Heat Transfer Part 2 Flashcards by Chris Wilfred Estrada

The Grashoff number is defined as the ratio of the
buoyancy to inertial forces.
buoyancy to viscous forces.
inertial to viscous forces.
buoyancy to surface tension forces.

buoyancy to viscous forces.

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According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
2f
f
f/2
f/4

f/2

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The absorptivity of a body is equal to its emissivity
at a particular temperature.
for circular bodies.
under thermal equilibrium.
none of these.

under thermal equilibrium.

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Absorptivity and refletivity of a perfect black body are respectively
1 and 0
0 and 1
1 and infinity
0 and 0.5

1 and 0

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In thermal radiation for a black body (where, E is emissivity and a is absorptivity)
a = 1; E ≠ 1
a ≠ 1, E = 1
a ≠ 1, E ≠ 1
a = 1; E = 1

a = 1; E = 1

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

S.I. unit of fouling factor is Watt/m2. K.

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When warm and cold liquids are mixed, the heat transfer is mainly by
conduction
convection
radiation
both(a) & (c)

convection

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A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300 C and allowed to cool in natural air. Which one will cool the slowest?
Cube
Plate
Sphere
All will cool at the same rate

Sphere

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Vent pipes are provided in a condenser to
remove non-condensable gases.
purge the condenser.
facilitate easy cleaning of tubes.
none of these.

remove non-condensable gases.

vent remove non-condensable

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Heat transfer by natural convection is enhanced in system with
high viscosity.
high co-efficient of thermal expansion.
low temperature gradients.
low density change with temperature.

high co-efficient of thermal expansion.

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In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
nucleate
pool
low pressure
none of these

pool

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A body cools down from 75 C to 70 C in 10 minutes. It will cool down from 70 C to 65 C in __________ minutes.
10
>10
<10
either (b) or (c), depends on the mass of the body

> 10

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In forced convection, the Nusselt number is a function of
Re and Pr
Re and Gr
Pr and Gr
Re and Sc

Re and Pr

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At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical?
0.5
1
1.5
80

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A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because
total heat transfer area of all the effects is -less than that in a single effect evaporator system.
total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
boiling point elevation in a single effect system is much higher than that in any effect in a multieffect system.
heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system.

total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.

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Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by
keeping the heat transfer surface clean.
enhancing the fluid pumping rate.
increasing the tube length.
none of these.

keeping the heat transfer surface clean.

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Trap is used to remove __________ from steam pipe lines.
steam
condensate
non-condensables
none of these.

condensate

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A composite flat wall of a furnace is made of two materials ‘A’ and ‘B’. The thermal conductivity of ‘A’ is twice of that of material ‘B’, while the thickness of layer of ‘A’ is half that of B. If the temperature at the two sides of the wall are 400 and 1200 K, then the temperature drop (in K) across the layer of material ‘A’ is
125
133
150
160

160

q = (TL-TR)/R1+R2.
q = (1200 - 400)/(XA/KA+XB /KB),
q= 800/(0.5 XB /2KB + XB /KB),
q= 800/ (1.25 XB /KB)—> (1)
q = T at A/( XA/KA)—> (2)

Sub eq. (1) in eq. (2) then;
800/ (1.25 XB /KB) =T at A/( XA/KA),
T at A = 160.

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Tube pitch is the __________ of tube diameters and the clearances.
sum
difference
ratio
none of these

sum

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With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
decreases
increases
increases exponentially
remains unaffected

decreases

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The equivalent diameter for the annulus of a double pipe heat exchanger, whose inner pipe has fins on the outside is __________ compared to the same size pipes without fins.
more
less
same
unpredictable

less

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Natural convection is characterised by
Grashoff number
Peclet number
Reynolds number
Prandtl number

Grashoff number

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Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
0.72
4.364
18
83

4.364

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Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
1
< 1
> 1
> 5

< 1

thermal hydrodynamic Prandtl <1

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The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe) K/h0 2K/h0 hi/K 2hi/K

K/h0 ## Footnote critical radius Kh0

The heat flux in the nucleate boiling regimes is proportional to (where, delta(T) = excess temperature) (ΔT)^2 (ΔT)^4 (ΔT)^3 sqrt(ΔT)

(ΔT)^3

In a 1-1 cocurrent heat exchanger, if the tube side fluid outlet temperature is equal to the shell side fluid outlet temperature, then the LMTD is infinity 0 equal to the difference between hot and cold fluids inlet temperature. equal to the difference between hot fluid inlet temperature and cold fluid outlet temperature.

A body is called grey if the monochromatic emissivity of the body is zero unity same for all wavelengths different for all wavelengths

same for all wavelengths

In an extended surface heat exchanger, fluid having lower co-efficient flows through the tube. flows outside the tubes. can flow either inside or outside the tubes. should not be used as it gives very high pressure drop.

flows outside the tubes.

In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is infinity. zero. same as average heat transfer co-efficient for tube side. none of these.

infinity.

Grashoff number, which is defined as g . beta(Ts - T(infinity)) rho2. L3/u2, is proportional to the ratio of buoyancy force to __________ force. viscous elastic inertial none of these.

viscous

Pick out the wrong statement. By increasing the number of shell passes, the temperature cross in a shell and tube heat exchanger can be prevented. An underdesigned steam trap will back up the condensate instead of discharging it out. Steam condensate is corrosive because of the presence of dissolved oxygen in it. Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.

Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop. ## Footnote wrong - film boiling is desirable

Graetz number is given by mCp/kL kL/mCp mCp/ku kL/mCp

mCp/kL

For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in increased economy decreased economy lower capacity no effect on economy

increased economy

Economy of an evaporator is influenced by the steam pressure feed temperature number of effect both(b) & (c)

both(b) & (c)

If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows __________ law. hyperbolic parabolic linear logarithmic

linear

In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred through a metallic wall. by direct contact of hot flue gas with air. by heating an intermediate material (like chequor bricks) and then heating the air from this hot material. none of these.

by heating an intermediate material (like chequor bricks) and then heating the air from this hot material. ## Footnote regenerative air preheater intermediate material

__________ equation relates the thermal conductivity of a solid to its temperature. Antoine Kopp's Lee's Kistyakowski

Lee's

For small temperature difference, the heat transfer rate as per Newton's law of cooling is proportional to (where, delta(t) = excess temperature) delta(t) delta(t)^2 delta(t)^3 sqrt(delta(t))

delta(t)

Pick out the wrong statement. The condensing film co-efficient is about 3 times lower for vertical condenser as compared to the equivalent horizontal condenser for identical situation. Film co-efficient for vaporisation decreases as a result of vapor binding. In industrial practice, sub-cooling of condensate is required, when the condensate is a volatile liquid and is to be transferred for storage. Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.

Overall heat transfer co-efficient in a heat exchanger is **controlled by the value of the film co-efficient**, which is **higher**. ## Footnote wrong - controlled by the value of film-coefficient,higher

Viscous & heat sensitive liquids are concentrated in __________ evaporators. open pan long tube agitated film none of these

agitated film

1 BTU/hr.ft. F is equal to __________ kcal/hr. m. C. 1.49 1 4.88 none of these

1.49

Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta C depends on T1 - T2 T1 - Ta T2 - Ta none of these

none of these

The ratio of momentum diffusivity to thermal diffusivity is the __________ number. Prandtl Nusselt Stanton Grashoff

Prandtl

For a perfectly transparent surface (like gases), the transmissivity = 1 reflectivity = 0 all (a), (b) & (c)

transmissivity = 1 ## Footnote TT1 transparent transmissivity 1

For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are same different never different linearly related

different

During crystallisation, formation of crystal can occur in __________ solution only. saturated supersaturated undersaturated all (a), (b) and (c)

supersaturated

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 4000 9000 4600 3000

4000

An insulator should have low thermal conductivity. high thermal conductivity. less resistance to heat flow. a porous structure.

low thermal conductivity.

In a backward feed multiple effect evaporator feed is introduced in the first effect. feed flows from low pressure to high pressure. no pumps are required between successive effects. none of these.

feed flows from low pressure to high pressure.

Choose the correct equation. Nu = (Re) (Pr) (Gz) Nu = (Re) (Pr) (St) Nu = (Re)(Pr) Nu = (Pr)(St)

Nu = (Re) (Pr) (St)

In a forward feed multiple effect evaporator unit viscosity of liquid is highest in first effect. transfer from effect to effect is done by pumps. no pump is required to withdraw the product from the last effect. none of these.

none of these.

In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is linear hyperbolic parabolic none of these.

parabolic

For heat flow through very thick walled cylinder, use____mean radius. arithmetic logarithmic geometric either (a) or (c)

logarithmic

Indirect contact heat exchangers are preferred over direct contact heat exchangers, because heat transfer co-efficient are high. there is no risk of contamination. there is no mist formation. cost of equipment is lower.

there is no risk of contamination.

Water is normally used as a coolant in the heat exchange equipments mainly because of its abundance & high heat capacity low density low viscosity high fluidity

abundance & high heat capacity

The rate of emission of radiation by a body does not depend upon the wavelength of radiation. surface temperature of the body. nature of the surface. shape and porosity of the body.

shape and porosity of the body.

Which of the following has maximum thermal conductivity at the same temperature? Steel Petrol Air All have the same conductivity

Steel

Maximum heat transfer rate is obtained in __________ flow. laminar turbulent creeping transition region

turbulent

In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface. nucleate local pool saturated

local

The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is 2 1/2 4 1/4

1/4

Terminal point temperature differences between fluids in case of a heat exchanger is termed as approach log mean temperature difference. arithmetic mean temperature difference. geometric mean temperature difference.

approach ## Footnote terminal approach

LMTD can't be used as such without a correction factor for the multipass heat exchanger. baffled heat exchanger. condensation of mixed vapour in a condenser. all (a) (b) and (c).

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

Fqra cold viscous feed, backward feed gives __________ than forward feed. a higher capacity a lower capacity lower economy none of these

a higher capacity

Walls of a cubical oven are of thickness l, and they are made of material of thermal conductivity k. The temperature inside the oven is 100 C and the inside heat transfer co-efficient is 3k/l. If the wall temperature on the outside is held at 25 C, what is the inside wall temperature in degree centigrade ? 35.5 43.75 81.25 48.25

81.25

Overall heat transfer co-efficient of a particular tube is U1. If the same tube with some dirt deposited on either side has coefficient U2, then U1 = U2 U2 > U1 U1 > U2 U1 = dirt factor - U2

U1 = U2

In SI units, thermal conductivity is expressed in Watt/m. K Watt/m2. K Watt/m2. K Watt/m4. K

Watt/m. K

Boiling point of a solution according to Duhring's rule is a linear function of the __________ of water. boiling point (at the same pressure) viscosity density thermal conductivity

boiling point (at the same pressure)

The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances. average geometric mean product sum

sum

At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe? 0.5 < 0.5 > 0.6 < 0.1

> 0.6

A sphere of radius 'R1' is enclosed in a sphere of radius 'R2'. The view (or shape) factor for radiative heat transfer of the outer sphere with respect to the inner sphere is 0 R2/(R1+R2) 1 (R1/R2)^2

R2/(R1+R2)

For a laminar flow of fluid in a circular tube, 'h1' is the convective heat transfer co-efficient at velocity 'V1'. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is 1.26 h1 0.794 h1 0.574 h1 1.741 h1

0.794 h1

The main purpose of providing fins on heat transfer surface is to increase the temperature gradient. mechanical strength of the equipment. heat transfer area. heat transfer co-efficient.

heat transfer area.

A BTU/hr.ft.2 F is equal to 1 kcal/hr. m2 C. 4.88 kcal/hr. m.2 C. 1 kcal/hr. m2. K. none of these.

4.88 kcal/hr. m.2 C.

Which of the following has the lowest Prandtl number? Molten sodium (a coolant used in fast breeder reactor) Water Transformer oil Dilute H2SO4

Molten sodium (a coolant used in fast breeder reactor) ## Footnote lowest Pr molten Na

In SI units, fouling factor is expressed in m2 K/W W/m2 K m2 K m K/W

m2 K/W

An ejector is used to increase pressure increase temperature remove condensate none of these

none of these ## Footnote ejector none

Extremely large or small volumes of fluids are generally best routed through the shell side of a shell and tube heat exchanger, because of the less corrosion problems. flexibility possible in the baffle arrangement. low pressure drop. high heat transfer co-efficient.

flexibility possible in the baffle arrangement.

All analogy equations connecting friction factor and heat transfer co-efficient apply only to wall or skin friction form friction both (a) and (b) turbulent flow

wall or skin friction

Electro-magnetic spectrum range, which is important for radiation varies from __________ microns. 1 to 100 0.5 to 50 10 to 100 100 to 1000

0.5 to 50

The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are Re, Gz Nu, Pr Nu, Pr, Re Nu, Gz

Nu, Gz ## Footnote NuGz Laminar

Heat transfer rate described by Fourier's law will decrease, if the __________ increases. thermal conductivity thickness temperature difference heat transfer area

thickness

A black body does not __________ radiation. absorb or emit refract reflect both(b) & (c)

both(b) & (c)

The capacity of double-effect evaporator is less than half of the capacity of two single effects, each of which is operating over same terminal temperature difference, when the solution has an elevation of boiling point. evaporators operate under vacuum. evaporators opreate at atmospheric pressure. none of these.

solution has an elevation of boiling point.

Peclet number (Pe) is given by Pe = Re.Pr Pe = Re/Pr Pe = Pr/Re Pe = Nu.Re

Pe = Re.Pr

Nusselt number is the ratio of the temperature gradient of the wall to that across the entire pipe. temperature difference to the temperature gradient at the wall. heat flux at the wall to that across the entire pipe. none of these

heat flux at the wall to that across the entire pipe.

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

momentum diffusivity to thermal diffusivity.

Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger? Tube side pressure drop and the heat transfer rate. Convective heat transfer co-efficient. Effective tube surface area for convective heat transfer. All (a) (b) and (c).

All (a) (b) and (c).

Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator? Long vertical evaporator Horizontal tube evaporator Agitated film evaporator Calenderia vertical tube evaporator

Agitated film evaporator ## Footnote Forced circulation Agitated film

"The ratio of the total emissive power to the absorptivity for all bodies is same at ther-mal equilibrium". This is __________ law. Kirchoffs Planck's Wien's displacement Stefan-Boltzman

Kirchoffs ## Footnote total emissive power/absorptivity Kirchoffs

View factor is important in heat transfer by steady state conduction natural convection forced convection radiation

radiation ## Footnote RVF Radiation View Factor

The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is lower tube side pressure drop. lower shell side pressure drop. higher tube side heat transfer co-efficient. higher shell side heat transfer co-efficient.

higher tube side heat transfer co-efficient.

For a multipass shell and tube heat exchanger, the LMTD correction factor is always 1 > 1 < 1 between 1 & 2

< 1

If heat transfer rate varies with the time, it is termed as forced convection steady state conduction monochromatic radiation none of these

none of these

Pick out the wrong statement. Fluid movement under the influence of buoyant forces resulting from change in density takes place in case of natural convection. The ratio NNu/NRe . Npr is called the Stanton number. The Peclet number is a measure of the ratio of energy transport by convection to that by conduction. The Colbum jH factor for heat transfer is given by Nst Npr.

The Colbum jH factor for heat transfer is given by Nst Npr. ## Footnote wrong Colbum jH factor

Tube expansion allowances exist in __________ heat exchanger. multipass fixed tube sheet U-tube single pass fixed tube sheet none of these

U-tube ## Footnote U-tube expansion allowance

Pick out the wrong statement. In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2. Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number. Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger. Hot gases at moderate pressure are usually in the shell side of shell and tube heat exchangers. At higher pressure, however, it is customary to put gas in the tube side.

Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger. ## Footnote wrong forced convection more effective schimdt larger

Agitated film evaporator is suitable for concentrating __________ liquids. foaming viscous very thin corrosive

viscous

The thickness of condensate layer in filmwise condensation depends on the condensation rate surface configuration liquid flow rate from the surface all (a), (b) and (c)

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

If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface tempeature follows __________ law. parabolic hyperbolic linear logarithmic

parabolic ## Footnote uniform surface temp parabolic

Intermittant tube cleaning is possible to be done in case of a __________ evaporator. basket type horizontal tube calendria none of these

basket type

Harmonic mean temperature difference is given by √ΔT1. ΔT2 (2.ΔT1.ΔT2)/(ΔT1 + ΔT2) (2.ΔT1.ΔT2)/(ΔT1 - ΔT2) (ΔT1 -ΔT2)/(ΔT1.ΔT2)

(2.ΔT1.ΔT2)/(ΔT1 + ΔT2)

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.

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

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

Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity? aluminum foil refractory bricks iron plate coke breeze

coke breeze

Thermal diffusivity is given by k/p.Cp p.Cp/k Cp.u/a u/hCp

k/p.Cp

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

Fourier's law of heat conduction applies to __________ surfaces. isothermal non-isothermal both (a) and (b) neither (a) and (b)

both (a) and (b)

__________ chart is known as transient heat conduction chart. Dirhing's Heisler's Mollier's Cox

Dirhing's ## Footnote DT Dirhing's Transient (heat conduction)

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.

Which of the following has the minimum thermal conductivity? Nitrogen Steel Carbon black Tar

Nitrogen

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.

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

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.

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

Circulation pump is located below the evaporater to avoid cavitation. avoid frequent priming. create more suction head. none of these.

create more suction head.

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

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)

At steady state the temperature variation in a plane wall, made of two different solids I & II is shown below : [image](https://www.indiabix.com/_files/images/chemical-engineering/heat-transfer/272-510-1.png) The thermal conductivity of material I is smaller than that of II. is greater than that of II. is equal to that of II. can be greater than or smaller than that of II.

is smaller than that of II.

Duhring rule is important in solving problems on distillation crystallisation evaporation humidification

evaporation

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 ## Footnote 25% cut 0.75 D

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

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

__________ 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

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

Transient heat conduction ## Footnote BTHC Biot 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. ## Footnote unsteady heat conduction 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 (delta(T)) between wall and outside bulk. The proportionality is given as (delta(T))^1/4 (delta(T))^1/2 (delta(T))^5/4 (delta(T))^3/4

(delta(T))^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.

both (a) and (b).

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/ v^-0.8 v^-0.8 v^-2 1/ v^-2

1/ v^-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 A total condenser vacuum evaporator partial condenser douple pipe heat exchanger

partial condenser ## Footnote dpc dephlegmator 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 1/U2 - 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 ## Footnote single converted 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. ## Footnote Fluid motion natural convection 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. ## Footnote wrong - filmwise non-wettable

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) ∝ T ∝ 1/T ∝ T^4 independent of T

∝ 1/T ## Footnote maximum monochromatic emissive power - ∝ 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 ## Footnote DO dropwise 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 ## Footnote PSBB Planck's Stefen's block body

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 ? Re^0.8, 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/? . 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 ## Footnote gases - viscosity temperature directly proportional; slow increase of viscosity liquids - viscosity temperature inversely proportional

What is the emissivity of a black body? 1 0 0.9 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 1

0.1 ## Footnote conduction buffer Pr 0.1