Chemical Engineering MCQs

Page No. 425

Pick out the wrong statement?


aFluid movement under the influence of buoyant forces resulting from change in density takes place in case of natural convection

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

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

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

✓


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

Pick out the wrong statement?


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

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

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

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dVaporisation of organic substances in evaporator mostly causes foam formation

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

Pick out the wrong statement ?


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

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

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cForced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger

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dHot 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

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View Answer Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger

Pick out the wrong statement/


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

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bThe emissivity of a surface increases with increase in surface roughness

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cThe emissivity of a polished surface is quite low

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

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

Pick out the wrong statement?


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

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bThe maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1

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cWhen 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

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

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View Answer The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1

Pick out the correct equation ?


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

✓


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

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

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

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

Pick out the wrong statement ?


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

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

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

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dFinned tube heat exchangers are suitable for heating air by steam

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

Pick out the correct statement ?


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

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

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

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

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

Pick out the wrong statement ?


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

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

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

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

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

Choose the correct equation ?


aNu = (Re) (Pr) (Gz)

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bNu = (Re) (Pr) (St)

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cNu = (Re) (Pr)

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dNu = (Pr) (St)

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View Answer Nu = (Re) (Pr) (St)

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Page 425 of 843

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