity = 0 d. all (a), (b) & (c)

transmissivity = 1

For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are a. same b. different c. never different d. linearly related

different

During crystallisation, formation of crystal can occur in __________ solution only. a. saturated b. supersaturated c. undersaturated d. all (a), (b) and (c)

supersaturated

An insulator should have a. low thermal conductivity. b. high thermal conductivity. c. less resistance to heat flow. d. a porous structure.

low thermal conductivity.

In a backward feed multiple effect evaporator a. feed is introduced in the first effect. b. feed flows from low pressure to high pressure. c. no pumps are required between successive effects. d. none of these.

feed flows from low pressure to high pressure.

In a forward feed multiple effect evaporator unit a. viscosity of liquid is highest in first effect. b. transfer from effect to effect is done by pumps. c. no pump is required to withdraw the product from the last effect. d. none of these.

none of these.

In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is a. linear b. hyperbolic c. parabolic d. none of these

parabolic

Indirect contact heat exchangers are preferred over direct contact heat exchangers, because a. heat transfer co-efficient are high. b. there is no risk of contamination. c. there is no mist formation. d. cost of equipment is lower.

there is no risk of contamination.

Water is normally used as a coolant in the heat exchange equipments mainly because ofits a. abundance & high heat capacity b. low density c. low viscosity d. high fluidity

abundance & high heat capacity

The rate of emission of radiation by a body does not depend upon the a. wavelength of radiation. b. surface temperature of the body. c. nature of the surface. d. shape and porosity of the body.

shape and porosity of the body.

Which of the following has maximum thermal conductivity at the same temperature? a. Steel b. Petrol c. Air d. All have the same conductivit

Steel

Maximum heat transfer rate is obtained in __________ flow. a. laminar b. turbulent c. creeping d. transition region

turbulent

In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface. a. nucleate b. local c. pool d. saturated

local

The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is a. 2 b. 1/2 c. 4 d. 1/4

1/4

Walls of a cubical oven are of thickness l, and they are made of material of thermal conductivity k. The temperature inside the oven is 100°C and the inside heat transfer co-efficient is 3k/l. If the wall temperature on the outside is held at 25°C, what is the inside wall temperature in degree centigrade? a. 35.5 b. 43.75 c. 81.25 d. 48.25

81.25

Terminal point temperature differences between fluids in case of a heat exchanger is termed as a. approach. b. log mean temperature difference. c. arithmetic mean temperature difference. d. geometric mean temperature difference.

approach

LMTD can't be used as such without a correction factor for the a. multipass heat exchanger. b. baffled heat exchanger. c. condensation of mixed vapour in a condenser. d. all (a) (b) and (c).

multipass heat exchanger.

Fqra cold viscous feed, backward feed gives __________ than forward feed. a. a higher capacity b. a lower capacity c. lower economy d. none of these

a higher capacity

Overall heat transfer co-efficient of a particular tube is U1. If the same tube with some dirt deposited on either side has coefficient U2, then a. U1 = U2 b. U2 > U1 c. U1 > U2 d. U1 = dirt factor - U2

U1 = U2

In SI units, thermal conductivity is expressed in a. Watt/m.°K b. Watt/m2. °K c. Watt/m2. °K d. Watt/m4. °K

Watt/m.°K

Boiling point of a solution according to Duhring's rule is a linear function of the __________ of water. a. boiling point (at the same pressure) b. viscosity c. density d. thermal conductivity

boiling point (at the same pressure)

The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances. a. average b. geometric mean c. product d. sum

sum

At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe? a. 0.5 b. < 0.5 c. > 0.6 d. < 0.1

> 0.6

A sphere of radius 'R1' is enclosed in a sphere of radius 'R2'. The view (or shape) factor for radiative heat transfer of the outer sphere with respect to the inner sphere is a. 0 b. R2/(R1+R2) c. 1 d. (R1/R2)2

R2/(R1+R2)

For a laminar flow of fluid in a circular tube, 'h1' is the convective heat transfer co-efficient at velocity 'V1'. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is a. 1.26 h1 b. 0.794 h1 c. 0.574 h1 d. 1.741 h1

0.794 h1

The main purpose of providing fins on heat transfer surface is to increase the a. temperature gradient. b. mechanical strength of the equipment. c. heat transfer area. d. heat transfer co-efficient.

heat transfer area.

Which of the following has the lowest Prandtl number? a. Molten sodium (a coolant used in fast breeder reactor) b. Water c. Transformer oil d. Dilute H2SO4

Molten sodium (a coolant used in fast breeder reactor)

An ejector is used to a. increase pressure b. increase temperature c. remove condensate d. none of these

none of these

Extremely large or small volumes of fluids are generally best routed through the shell side of a shell and tube heat exchanger, because of the a. less corrosion problems. b. flexibility possible in the baffle arrangement. c. low pressure drop. d. high heat transfer co-efficient.

flexibility possible in the baffle arrangement.

Electro-magnetic spectrum range, which is important for radiation varies from __________ microns. a. 1 to 100 b. 0.5 to 50 c. 10 to 100 d. 100 to 1000

0.5 to 50

The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are a. Re, Gz b. Nu, Pr c. Nu, Pr, Re d. Nu, Gz

Nu, Gz

Heat transfer rate described by Fourier's law will decrease, if the __________ increases. a. thermal conductivity b. thickness c. temperature difference d. heat transfer area

thickness

The capacity of double-effect evaporator is less than half of the capacity of two single effects, each of which is operating over same terminal temperature difference, when the a. solution has an elevation of boiling point. b. evaporators operate under vacuum. c. evaporators opreate at atmospheric pressure. d. none of these.

solution has an elevation of boiling point.

Nusselt number is the ratio of the a. temperature gradient of the wall to that across the entire pipe. b. temperature difference to the temperature gradient at the wall. c. heat flux at the wall to that across the entire pipe. d. none of these

heat flux at the wall to that across the entire pipe.

Prandtl number is the ratio of a. momentum diffusivity to mass diffusivity. b. momentum diffusivity to thermal diffusivity. c. thermal diffusivity to mass diffusivity. d. thermal diffusivity to momentum diffusivity.

momentum diffusivity to thermal diffusivity.

Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger ? a. Tube side pressure drop and the heat transfer rate. b. Convective heat transfer co-efficient. c. Effective tube surface area for convective heat transfer. d. All (a) (b) and (c).

all

Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator? a. Long vertical evaporator b. Horizontal tube evaporator c. Agitated film evaporator d. Calenderia vertical tube evaporator

Agitated film evaporator

"The ratio of the total emissive power to the absorptivity for all bodies is same at thermal equilibrium". This is __________ law. a. Kirchoffs b. Planck's c. Wien's displacement d. Stefan-Boltzman

Kirchoffs

View factor is important in heat transfer by a. steady state conduction b. natural convection c. forced convection d. radiation

radiation

The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is a. lower tube side pressure drop. b. lower shell side pressure drop. c. higher tube side heat transfer co-efficient. d. higher shell side heat transfer co-efficient.

higher tube side heat transfer co-efficient.

For a multipass shell and tube heat exchanger, the LMTD correction factor is always a. 1 b. >1 c. < 1 d. between 1 & 2

< 1

If heat transfer rate varies with the time, it is termed as a. forced convection b. steady state conduction c. monochromatic radiation d. none of these

none of these

Pick out the wrong statement. a. Fluid movement under the influence of buoyant forces resulting from change in density takes place in case of natural convection. b. The ratio NNu/NRe . Npr is called the Stanton number. c. The Peclet number is a measure of the ratio of energy transport by convection to that by conduction. d. The Colbum jH factor for heat transfer is given by Nst Npr.

The Colbum jH factor for heat transfer is given by Nst Npr.

Tube expansion allowances exist in __________ heat exchanger. a. multipass fixed tube sheet b. U-tube c. single pass fixed tube sheet d. none of these

single pass fixed tube sheet

Pick out the wrong statement. a. In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2. b. Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number. c. Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger. d. Hot gases at moderate pressure are usually in the shell side of shell and tube heat exchangers. At higher pressure, however, it is customary to put gas in the tube side.

Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.

Agitated film evaporator is suitable for concentrating __________ liquids. a. foaming b. viscous c. very thin d. corrosive

viscous

The thickness of condensate layer in filmwise condensation depends on the a. condensation rate b. surface configuration c. liquid flow rate from the surface d. all (a), (b) and (c)

all

If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface tempeature follows __________ law. a. parabolic b. hyperbolic c. linear d. logarithmic

parabolic

Intermittant tube cleaning is possible to be done in case of a __________ evaporator. a. basket type b. horizontal tube c. calendria d. none of these

basket type

Three solid objects of the same material and of equal mass-a sphere, a cylinder (length = diameter) and a cube are at 500°C initially. These are dropped in a quenching bath containing a large volume of cooling oil each attaining the bath temperature eventually. The time required for 90% change in temperature is the smallest for a. cube b. cylinder c. sphere d. equal for all the three

cube

Overall heat transfer co-efficient for cooling of hydrocarbons by water is about a. 50 -100 Kcal/hr.m2.°C b. 50 -100 W/m2 . °K c. 50 -100 BTU/hr. ft.2°F d. 1000 - 1500 BTU/hr. ft.2°F

50 -100 BTU/hr. ft.2°F

Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity? a. aluminium foil b. refractory bricks c. iron plate d. coke breeze

coke breeze

At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical ? a. 1 b. < 1 c. > 1 d. none of these

_________ chart is known as transient heat conduction chart. a. Dirhing’s b. Heisler's c. Mollier's d. Cox

Dirhing’s

Choose the most important factor on which the heat conducted through a wall in a unit time will depend on? a. Thickness of the wall. b. Area of the wall perpendicular to heat flow. c. Material of the wall. d. Temperature difference between the two surfaces of the wall.

Temperature difference between the two surfaces of the wall.

Which of the following has the minimum thermal conductivity? a. Nitrogen b. Steel c. Carbon black d. Tar

Nitrogen

For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then a. hf = hd b. hf > hd c. hf < hd d. hf could be greater or smaller than hd

hf < hd

Pick out the wrong statement: a. The capacity of an evaporator is reduced by the boiling point elevation. b. Corrosive liquid is normally passed through the tubes in a shell and tube heat exchanger. c. Steam jet ejector is used for vapor compression in a thermal recompression evaporator. d. Heat sensitive materials should be concentrated in high pressure evaporators.

Heat sensitive materials should be concentrated in high pressure evaporators.

For concentrating an aqueous solution of a material like anhydrous Na2SO4, whose solubility decreases with rise in temperature, the most suitable evaporator is a __________ evaporator. a. high pressure b. vacuum c. backward feed d. none of these

vacuum

Circulation pump is located below the evaporater to a. avoid cavitation. b. avoid frequent priming. c. create more suction head. d. none of these.

create more suction head.

The statement that "maximum wavelength of radiation is inversely proportional to the temperature" is __________ law. a. Stefan-Boltzman's b. Planck's c. Wien's displacement d. none of these

Wien's displacement

Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low a. hardness b. dissolved soilds content c. suspended solids content d. all (a), (b) and(c)

all

Duhring rule is important in solving problems on a. distillation b. crystallisation c. evaporation d. humidification

evaporation

In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell) a. 0.25 D b. 0.75 D c. 0.50 D d. none of these

0.75 D

Multiple effect evaporation is generally recommended, when the a. large scale evaporation of liquor is needed. b. corrosive liquids are to be concentrated. c. fuel is cheaply available. d. evaporation on small scale is to be done.

large scale evaporation of liquor is needed.

_________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side. a. Single pass, fixed tube sheet b. U-tube c. Three pass, fixed tube sheet d. none of these

U-tube

In which mode of heat transfer, the Biot number is important? a. Transient heat conduction b. Natural convection c. Forced convection d. Radiation

Transient heat conduction

________ paint has the minimum absorption co-efficient. a. Black b. White lead c. Grey d. Light cream

White lead

Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube a. exit-fluid temperature > wall temperature. b. exit fluid temperature < wall temperature. c. exit fluid temperature = wall temperature. d. Graetz number > 100.

exit fluid temperature = wall temperature.

Which is the most suitable for the con-certration of highly concentrated solution? a. Open pan evaporation b. Long tube vertical evaporator c. Agitated film evaporator d. None of these

Open pan evaporation

Which is the most suitable for the con-certration of highly concentrated solution? a. Open pan evaporation b. Long tube vertical evaporator c. Agitated film evaporator d. None of these

Open pan evaporation

Unsteady state heat conduction occurs, when a. temperature distribution is independent of time. b. temperature distribution is dependent on time. c. heat flows in one direction only. d. three dimensional heat flow is concerned.

temperature distribution is dependent on time.

Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called a. radiation b. natural convection c. forced convection d. conduction

forced convection

Baffle spacing a. is not the same as baffle pitch. b. should be less than one fifth the diameter of the shell. c. should be less than the inside diameter of the shell. d. none of these.

should be less than the inside diameter of the shell.

The advantage of backward feed multiple effect evaporators over forward feed units is that a. heat sensitive material can be handled. b. there is no additional cost of pumping. c. most concentrated liquid is at highest temperature. d. equal heat transfer co-efficients exist in various effects.

most concentrated liquid is at highest temperature.

Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface? a. Liquid density b. Liquid viscosity c. Hot surface temperature d. Interfacial tension

Interfacial tension

Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger? a. Corrosiveness b. Fouling characteristic c. Viscosity d. None of these

None of these

Thermal conductivities of most of the liquids __________ with rise in temperature. a. increases b. decreases c. remains unchanged d. may increase or decrease; depends on the liquid

decreases

A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m3 and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m2.K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is a. 555 b. 55.5 c. 0.55 d. 0.15

0.15

The purpose of floating head in a heat exchanger is to a. avoid buckling of tubes. b. provide support for tubes. c. decrease the pressure drop. d. facilitate its lengthening, if needed.

avoid buckling of tubes.

1000 Kg of liquid at 30°C in a well stirred vessel has to be heated to 120°C, using immersed coils carrying condensing steam at 150°C. The area of the steam coils is 1.2 m2 and the overall heat transfer co-efficient to the liquid is 1500 W/m2.°C. Assuming negligible heat loss to the surrounding and specific heat capacity of the liquid to be 4 kJ/kg.°C, the time taken for the liquid to reach desired temperature will be a. 15 min b. 22 min c. 44 min d. 51 min

51 min

Double pipe heat exchangers are preferably useful, when a. high viscosity liquid is to be cooled. b. requirement of heat transfer area is low. c. overall heat transfer co-efficient is very high. d. a corrosive liquid is to be heated.

requirement of heat transfer area is low.

If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor. a. increase b. decrease c. not affect d. increase the condensing film co-efficient as well as

decrease

The outlet temperature of cooling water in a heat exchanger is generally not allowed to exceed above 50°C in industrial practice mainly to avoid a. its evaporation loss b. excessive corrosion c. uneconomic LMTD d. decrease in heat exchanger efficiency

excessive corrosion

The type of liquor circulation system to be ' employed in evaporators (viz. natural or forced circulation) is determined mainly by the __________ of the liquid. a. viscosity b. density c. thermal conductivity d. corrosive nature

viscosity

Thermal diffusivity is the most important in heat transfer by a. conduction b. radiation c. condensation d. natural convection

conduction

(NGr x NPr) is called the __________ number. a. Graetz b. Reyleigh c. Nusselt d. Stanton

Reyleigh

A dephlegmator is a a. total condenser b. vacuum evaporator c. partial condenser d. double pipe heat exchanger

partial condenser

A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun? a. Yellow paint b. White paint c. Black paint d. Grey paint

Black paint

The separation of liquid droplets from the vapor is done by a/an __________, in the evaporators. a. steam ejector b. entrainment separator c. compressor d. vacuum pump

entrainment separator

If a single tube pass heat exchanger is converted to two pass, then for the same flow rate, the pressure drop per unit length in tube side will a. increase by 1.8 times b. decrease by 2^2 c. increase by 2^16 d. remain unchanged

increase by 2^16

A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions? Assume that the conductive heat transfer co-efficient is infinitely large. a. 5 minutes b. 10 minutes c. 20 minutes d. 40 minutes

5 minutes

Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the a. existence of thermal boundary layer. b. temperature gradient produced due to density difference c. buoyancy of the bubbles produced at active nucleation site. d. none of these.

none of these.

Evaporator tubes are generally a. horizontal b. vertical c. inclined d. random

vertical

In a boiling curve, the peak heat flux is called the __________ point. a. Nusselt b. Leidenfrost c. boiling d. burnout

burnout

Pick out the wrong statement. a. Heat transfer from a hot body to cold body by the emission of heat waves is called radiation. b. Filmwise condensation takes place on non wettable surfaces. c. The boiling point of a solution is affected by liquid head as well as boiling point elevation. d. none of these.

Filmwise condensation takes place on non wettable surfaces.

Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.) a. x/KA b. KA/x c. K/Ax d. A/Kx

KA/x

Heat flux, as defined in heat flow is analogous to __________ in electricity flow. a. current b. voltage c. resistance d. none of these

current

The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body) a. αT b. α 1/T c. α T4 d. independent of T

α 1/T

In a shell and tube heat exchanger, putting a longitudinal baffle across the shell, forces the shell side fluid to pass __________ through the heat exchanger. a. once b. twice c. thrice d. four times

twice

When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is a. free convection b. entry length problem in laminar forced conduction (developing thermal boundary layer) c. mixed convection (both free and forced) d. forced convection

mixed convection (both free and forced)

The purpose of providing bleed points in the evaporator is to a. admit the feed b. remove the product c. facilitate removal of non-condensable gases d. create vacuum

facilitate removal of non-condensable gases

Water always boils when its a. temperature reaches 100° C. b. vapour pressure equals 76 cm of Hg. c. saturated vapour pressure equals the external pressure on its surface. d. saturated vapour pressure is less than the atmospheric pressure.

saturated vapour pressure equals the external pressure on its surface

A multiple effect evaporator as compared to a single effect evaporator of the same capacity has a. lower heat transfer area. b. lower steam economy. c. higher steam economy. d. higher solute concentration in the product.

higher steam economy.

Which is the most suitable for cold viscous feed? a. Forward feed b. Backward feed c. Mixed feed d. Parallel feed

Backward feed

Dropwise condensation is promoted on a/an __________ surface. a. glazed b. oily c. coated d. smooth

oily

In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator. a. increases b. decreases c. remains constant d. may increase or decrease, depends on the vacuum

increases

Nucleate boiling is promoted a. on polished surfaces b. on rough surfaces c. in the absence of agitation d. none of these

on rough surfaces

If all the conditions and dimensions are same, then the ratio of velocity through the tubes of a double pass heat exchanger to that through the single pass heat exchanger is a. 1 b. 2 c. 1/2 d. 4

The equation, Nst = f/2, is the __________ analogy. a. Colburn b. Reynolds c. Prandtl d. none of these

Reynolds

Boiling point elevation for a strong and concentrated solution is found by Duhring's rule, which states that at the same pressure, the boiling point of a solution is a linear function of the __________ of pure water. a. boiling point b. dynamic viscosity c. kinematic viscosity d. density

boiling point

Rate of crystallisation does not depend upon the a. extent of supersaturation. b. turbulence within the solution. c. number and active surface area of the crystals. d. none of these.

none of these.

Stefen's block body radiation law can also be derived from __________ law. a. Kirchoffs b. Planck's c. Fourier's d. none of these

Planck's

Multipass heat exchangers are used a. because of simplicity of fabrication. b. for low heat load. c. to obtain higher heat transfer co-efficient and shorter tube. d. to reduce the pressure drop.

to obtain higher heat transfer co-efficient and shorter tube.

Condensing film co-efficient for steam on horizontal tubes ranges from 5000 to 15000 Kcal/hr.m2 .°C. Condensation of vapor is carried out inside the tube in a shell and tube heat exchanger, when the a. higher condensing film co-efficient is desired. b. condensate is corrosive in nature. c. lower pressure drop through the exchanger is desired. d. temperature of the incoming vapor is very high.

condensate is corrosive in nature.

When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids? a. At high fluid velocity b. At low velocity c. When fluid flows past a smooth surface d. None of these

At high fluid velocity

A long iron rod initially at a temperature of 20°C has one end dipped in boiling water (100°C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40°C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time a. t = 283 s b. t = 356 s c.t = 400 s d. t = 800 s

t = 800 s

LMTD for evaporators & condensers for a given terminal parameters & set of conditions for counterflow is equal to that for parallel flow. In such heat exchangers, with one of the fluids condensing or evaporating, the surface area required is the least in the __________ flow. a. parallel b. mixed c. counterflow d. same in either 'a', b' or 'c'

same in either 'a', b' or 'c'

Thermal diffusivity of a material a. has the unit m2 /sec. b. is defined as K/ρ . Cp. c. is the ratio of thermal conductivity to thermal capacity. d. all (a), (b) and (c).

all

Viscosity of gases __________ with increase in temperature. a. increase very rapidly b. increase slowly c. decrease slowly d. remain unaffected

increase slowly

Steam is routed through the tube in case of a __________ evaporator. a. basket type b. horizontal tube c. short tube vertical d. long tube vertical

horizontal tube

Dropwise condensation of steam on cooling surface is promoted a. an oily surface. b. when both the steam and the tube are clean. c. only in presence of air. d. none of these.

an oily surface.

In a heat exchanger, the rate of heat transfer from the hot fluid to the cold fluid a. varies directly as the area and the LMTD. b. directly proportional to LMTD and inversely proportional to the area. c. varies as square of the area. d. none of these.

varies directly as the area and the LMTD.

Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is a. 0.1 b. >1 c. < 1 d. 1

0.1

varies directly as the area and the LMTD.

With increase in temperature, the total emissivity of conductors a. increases b. decreases c. remains same d. decreases linearly

increases

Loss of heat from untagged steam pipe to the ambient air is by a. conduction b. convection c. radiation d. all (a), (b) & (c)

all

Fruit juice (a heat sensitive material) can be concentrated in a __________ evaporator. a. long tube b. falling film c. high pressure d. none of these

falling film

Tubes are held between top and bottom tube sheets in Calenderia type evaporator by keeping a. both the tube sheets fixed. b. both the tube sheets floating. c. the top tube sheet floating and bottom tube sheet fixed. d. the top tube sheet fixed and the bottom tube-sheet floating.

both the tube sheets fixed.

Heat sensitive materials can be concentrated in an evaporator employing a. vacuum b. high pressure c. high residence time d. none of these

vacuum

A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires a. less heating surface. b. more space. c. lower maintenance cost. d. none of these.

more space.

In a shell and tube heat exchanger, floating head is used for a. large temperature differentials. b. high heat transfer co-efficient. c. low pressure drop. d. less corrosion of tubes.

large temperature differentials.

Steam trap is used to a. condense the steam flowing in the pipeline. b. remove water resulting from partial condensation of steam. c. stop the supply of steam. d. none of these.

remove water resulting from partial condensation of steam

The average heat transfer co-efficient over the entire length of the plate (ha) and the local heat transfer co-efficient (hL), in case of heat transfer over a flat plate in laminar zone is related as a. ha = 0.8hL b. ha = 2hL c. ha = hL d. ha = 5hL

ha = 2hL

Maximum heat transfer rate is achieved in __________ flow. a. co-current b. counter-current c. turbulent d. laminar

turbulent

Wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. This is __________ law. a. Stefan's b. Dalton's c. Wien's d. Kirchoffs

Wien's

Three material A, B and C of equal thick-nes and of thermal conductivity of 20, 40 & 60 kcal/hr. m. °C respectively are joined together. The temperature outside of A and C are 30°C and 100°C respectively. The interface between B and C will be at a temperature of __________ °C. a. 40 b. 95 c. 70 d. 50

Which of the following parameters of the fluid is not very important, while deciding its route in a shell and tube heat exchanger? a. Corrosiveness & fouling characteristics b. pressure c. viscosity d. density

density

The equation, (NSt x N2/3Pr) = f/2, is the __________ analogy. a. Colburn b. Reynolds c. Prandtl d. none of these.

Colburn

The driving potential for the crystal growth during crystallisation is the __________ of the solution. a. concentration b.viscosity c. super-saturation d. density

super-saturation

jH factor for heat transfer depends upon the __________ number. a. Biot b. Nusselt c. Reynolds d. Prandtl

Reynolds

Kg of liquid evaporated per hour in an evaporator is defined as its a. capacity b. economy c. steam load d. none of these

capacity

Pick out the correct statement. a. 1 kcal/hr.m.°C is equal to 1 BTU/hr. ft.°F. b. In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity. c. In unsteady state heat conduction, heat flows in the direction of temperature rise. d. In heat transfer by forced convection, Grashoff number is very important.

In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity.

Open pan evaporators are preferred to be used, when the solution to be concentrated is a. scaling b. highly viscous c. corrosive d. salty

highly viscous

The maximum heat transfer co-efficient from steam heating will be attained when the steam is a. supersaturated b. saturated c. wet d. none of these

saturated

In a forward feed multiple effect evaporator, the pressure is a. highest in last effect b. lowest in last effect c. same in all effects d. dependent on the number of effects

lowest in last effect

In a co-current double pipe heat exchanger used for condensing saturated steam over the inner tube, if the entrance and exit conditions of the coolant are interchanged, then the rate of condensation will a. increase b. decrease c. remain unchanged d. either increase or decrease; depends on the coolant flow rate

remain unchanged

Which of the following situations can be approximated to a steady state heat transfer system? a. A red hot steel slab (having outside surface temperature as 1300°C) exposed to the atmosheric air at 35°C. b. 10 kg of dry saturated steam at 8 kgf/cm2 flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C. c. Boiling brine kept in open vessel when the bottom surface temperature of the vessel is maintained constant at 180°C. d. A sub-cooled refrigerant liquid at 8°C flowing at the rate of 6 Kg/minute through a copper pipe exposed to atmospheric air at 35°C.

10 kg of dry saturated steam at 8 kgf/cm2 flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C.

In a shell and tube heat exchanger, square pitch compared to triangular pitch a. gives a higher shell side pressure drop. b. gives a lower shell side pressure drop. c. can pack more surface area into a shell of given diameter. d. none of these.

gives a lower shell side pressure drop.

Which of the following has maximum thermal conductivity ? a. Iron b. Coal c. Nitrogen d. Tar

Iron

The film co-efficient between condensing vapour and metal wall increases with a. increasing temperature of the vapour. b. decreasing temperature of the vapour. c. increasing viscosity of the film of condensate. d. increasing temperature drop.

increasing temperature of the vapour.

The thermal boundary layer at NPr > 1 a. is thicker than hydrodynamic boundary layer. b. is thinner than hydrodynamic boundary layer. c. and the hydrodynamic boundary layer are identical. d. disappears.

is thinner than hydrodynamic boundary layer.

With increase in temperature, the thermal conductivity of fresh lubricating oil a. increases b. decreases c. remains unchanged d. may increase or decrease; depends on its composition

decreases

Nusselt number is the ratio of the temperature gradient at the wall to a. temperature difference b. heat flux c. that across the entire pipe d. none of these

that across the entire pipe

The Stefan-Boltzman constant depends on the a. medium b. temperature c. surface d. none of these

none of these

Thermal conductivity of a gas at low density, __________ with an increase in temperature. a. decreases b. increases c. remains unchanged d. may increase or decrease; depends on the gas

increases

Nusselt number is a function of Prandtl number and __________ number of fluid in natural convection heat transfer. a. Grashoff b. Biot c. Stantan d. Reynolds

Grashoff

_________ heat exchanger is used for chilling oil to be dewaxed. a. U-tube b. Double pipe c. Fixed tube d. Floating head

Double pipe

It is not recommended to use a 1-2 shell and tube heat exchanger for a particular heat duty, whenever the LMTD correction factor is a. > 0.75 b. < 0.75 c. < 0.50 d. < 0.25

< 0.75

Pick out the wrong statement. a. Economy of a multiple effect evaporator is not influenced by the b. boiling point elevation. b. Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature. c. Double pipe heat exchangers are mostly used in the field of refrigeration. d. Finned tube heat exchangers are suitable for heating air by steam.

Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature.

In sub-cooled boiling, a. temperature of the heating surface is less than the boiling point of the liquid. b. temperature of the heating surface is more than the boiling point of the liquid. c. bubbles from heating surface are absorbed by the mass of the liquid. d. very large vapour space is necessary.

temperature of the heating surface is more than the boiling point of the liquid.

With increase in temperature, the thermal conductivity of a gas a. increases. b. decreases. c. remains same. d. may increase or decrease depending on the type of gas

increases

A __________ evaporator employs an annular downtake. a. basket type b. horizontal c. long tube vertical d. none of these

basket type

In natural convection heat transfer, the correlating parameter is the a. Graetz number b. Eckert number c. Grashoff number d. Bond number

Grashoff number

Boiling of milk in an open vessel is an example of __________ boiling. a. film b. sub-cooled c. saturated nucleate d. none of these

film

Pick out the wrong statement. a. In process heat exchangers, saturated steam is preferred over the superheated steam. b. The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1. c. When a vertical plate is heated in infinite air environmental under natural convection conditions, the velocity profile in air, normal to the plate, exhibits a maximum. d. A body at 925 K emits an energy of 1.42 x 1011σW/m2 (σ is the Stefan-Boltzman constant) in the wavelength band between 3 μm to 4 μm. The fraction of this energy in the total energy emitted over the entire wavelength range is equal to emissivity.

The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1.

Mode of heat transfer involved in the cooling of air cooled internal combustion engine is a. conduction b. natural convection c. forced convection d. none of these

natural convection

Steady state one dimensional heat flow by conduction as given by Fourier's low does not assume that a. there is no internal heat generation. b. boundary surfaces are isothermal. c. material is anisotropic. d. constant temperature gradient exists.

material is anisotropic.

The left face of a one dimensional slab of thickness 0.2 m is maintained at 80°C and the right face is exposed to air at 30°C. The thermal conductivity of the slab is 1.2 W/m.K and the heat transfer co-efficient from the right face is 10 W/m2.K. At steady state, the temperature of the right face in °C is a. 77.2 b. 71.2 c. 63.8 d. 48.7

48.7

In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the a. tube layout b. tube diameter c. tube pitch d. no. of baffles

tube diameter

To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is a. 1-2 heat exchanger b. 1-1 heat exchanger c. 3-2 heat exchanger d. 2-4 heat exchanger

1-1 heat exchanger

Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2°C. a. 10-50 b. 100-500 c. 1000-1500 d. 5000-15000

5000-15000

Correction is applied to LMTD for __________ flow. a. parallel b. counter c. cross d. none of these

cross

If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is a. always less than h1 b. always between h1 and h2 c. always higher than h2 d. dependent on metal resistance

always between h1 and h2

One kilogram of water at 0°C is changed to superheated steam of one atm pressure and 300° C. The major heat consumption in the process will be to a. heat the water from 0°C to 100°C. b. evaporate the water. c. to superheat the steam. d. data insufficient, can't be predicted

evaporate the water.

For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone. a. extremely low b. low c. high d. no

high

The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is 8 kW/m2. The flux, when the element temperature is increased to 1000°C for the same furnace temperature is a. 11.2 kW/m2 b. 12.0 kW/m2 c. 14.6 kW/m2 d. 16.5 kW/m2

16.5 kW/m2

Heat exchangers operating, when the asymptotic range is reached, a. provide very large heat transfer co-efficient. b. results in making part of the heating surface inactive. c. results in abruptly increased velocity. d. none of these.

results in making part of the heating surface inactive.

In case of a shell and tube heat exchanger, the minimum and maximum baffle spacing is respectively (where, D = inside diameter of the shell) a. D/5 and D b. D/2 and 2 D c. D/4 and 2 D d. D and 2 D

D/5 and D

In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate. a. geometric b. arithmetic c. logarithmic d. either (a), (b) or (c)

logarithmic

In a gas-liquid shell and tube heat exchanger, the a. presence of a non-condensible gas decreases the condensing film co-efficient. b. gases under high pressure are routed through the tube side, because high pressure gases are corrosive in nature. c. gases to be heated/cooled is normally routed through the shell side, because the corrosion caused by the cooling water or steam condensate remain localised to the tubes. d. all 'a', 'b' & 'c'

all

Which is the most suitable for the concentration of foamy & frothy liquors? a. Agitated film evaporator b. Long tube vertical evaporator c. Open pan evaporator d. None of these

Long tube vertical evaporator

The rate of heat transfer is a product of overall heat transfer co- efficient, the difference in temperature and the a. heating volume b. heat transfer area c. Nusselt number d. none of these

heat transfer area

The rate of heat transfer is a product of overall heat transfer co- efficient, the difference in temperature and the a. heating volume b. heat transfer area c. Nusselt number d. none of these

heat transfer area

Which of the following has the lowest overall heat transfer co-efficient? a. Dowtherm b. Molten sodium c. Water d. Air

Air

Which of the following has the minimum absorptivity? a. Aluminium foil b. Coal dust c. Refractory bricks d. Iron plates

Aluminium foil

A black body when hot, emits heat radiation of __________ wavelengths. a. small b. large c. all d. one fixed

all

Vibrations in the tubes of a shell and tube heat exchanger is induced due to the a. flow of fluid on the tube and shell sides. b. oscillations in the flow of shell/tube sides fluid. c. vibrations transmitted through piping and/or supports due to external reasons. d. all (a), (b) and (c)

all

Heat transfer by conduction in the turbulent core of a fluid flowing through a heated pipe is negligible, if the value of Prandtl number is a. 0.2 b. 0.4 c. 0.6 d. 0.8

0.6

The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is a. 1.66 b. 88.66 c. 3.66 d. dependent on NRe only

3.66

Sensible heat of hot industrial flue gases can not be recovered by a/an a. economiser b. regenerator c. ceramic recuperator d. none of these

none of these

The critical radius of insulation for a spherical shell is (where, K = thermal conductivity of insulating material h0 = heat transfer coefficient at the outer surface) a. K/h0 b. 2K/h0 c. h0/K d. h0/2K

2K/h0

Radiator of an automobile engine is a __________ type of heat exchanger. a. co-current b. cross-current c. counter-current d. direct contact

direct contact

In pipe flow, heat is transferred from hot wall to the liquid by a. conduction only. b. forced convection only. c. forced convection and conduction. d. free and forced convection.

forced convection and conduction.

For turbulent flow in a tube, the heat transfer co-efficient is obtained from the Dittus-Boelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer co- efficient will change by a factor of a. 1 b. 1.74 c. 6.1 d. 37

6.1

The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 W.m-1.°C-1 respectively. The ambient air is at 30°C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m-2.°C-1. The rate of heat loss from the outer surface in W.m-2 is a. 165.4 b. 167.5 c. 172.5 d. 175

165.4

Low thermal conductivity of heat insulating materials is due to its a. dense structure. b. high proportion of air space. c. high specific heat. d. none of these.

high proportion of air space.

In an interphase heat transfer process, the equilibrium state corresponds to equality of temperature in the two phases, while the condition for equilibrium in an interphase mass transfer process is equality of a. concentrations b. chemical potentials c. activity co-efficients d. mass transfer co-efficients

concentrations

Calburn analogy is applicable for the value of Prandtl number from a. 0.001 to 1 b. 0.6 to 120 c. 0.5 to 5 d. 120 to 400

0.6 to 120

Multiple effect evaporators ar used to a. increase the steam economy & decrease the capacity. b. increase the steam economy & the capacity. c. decrease the steam economy & the capacity. d. decrease the steam economy & increase the capacity.

increase the steam economy & the capacity.

Small scale evaporation is done in a a. heat exchanger b. condenser c. multiple effect evaporator d. steam jacketed kettle

steam jacketed kettle

Heat exchanger tubes are never made of a. plain carbon steel b. stainless steel c. lead d. copper

lead

At constant temperature, the thermal conductivities of gases __________ with rise in pressure. a. decrease b. increase c. remain unchanged d. may increase or decrease; depends on the pressure

decrease

In a cooling tower, water becomes cool by a. loosing sensible heat. b. heat transfer to surroundings. c. vaporisation due to heat loss to air. d. loosing latent heat.

loosing sensible heat.

In Fourier's law, the proportionality constant is called the a. heat transfer co-efficient b. thermal diffusivity c. thermal conductivity d. Stefan-Boltzman constant

thermal conductivity

The heat transfer co-efficient in film type condensation is __________ that for dropwise condensation. a. greater than b. lower than c. is same as d. half

lower than

Pick out the wrong statement. a. Bubble size increases with the dynamic viscosity of the liquid in case of nucleate pool boiling. b. Thermal conductivity of a dry material is more than that of the damp material. c. Ratio of its capacity to economy equals the steam consumption in kg/hr in an evaporator. d. Vaporisation of organic substances in evaporator mostly causes foam formation.

Thermal conductivity of a dry material is more than that of the damp material.

Which has the lowest Prandtl number ? a. Liquid metal b. Aqueous solution c. Water d. Lube oil

Liquid metal

Pick out the wrong statement. a. The value of hydrostatic head increases with increase in vacuum in the effect in a multiple effect evaporator system. b. Entering velocity of the liquid in the tubes of natural circulation evaporators is in the range of 0.3 to 0.9 metre/second. c. Duhring's plot is used for calculating the concentration of solution. d. In a multiple effect evaporation system, the number of effects is limited by the total boiling point rise.

Duhring's plot is used for calculating the concentration of solution.

In a heat exchanger with steam outside the tubes, a liquid gets heated to 45°C, when its flow velocity in the tubes is 2 m/s. If the flow velocity is reduced to 1 m/s, other things remaining the same, the temperature of the exit liquid will be a. less than 45°C b. more than 45°C c. equal to 45°C d. initially decreases and remains constant thereafter.

more than 45°C

Multiple effect evaporation accounts for a. steam economy b. lower operating costs c. investment economy d. none of these

steam economy

In case of a supercooled solution, which is on the verge of crystallisation, the free energy of the solution as compared to that of the solid is a. more b. less c. same d. more or less; depends on the nature of solution

Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second. a. 1 b. 10 c. 20 d. 30

In Biot number, the characteristic length used is the ratio of the __________ of the solid. a. volume to surface area b. perimeter to surface area c. surface area to volume d. surface area to perimeter

volume to surface area

Which of the following is the most widely used heat insulating material for pipelines carrying steam ? a. Tar dolomite bricks followed by asbestos. b. Fireclay refractory followed by aluminium sheet. c. Cotton followed by aluminium foil. d. 85% magnesia cement and glass wool.

85% magnesia cement and glass wool.

Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re 0.8. Pr^n, is not valid, if the value of a. n = 0.4 is used for heating. b. n = 0.3 is used for cooling. c. Reynolds number for the flow involved is > 10000. d. Reynolds number for the flow involved is < 2100.

Reynolds number for the flow involved is < 2100.

Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling? a. Square pitch b. Triangular pitch c. Diagonal square pitch d. None of these

Triangular pitch

In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid a. is inversely proportional to the resistance across which the drop occurs. b. and the wall are proportional to individual resistances. c. and the wall is not related. d. none of these.

and the wall are proportional to individual resistances.

The purpose of providing expansion bellows in the shell of tubular exchanger is to a. increase the heating load. b. impart structural strength. c. account for the uneven expansion of shell and tube bundles. d. facilitate increase of shell length, if needed.

account for the uneven expansion of shell and tube bundles.

Pick out the wrong statement. a. 'Solvates' are chemical compounds formed by solute with their solvents. When water is the solvent, then it is called a 'hydrate'. b. In heat exchanger calculations (Δt)weighted is used in place of Δt, when it involves more than one sequence of heating or cooling i.e., desuperheating & condensation or condensation & sub-cooling. c. Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted. d. In case of short tube vertical evaporators, area of central downtake is about 50 to 100% of the total tube cross-sectional area.

Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.

Steam economy in case of a triple effect evaporator will be a. 1 b. < 1 c. >1 d. between 0 and 1

Heat Transfer Flashcards

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