Chemical Heat Transfer Quiz 6

1 / 50

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

Salty

Scaling

Corrosive

Highly viscous

2 / 50

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

50 -100 W/m2.?K

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

50 -100 Kcal/hr.m2.?C

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

3 / 50

Pick out the wrong statement ?

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

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

4 / 50

Multiple effect evaporators ar used to_____?

Decrease the steam economy & the capacity

Decrease the steam economy & increase the capacity

Increase the steam economy & decrease the capacity

Increase the steam economy & the capacity

5 / 50

Natural convection is characterised by___________?

Peclet number

Grashoff number

Reynolds number

Prandtl number

6 / 50

Prandtl number is given by___________?

hD/k

CP ?/k

?/h CP

CP ?/a

7 / 50

Mechanical recompression evaporation is used in the production of______?

Distilled water

Fruits jam

Alcohol

Salt

8 / 50

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

9 / 50

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

0.5

10 / 50

Pick out the wrong statement?

Unit of thermal conductance is W/?K

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

11 / 50

Pick out the wrong statement ?

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

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

12 / 50

Pick out the wrong statement?

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

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

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

13 / 50

Pick out the wrong statement ?

Distillation process is not the same as evaporation

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

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

14 / 50

Pick out the wrong statement ?

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

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

15 / 50

Prandtl number is the ratio of_________?

Momentum diffusivity to thermal diffusivity

Thermal diffusivity to momentum diffusivity

Thermal diffusivity to mass diffusivity

Mass diffusivity to thermal diffusivity

16 / 50

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

0.5 D

0.80 D

0.2 D

0.66 D

17 / 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 Peclet number is a measure of the ratio of energy transport by convection to that by conduction

The ratio NNu/NRe . Npr is called the Stanton number

The Colburn jH factor for heat transfer is given by Nst Npr

18 / 50

Pick out the wrong statement ?

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

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

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

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

19 / 50

Peclet number (Pe) is given by______?

Pe = Nu.Re

Pe = Re/Pr

Pe = Pr/Re

Pe = Re.Pr

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

Forced convection

Conduction

21 / 50

Pick out the correct statement ?

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

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

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

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

22 / 50

Nusselt number is the ratio of the ______?

Temperature difference to the temperature gradient at the wall

Temperature gradient of the wall to that across the entire pipe

None of these

Heat flux at the wall to that across the entire pipe

23 / 50

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

Forced convection

None of these

Natural convection

Conduction

24 / 50

Pick out the wrong statement?

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

Heat sensitive materials should be concentrated in high pressure evaporators

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

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

25 / 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

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

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

Heat transfer by radiation cannot occur across an absolute volume

26 / 50

Pick out the wrong statement?

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

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

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

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

27 / 50

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

28 / 50

Nucleate boiling is promoted _____?

On polished surfaces

In the absence of agitation

On rough surfaces

None of these

29 / 50

Pick out the wrong statement ?

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

Finned tube heat exchangers are suitable for heating air by steam

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

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

30 / 50

Prandtl number is the reciprocal of______?

Thermal diffusivity ? Mass diffusivity

Mass diffusivity ? Momentum diffusivity

Thermal diffusivity/Momentum diffusivity

Thermal diffusivity ? Momentum

31 / 50

Planck?s distribution law is valid for ______ bodies?

Black

White

Coloured

All A., B. & C.

32 / 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__________?

U2 > U1

U1 = dirt factor ? U2

U1 > U2

U1 = U2

33 / 50

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

Reynolds

Neither A. nor B.

Prandtl

Both A. & B.

34 / 50

Multiple effect evaporation is generally recommended, when the________?

Corrosive liquids are to be concentrated

Large scale evaporation of liquor is needed

Evaporation on small scale is to be done

Fuel is cheaply available

35 / 50

Pick out the wrong statement/

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

The emissivity of a surface increases with increase in surface roughness

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

The emissivity of a polished surface is quite low

36 / 50

Pick out the wrong statement ?

Absorptivity of a body approaches unity in case of diffuse reflection

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 perfectly black body is unity

37 / 50

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

Heat flux

That across the entire pipe

None of these

Temperature difference

38 / 50

Prandtl number for most of dry gases is about_________?

0.72

150

0.001

39 / 50

Prandtl number is the ratio of_________?

Thermal diffusivity to momentum diffusivity

Thermal diffusivity to mass diffusivity

Momentum diffusivity to thermal diffusivity

Momentum diffusivity to mass diffusivity

40 / 50

Pick out the correct equation ?

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

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

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

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

41 / 50

Pick out the wrong statement?

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

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

42 / 50

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

> 2

1.5

0.5

43 / 50

Pick out the wrong statement ?

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

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

44 / 50

Multipass heat exchangers are used________?

Because of simplicity of fabrication

To obtain higher heat transfer co-efficient and shorter tube

For low heat load

To reduce the pressure drop

45 / 50

Multiple effect evaporation accounts for_____?

Lower operating costs

Investment economy

None of these

Steam economy

46 / 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__________?

0.35

0.85

0.55

0.45

47 / 50

Pick out the correct statement ?

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

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

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

48 / 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 rate of fluid flow

Constant specific heat

Constant overall heat transfer co-efficient.

49 / 50

Pick out the wrong statement?

Vaporisation of organic substances in evaporator mostly causes foam formation

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

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

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

50 / 50

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

Grashoff

Stanton

Reynolds

Biot

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