IBX - Heat Transfer Sec. 7 to 11 Flashcards by Vince Joell Feratero

Intermittant tube cleaning is possible to be done in case of a __________ evaporator.

basket type
horizontal tube
calendria
none of these

basket type

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Harmonic mean temperature difference is given by

sqrt(ΔT1.ΔT2)
(2.ΔT1.ΔT2)/(ΔT1+ΔT2)
(2.ΔT1.ΔT2)/(ΔT1-ΔT2)
(ΔT1-ΔT2)/(ΔT1.ΔT2)

B. (2.ΔT1.ΔT2)/(ΔT1+ΔT2)

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Arithmetic mean area can be used in heat transfer problem to calculate the heat flow by conduction through a cylinder which is

thin walled having the value of Ao Ai/< 2.
thick walled.
having the value of Ao/Ai > 2.
both (b) and (c).

thin walled having the value of Ao Ai/< 2.

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Three solid objects of the same material and of equal mass-a sphere, a cylinder (length = diameter) and a cube are at 500°C initially. These are dropped in a quenching bath containing a large volume of cooling oil each attaining the bath temperature eventually. The time required for 90% change in temperature is the smallest for

cube
cylinder
sphere
equal for all the three

cube

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Overall heat transfer co-efficient for cooling of hydrocarbons by water is about

50 -100 Kcal/hr.m2.°C
50 -100 W/m2 . °K
50 -100 BTU/hr. ft.2°F
1000 - 1500 BTU/hr. ft.2°F

50 -100 BTU/hr. ft.2°F

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

aluminium foil
refractory bricks
iron plate
coke breeze

coke breeze

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Thermal diffusivity is given by

k/ (ρ. Cp)
(ρ. Cp)/k
(Cp. µ)/a
µ/hCp

k/ (ρ. Cp)

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

1
< 1
> 1
none of these

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Fourier’s law of heat conduction applies to __________ surfaces.

isothermal
non-isothermal
both (a) and (b)
neither (a) and (b)

both (a) and (b)

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__________ chart is known as transient heat conduction chart.

Dirhing’s
Heisler’s
Mollier’s
Cox

Dirhing’s

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

Thickness of the wall.
Area of the wall perpendicular to heat flow.
Material of the wall.
Temperature difference between the two surfaces of the wall.

Temperature difference between the two surfaces of the wall.

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Which of the following has the minimum thermal conductivity ?

Nitrogen
Steel
Carbon black
Tar

Nitrogen

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In a heat exchanger, one transfer unit means

A section of the exchanger in which change in temperature of one stream equals the average driving force in the section.
The size of the exchanger in which heat transfer rate is 1 kcal/hr.
Both (a) and (b)
None of these

A section of the exchanger in which change in temperature of one stream equals the average driving force in the section.

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

hf = hd
hf > hd
hf < hd
hf could be greater or smaller than hd

hf < hd

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Pick out the wrong statement:

The capacity of an evaporator is reduced by the boiling point elevation.
Corrosive liquid is normally passed through the tubes in a shell and tube heat exchanger.
Steam jet ejector is used for vapor compression in a thermal recompression evaporator.
Heat sensitive materials should be concentrated in high pressure evaporators.

Heat sensitive materials should be concentrated in high pressure evaporators.

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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.

high pressure
vacuum
backward feed
none of these

vacuum

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Circulation pump is located below the evaporater to

avoid cavitation.
avoid frequent priming.
create more suction head.
none of these.

create more suction head.

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The statement that “maximum wavelength of radiation is inversly proportional to the temperature” is __________ law.

Stefan-Boltzman’s
Planck’s
Wien’s displacement
none of these

Wien’s displacement

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

hardness
dissolved soilds content
suspended solids content
all (a), (b) and(c)

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

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Duhring rule is important in solving problems on

distillation
crystallisation
evaporation
humidification

evaporation

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In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)

0.25 D
0.75 D
0.50 D
none of these

0.75 D

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Multiple effect evaporation is generally recommended, when the

large scale evaporation of liquor is needed.
corrosive liquids are to be concentrated.
fuel is cheaply available.
evaporation on small scale is to be done.

large scale evaporation of liquor is needed.

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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.

2^0.8
4^0.8
4^0.4
2^0.4

4^0.8

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

Single pass, fixed tube sheet
U-tube
Three pass, fixed tube sheet
none of these

U-tube

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In which mode of heat transfer, the Biot number is important ? Transient heat conduction Natural convection Forced convection Radiation

Transient heat conduction

__________ paint has the minimum absorption co-efficient. Black White lead Grey Light cream

White lead

Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube exit-fluid temperature > wall temperature. exit fluid temperature < wall temperature. exit fluid temperature = wall temperature. Graetz number > 100.

exit fluid temperature = wall temperature.

1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is 520 200 400 500

400

Which is the most suitable for the con-certration of highly concentrated solution? Open pan evaporation Long tube vertical evaporator Agitated film evaporator None of these

Open pan evaporation

Unsteady state heat conduction occurs, when temperature distribution is independent of time. temperature distribution is dependent on time. heat flows in one direction only. three dimensional heat flow is concerned.

temperature distribution is dependent on time

What is the absorptivity of a black body ? 1 0 0.78 0.95

Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called radiation natural convection forced convection conduction

forced convection

Baffle spacing is not the same as baffle pitch. should be less than one fifth the diameter of the shell. should be less than the inside diameter of the shell. none of these.

should be less than the inside diameter of the shell.

Boiling point elevation of a solution of NaOH increases rapidly with temperature rise. is almost independent of temperature. is almost independent of pressure. both (b) and (c).

both (b) and (c).

The advantage of backward feed multiple effect evaporators over forward feed units is that heat sensitive material can be handled. there is no additional cost of pumping. most concentrated liquid is at highest temperature. equal heat transfer co-efficients exist in various effects.

most concentrated liquid is at highest temperature

Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface ? Liquid density Liquid viscosity Hot surface temperature Interfacial tension

Interfacial tension

Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger ? Corrosiveness Fouling characteristic Viscosity None of these

None of these

Thermal conductivities of most of the liquids __________ with rise in temperature. increases decreases remains unchanged may increase or decrease; depends on the liquid

decreases

A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m3 and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m2.K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is 555 55.5 0.55 0.15

0.15

The purpose of floating head in a heat exchanger is to avoid buckling of tubes. provide support for tubes. decrease the pressure drop. facilitate its lengthening, if needed.

avoid buckling of tubes.

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 15 min 22 min 44 min 51 min

51 min

Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the maximum resistance in the series. sum of all resistances. average of all resistances. minimum resistance presents in the series.

sum of all resistances.

Double pipe heat exchangers are preferably useful, when high viscosity liquid is to be cooled. requirement of heat transfer area is low. overall heat transfer co-efficient is very high. a corrosive liquid is to be heated.

requirement of heat transfer area is low.

Removal of __________ heat is involved in the condensation of a vapor under saturated conditions. super sensible latent both(b) & (c)

latent

The rate of heat transfer from a vertical plate by natural convection depends upon the temperature differences (ΔT) between wall and outside bulk. The proportionality is given as (ΔT)^1/4 (ΔT)^1/2 (ΔT)^5/4 (ΔT)^3/4

(ΔT)^3/4

If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor. increase decrease not affect increase the condensing film co-efficient as well as

decrease

Prandtl and Reynold's analogies are identical for Prandtl number value of 0 0.5 1 5

The outlet temperature of cooling water in a heat exchanger is generally not allowed to exceed above 50°C in industrial practice mainly to avoid its evaporation loss excessive corrosion uneconomic LMTD decrease in heat exchanger efficiency

excessive corrosion

In evaporators, lowering the feed temperature increases the heating area required. reduces the economy. both (a) and (b). decreases the heating area required.

reduces the economy

The type of liquor circulation system to be ' employed in evaporators (viz. natural or forced circulation) is determined mainly by the __________ of the liquid. viscosity density thermal conductivity corrosive nature

viscosity

In case of surface condensers, a straight line is obtained on plotting 1/U vs __________ on an ordinary graph paper. 1/ v̅^-0.8 v̅^-0.8 v̅^-2 1/ v̅^2

1/ v̅^-0.8

Thermal diffusivity is the most important in heat transfer by conduction radiation condensation natural convection

conduction

(NGr x NPr) is called the __________ number. Graetz Reyleigh Nusselt Stanton

Reyleigh

A dephlegmator is a total condenser vacuum evaporator partial condenser double pipe heat exchanger

partial condenser

A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun? Yellow paint White paint Black paint Grey paint

Black paint

The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators. steam ejector entrainment separator compressor vacuum pump

entrainment separator

Fouling factor for a heat exchanger is given by (where, U1 = heat transfer co-efficient of dirty surface U2 = heat transfer co-efficient of clean surface) U1 - U2 (1/U1)-(1/U2) (1U2)-(1/U1) U2 - U1

(1/U1)-(1/U2)

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 increase by 1.8 times decrease by 2^2 increase by 2^16 remain unchanged

increase by 2^16

A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions ? Assume that the conductive heat transfer co-efficient is infinitely large. 5 minutes 10 minutes 20 minutes 40 minutes

5 minutes

Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the Existence of thermal boundary layer. Temperature gradient produced due to density difference Buoyancy of the bubbles produced at active nucleation site. None of these.

None of these.

Evaporator tubes are generally horizontal vertical inclined random

vertical

For a counter current heat exchanger with Tih = 80°C, T°c = 60°C, T°h = 50°C and Tic = 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 (d^2T/dZ^2) > 0 (d^2T/dZ^2) = 0 (d^2T/dZ^2) < 0 dT/dZ = 0

(d^2T/dZ^2) = 0

In a boiling curve, the peak heat flux is called the __________ point. Nusselt Leidenfrost boiling burnout

burnout

Pick out the wrong statement. Heat transfer from a hot body to cold body by the emission of heat waves is called radiation: Filmwise condensation takes place on non-wettable surfaces. The boiling point of a solution is affected by liquid head as well as boiling point elevation. none of these.

Filmwise condensation takes place on non-wettable surfaces.

h.D/K is called the __________ number. Nusselt Peclet Rayleigh Grashoff

Nusselt

Prandtl number is the ratio of mass diffusivity to thermal diffusivity. momentum diffusivity to thermal diffusivity. thermal diffusivity to mass diffusivity. thermal diffusivity to momentum diffusivity.

momentum diffusivity to thermal diffusivity.

Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.) x/KA KA/x K/Ax A/Kx

KA/x

Heat flux, as defined in heat flow is analogous to __________ in electricity flow. current voltage resistance none of these

current

The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body) αT α 1/T α T4 independent of T

α 1/T

In a shell and tube heat exchanger, putting a longitudinal baffle across the shell, forces the shell side fluid to pass __________ through the heat exchanger. once twice thrice four times

twice

When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is Free convection Entry length problem in laminar forced conduction (developing thermal boundary layer) Mixed convection (both free and forced) Forced convection

Mixed convection (both free and forced)

The purpose of providing bleed points in the evaporator is to admit the feed remove the product facilitate removal of non-condensable gases create vacuum

facilitate removal of non-condensable gases

Water always boils when its Temperature reaches 100° C. Vapour pressure equals 76 cm of Hg. Saturated vapour pressure equals the external pressure on its surface. Saturated vapour pressure is less than the atmospheric pressure.

Saturated vapour pressure equals the external pressure on its surface.

A multiple effect evaporator as compared to a single effect evaporator of the same capacity has lower heat transfer area. lower steam economy. higher steam economy. higher solute concentration in the product.

higher steam economy.

Which is the most suitable for cold viscous feed? Forward feed Backward feed Mixed feed Parallel feed

Backward feed

Dropwise condensation is promoted on a/an __________ surface. glazed oily coated smooth

oily

At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the buffer zone turbulent core both (a) and (b) viscous sub-layer

viscous sub-layer

In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator. increases decreases remains constant may increase or decrease, depends on the vacuum

increases

(Nre . NPr) (D/L) is called the ______ number Peclet Stanton Graetz none of these

Graetz

Nucleate boiling is promoted on polished surfaces on rough surfaces in the absence of agitation none of these

on rough surfaces

In forced circulation, the heating element is injected internally externally both (a) and (a) neither (a) nor (b)

internally

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 1 2 1/2 4

The equation, Nst = f/2, is the __________ analogy. Colburn Reynolds Prandtl none of these

Reynolds

Boiling point elevation for a strong and concentrated solution is found by Duhring's rule, which states that at the same pressure, the boiling point of a solution is a linear function of the __________ of pure water. boiling point dynamic viscosity kinematic viscosity density

boiling point

Rate of crystallisation does not depend upon the extent of supersaturation. turbulence within the solution. number and active surface area of the crystals. none of these.

none of these.

Stefen's block body radiation law can also be derived from __________ law. Kirchoffs Planck's Fourier's none of these

Planck's

Multipass heat exchangers are used because of simplicity of fabrication. for low heat load. to obtain higher heat transfer co-efficient and shorter tube. to reduce the pressure drop.

to obtain higher heat transfer co-efficient and shorter tube.

A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case, The cooling rate will be the same for the two spheres and hence the two spheres will have equal temperatures at any instant. Both the spheres will emit equal amount of radiation per unit time in the beginning. Both will absorb equal amount of radiation from the surrounding in the beginning. Both(b) & (c).

Both(b) & (c).

Condensing film co-efficient for steam on horizontal tubes ranges from 5000 to 15000 Kcal/hr.m2 .°C. Condensation of vapor is carried out inside the tube in a shell and tube heat exchanger, when the Higher condensing film co-efficient is desired. Condensate is corrosive in nature. Lower pressure drop through the exchanger is desired. Temperature of the incoming vapor is very high.

Condensate is corrosive in nature.

When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids ? At high fluid velocity At low velocity When fluid flows past a smooth surface None of these

At high fluid velocity

The Sieder-Tate correlation for heat transfer in turbulent flow in pipe gives Nu α Re0.8, where, Nu is the Nusselt number and Re is the Reynolds number for the flow. Assuming that this relation is valid, the heat transfer co-efficient varies with the pipe diameter (D) as (D)^-1.8 (D)^-0.2 (D)^0.2 (D)^1.8

(D)^-0.2

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 t = 283 s t = 356 s t = 400 s t = 800 s

t = 800 s

LMTD for evaporators & condensers for a given terminal parameters & set of conditions for counterflow is equal to that for parallel flow. In such heat exchangers, with one of the fluids condensing or evaporating, the surface area required is the least in the __________ flow. parallel mixed counterflow same in either 'a', b' or 'c'

same in either 'a', b' or 'c'

Thermal diffusivity of a material has the unit m2 /sec. is defined as K/ρ . Cp. is the ratio of thermal conductivity to thermal capacity. all (a), (b) and (c).

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

Viscosity of gases __________ with increase in temperature. increase very rapidly increase slowly decrease slowly remain unaffected

increase slowly

What is the emissivity of a black body ? 1 0 0.90 0.5

Steam is routed through the tube in case of a __________ evaporator. basket type horizontal tube short tube vertical long tube vertical

horizontal tube

Dropwise condensation of steam on cooling surface is promoted an oily surface. when both the steam and the tube are clean. only in presence of air. none of these.

an oily surface

Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is 0.1 >1 < 1 l

0.1