# Heat Transfer Questions and Answers

## Multiple Choice Questions (MCQ) and Answers on Heat Transfer

Question 1 : Bulk of the convective heat transfer resistance for a hot tube surface  to the fluid flowing in it, is

1. in the central core of the fluid
2. uniformly distributed throughout the fluid
3. mainly confined to a thin film of fluid near the surface.
4. none of these.

Question 2 : Which of the following is concerned with both heat and mass transfer ?

1. Lewis relationship
2. Nusselt number
4. Froude number

Question 3 : A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides

1. higher economy
2. lower capacity
3. both (1) & (2)
4. lower economy

Question 4 : 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

1. greater than that for uninsulated steam pipe.
2. less than that of the uninsulated steam pipe.
3. equal to that of the uninsulated steam pipe.
4. less than the steam pipe with 5 cms insulation.

Question 5 : The equivalent diameter for pressure drop is __________ that for heat transfer.

1. smaller than
2. greater than
3. equal to
4. not related with

Question 6 : Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.

1. no
2. the least
3. the largest
4. lower

Question 7 : 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

1. 25°C
2. 30°C
3. 35°C
4. 40°C

Question 8 : Thermal conductivity of a conducting solid material depends upon its

1. temperature
2. porosity
3. both (1) & (2)
4. neither (1) nor (2)

Question 9 : Pick out the wrong statement.

1. The controlling resistance in case of heating of air by condensing steam is in the air film.
2. 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.
3. In case of a 1 – 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.
4. Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice-versa occurs at saturation temperature.

Question 10 : The average heat transfer co-efficient for laminar film condensation on vertical surface is inversely proportional to (where, ΔT = Temperature drop across condensate film )

1. (ΔT)2

2. $\sqrt{∆T}$

3. (ΔT)1/4
4. (ΔT)3/2

Question 11 : The unit of heat transfer co-efficient in SI unit is

1. J/M2°K
2. W/m2°K
3. W/m°K
4. J/m°K

Question 12 : The number of kg vaporised per kg of steam fed to the evaporator is defined as

1. capacity
2. rate of evaporation
3. economy
4. rate of vaporisation

Question 13 : Pick out the wrong statement.

1. In case of heat transfer by purely forced convection, GR/Re2 ≤ 1.
2. The equivalent diameter of heat transfer for a duct of square cross-section (having each side as ‘x’) is equal to 4x.
3. Distillation process is not the same as evaporation.
4. The effectiveness of nucleate boiling depends basically on the ease with which the bubbles are formed and detached from the heating surface.

Question 14 : Economy of a multiple effect evaporator is not influenced much by the

1. boiling point elevations
2. temperature of the feed
3. rate of heat transfer
4. ratio of the weight of the thin liquor to thick liquor

Question 15 : Increasing the liquor level in the evaporator results in the

1. decreased capacity.
2. increase in liquor film co-efficient.
3. decreased effect of hydrostatic head.
4. increased true temperature drop.

Question 16 : Kirchoff s law applies to __________ radiation.

1. total
2. monochromatic
3. both (1) & (2)
4. neither (1) nor (2)

Question 17 : With the increase of temperature, the Col-burn jH factor

1. increases.
2. decreases.
3. remains unchanged.
4. may increase or decrease ; depending on temperature.

Question 18 : In case of evaporators, liquid entrainment results primarily due to

1. high vacuum in the evaporator.
2. high evaporation rate.
3. foaming of the solution.
4. high heat transfer rate.

Question 19 : Baffles in the shell side of a shell and tube heat exchanger

1. increase the cross-section of the shell side liquid.
2. force the liquid to flow parallel to the bank.
3. increase the shell side heat transfer co-efficient.
4. decrease the shell side heat transfer co-efficient.

Question 20 : 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

1. finned tube heat exchanger with air inside and steam outside.
2. finned tube heat exchanger with air outside and steam inside.
3. shell and tube heat exchanger with air inside tubes and steam on shell side.
4. shell and tube heat exchanger with air on shell side and steam inside tubes.

Question 21 : Pick out the wrong statement.

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

Question 22 : Convective heat transfer co-efficient in case of fluid flowing in tubes is not affected by the tube length/diameter ratio, if the flow is in the __________ zone.

1. laminar
2. transition
3. both ‘1’ & ‘2’
4. highly turbulent

Question 23 : The actual temperature drop across the heating surface in an evaporator depends on the

1. feed.
2. depth of liquid over heating surface.
3. pressure difference between steam chest and vapour space.
4. all (1), (2) and (3).

Question 24 : Which of the following is not used as a medium for high temperature heating ?

1. Dowtherm
2. mercury
3. liquid metal (e.g. molten sodium)
4. fused salts (e.g., an eutectic mixture of 53% KNO3, 40% NaNO2and 7% NaNO3)

Question 25 : Presence of a non-condensing gas in a condensing vapour

1. increases the rate of condensation.
2. decreases thermal resistance.
3. is desirable to increase the film co-efficient.
4. none of these.

Question 26 : 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

1. 32
2. 37
3. 45
4. 50

Question 27 : In a heat exchanger, floating head is provided to

1. facilitate cleaning of the exchanger.
2. increase the heat transfer area.
3. relieve stresses caused by thermal expansion.
4. increase log mean temperature gradient.

Question 28 : Leidenfrost point is a term concerned with the

1. condensation of the saturated vapor on a cold surface.
2. concentration of a corrosive solution by evaporation.
3. heat transfer between two highly viscous liquids.
4. boiling of a liquid on a hot surface.

Question 29 : 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

1. counter flow with process stream on shell side.
2. counter flow with process stream on tube side.
3. parallel flow with process stream on shell side.
4. parallel flow with process stream on tube side.

Question 30 : Heat produced when a steady state current, I passes through an electrical conductor having resistance, ‘R‘ is

1. IR
2. I2R
3. IR2
4. I2R2

Question 31 : Fouling factor

1. is a dimensionless quantity.
2. does not provide a safety factor for design.
3. accounts for additional resistances to heat flow.
4. none of these.

Question 32 : In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient

1. increases
2. decreases
3. is not affected
4. may increase or decrease; depends on the feed

Question 33 : The steam ejector is used to

1. remove condensate from the steam pipelines.
2. create vacuum.
3. superheat the steam.
4. none of these.

Question 34 : For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area

1. increases
2. decreases
3. remains constant
4. passes through a maxima

Question 35 : 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

1. (COP)R = (COP)HP = 0.6
2. (COP)R = 2.5; (COP)HP = 1.5
3. (COP)R = 1.5; (COP)HP = 2.5
4. (COP)R = (COP)HP = 2.5

Question 36 : Shell side pressure drop in a shell and tube heat exchanger does not depend upon the

1. baffle spacing & shell diameter.
2. tube diameter & pitch.
3. viscosity, density & mass velocity of shell side fluid.
4. none of these.

Question 37 : In the equation Q = UAΔt; Δt is

1. geometric mean temperature difference.
2. arithmetic mean temperature difference.
3. logarithmic mean temperature difference.
4. the difference of average bulk temperatures of hot and cold fluids.

Question 38 : With increase in porosity, the thermal conductivity of a solid substance

1. increases
2. decreases
3. remains unchanged
4. may increase or decrease; depends on the solid

Question 39 : For an ideal black body

1. absorptivity = 1
2. reflectivity = 1
3. emissivity = 0
4. transmissivity = 1

Question 40 : 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

1. 40.5
2. 34.4
3. 26.8
4. 25

Question 41 : When vaporisation takes place directly at the heating surface, it is called

1. film boiling
2. nucleate boiling
3. vapour binding
4. none of these

Question 42 : Steam consumption in kg/hr in case of an evaporator is given by (where, C & E are capacity the economy of the evaporator respectively)

1. C/E
2. E/C
3. CE
4.  1/CE

Question 43 :  Reynold’s analogy states that

1. Nst α f
2. Nst α NRe
3. NNu α f
4. NRe α f

Question 44 : With increase in temperature, the thermal conductivity of non-metallic amorphous solids

1. decreases
2. increases
3. remains constant
4. first decreases upto certain temperature and then increases

Question 45 : Fourier’s law applies to the heat transfer by

1. convection
3. conduction
4. all (1), (2) & (3)

Question 46 : Crystal size in a continuous crystalliser depends upon the

1. rate of heat transfer.
2. degree of turbulence.
3. degree of supersaturation.
4. all (1), (2) and (3).

Question 47 : Heat waves

1. can not pass through vacuum.
2. travel in straight line.
3. can be reflected by a mirror.
4. both (2) and (3).

Question 48 : Air is best heated with steam in a heat exchanger of

1. plate type.
2. double pipe type with fin on steam side.
3. double pipe type with fin on air side.
4. shell and tube type.

Question 49 : Which of the following is generally considered as opaque surface towards radiations?

1. Gases
2. Solids
3. Liquids
4. Both (2) and (3)