TRANSPORT PROCESSES (HEAT TRANSFER, DIFFUSION, GAS ABSORPTION)

The driving force in heat transfer is

a. concentration gradient

b. temperature gradient

c. viscosity gradient

d. thickness of the solid in question

b. temperature gradient

Thermal diffusivity of a material

a. has the unit m2/sec.

b. is defined as k/ρ x Cp.

c. is the ratio of thermal conductivity to thermal capacity.

d. all of these

d. all of these

Under the same temperature gradient across various metal

plates of the same thickness, the heat conducted per unit of

surface area will be largest across

a. lead c. iron

b. copper d. nickel

b. copper

If a man touches two metals which were kept together at room

temperature, why would one metal feel colder than the other

a. one has a high heat transfer coefficient

b. one has a high thermal conductivity

c. one has a lower temperature

d. one has a higher heat capacity

b. one has a high thermal conductivity

Cork is a good insulator because it has

a. free electrons

b. atoms colliding frequency

c. low density

d. porous body

d. porous body

The insulation ability of an insulator with the presence of

moisture would

a. increase c. remains unaffected

b. decrease d. none of the above

b. decrease

The critical radius ‘r’ of insulation on a pipe is given by

a. r = 2k/h c. r = k/h

b. r = k/2h d. r = h/k

c. r = k/h

The critical radius of insulation for a spherical shell is (where k

= thermal conductivity of insulating material and ho = heat

transfer coefficient of the outer surface)

a. k/ho c. ho/k

b. 2k/ho d. ho/2k

b. 2k/ho

Heat transfer occurs by natural convection because change in

temperature causes differences in

a. viscosity c. thermal conductivity

b. density d. heat capacity

b. density

In a heat exchanger with steam outside the tubes, a liquid gets

heated to 45°C, when its flow velocity in the tubes is 2 m/s. If

the flow velocity is reduced to 1 m/s, other things remaining

the same, the temperature of the exit liquid will be

a. less than 45°C

b. more than 45°C

c. equal to 45°C

d. initially decreases and remains constant thereafter

b. more than 45°C

Heat transfer in turbulent flow may be described by an

empirical equation correlating

a. Nusselt, Peclet, Prandtl numbers

b. Nusselt, Prandtl, Stanton numbers

c. Nusselt, Prandtl, Reynolds numbers

d. Nusselt, Graetz, Schimdt numbers

c. Nusselt, Prandtl, Reynolds numbers

When heat is transferred from hot body to cold body, in a

straight line, without affecting the intervening medium, it is

referred as heat transfer by

a. conduction c. radiation

b. convection d. convection and radiation

c. radiation

An ideal surface that absorbs all incident radiation, regardless

of the wavelengths and direction and is also considered to be

a perfect emitter is referred to as a

a. gray body c. black body

b. black hole d. pin hole

c. black body

In thermal radiation, for a black body

a. α= 1 and ε is not equal to 1

b. α is not equal to 1 and ε= 1

c. α and ε are not equal to 1

d. α= 1 and ε= 1 where α is absorptivity and ε is emissivity.

d. α= 1 and ε= 1

where α is absorptivity and ε is emissivity.

37. In what flow region is the Dittus-Boelter equation valid?

A. Natural convection
B. Laminar
C. Turbulent
D. Forced convection

C. Turbulent

38. What property in heat transfer accounts for the thermal conductivity, density, and specific heat of a material?

A. Heat transfer coefficient
B. Thermal conductivity
C. Thermal expansivity
D. Thermal diffusivity

D. Thermal diffusivity

40. What happens to liquid water if dropped on a pan at 193°C (Leidenfrost point)?

A. Will vaporize instantly
B. Will bounce like balls
C. Will freeze temporarily
D. Will fall flat

B. Will bounce like balls

The Fick’s Law of Diffusion gives the rate of diffusion based on

a. pressure driving force

b. temperature driving force

c. concentration driving force

d. all of these

c. concentration driving force

In steady-state equimolal counterdiffusion, the following are

true:

a. NB = 0; NA is constant

b. NA = 0; NB is nonzero

c. NA = - NB; NA + NB = 0

d. NA + NB = constant

NA = - NB; NA + NB = 0

The enrichment of ethyl alcohol in the vapor phase from an aqueous ethyl alcohol solution during rectification is an example of

a. unicomponent diffusion

b. equimolal counterdiffusion

c. phase drift

d. convective diffusion

b. equimolal counterdiffusion

Ammonia (A) being absorbed from air (B) into water is an example of

a. steady-state equimolal diffusion

b. steady-state unicomponent

c. steady-state multicomponent diffusion

d. unsteady state equimolal diffusion

b. steady-state unicomponent

For a gas phase diffusion, unicomponent diffusion through a gas liquid interface will likely to occur if

a. one component is soluble and the other is not

b. both components are soluble in the solvent

c. both components are insoluble in the solvent

d. none of these

a. one component is soluble and the other is not

Which of the following is true about rate of mass transfer in liquid?

a. NA = kG (Pab – Pai) c. NA = kC (Pab – Pai)

b. NA = kG (Cab – Cai) d. NA = kC (Cab – Cai)

NA = kC (Cab – Cai)

It represents the total mass transferred to mass transferred by

molecular diffusion.

a. Schmidt Number c. Reynolds Number

b. Sherwood Number d. Nusselt Number

b. Sherwood Number

For turbulent mass transfer in pipes, the Sherwood number

depends upon the Reynolds number (Re) as

a. Re^0.33 c. Re^0.53

b. Re^0.83 d. Re

b. Re^0.83

A dimensionless number that represents the ratio of the

molecular momentum diffusion to the molecular mass

diffusivity

a. Sherwood Number c. Schmidt Number

b. Reynolds Number d. Rayleigh Number

c. Schmidt Number

In a gas absorption tower, mass transfer occurs primarily

a. from the liquid phase to the gas phase

b. from the liquid phase to the packing

c. from the gas phase to the liquid phase

d. from the gas phase to the packing

c. from the gas phase to the liquid phase

Separation of a mixture of two gases by absorption in the liquid

solvent depends upon the difference in their

a. viscosity c. density

b. solubility d. relative volatility

b. solubility

In gas absorption, one solute is being absorbed while the restof the components are assumed to be ______ and the liquid is ________.

a. inert, volatile

b. non-soluble, non-volatile

c. non-soluble, volatile

d. soluble, volatile

a. inert, volatile

What could be the effect of lowering the temperature of

stripping?

a. number of stages increases

b. number of stages decreases

c. either increase or decrease

d. no effect

a. number of stages increases

The reverse is true for stripping. However, the operating pressure should not be too high, and the operating temperature should not be too low as to ___ the feed gas.

a. vaporize c. condense

b. compress d. none of these

c. condense

Flooding results in

a. high tray efficiency

b. low tray efficiency

c. high gas velocity

d. good contact between the fluids

b. low tray efficiency

Operating velocity in a packed tower is usually

a. half the flooding velocity

b. twice the flooding velocity

c. equal to flooding velocity

d. more than the flooding velocity

a. half the flooding velocity

Flooding in a column results due to

a. high pressure drop

b. low pressure drop

c. low velocity of the liquid

d. high temperature

a. high pressure drop

Which of the following same diameter columns gives lowest

pressure drop per unit height?

a. Bubble-cap column

b. Packed column (stacked)

c. Sieve-plate column

d. Randomly packed column

b. Packed column (stacked)

Which of the following provides maximum contact surface for a liquid-vapor system?

a. Packed tower c. Bubble-cap plate column

b. Sieve-plate column d. Wetted wall column

a. Packed tower

Which of the following liquid-vapor contacting devices provides maximum contact surface area for a particular duty?

a. Sieve plate column

b. Randomly packed column

c. Bubble cap column

d. Wetted wall column

d. Wetted wall column

__________ column is preferred to be used, when a high liquid hold up is required in a reactor for gas-liquid reaction.

a. Packed c. Spray

b. Bubble d. Tray

b. Bubble

Very tall packed towers are divided into series of beds to

a. reduce the overall pressure drop

b. avoid channeling

c. reduce liquid hold-up

d. avoid flooding

b. avoid channeling

Channeling in a packed tower results from

a. high pressure drop

b. non-uniformity of packing

c. maldistribution of liquid

d. none of these

c. maldistribution of liquid

For the same system, if the same liquid used in absorber is decreased, the tower height will

a. decrease c. remains the same

b. increase d. decrease or increase depends on the type of liquid

b. increase

What is the degree of freedom of an absorption process in which only one component is transferred between phases?

a. 0 c. 2

b. 1 d. 3

d. 3

The dimensionless number relates bouyancy forces to viscous forces
a. Archimedes number
b. Grashof number

c. Graetz number

d. Rayleigh number

b. Grashof number

The dimensionless group in mass transfer taht is equivalent to Prantld number in heat transfer is
a. Sherwood number

b. Nusselt number

c. Schmidt number

d. Stanton number

c. Schmidt number

Pick out the wrong statement

a. the controlling resistance in the case of heating of air by condensing steam is in the air film

b. in case of a 1-2 shell and tube heat exchanger, the LMTD correction factor value increases sharply when a temperature cross occurs.

c. Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice versa occurs at saturation temperature
d. the log mean temperature difference (LMTD) for counter flow and parallel flow can be theoretically same when anu one of the fluids, hot or cold fluid, passes through the heat exchanger at constant temperature

b. in case of a 1-2 shell and tube heat exchanger, the LMTD correction factor value increases sharply when a temperature cross occurs.

Air is best heated with steam in a heat exchanger of
a. Shell and tube type

b. Plate type

c. double pipe type with fin on air side

d. double pipe type with fin on steam side

c. double pipe type with fin on air side

Hot water (0.01 m³/min) enters the tube side of a countercurrent shell and tube heat exchanger at 80 deg C and leaves at 50 deg C. Cold oil (0.05 m³/min) of density 800 kg/m³ and specific heat of 2kj/kg-k enters at 20 deg C. The log mean temperature difference in C is apporximately
a. 45

b. 50

c. 32

d. 37

c. 32