Chemical Heat Transfer Quiz 6

1 / 50

Pick out the wrong statement ?

In industrial practice, sub-cooling of condensate is required, when the condensate is a volatile liquid and is to be transferred for storage

Film co-efficient for vaporisation decreases as a result of vapor binding

The condensing film co-efficient is about 3 times lower for vertical condenser as compared to the equivalent horizontal condenser for identical situation

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

2 / 50

Pick out the wrong statement ?

The boiling point of a solution is affected by liquid head as well as boiling point elevation

None of these

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

3 / 50

Nusselt number is the ratio of the temperature gradient at the wall to__________?

None of these

That across the entire pipe

Temperature difference

Heat flux

4 / 50

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

5 / 50

Pick out the wrong statement ?

The controlling resistance in case of heating of air by condensing steam is in the air fil

Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice-versa occurs at saturation temperature

The log mean temperature difference (LMTD) for counter flow and parallel flow can be theoretically same when any one of the fluids (hot or cold fluid) passes through the heat exchanger at constant temperature

In case of a 1 ? 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs

6 / 50

Prandtl and Reynold?s analogies are identical for Prandtl number value of___________?

0.5

7 / 50

Pick out the correct statement ?

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

In unsteady state heat conduction, heat flows in the direction of temperature rise

In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity

In heat transfer by forced convection, Grashoff number is very important

8 / 50

Nucleate boiling is promoted _____?

On polished surfaces

None of these

In the absence of agitation

On rough surfaces

9 / 50

Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the______?

Minimum resistance presents in the series

Maximum resistance in the series

Average of all resistances

Sum of all resistances

10 / 50

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 Colburn jH factor for heat transfer is given by Nst Npr

11 / 50

Pick out the wrong statement?

In process heat exchangers, saturated steam is preferred over the superheated steam

The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of ?1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 ?1

When a vertical plate is heated in infinite air environmental under natural convection conditions, the velocity profile in air, normal to the plate, exhibits a maximum

A body at 925 K emits an energy of 1.42 x 1011?W/m2 (? is the Stefan-Boltzmann constant) in the wavelength band between 3 ?m to 4 ?m. The fraction of this energy in the total energy emitted over the entire wavelength range is equal to emissivity

12 / 50

Pick out the wrong statement ?

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

Propagation velocity for travel of heat radiation through vacuum is equal to the velocity of the light

The amount of heat involved in the condensation or vaporisation of 1 kg of a fluid is the same

Heat transfer by radiation cannot occur across an absolute volume

13 / 50

Natural convection is characterised by___________?

Peclet number

Prandtl number

Reynolds number

Grashoff number

14 / 50

Prandtl number is the ratio of_________?

Thermal diffusivity to momentum diffusivity

Momentum diffusivity to thermal diffusivity

Thermal diffusivity to mass diffusivity

Momentum diffusivity to mass diffusivity

15 / 50

Pick out the wrong statement?

Vaporisation of organic substances in evaporator mostly causes foam formation

Ratio of its capacity to economy equals the steam consumption in kg/hr in an evaporator

Bubble size increases with the dynamic viscosity of the liquid in case of nucleate pool boiling

Thermal conductivity of a dry material is more than that of the damp material

16 / 50

Mechanical recompression evaporation is used in the production of______?

Alcohol

Distilled water

Fruits jam

Salt

17 / 50

Pick out the wrong statement ?

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

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

In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2

18 / 50

Nusselt number is a function of Prandtl number and _______ number of fluid in natural convection heat transfer?

Grashoff

Stanton

Reynolds

Biot

19 / 50

Prandtl number is the ratio of_________?

Thermal diffusivity to mass diffusivity

Momentum diffusivity to thermal diffusivity

Mass diffusivity to thermal diffusivity

Thermal diffusivity to momentum diffusivity

20 / 50

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

Conduction

Forced convection

21 / 50

Prandtl number for most of dry gases is about_________?

0.72

150

0.001

22 / 50

Nusselt number is the ratio of the ______?

Heat flux at the wall to that across the entire pipe

Temperature gradient of the wall to that across the entire pipe

Temperature difference to the temperature gradient at the wall

None of these

23 / 50

Peclet number (Pe) is given by______?

Pe = Pr/Re

Pe = Re.Pr

Pe = Re/Pr

Pe = Nu.Re

24 / 50

Multiple effect evaporation accounts for_____?

Lower operating costs

None of these

Investment economy

Steam economy

25 / 50

Multiple effect evaporation is generally recommended, when the________?

Corrosive liquids are to be concentrated

Evaporation on small scale is to be done

Large scale evaporation of liquor is needed

Fuel is cheaply available

26 / 50

Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)?

0.5 D

0.66 D

0.80 D

0.2 D

27 / 50

Prandtl and Reynolds analogy are same, when Prandtl number is______?

> 2

0.5

1.5

28 / 50

Nusselt number (for forced convection heat transfer) is a function of the ______ number?

Both A. & B.

Neither A. nor B.

Reynolds

Prandtl

29 / 50

Pick out the wrong statement?

None of these

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

Reciprocal of the resistance to heat flow is called thermal conductance

Unit of thermal conductance is W/?K

30 / 50

Pick out the wrong statement ?

In case of short tube vertical evaporators, area of central downtake is about 50 to 100% of the total tube cross-sectional area

Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted

?Solvates? are chemical compounds formed by solute with their solvents. When water is the solvent, then it is called a ?hydrate?

In heat exchanger calculations (?t) weighted is used in place of ?t, when it involves more than one sequence of heating or cooling i.e., desuperheating & condensation or condensation & sub-cooling

31 / 50

Multipass heat exchangers are used________?

To obtain higher heat transfer co-efficient and shorter tube

Because of simplicity of fabrication

To reduce the pressure drop

For low heat load

32 / 50

Mode of heat transfer involved in the cooling of air cooled internal combustion engine is__________?

None of these

Forced convection

Natural convection

Conduction

33 / 50

Prandtl number is the reciprocal of______?

Mass diffusivity ? Momentum diffusivity

Thermal diffusivity ? Momentum

Thermal diffusivity ? Mass diffusivity

Thermal diffusivity/Momentum diffusivity

34 / 50

Pick out the wrong statement ?

Absorptivity of a perfectly black body is unity

With change in temperature, the radiant energy emitted by a black body remains unchanged

Value of Stefan-Boltzmann constant is 4.876 ? 10-8 KCal/m2.hr.?K4

Absorptivity of a body approaches unity in case of diffuse reflection

35 / 50

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

0.85

0.35

0.45

0.55

36 / 50

Pick out the correct equation ?

jH = (St)2/3(Pr) = f/2

jH = (St)(Pr)2/3 = f/2

jH = (St)(Pr)1/3 = f/2

jH = (St)1/3(Pr) = f/2

37 / 50

Pick out the wrong statement ?

In a shell and tube heat exchanger, the shell pressure drop is maximum for orifice baffles

Superheated steam is preferably not used for process heating because of its low heat transfer film co-efficient

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

38 / 50

Pick out the wrong statement?

Pressure drop on the shell side of a heat exchanger depends upon tube pitch also

In a horizontal tube evaporator, surface blanketing by air is avoided

Orifice baffles are never used in a shell and tube heat exchanger

Split ring type and pull through type floating heads are two commonly used floating heads is heat exchangers

39 / 50

Pick out the wrong statement?

Super saturation of the solution is the driving potential for a crystal growth

The first stage of crystal formation is called nucleation

Swenson-Walker crystalliser is a batch crystalliser

The liquor left after the removal of crystals from a solution is called mother liquor

40 / 50

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

1000 ? 1500 BTU/hr. ft.2?F

50 -100 W/m2.?K

50 -100 BTU/hr. ft.2?F

50 -100 Kcal/hr.m2.?C

41 / 50

Out of the following four assumptions used in the derivation of the equation for LMTD [LMTD = (?t1 ? ?t2)/ln(?t1/?t2)], which one is subject to the largest deviation in practice ?

No partial phase change in the system

Constant specific heat

Constant overall heat transfer co-efficient.

Constant rate of fluid flow

42 / 50

Pick out the wrong statement/

The emissivity of a surface increases with increase in surface roughness

The emissivity of a polished surface is quite low

The emissivity of a non-metallic surface decreases with increase in the temperature

The emissivity of a surface decreases, if it gets corroded by atmospheric environment

43 / 50

Pick out the correct statement ?

The wavelength corresponding to maximum mono-chromatic emissive power increases with rise in temperature

Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder

Solid angle subtended by the finite surface at the radiating element is called the angle of incidence

Higher is the temperature of the radiating body, higher is the wavelength of radiation

44 / 50

Pick out the wrong statement ?

Double pipe heat exchangers are mostly used in the field of refrigeration

Finned tube heat exchangers are suitable for heating air by steam

Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature

Economy of a multiple effect evaporator is not influenced by the boiling point elevation

45 / 50

Pick out the wrong statement ?

In case of heat transfer by purely forced convection, GR/Re 2 ? 1

Distillation process is not the same as evaporation

The equivalent diameter of heat transfer for a duct of square cross-section (having each side as ?x?) is equal to 4x

The effectiveness of nucleate boiling depends basically on the ease with which the bubbles are formed and detached from the heating surface

46 / 50

Pick out the wrong statement?

Corrosive liquid is normally passed through the tubes in a shell and tube heat exchanger

Heat sensitive materials should be concentrated in high pressure evaporators

The capacity of an evaporator is reduced by the boiling point elevation

Steam jet ejector is used for vapor compression in a thermal recompression evaporator

47 / 50

Open pan evaporators are preferred to be used, when the solution to be concentrated is__________?

Salty

Highly viscous

Scaling

Corrosive

48 / 50

Prandtl number is given by___________?

hD/k

CP ?/a

CP ?/k

?/h CP

49 / 50

Multiple effect evaporators ar used to_____?

Decrease the steam economy & the capacity

Increase the steam economy & decrease the capacity

Increase the steam economy & the capacity

Decrease the steam economy & increase the capacity

50 / 50

Planck?s distribution law is valid for ______ bodies?

All A., B. & C.

Black

White

Coloured

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