Heat Transfer

1.Fourier’s Law described heat transfer by

(a) conduction

(b) convection

(c) radiation

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

2.The heat equation is known as

(a) Reynolds equation

(b) Prandtl equation

(c) Van’t Hoff’s general equation

(d) Fourier’s general equation

3. In SI system the unit of thermal conductivity is

(a) W/ m

^{2}. K
(b) W/ m. K

(c) W/K

(d) W/ m

^{2}.
4. Flow is proportional to

(a) potential X resistance

(b) potential/conductance

(c) potential X conductance

(d) none of the above

5. Heat flows by conduction through a rod 1 m long and having a cross-sectional area of 10 cm

^{2}. The thermal conductivity of the rod material is K W/m.K. The thermal resistance for the process is
(a) 1000/K

(b) K/1000

(c) 1000 * K

(d) none of the above

6.Thermal conductivity of a material is defined as

(a) the resistance to heat transfer for a quantity of material 1 m thick with heat flow area 1m

^{2}, time unit 1 hr and temperature difference 1K
(b) the conductance for a quantity of material 1 m thick with heat flow area 1m

^{2}, time unit 1 sec and temperature difference 1 K
(c) the resistance to heat transfer for a quantity of material 1 m thick with heat flow area 100 me, time unit 1 sec and temperature difference 100 K.

(d) none of the above

7.For heat transfer through a composite wall, overall resistance to heat transfer is

(a) sum of the resistance

(b) product of the resistance

(c) a ratio of (sum of the resistances)/(product of the resistances)

(d) (sum of the resistances) + (product of the resistances)

8.For a body with very high thermal conductivity,

(a) heat transfer will be very rapid

(b) heat transfer will be very slow

(c) there will be no heat transfer at all

(d) none of the above is true

9.An insulator should have

(a) very high thermal conductivity

(b) very high thermal diffusivity

(c) very low thermal conductivity

(d) none of the above

10.Air is

(a) an excellent heat conductor

(b) a poor heat conductor

(c) is a better heat conductor than steel

(d) none of these

11.Thermal diffusivity is defined as

(a) K Cp/r

(b) K r/Cp

(c) K /rCp

(d) K rCp

12. The unit of thermal diffusivity is

(a) m/s

^{(b)}

^{ }m/s

^{2}

^{(c)}

^{ }m

^{2}/s

^{2}

(d) m

^{2}/s
13.Fourier’s law of heat conduction is very similar to

(a) Fick’s law for mass diffusion

(b) Newton’s law of viscosity

(c) Both (a) and (b)

(d) Neither (a) nor (b)

14.A pipe having an outside diameter do and an inside diameter di is used to transport a hot fluid. Heat transfer occurs radially outwards. The area for heat transfer per unit length of the pipe in given by

15.For heat transfer by conduction through a uniformly tapered rod, which of the following is true ?

(a) Flux = constant

(b) (Flux)(area) = constant

(c) none of the above

16.A hot fluid is stored in a spherical container which has an outside diameter do and an inside diameter di. Heat flows through the wall of the container radially outwards. The area for heat transfer is

(a)

(b)

(c)

(d)

17. The wall of an oven consists of two layers of insulating bricks. In between the layers there is an air gap. The presence of air gap will

(a) cause an increase in heat transfer rate through the composite wall

(b) cause a decrease in heat transfer rate through the composite wall

(c) have no influence on the rate of heat transfer

18.Consider the composite wall with an air gap described in problem (17). Heat transfer through the composite wall occurs by (assume that radiative heat transfer is not important)

(a) conduction only

(b) convection only

(c) both conduction and convection

19.A steel pipe having an outside diameter of 1.32 inch and an inside diameter of 1.049 inch is used to transport a hot fluid. Heat transfer occurs radially outwards. The area for heat transfer per unit length of the pipe is

(a) 0.739 p in

^{2}
(b) 1.038 p in

^{2}
(c) 1.179 p in

^{2}
(d) 2.41 p in

^{2}
20.Thermal conductivity of a solid varies with temperature according to the relation K = a + bT. Between 30oC and 60oC mean thermal conductivity of the solid is equal to

(a) a + 30 b

(b) a + 45 b

(c) a + 60b

(d) a + 90 b

21.Consider a composite wall consisting of three layers of insulation of length L1, L2 and L3, and thermal conductivities k

_{1}, k_{2}and k_{3}respectively. The insulating layers are placed in sequence 1, 2 and 3 and a certain rate of heat transfer results. If the order in now reversed to 3, 2, 1, rate of heat transfer through the wall under otherwise uniform conditions.
(a) will decrease

(b) will increase

(c) will remain unchanged

(d) cannot be predicted, more information required

22.The thermal resistance of a single sheet of glass (thermal conductivity = 0.63 Btu/(ft) (hr) (oF)) of thickness 1/8 inch and area 1 ft

^{2}is^{(a)}

^{ }0.0165 (Btu/hr,

^{o}F)

^{-1 }

^{(b)}

^{ }0.0278 (Btu/hr,

^{o}F)

^{-1}

(c) 0.0379 (Btu/hr,

^{o}F)^{-1}
(d) none of the above

23. Consider a uniformly tapered steel rod of circular cross-section of 1 m length. The diameter of the rod at one end is 5 cm and that at the other and is 2.5 cm. If the heat flux at the end of larger cross-section is 2500 Kcal/m

^{2}. hr the heat flux at the other end is equal to
(a) 2500 kcal/m

^{2}.h
(b) 5000 kcal/m

^{2}.h
(c) 7500 kcal/m

^{2}.h
(d) 10,000 kcal/m

^{2}.h
24.Heat transfer by convection may be of two kinds : free and forced. When the fluid motion is mechanically produced, the convection is said to be

(a) free convention

(b) forced convection

25.Forced convection is unimportant when

(a) (Re. Pr)

^{½ }>> 1
(b) (Re. Pr)

^{½ }<< 1
(c) (Re. Pr)

^{½ }® 1
(d) none of the above

26.Natural convection is negligible when

(a) (Ra. Pr)

^{½ }>> ¥
(b) (Ra. Pr)

^{1/4}>> 1
(c) (Ra. Pr)

^{1/4}<< 1
(d) none of the above

27.Consider an isolated, isothermal solid sphere suspended in an infinite medium. In the limit of negligible forced or natural covection

(a) Nusselt number, Nu = 0

(b) Nu = 1

(c) Nu = 2

(d) Nu = infinity

28.Consider heat loss through pipe insulation. At the critical radius r = r

_{c}, heat loss is
(a) Maximum

(b) minimum

(c) zero

(d) none of the above

29.The critical radius of insulation is given by the equation

_{ }

30.Consider a pipe which has to be insulated. The inside diameter of the pipe is di and the outside diameter is do. The insulation should be so designed that

_{(a)}

_{ }2 r

_{c}< d

_{o}

_{(b)}

_{ }2 r

_{c}> d

_{o}

_{(c)}

_{ }2 r

_{c}= d

_{o}

(d) either (b) or (c)

31.For calculation of heat flow through a pipe wall, area based on logarithmic mean diameter is used as heat transfer area. However if the inside diameter of a pipe is greater than 0.75 of the outside diameter, area based on arithmetic mean diameter may be used without any significant error. Consider now the following problem. A steel pipe having an outside diameter of 2.38 inch and an inside diameter of 2.067 inch is used to transport a hot fluid. The inside wall temperature is 100oC and the outside wall temperature is 95oC. The thermal conductivity of steel is 26 Btu/ft.hr.oF. Under these conditions the heat-flow rate for heat transfer radically outwards, based on logarithmic mean area, is 10435 Btu/(lin ft)(hr). The heat flow rate based on arithmetic mean area is --

(a) 10435 Btu/(lin.ft.) (hr.)

(b) 10444 Btu/(lin.ft) (hr)

(c) 10500 Btu/(lin.ft.) (hr.)

(d) 10970 Btu/(lin.ft.) (hr.)

32.A steel pipe, 2.38 inch O.D. and 2.067 inch I.D., is being used to transport a cold fluid. The pipe is insulated with ½ inch Kapok. The outer wall temperature of the pipe is O.C. If the heat transfer rate per linear ft. of the pipe is 19.35 Btu/hr, what will be the outer surface temperature of Kapok. The thermal conductivity of Kapok is 0.020 Btu/ft.hr.oF.

(a) 10

^{ o }C
(b) 20

^{o }C
(c) 30

^{o}C
(d) 40

^{o}C
33.Prandtl number is defined as

(a) m/k Cp

(b) m/r Cp

(c) m/r D

_{AB}
(d) mCp/K

34.Prandtl number is a ratio of

(a) molecular diffusivity to thermal diffusivity

(b) molecular diffusivity to momentum diffusivity

(c) momentum diffusivity to thermal diffusivity

(d) none of the above

35.When Prandtl number is greater than one,

(a) the thermal boundary layer will be outside the momentum B.L.

(b) the thermal and momentum B.L.s will merge together

(c) the momentum B.L. will be outside the thermal B.L.

(d) the momentum and concentration B.L.s will merge together.

36.Prandtl number for dry gases is of the order of

(a) 10

^{-5}
(b) 1

(c) 100

^{(d)}

^{ }10

^{5}

37.Fourier number is defined as

(a) at

^{2}/R
(b) aR

^{2}/t
(c) a

^{2}R/t^{(d)}

^{ }at/R

^{2}

38.Stanton number is defined as

(a) Pr/ (Nu. Re)

(b) Nu. Re/ Pr

(c) Nu. Pr/ Re

(d) Nu/Re. Pr

39.Reynolds analogy, St = f/2, holds for

(a) Pr << 1

(b) Pr >> 1

(c) Pr = 1

(d) all (a), (b), (c)

40.Grashof number is important in

(a) heat conduction

(b) forced convection

(c) natural convection

(d) radiative heat transfer

(e) all (a), (b), (c), (d)

41.For free convection heat transfer, Nusselt number depends on

(a) Reynolds number and Prandtl number

(b) Reynolds number, Prandtl number and Grashof number

(c) Reynolds number and Grashof number

(d) Grashof number and Prandtl number

42.“Brinkman number” is important in problems related to

(a) heat conduction with a nuclear heat source

(b) heat conduction with a viscous heat source

(c) heat conduction with a chemical heat source

(d) heat conduction with an electric heat source

43.Nusselt number for forced convection heat transfer in pipe of diameter D is defined as

(a) h D/K

(b) h K/D

(c) K D/h

(d) K D/h

^{2}
44.When the ratio of Grashof number to the square of the Reynolds number is unity, the dominant mechanism for heat transfer is

(a) Free convection

(b) Entry length problem in laminar forced convection ( developing thermal boundary layer)

(c) Mixed convection( both free and forced)

(d) Forced convection

45.A small metal ball at 30oC is placed in a hot liquid (taken in a vessel) at 150oC. The liquid is stirred slowly and liquid volume is much larger compared to the ball volume. Without any other numerical data what can you say about the Biot number ?

(a) Biot number will be large

(b) Biot number will be small

(c) Nothing can be said about the Biot number ; data required for even crude prediction.

46.Colburn J. factor for heat transfer is given by

(a) St. Pr (mw/m)

^{0.14}
(b) St

^{2/3}. Pr
(c) St. Pr

^{2/3}(mw/m)^{0.14}^{(d)}

^{ }(St. Pr)

^{2/3}

47.Greatz number is defined by the equation

48.Greatz number is important in problem related to

(a) heat transfer in a well-agitated vessel

(b) heat transfer in laminar tube flow

(c) heat transfer in turbulent flow in pipes

(d) both (a) and (c)

49.For heat transfer in tube flow Graetz number and Fourier number are related by equation

(a) N

_{GZ}. N_{FO}= J_{H}
(b) N

_{GZ }. N_{FO}= Öp
(c) N

_{GZ }.N_{FO}= N_{Re}
(d) N

_{GZ }.N_{FO}= p
50. Brinkman number is a ratio of

(a) inertial forces to gravity forces

(b) buoyancy forces to inertial forces

(c) heat transport by convection to heat transport by conduction

(d) heat production by viscous dissipation to heat transport by conduction

51. Grashof number is a ratio of

(a) buoyancy force to viscous force

(b) buoyancy force to gravity force

(c) inertial force to gravity force

(d) none of these

52. Biot number is a ratio of

(a) inertial forces to gravity forces

(b) heat transport by convection to heat production by viscous dissipation

(c) internal thermal resistance to heat transfer to the external (fluid film) resistance to heat transfer

(d) none of the above

53. In fin-tube heat exchanger; fins are provided on the tubes to increase heat transfer area. Now large fin effectiveness results from

(a) large values of heat transfer coefficient

(b) long fins (length measured in the direction of heat flow)

(c) high values of thermal conductivity

(d) all (a), (b), (c)

54.Prandtl numbers for liquid metals are

(a) higher than those for gases

(b) higher than those for liquids

(c) much lower than those for gases

(d) in between those for gases and liquids

55.Eckert number is given by the ratio of

(a) Brinkman number / Prandtl number

(b) Reynolds number / Grashof number

(c) Froude number / Grashof number

(d) Peclet number / Stanton number

56.In order to get large fin effectiveness, fins are so designed that

(a) most of the fin operates at a surface temperature very different from the fin root temperature

(b) most of the fin operates at a surface temperature not very different from the fin root temperature

(c) they provide large surface area for heat transfer, the operating temperature mentioned in part (a) and (b) in not important.

58.In a fin-tube heat exchanger, fins are placed on the side having

(a) maximum heat conductance

(b) minimum thermal resistance

(c) minimum heat conductance

(d) either (a) or (b)

59.The rate of diffusion of momentum relative to the rate of diffusion of heat is

(a) proportional to Prandtl number

(b) inversely proportional to Prandtl number

(c) proportional to Fourier number

(d) proportional to Graetz number

60.In a fin tube heat exchanger,

(a) only heat transfer area is augmented

(b) only heat transfer coefficient is augmented

(c) both heat transfer area and heat transfer coefficient are augmented

(d) none of the above

61.Biot number is defined as Bi = hL/K, where L is the characteristic length. In this definition, K is

- the thermal conductivity of the solid
- the thermal conductivity of the fluid present around the solid
- an average of the thermal conductivity of the solid and the fluid
- a constant whose value depends on the geometry of the solid

62.All substances at temperatures above … …emit radiation.

- room temperature
- absolute zero
- 373 K temperature
- 1000 K temperature

63.Let us represent absorptivity by a , reflectivity by r and transmissivity by t . Now for a body emitting radiation.

- a + r + t = 0
- a + r + t = 0.5
- a + r + t = 1.0
- a + r + t = 100

64.Absorptivity for a blackbody is

- zero
- 0.5
- 1.0
- none of the above

65.Reflectivity for a black body is

- zero
- 0.5
- 0.75
- 1.0

66.Transmissivity for a blackbody is

- negative
- zero
- 0.75
- 1.0

67.Monochromatic radiation is defined as radiation

- having a single wavelength
- having more than one wavelength

68.Electromagnetic radiation that is of importance in heat flow has wavelength in the range of

- 5 to 10 m
- 100 to 500 m
- 0.5 to 50 m m
- none of the above

69.Visible light has wavelength in the range of

- 0.38 to 0.78 m m
- 1 to 5 m
- 100 to 500 m
- 500 m and above

70.As the temperature of a body emitting radiation increases, the wavelength of the predominant thermal radiation

- increases
- decreases
- remains unchanged

71.If the total emissive power of a body is W and that of a blackbody is W

_{b}, the emissivity of the body is defined as- W
_{b}– W - W
_{b}+ W __W/__(W_{b}+ W__)__- W/W
_{b}

72.If the monochromatic emissivity of a body is the same for all wavelengths, the body is called

- a gray body
- a black body
- none of these

73.Emissivity of solids usually

- decreases with temperature
- increases with temperature
- remains essentially unchanged with changes in temperature

74.An isothermal enclosure containing a small peephole is an experimental equivalent of

- a gray body
- a black body
- none of these

75. Stefan–Boltzmann law for black body radiation states that the total emissive power of a blackbody is directly proportional to

- T
^{ ½} - T
- T
^{2} - T
^{4}

76.According to Wien’s displacement law, l max T is equal to

- 0.0121 cm
^{o}K - 0.2884 cm
^{o}K - 0.9774 cm
^{o}K - 2.3721 cm
^{o}K

77.A gray body is a substance having absorptivity

- greater than one
- less than one and independent of temperature
- less than one and dependent on temperature
- equal to one

78.When any body is at temperature equilibrium with its surroundings, the emissivity of the body is

- greater than absorptivity
- lower than absorptivity
- equal to absorptivity

79.At temperature equilibrium, the ratio of the total radiating power of any body to the absorptivity of that body depends only on the temperature of the body. This is the statement of

- Planck’s law
- Stafan – Boltzmann law
- Wien’s displacemnet law
- Kirchhoff’s law

80.All analogy equations relating friction factor and heat transfer coefficient apply

- only to skin friction
- only to form frcition
- to both skin and form frcition
- to situation where there is neither skin nor form friction

81.In the laminar sublayer, heat transfer

- by conduction is of major importance
- by convection is of major importance
- by radiation is of major importance
- by conduction, convection and rediation are of equal importance

82.Peclet number for heat transfer is given by

- Re. Gr
- Re. Fo
- Re. Pr
- Re. Nu

83.Colburn analogy applies over a range of Prandtl number

- from 0.01 to 0.5
- from 0.06 to 6
- from 0.1 to 16
- from 0.6 to 120

84.In common fluids flowing through tubes,heat transfer by conduction is limited to the viscous sublayer. In liquid metals flowing through tubes,

- likewise, heat transfer by conduction is limited to the viscous sublayer
- heat transfer by conduction is important in the viscous sublayer as well as in the buffer layer
- heat transfer by conduction is important throughout the entire turbulent core
- heat transfer by convection only is important; heat transfer by conduction is negligible even in the viscous sublayer.

85.Consider laminar flow heat transfer to a flat plate of length L. The plate is heated over its entire length, and local Nusselt number at the end of the plate given by Nu,L= 0.332 (Pr)1/3 (Re, L) ½ . The average Nussent number for the entire plate is

- Nu, av = 0.332 (Pr)
^{2/3}(Re, L)^{ ½} - Nu, av = 0.332
- Nu, av = 0.446 (Pr)
^{1/3}(Re, L)^{ ½} - Nu, av = 0.664 (Pr)
^{1/3}(Re, L)^{ ½}

86.A hot fluid at 150oC is to be cooled to 100oC in a double-pipe heat exchanger by using a coolant which will be heated from 40oC to 80oC. If the hot and cold streams flow counter-currently, the driving force for heat transfer is

- 52.9
^{o}C - 64.9
^{o}C - 75.0
^{o}C - none of these

87. In question no. 86, if the hot and cold streams flow cocurrently, the driving force for heat transfer is

- 43.7
^{o}C - 51.2
^{o}C - 52.9
^{o}C - 64.9
^{o}C

88.A cold fluid at 30oC is to be heated to 75oC in a double – pipe heat exchanger by condensing saturated steam. The driving force for heat transfer for counter-current flow arrangement of the hot and cold streams

- is greater than that for parallel flow
- is less than that for parallel flow
- is equal to that for parallel flow
- cannot be predicted, more information required.

89.A hot fluid at 100oC is to be cooled to 60oC in a double-pipe heat exchanger by using a coolant which will be heated from 30oC to 60oC. If the hot and cold streams flow concurrently, the driving force for heat transfer ( by mean temperature difference ) is equal to

- 0
^{ o}C - 30
^{o}C - 40
^{o}C - none of these

90.Refer to question no. 89. In order to achieve the temperature drop of the hot fluid as mentioned, the heat transfer area requirement will be

- finite and small
- finite and large
- infinite (theoretically)
- zero

91.In parallel flow of the hot and cold fluids, the lowest temperature theoretically attainable by the hot fluid is

- that of the outlet temperature of the cold fluid
- less than that of the outlet temperature of the cold fluid

92.Double-pipe heat exchangers are used for cases where the heat transfer area requirement is around

- 10 to 20 m
^{2} - 100 to 200 m
^{2} - 500 to 1000 m
^{2} - 5000 to 10000 m
^{2}#9;

93.Heat transfer coefficients for fluids flowing through pipes in laminar region are calculated by using the correlation of Sieder and Tate. Which one of the following is the correlation proposed by Sieder and Tate ?

- Nu = 0.023 (Re)
^{0.8}(Pr)^{0.33}(__m____/____m___{w}^{0.14} - Nu = 1.86 [(Re) (Pr) (D/L)]
^{1/3}(m_{/}_{m}_{ w})^{0.14} - Nu = 2.0 + 0.60 (Re)
^{0.50}(Pr)^{1/3} - None of the above

94.In a double-pipe heat exchanger, the tube-side heat transfer coefficient hi = 10 Btu/ft2. hr. oF and the annules – side heat – transfer coefficient ho = 1000 Btu/ft2. hr. oF. The wall thickness of the inner tube is small. The overall heat transfer coefficient for this situation is close to

- 1000 Btu/ft
^{2}. hr.^{o}F - 500 Btu/ft
^{2}. hr.^{o}F - 10 Btu/ft
^{2}. hr.^{o}F - 1 Btu/ft
^{2}. hr.^{o}F

95.For a certain heat exchanger, when the dirt factor (deposited) is greater than the dirt factor (allowed),

- the heat exchanger delivers a quantity of heat more than that required in the process
- the heat exchanger delivers a quantity of heat exactly equal to that required in the process
- the heat exchanger no longer delivers a quantity of heat equal to the process requirements and must be cleaned
- none of the above is ture; dirt factor is a fictitious thing; under no circumstances can be dirt factor (deposited) is greater than the dirt factor (allowed)

96.In a certain process two double-pipe heat exchangers are connected in series and the hot and cold streams flow counter currently. For this situation which of the following is true ?

- Both exchangers transfer equal quantities of heat
- Both exchangers do not transfer equal quantities of heat
- Double – pipe heat exchangers are never connected in series. The question, therefore, is meaningless.

97.A shell – and – tube heat exchanger is used to heat sugar solution by using steam. No data about the exchanger is given. Can you say which film resistance is likely to control the overall heat transfer process ?

- steam film resistance
- sugar solution film resistance
- both steam and sugar solution film resistances
- nothing can be said about the controlling resistance

98.Heat transfer coefficient for codensation of steam is around

- 1 Btu/ft
^{2}. hr.^{o}F - 100 Btu/ft
^{2}. hr.^{o}F - 1500 Btu/ft
^{2}. hr.^{o}F - none of the above

99.In a shell-and – tube heat exchanger, baffles are provided on the shell side

- to increase heat transfer area
- to induce turbulence in the shell-side liquid, thereby increasing the shell side heat transfer coefficient
- to give structural support to the tubes
- all of the foregoing

100.Thickness of heat exchanger tubes is specified in terms of BWG (Birmingham Wire Gage). For a fixed tube OD (outside diameter) higher BWG means

- increased wall thickness
- reduced wall thickness

101. Which of the following tubes are most common in heat exchanger design ?

- ½ and 1½ inch OD tubes
- ¾ and 1 inch OD tubes
- 1 and 2 inch OD tubes

102.In a shell – and – tube heat exchanger, the baffle spacing is usually between

- one tenth the inside diameter of the shell and one fifth the inside diameter of the shell
- one fifth the inside diameter of the shell and the inside diameter of the shell
- one half the inside diameter of the shell and twice the inside diameter of the shell
- none of the foregoing

103.Tube pitch is equal to

- inside diameter of tube – clearance
- outside diameter of tube – clearance
- outside diameter of tube + clearance
- inside diameter of tube + clearance

104. In a 1 – 2 shell – and – tube heat exchanger, relative to the shell fluid

- one tube pass is in counterflow and the other in parallel flow
- both tube passes are in counterflow
- both tube passes are in parallel flow
- none of the above is ture; in a 1-2 shell-and-tube heat exchanger, there are two passes for shell-side fluid and one pass for tube-side fluid.

105.In a 1 – 2 shell- and – tube heat exchanger, the total heat transferred is given by the equation

- Q = UA [LMTD]
- Q = UA F
_{T}[ LMTD]( F_{T}> 1) - Q = UA F
_{T}[ LMTD]( F_{T}< 1) - Q = UA [F
_{T}+ LMTD]( F_{T}> 1)

106.When a temperature cross occurs in a 1 – 2 exchanger, the value of F

_{T}- drops sharply
- increases rapidly
- becomes zero
- becomes negative

107. Larger temperature crosses are permissible in

- 1 – 2 shell – and – tube exchangers
- 2 – 4 shell – and – tube exchangers
- same temperature crosses are permissible in both 1 – 2 and 2 – 4 exchangers; the question is, therefore, meaningless.

108. A 2 – 4 shell – and – tube heat exchanger

- is thermally superior to two 1 – 2 exchangers in series
- is thermally inferior to two 1 – 2 exchangers in series
- is thermally identical with two 1 – 2 exchangers in series
- may be any one of (a), (b), (c) depending on the fluid flow characteristics

109. When the temperature cross is too large to be allowed,

- a 1- 1 true counterflow shell – and – tube exchanger should be employed
- a 1 – 2 shell – and – tube heat exchanger should be employed
- a 3 – 6 exchanger should be employed
- a 4 – 8 exchanger should be employed

110.Prandtl number for gases

- are strong functions of temperature
- show little dependence on temperature except near the critical temperature
- are of the order of one
- all of above (a,b and c)

111.For the same value of jH , film coefficients for gases

- will be smaller than those for liquids
- will be the same as those for liquids
- will be greater than those for liquids
- will be smaller at low temperatures; greater at high temperatures.

112.A gas at normal pressure is to be heated by steam in a 1-2 shell – and – tube heat exchanger. For this situation

- gas should be placed on the tube – side
- gas should be placed on the shell – side
- steam should be placed on the shell – side
- any one of the arrangements of (a), (b), (c) will be equally effective and will have the same advantages/disadvantages

113.A gas at high pressure is to be cooled by cooling water in a 1 – 2 shell – and – tube heat exchanger. For this situation,

- gas should be placed on the tube – side
- gas should be placed on the shell – side
- either of the two arrangements will be equally effective

114. Vapour of a pure substance is to be cooled from a certain temperature (higher than its dew point) to a temperature lower than its dew point. Heat transferred in this process

- is only sensible heat
- is only latent heat
- are both sensible and latent heat

115.Under otherwise uniform conditions rate of heat transferred in dropwise condensation is

- more than that in film – wise condensation
- less than that in filmwise condensation
- equal to that in filmwise condensation
- any of the foregoing, depending on the condensing vapour

116. The heat transfer coefficient in dropwise condensation is usually

- twice that in filmwise condensation
- four to eight times the heat transfer coefficient in filmwise condensation

- twenty to thirty times the heat transfer coefficient in filmwise condensation
- hundred to thousand times the heat transfer coefficient in filmwise condensation

117. Nusselt’s theory for condensation is derived for

- dropwise condensation
- filmwise condensation
- both dropwise and filmwise condensation
- there is nothing called Nusselt’s theory (for condensatin), there is a dimensionless group called Nusselt number. The question, therefore, is meaningless.

118. For condensation of vapour (of a pure substance) on a vertical surface, the thickness of the condensate film cumulatively increases from top to bottom. The heat transfer coefficient for condensation, of vapour on a vertical surface, therefore,

- decreases from top to bottom
- increases from top to bottom
- remain equal at all locations from top to bottom

119. Under otherwise uniform conditions the heat transfer coefficient for condensation

- decreases with increasing condensate temperature
- increases with increasing condensate temperature
- remains essentially constant for varying condensate temperatures
- cannot be predicted, more information required for correct prediction

120. The heat transfer coefficient for condensation of a pure vapour on a vertical tube is

- more than the condensing coefficient on a horizontal tube
- equal to the condensing coefficient on a horizontal tube
- less than the condensing coefficient on a horizontal tube

121.Steam condenses inside the tubes of a vertical condenser. Condensate flowrate in the tubes is such that flow is laminar in the upper portion of the tubes and turbulent afterwards. The local condensing coefficnet for this situation will

- increase continuously from the top of the tube to the bottom
- decrease continuously from the top of the tube of the bottom
- decrease continuously from the top downward, after the flow becomes turbulent the coefficient will increase in accordance with the usual behaviour of forced convection
- remain uniformly constant throughout the tube

122.A vertical condenser operates with condensation inside the tubes. For this situation

- only one tube pass is possible
- at the most two tube passes are possible
- any number of tube passes is possible

123.Condenser of a certain distillation column has been so installed that reflux to the column flows by gravity flow. For this system the allowable vapour pressure drop in the condenser will be usually

- 1 to 2 psi
- 4 to 5 psi
- around 10 psi
- around 25 psi

124.Reflux from a condenser to a distillation column may flow either by gravity or reflux may be pumped to the column from the condensate accumulator. For pumped return of reflux the allowable vapour pressure drop in the condenser is usually around

- 0.5 psi
- psi
- 5.0 psi
- 10.0 psi

125. Presence of a noncondensable gas, such as air, in steam

- increases the condensation heat transfer coefficient
- reduces the condensation heat transfer coefficient
- does not influence the condensation heat transfer coefficient at all
- may increase or reduce the condensation heat transfer coefficient, depending on the volume percent of air insteam. If the volume % of air in steam is less than 10, condensation heat transfer coefficient is increased. If the volume % of air is more than 10, condensation heat transfer coefficient is reduced. The condensation heat transfer coefficient is maximum when the volume percent of air in steam is around 9.9.

126. Consider condensation of a mixture of vapour of N miscible components. The degrees of freedom for such a process are

- N – 1
- 2N – 1
- N/2
- N

127. Consider condensation of a vapour – gas mixture of pentane + hexane + steam + air. The degrees of freedom for such a process is

- zero
- one
- two
- three

128 In a counter-current heat exchanger which has been in service for quite some time, due to formation of scale, the heat transfer rate is reduced to 90% of its original value based on clean surface. Assume that terminal temperatures of fluids are the same in both cases, and the effective heat transfer area does not change appreciably due to scale formation. The overall clean heat transfer coefficient is 300 Btu/ft2.hr.oF. What is the overall dirt factor ?

- 0.054 hr.ft
^{2},^{o}F /Btu #9; #9; - 1.1 hr.ft
^{2},^{o}F /Btu - 3.7 x 10
^{-4}hr.ft^{2},^{o}F /Btu - 10.8 hr.ft
^{2},^{o}F /Btu

129. For a counter-current heat exchanger the clean overall heat transfer coefficient is 500 W/m2.K and the overall fouling factor is 0.00035 m2 K/W. What will be the value of the design overall heat transfer coefficient ?

- 485.6W/m2.k
- 425.5 W/m
^{2}.k - 392.8 W/m
^{2}.k - none of the foregoing