Heat Transfer (Set 2)

Question 1

What is the steam economy in case of a single effect evaporator system?

Answer 1

<1

Question 2

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

Answer 2

ha = 2hl

Question 3

jH factor for heat transfer is not a function of the ____ number.

Answer 3

both (b) & (c)

Nusselt & Grashoff

Question 4

The Fourier number (defined as a.t/L2) is used in the analysis of problem involving heat transfer by

Answer 4

transient conduction

Question 5

The energy radiated from a surface Q at absolute temperature T is related as

Answer 5

Q ∝ T^4

Question 6

Baffles are provided in heat exchangers to increase the

Answer 6

heat transfer co-efficient

Question 7

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)

Answer 7

0.1

Question 8

In case of a vertical tube evaporator, with increase in the liquor level, the

Answer 8

capacity of the evaporator is decreased.

Question 9

The ratio of total radiating power to the absorptivity of the body depends upon the ____ as per Kirchoffs law.

Answer 9

temperature of the body

Question 10

For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally

Answer 10

decreases

Question 11

A 2-4 heat exchanger involves

Answer 11

both counter and parallel-flow of the fluids.

Question 12

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

Answer 12

< 1

Question 13

With increase in temperature, the thermal conductivity of steel

Answer 13

decreases

Question 14

Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids?

Answer 14

Sieder-Tate equation

Question 15

The main function of baffles provided in a shell and tube heat exchanger is to

Answer 15

enhance turbulence.

Question 16

In a single effect evaporator, the economy is

Answer 16

< 1

Question 17

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

Answer 17

<1

Question 18

Which of the following is not concerned with the heat transfer?

Answer 18

Schmidt number

Question 19

Latent heat absorbed by 1 lb of water at 212°F, when it is changed to steam at 212°F, may be around ____ BTU.

Answer 19

180

Question 20

In counter flow compared to parallel flow,
a. LMTD is greater.
b. less surface area is required for a given heat transfer rate.

Answer 20

both (a) and (b).

Question 21

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)

Answer 21

1000

Question 22

Crystallisation of solids from a homogeneous solution is a/an ____ process.

Answer 22

exothermic

Question 23

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

Answer 23

24000

Question 24

Value of Prandtl number for water ranges from

Answer 24

5 to 10

Question 25

Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by

Answer 25

conduction

Question 26

Among liquids, water has a comparatively high value of thermal conductivity, due to its

Answer 26

partial ionisation

Question 27

Reynold's analogy states that (where, St = Stanton number f = friction factor)

Answer 27

St = f/2

Question 28

For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is

Answer 28

backward

Question 29

The sum of reflectivity and absorptivity for an opaque body is equal to

Answer 29

1

Question 30

The film thickness for laminar film condensation on vertical surface ____ from top to bottom.

Answer 30

cummulatively increases

Question 31

In case of a multipass shell and tube heat exchanger, providing a baffle on the shell side ____ the heat transfer rate.

Answer 31

increases

Question 32

The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)

Answer 32

∝ 1/t

Question 33

Heat transfer co-efficient (h1) for liquids increases with

Answer 33

increasing temperature.

Question 34

A diathermaneous substance ____ the thermal radiation completely.

Answer 34

transmits

Question 35

A hot body will radiate heat most rapidly, if its surface is

Answer 35

black & rough

Question 36

Evaporation of 1kg of water from a solution in a single effect evaporator requires about ____ kg of steam.

Answer 36

1-1.3

Question 37

A metal wire of 0.01 m dia and thermal conductivity 200 W/m.K is exposed to a fluid stream with a convective heat transfer coefficient of 100 W/m2.K. The Biot number is

Answer 37

0.025

Question 38

Analogy between mass and heat transfer is not applicable in case of a. same velocity profile or equal eddy diffusivities. b. thermal or pressure mass diffusion. c. viscous heating or chemical reaction.

Answer 38

both (b) and (c).

Question 39

A ____ surface has the maximum thermal emissivity out of the following.

Answer 39

black & rough

Question 40

Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.)

Answer 40

d^2

Question 41

Mechanical recompression evaporation is used in the production of

Answer 41

distilled water

Question 42

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.

Answer 42

Brinkman

Question 43

Out of 100 kcal/second of incident radiant energy on the surface of a thermally transparent body, 300 kcal/second is reflected back. If the transmissivity of the body is 0.25, the emissivity of the surface will be

Answer 43

0.45

Question 44

The purpose of providing a 'catchall' in the vapor line of an evaporator is to

Answer 44

arrest the entrained liquid

Question 45

What is the thermal conductivity of a perfect heat insulator?

Answer 45

zero

Question 46

Fouling factor' used in the design of a multipass shell and tube heat exchanger is a

Answer 46

factor of safety.

Question 47

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

Answer 47

d^-0.2

Question 48

Prandtl and Reynolds analogy are same, when Prandtl number is

Answer 48

1

Question 49

Heat transfer occurs by natural convection because change in temperature causes difference in

Answer 49

density

Question 50

The ratio of kinematic viscosity to thermal diffusivity is called the ____ number.

Answer 50

Prandtl

Question 51

The critical radius 'r' of insulation on a pipe is given by

Answer 51

r = k/h

Question 52

As per Kirchoffs law, the ratio of the total radiating power to the absorptivity of a body depends on the

Answer 52

temperature of the body only.

Question 53

If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid

Answer 53

decreases.

Question 54

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

Answer 54

painting the surface white (with aluminium paint).

Question 55

____ paint has the maximum absorption coefficient.

Answer 55

Black

Question 56

What is Nusselt number?

Answer 56

hD/k

Question 57

Steam traps are provided in steam carrying pipelines to

Answer 57

remove condensate and inert gases.

Question 58

The Dietus-Boelter equation for convective leat transfer https://www.indiabix.com/_files/images/chemical-engineering/heat-transfer/245-165-1.png cannot be used for (See picture for equation)

Answer 58

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

Question 59

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

Answer 59

straight line

Question 60

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.

Answer 60

none of these.

Question 61

Which of the following is correct?

Answer 61

Driving force = Rate x Resistance

Question 62

Convective heat transfer, in which heat is transferred by movement of warmed matter is described by

Answer 62

Newton's law of cooling

Question 63

As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient

Answer 63

goes through a minimum.

Question 64

Heat transfer rate per unit area is called

Answer 64

heat flux

Question 65

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.

Answer 65

none of these.

Question 66

In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to ____ the velocity head.

Answer 66

four times

Question 67

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

Answer 67

first increase and then decrease

Question 68

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

Answer 68

zero

Question 69

For what value of Prandtl number, the Col-burn analogy is valid?

Answer 69

0.6 to 120

Question 70

In a forward feed multiple effect, the pressure build up will be

Answer 70

least at the outlet of the last effect.

Question 71

Film boiling is usually not desired in commercial equipments, because

Answer 71

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

Question 72

For the same heat transfer area and the terminal conditions, the ratio of the capacities of a single effect evaporator to a triple effect evaporator is

Answer 72

1

Question 73

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

Answer 73

W/m.k2

Question 74

Film condensation is promoted on a/an ____ surface.

Answer 74

clean & smooth

Question 75

Dietus-Boelter equation cannot be used for molten metals mainly due to its very low

Answer 75

Prandtl number

Question 76

If Prandtl number is greater than the Schmidt number, then the

Answer 76

thermal boundary layer lies inside the concentration boundary layer.

Question 77

Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid?

Answer 77

Catchall

Question 78

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.

Answer 78

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

Question 79

Multiple effect evaporators are commonly used in the manufacture of P. Paper Q. Superphosphate R. Sugar S. Fats

Answer 79

P and S

Question 80

Which of the following has the highest thermal conductivity?

Answer 80

Silver

Question 81

Which of the following is unimportant in forced convection?

Answer 81

Grashoff number

Question 82

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

Answer 82

vertical heat exchanger with steam on shell side.

Question 83

In a multiple effect evaporator, the effect of boiling point elevation is to

Answer 83

reduce the capacity

Question 84

The rate of heat transfer through a pipe wall is given by, https://www.indiabix.com/_files/images/chemical-engineering/heat-transfer/271-499-1.png . For cylinder of very thin wall, q can be approximated by

Answer 84

https://www.indiabix.com/_files/images/chemical-engineering/heat-transfer/271-499-5.png

Question 85

Dropwise condensation occurs on ____ surfaces.

Answer 85

contaminated cooling

Question 86

For specified tube outside diameter, higher BWG means higher

Answer 86

cross-sectional area

Question 87

Pick out the wrong statement.

Answer 87

Swenson-Walker crystalliser is a batch crystalliser.

Question 88

In case of a vertical tube evaporator, with increase in the liquor level, the ____ is increased.

Answer 88

neither (a) and (b)

Question 89

Electromagnetic radiations propagate in vacuum with a velocity of ____ metre/second.

Answer 89

3 x l08

Question 90

Heat flux is the time rate of heat transfer per unit

Answer 90

area

Question 91

Film boiling occurs at ____ pressure.

Answer 91

very high

Question 92

Heat flux through several resistances in series in analogous to the current flowing through several

Answer 92

resistances in series.

Question 93

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

Answer 93

constant temperature equal to that of the base

Question 94

Vacuum is generally maintained in the vapour space of an evaporator mainly to

Answer 94

get economical temperature difference by using moderate pressure steam.

Question 95

Double pipe heat exchangers are used

Answer 95

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

Question 96

Nusselt number (for forced convection heat transfer) is a function of the ____ number.

Answer 96

both (a) & (b)

Question 97

Pick out the wrong statement.

Answer 97

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

Question 98

Which type of heat exchanger is preferred for heavy heat loads ?Which type of heat exchanger is preferred for heavy heat loads?

Answer 98

Series and parallel set of shell and tube

Question 99

Pick out the wrong statement.

Answer 99

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

Question 100

While the total emissivity of a perfect black body is unity, the same for a real body is

Answer 100

between 0 and 1