Deck 11: Heat Exchangers and Condensation

Question 1

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

A)Double pipe
B)Plate fine
C)Series and parallel set of shell and tube
D)None of these

Answer 1

Series and parallel set of shell and tube

Question 2

For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then

A)hf = hd
B)hf > hd
C)hf < hd
D)hf could be greater or smaller than hd

Answer 2

hf < hd

Question 3

The unit of heat transfer co-efficient in SI unit is

A)J/m²°K
B)W/m²°K
C)W/m°K
D)J/m°K

Answer 3

W/m²°K

Question 4

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

A)Eddies current
B)Conduction
C)Convection
D)None of these

Question 5

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

A)Remains unchanged
B)Increases
C)Increases or decreases depending on number of shell passes
D)Decreases

Question 6

__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.

A)Single pass, fixed tube sheet
B)U-tube
C)Three pass, fixed tube sheet
D)None of these

Question 7

Fouling factor

A)Is a dimensionless quantity
B)Does not provide a safety factor for design
C)Accounts for additional resistances to heat flow
D)None of these

Question 8

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

A)0.4 - 0.6
B)1-1.3
C)1.8-2
D)2 - 2.4

Question 9

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

A)Continues to increase
B)Continues to decrease
C)Goes through a minimum
D)Goes through a maximum

Question 10

Thermal conductivities of most of the liquids __________ with rise in temperature.

A)Increases
B)Decreases
C)Remains unchanged
D)May increase or decrease; depends on the liquid

Question 11

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

A)Smaller than
B)Greater than
C)Equal to
D)Not related with

Question 12

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

A)Create vacuum
B)Regulate the vapor flow
C)Vent the non-condensable gases
D)Arrest the entrained liquid

Question 13

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

A)Bleed point
B)Vent
C)Catchall
D)Baffle

Question 14

Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.

A)Super
B)Sensible
C)Latent
D)Both (B) & (C)

Question 15

In a heat exchanger, floating head is provided to

A)Facilitate cleaning of the exchanger
B)Increase the heat transfer area
C)Relieve stresses caused by thermal expansion
D)Increase log mean temperature gradient

Question 16

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

A)Q ? T2
B)Q ? T4
C)Q ? T3
D)None of these

Question 17

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

A)3 × 10 ⁵
B)3 × 10⁸
C)3 × 10¹⁰
D)3 × 10¹²

Question 18

Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity?

A)Aluminium foil
B)Refractory bricks
C)Iron plate
D)Coke breeze

Question 19

With the increase of temperature, the Col-burn jH factor

A)Increases
B)Decreases
C)Remains unchanged
D)May increase or decrease; depending on temperature

Question 20

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

A)Exothermic
B)Mildly endothermic
C)Highly endothermic
D)None of these

Question 21

Pick out the wrong statement.

A)Heat transfer by radiation cannot occur across an absolute volume
B)In case of a shell and tube heat exchanger, the pressure drop through the shell is proportional to the number of times the fluid crosses the bundle between baffles
C)Propagation velocity for travel of heat radiation through vacuum is equal to the velocity of the light
D)The amount of heat involved in the condensation or vaporisation of 1 kg of a fluid is the same

Question 22

Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low

A)Hardness
B)Dissolved solids content
C)Suspended solids content
D)All (A), (B) and (C)

Question 23

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

A)Increases
B)Decreases
C)Does not affect
D)May increase or decrease, depends on the type of baffle

Question 24

__________ paint has the maximum absorption coefficient.

A)Black
B)Yellow
C)White
D)Grey

Question 25

The advantage of backward feed multiple effect evaporators over forward feed units is that

A)Heat sensitive material can be handled
B)There is no additional cost of pumping
C)Most concentrated liquid is at highest temperature
D)Equal heat transfer co-efficients exist in various effects

Question 26

Baffles are provided in heat exchangers to increase the

A)Fouling factor
B)Heat transfer area
C)Heat transfer co-efficient
D)Heat transfer rate

Question 27

Film boiling is usually not desired in commercial equipments, because

A)The heat transfer rate is low in view of the large temperature drop
B)It is difficult to maintain
C)It is not economic
D)None of these

Question 28

Pick out the wrong statement.

A)Superheated steam is preferably not used for process heating because of its low heat transfer film co-efficient
B)In a shell and tube heat exchanger, the shell pressure drop is maximum for orifice baffles
C)S.I. unit of fouling factor is Watt/m² .°K
D)Longitudinal fins are used in extended surface heat exchangers, when the direction of fluid flow is parallel to the axis of the tube

Question 29

A dephlegmator is a

A)Total condenser
B)Vacuum evaporator
C)Partial condenser
D)Double pipe heat exchanger

Question 30

Prandtl and Reynold's analogies are identical for Prandtl number value of

A)0
B)0.5
C)1
D)5

Question 31

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

A)St = f/2
B)St = f/4
C)St = 4f
D)St = f1/2

Question 32

The rate of heat transfer through a pipe wall is given by, q = 2? k (Ti- T0)/ln (ri /r0). For cylinder of very thin wall, q can be approximated by

A)q = [2? k (Ti + T0)/2]/ln (ri /r0)
B)q = 2? ri k (Ti- T0)/(r0/ri)
C)q = 2? k (Ti- T0)/(r0/ri)
D)q = 2? k (Ti- T0)/[(r0 + ri)/2]

Question 33

Heat transfer by natural convection is enhanced in system with

A)High viscosity
B)High co-efficient of thermal expansion
C)Low temperature gradients
D)Low density change with temperature

Question 34

If a single tube pass heat exchanger is converted to two pass, then for the same flow rate, the pressure drop per unit length in tube side will

A)Increase by 1.8 times
B)Decrease by 22
C)Increase by 216
D)Remain unchanged

Question 35

At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical?

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

Question 36

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.

A)Condensation
B)Grashoff
C)Stanton
D)Brinkman

Question 37

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

A)Resistances in parallel
B)Capacitors in series
C)Resistances in series
D)None of these

Question 38

Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by

A)Keeping the heat transfer surface clean
B)Enhancing the fluid pumping rate
C)Increasing the tube length
D)None of these

Question 39

For a counter current heat exchanger with Ti h = 80°C, T°c = 60°C, T°h = 50°C and Ti c = 30°C, and the temperature difference between the two streams being the same everywhere along Z, the direction of flow of hot fluid. The temperature profile should satisfy

A)d ² T/dZ² > 0
B)d ² T/dZ² = 0
C)d ² T/dZ² < 0
D)dT/dZ = 0

Question 40

Which is the most suitable for the concert-ration of highly concentrated solution?

A)Open pan evaporation
B)Long tube vertical evaporator
C)Agitated film evaporator
D)None of these

Question 41

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

A)Decreases
B)Increases
C)Remains unchanged
D)Increases exponentially

Question 42

A hot liquid is kept in a big room. The logarithm 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

A)Ellipse
B)Straight line
C)Parabola
D)Circular arc

Question 43

With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer

A)Decreases
B)Increases
C)Increases exponentially
D)Remain unaffected

Question 44

Prandtl number is the ratio of

A)Mass diffusivity to thermal diffusivity
B)Momentum diffusivity to thermal diffusivity
C)Thermal diffusivity to mass diffusivity
D)Thermal diffusivity to momentum diffusivity

Question 45

Double pipe heat exchangers are preferably useful, when

A)High viscosity liquid is to be cooled
B)Requirement of heat transfer area is low
C)Overall heat transfer co-efficient is very high
D)A corrosive liquid is to be heated

Question 46

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

A)White & rough
B)Black & rough
C)White & polished
D)Black & polished

Question 47

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

Question 48

Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is

A)1
B)< 1
C)> 1
D)> 5

Question 49

The purpose of providing bleed points in the evaporator is to

A)Admit the feed
B)Remove the product
C)Facilitate removal of non-condensable gases
D)Create vacuum

Question 50

Choose the most important factor on which the heat conducted through a wall in a unit time will depend on?

A)Thickness of the wall
B)Area of the wall perpendicular to heat flow
C)Material of the wall
D)Temperature difference between the two surfaces of the wall

Question 51

With increase in temperature, the thermal conductivity of steel

A)Increases
B)Decreases
C)Remains unchanged
D)Increases exponentially

Question 52

Film boiling occurs at __________ pressure.

A)Atmospheric
B)Sub-atmospheric
C)Negative
D)Very high

Question 53

In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is

A)Infinity
B)Zero
C)Same as average heat transfer co-efficient for tube side
D)None of these

Question 54

At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the

A)Buffer zone
B)Turbulent core
C)Both (A) and (B)
D)Viscous sub-layer

Question 55

Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface?

A)Liquid density
B)Liquid viscosity
C)Hot surface temperature
D)Interfacial tension

Question 56

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

A)Non-dimensional factor
B)Factor of safety
C)Conversion factor for individual film heat transfer co-efficient to overall heat transfer coefficient
D)None of these

Question 57

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
D)None of these

Question 58

Economy of an evaporator is influenced by the

A)Steam pressure
B)Feed temperature
C)Number of effect
D)Both (B) & (C)

Question 59

If all the conditions and dimensions are same, then the ratio of velocity through the tubes of a double pass heat exchanger to that through the single pass heat exchanger is

A)1
B)2
C)1/2
D)4

Question 60

Circulation pump is located below the evaporator to

A)Avoid cavitation
B)Avoid frequent priming
C)Create more suction head
D)None of these

Question 61

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

A)Brinkman number
B)Stanton number
C)Schmidt number
D)Peclet number

Question 62

__________ equation relates the thermal conductivity of a solid to its temperature.

A)Antoine
B)Kopp's
C)Lee's
D)Kistyakowsky

Question 63

A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,

A)The cooling rate will be the same for the two spheres and hence the two spheres will have equal temperatures at any instant
B)Both the spheres will emit equal amount of radiation per unit time in the beginning
C)Both will absorb equal amount of radiation from the surrounding in the beginning
D)Both (B) & (C)

Question 64

Unsteady state heat conduction occurs, when

A)Temperature distribution is independent of time
B)Temperature distribution is dependent on time
C)Heat flows in one direction only
D)Three dimensional heat flow is concerned

Question 65

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

A)Black & smooth
B)Black & rough
C)White & smooth
D)White & rough

Question 66

For specified tube outside diameter, higher BWG means higher

A)Tube thickness
B)Cross-sectional area
C)Weight per unit length
D)None of these

Question 67

1 BTU/hr.ft.°F is equal to __________ kcal/hr. m.°C.

A)1.49
B)1
C)4.88
D)None of these

Question 68

A long iron rod initially at a temperature of 20°C has one end dipped in boiling water (100°C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40°C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time

A)t = 283 s
B)t = 356 s
C)t = 400 s
D)t = 800 s

Question 69

Multiple effect evaporation is generally recommended, when the

A)Large scale evaporation of liquor is needed
B)Corrosive liquids are to be concentrated
C)Fuel is cheaply available
D)Evaporation on small scale is to be done

Question 70

Value of Prandtl number for water ranges from

A)1 to 2
B)5 to 10
C)100 to 500
D)1000 to 2000

Question 71

Which of the following is correct?

A)Rate = Driving force × Resistance
B)Driving force = Rate × Resistance
C)Resistance = Driving force × Rate
D)Rate = Resistance/Driving force

Question 72

For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are

A)Same
B)Different
C)Never different
D)Linearly related

Question 73

Steam is routed through the tube in case of a __________ evaporator.

A)Basket type
B)Horizontal tube
C)Short tube vertical
D)Long tube vertical

Question 74

1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is

A)520
B)200
C)400
D)500