Heat Transfer 2

Question 1

In natural convection heat transfer, the correlating parameter is the

Answer 1

Grashoff number

Question 2

Boiling of milk in an open vessel is an example of __________ boiling.

Answer 2

film

Question 3

Pick out the wrong statement.

Answer 3

The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of ?1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 ?1.

Question 4

Mode of heat transfer involved in the cooling of air cooled internal combustion engine is

Answer 4

natural convection

Question 5

Steady state one dimensional heat flow by conduction as given by Fourier’s low does not assume that

Answer 5

material is anisotropic.

Question 6

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

Answer 6

48.7

Question 7

In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the

Answer 7

tube diameter

Question 8

To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is

Answer 8

1-1 heat exchanger

Question 9

Heat transfer co-efficient (h) for a fluid flowing inside a clean pipe is given by
This is valid for the value of NRe equal to

Answer 9

> 10000

Question 10

Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2�C.

Answer 10

5000-15000

Question 11

Correction is applied to LMTD for __________ flow.

Answer 11

cross

Question 12

If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is

Answer 12

always between h1 and h2

Question 13

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

Answer 13

evaporate the water.

Question 14

For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.

Answer 14

high

Question 15

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

Answer 15

16.5 kW/m2

Question 16

Heat exchangers operating, when the asymptotic range is reached,

Answer 16

results in making part of the heating surface inactive.

Question 17

In case of a shell and tube heat exchanger, the minimum and maximum baffle spacing is respectively (where, D = inside diameter of the shell)

Answer 17

D/5 and D

Question 18

In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate.

Answer 18

logarithmic

Question 19

In a gas-liquid shell and tube heat exchanger, the

Answer 19

all ‘a’, ‘b’ & ‘c’

Question 20

Which is the most suitable for the concentration of foamy & frothy liquors?

Answer 20

Long tube vertical evaporator

Question 21

Boiling point elevation of an ideal solution

Answer 21

increases rapidly with temperature rise.

Question 22

The rate of heat transfer is a product of overall heat transfer co-efficient, the difference in temperature and the

Answer 22

heat transfer area

Question 23

Which of the following has the lowest overall heat transfer co-efficient?

Answer 23

Air

Question 24

Which of the following has the minimum absorptivity?

Answer 24

Aluminum foil

Question 25

A black body when hot, emits heat radiation of __________ wavelengths.

Answer 25

all

Question 26

A composite wall consists of two plates A and B placed in series normal to the flow of heat. The thermal conductivities are kA and kB and the specific heat capacities are CPA and CPB for plates A and B respectively. Plate B has twice the thickness of plate A. At steady state, the temperature difference across plate A is greater than that across plate B, when

Answer 26

kA < 0.5kB

Question 27

Vibrations in the tubes of a shell and tube heat exchanger is induced due to the

Answer 27

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

Question 28

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

Answer 28

0.6

Question 29

The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is

Answer 29

3.66

Question 30

Sensible heat of hot industrial flue gases can not be recovered by a/an

Answer 30

none of these

Question 31

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)

Answer 31

2K/h0

Question 32

Radiator of an automobile engine is a __________ type of heat exchanger.

Answer 32

direct contact

Question 33

In pipe flow, heat is transferred from hot wall to the liquid by

Answer 33

forced convection and conduction

Question 34

The value of Stefan-Boltazman constant in SI unit is

Answer 34

5.6697 x 10-8W/m2.�K4

Question 35

The unit of heat transfer co-efficient is

Answer 35

BTU/hr. ft2�F

Question 36

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

Answer 36

6.1

Question 37

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-2is

Answer 37

165.4

Question 38

Value of Nusselt number [Nu = (hD/k)] for the heat transfer by conduction from a droplet or a spherical particle to a surrounding stagnant film is

Answer 38

2

Question 39

Low thermal conductivity of heat insulating materials is due to its

Answer 39

high proportion of air space

Question 40

The most conducive surface for dropwise condensation to occur is the __________ surface.

Answer 40

oily

Question 41

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

Answer 41

concentrations

Question 42

Calburn analogy is applicable for the value of Prandtl number from

Answer 42

0.6 to 120

Question 43

Pick out the correct equation.

Answer 43

jH = (St)(Pr)^2/3 = f/2

Question 44

Multiple effect evaporators ar used to

Answer 44

increase the steam economy & the capacity.

Question 45

Prandtl number is given by

Answer 45

(Cp � ?)/k

Question 46

Small scale evaporation is done in a

Answer 46

steam jacketed kettle

Question 47

Heat exchanger tubes are never made of

Answer 47

lead

Question 48

At constant temperature, the thermal conductivities of gases __________ with rise in pressure.

Answer 48

decrease

Question 49

Reason for operating an evaporator in multiple effect is to secure

Answer 49

both (a) and (b).

Question 50

In a cooling tower, water becomes cool by

Answer 50

loosing sensible heat.

Question 51

In Fourier's law, the proportionality constant is called the

Answer 51

thermal conductivity

Question 52

The heat transfer co-efficient in film type condensation is __________ that for dropwise condensation.

Answer 52

lower than

Question 53

Pick out the wrong statement.

Answer 53

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

Question 54

Duhring's plot' is of use in

Answer 54

evaporation

Question 55

Which has the lowest Prandtl number?

Answer 55

Liquid metal

Question 56

Pick out the wrong statement.

Answer 56

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

Question 57

Prandtl number for most of dry gases is about

Answer 57

0.72

Question 58

Prandtl number is the reciprocal of

Answer 58

Thermal diffusivity/Momentum diffusivity

Question 59

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

Answer 59

more than 45�C

Question 60

For a liquid in laminar flow through a very long tube, when the exit fluid temperature approaches the wall temperature, the equation to be used is

Answer 60

Nu = 2/pi x Gz

Question 61

Multiple effect evaporation accounts for

Answer 61

steam economy

Question 62

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

Answer 62

more

Question 63

Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.

Answer 63

1

Question 64

In Biot number, the characteristic length used is the ratio of the __________ of the solid.

Answer 64

volume to surface area

Question 65

Which of the following is the most widely used heat insulating material for pipelines carrying steam?

Answer 65

85% magnesia cement and glass wool.

Question 66

Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re^0.8 . Pr^n, is not valid, if the value of

Answer 66

Reynolds number for the flow involved is < 2100.

Question 67

Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling?

Answer 67

Triangular pitch

Question 68

In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid

Answer 68

and the wall are proportional to individual resistances.

Question 69

The purpose of providing expansion bellows in the shell of tubular exchanger is to

Answer 69

account for the uneven expansion of shell and tube bundles.

Question 70

A graph between __________ is called Wilson plot.

Answer 70

1/U Vs. 1/V^0.8

Question 71

Grashhoff number is given by

Answer 71

gD^3.?.?t?2/?^2

Question 72

The overall heat transfer co-efficient for a shell and tube heat exchanger for clean surfaces is U0 = 400 W/m2.K. The fouling factor after one year of operation is found to be hd0 = 2000 W/m2.K. The overall heat transfer co-efficient at this time is

Answer 72

333W/m2.K

Question 73

In a laboratory test run, the rate of drying was found to be 0.5 x 10-3 kg/m2.s, when the moisture content reduced from 0.4 to 0.1 on dry basis. The critical moisture content of the material is 0.08 on a dry basis. A tray dryer is used to dry 100 kg (dry basis) of the same material under identical conditions. The surface area of the material is 0.04 m2/kg of dry solid. The time required (in seconds) to reduce the moisture content of the solids from 0.3 to 0.2 (dry basis) is

Answer 73

5000

Question 74

Pick out the wrong statement.

Answer 74

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

Question 75

What is the geometric mean of two heat transfer areas A1 and A2?

Answer 75

sqrt*(A1 . A2)

Question 76

Evaporation by thermocompression results in the

Answer 76

both (a) and (b).

Question 77

Forced circulation evaporators are normally used for concentrating liquids having

Answer 77

both(a) & (b)

Question 78

Steam economy in case of a triple effect evaporator will be

Answer 78

> 1