Deck 8: Heat Transfer and Crystallisation

Question 1

Crystal size in a continuous crystalliser depends upon the

A)Rate of heat transfer
B)Degree of turbulence
C)Degree of super-saturation
D)All (A), (B) and (C)

Answer 1

All (A), (B) and (C)

Question 2

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

Answer 2

r = k/h

Question 3

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

A)Film
B)Sub-cooled
C)Saturated nucleate
D)None of these

Answer 3

Film

Question 4

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

Question 5

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

Question 6

What is Nusselt number?

A)CP. µ/k
B)hD/k
C)h. CP/µ
D)CP. µ/h

Question 7

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

Question 8

In the equation Q = UA?t; ?t 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

Question 9

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

Question 10

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

Question 11

Which of the following has the minimum absorptivity?

A)Aluminium foil
B)Coal dust
C)Refractory bricks
D)Iron plates

Question 12

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

Question 13

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

Question 14

Graetz number is given by

A)mCp/kL
B)kL/mCp
C)mCp/kµ
D)kµ/mCp

Question 15

Thermal diffusivity is given by

A)k/?Cp
B)?Cp/k
C)µCp/a
D)µ/hCp

Question 16

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

Question 17

Reynold's analogy states that

A)Nst ? f
B)Nst ? NRe
C)NNu ? f
D)NRe ? f

Question 18

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

A)Capacity of the evaporator is decreased
B)Capacity of the evaporator is increased
C)True temperature drop increases
D)Both (B) and (C)

Question 19

Which of the following has the minimum thermal conductivity?

A)Nitrogen
B)Steel
C)Carbon black
D)Tar

Question 20

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

Question 21

For an ideal black body

A)Absorptivity = 1
B)Reflectivity = 1
C)Emissivity = 0
D)Transmissivity = 1

Question 22

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)?

Question 23

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

Question 24

Small scale evaporation is done in a

A)Heat exchanger
B)Condenser
C)Multiple effect evaporator
D)Steam jacketed kettle

Question 25

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

Question 26

In a single effect evaporator, the economy is

A)1
B)< 1
C)> 1
D)None of these

Question 27

At steady state the temperature variation in a plane wall, made of two different solids I & II is shown below: The thermal conductivity of material „I?

A)Is smaller than that of II
B)Is greater than that of II
C)Is equal to that of II
D)Can be greater than or smaller than that of II

Question 28

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

Question 29

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% NaNO₂ and 7% NaNO₃)

Question 30

In counter flow compared to parallel flow,

A)LMTD is greater
B)Less surface area is required for a given heat transfer rate
C)Both (A) and (B)
D)More surface area is required for a given heat transfer rate

Question 31

The inside heat transfer co-efficient in case of turbulent flow of liquid in the tube side in a 1-2 shell and tube heat exchanger is increased by __________ times, when the number of tube passes is increased to 8.

A)20.8
B)40.8
C)40.4
D)20.4

Question 32

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

Question 33

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

Question 34

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

Question 35

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

Question 36

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

Question 37

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

Question 38

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

Question 39

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

A)Forward
B)Backward
C)Parallel
D)None of these

Question 40

What is the absorptivity of a black body?

A)1
B)0
C)0.78
D)0.95

Question 41

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

Question 42

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

A)(?T)²
B)?(?T)
C)(?T^)1/4
D)(?T)^3/2

Question 43

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

A)Increasing temperature
B)Decreasing temperature
C)Decreasing Reynolds number
D)None of these

Question 44

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

Question 45

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

Question 46

An evaporator while concentrating an aqueous solution from 10 to 40% solids evaporates 30000 kg of water. The amount of solids handled by the system in kg is

A)4000
B)9000
C)4600
D)3000

Question 47

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

A)Higher economy
B)Lower capacity
C)Both (A) & (B)
D)Lower economy

Question 48

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
D)Both (B) and (C)

Question 49

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

Question 50

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