Chemical Heat Transfer Quiz 6

1 / 50

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

Prandtl

Both A. & B.

Reynolds

Neither A. nor B.

2 / 50

Pick out the correct equation ?

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

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

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

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

3 / 50

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

0.5

> 2

1.5

4 / 50

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

Temperature difference

Heat flux

That across the entire pipe

None of these

5 / 50

Prandtl number is the ratio of_________?

Thermal diffusivity to momentum diffusivity

Momentum diffusivity to thermal diffusivity

Mass diffusivity to thermal diffusivity

Thermal diffusivity to mass diffusivity

6 / 50

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

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

50 -100 Kcal/hr.m2.?C

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

50 -100 W/m2.?K

7 / 50

Prandtl number is the ratio of_________?

Momentum diffusivity to thermal diffusivity

Momentum diffusivity to mass diffusivity

Thermal diffusivity to mass diffusivity

Thermal diffusivity to momentum diffusivity

8 / 50

Pick out the wrong statement?

Reciprocal of the resistance to heat flow is called thermal conductance

Unit of thermal conductance is W/?K

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

None of these

9 / 50

Multiple effect evaporators ar used to_____?

Increase the steam economy & decrease the capacity

Decrease the steam economy & increase the capacity

Increase the steam economy & the capacity

Decrease the steam economy & the capacity

10 / 50

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

11 / 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 ?

Constant rate of fluid flow

Constant overall heat transfer co-efficient.

Constant specific heat

No partial phase change in the system

12 / 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__________?

0.55

0.85

0.35

0.45

13 / 50

Pick out the wrong statement ?

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

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

14 / 50

Prandtl number for most of dry gases is about_________?

0.72

150

0.001

15 / 50

Planck?s distribution law is valid for ______ bodies?

Coloured

All A., B. & C.

Black

White

16 / 50

Pick out the wrong statement/

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 increases with increase in surface roughness

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

17 / 50

Nucleate boiling is promoted _____?

On rough surfaces

On polished surfaces

In the absence of agitation

None of these

18 / 50

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

Average of all resistances

Sum of all resistances

Maximum resistance in the series

Minimum resistance presents in the series

19 / 50

Pick out the wrong statement ?

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

Distillation process is not the same as evaporation

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

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

20 / 50

Pick out the wrong statement?

The first stage of crystal formation is called nucleation

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

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

Swenson-Walker crystalliser is a batch crystalliser

21 / 50

Pick out the wrong statement ?

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

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

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

22 / 50

Pick out the wrong statement?

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

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

23 / 50

Mechanical recompression evaporation is used in the production of______?

Distilled water

Fruits jam

Alcohol

Salt

24 / 50

Pick out the wrong statement?

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

Vaporisation of organic substances in evaporator mostly causes foam formation

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

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

25 / 50

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

None of these

Conduction

Forced convection

Natural convection

26 / 50

Pick out the correct statement ?

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

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

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

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

27 / 50

Prandtl number is the reciprocal of______?

Thermal diffusivity/Momentum diffusivity

Thermal diffusivity ? Mass diffusivity

Mass diffusivity ? Momentum diffusivity

Thermal diffusivity ? Momentum

28 / 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

Conduction

Natural convection

Forced convection

29 / 50

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

Stanton

Grashoff

Reynolds

Biot

30 / 50

Pick out the wrong statement ?

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

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

31 / 50

Pick out the wrong statement ?

Absorptivity of a perfectly black body is unity

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

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

32 / 50

Natural convection is characterised by___________?

Reynolds number

Grashoff number

Prandtl number

Peclet number

33 / 50

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

Highly viscous

Salty

Scaling

Corrosive

34 / 50

Pick out the wrong statement?

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

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

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

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

35 / 50

Pick out the wrong statement ?

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

Heat transfer from a hot body to cold body by the emission of heat waves is called radiation

None of these

36 / 50

Nusselt number is the ratio of the ______?

Temperature difference to the temperature gradient at the wall

Heat flux at the wall to that across the entire pipe

None of these

Temperature gradient of the wall to that across the entire pipe

37 / 50

Pick out the wrong statement?

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

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 Colburn jH factor for heat transfer is given by Nst Npr

38 / 50

Peclet number (Pe) is given by______?

Pe = Re/Pr

Pe = Re.Pr

Pe = Nu.Re

Pe = Pr/Re

39 / 50

Prandtl number is given by___________?

hD/k

CP ?/k

?/h CP

CP ?/a

40 / 50

Multiple effect evaporation is generally recommended, when the________?

Evaporation on small scale is to be done

Large scale evaporation of liquor is needed

Corrosive liquids are to be concentrated

Fuel is cheaply available

41 / 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 = U2

U1 > U2

U1 = dirt factor ? U2

42 / 50

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

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

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

43 / 50

Pick out the wrong statement?

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

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

Heat sensitive materials should be concentrated in high pressure evaporators

44 / 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

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

45 / 50

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

0.5

46 / 50

Multiple effect evaporation accounts for_____?

Investment economy

None of these

Lower operating costs

Steam economy

47 / 50

Pick out the correct statement ?

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

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

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

48 / 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

Heat transfer by radiation cannot occur across an absolute volume

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

49 / 50

Pick out the wrong statement ?

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

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

Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number

50 / 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

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