Deck 19: Heat Transfer and Evaporation

Question 1

Pick out the wrong statement.

A)'Solvates' are chemical compounds formed by solute with their solvents. When water is the solvent, then it is called a 'hydrate'
B)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
C)Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted
D)In case of short tube vertical evaporators, area of central downtake is about 50 to 100% of the total tube cross-sectional area

Answer 1

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

Question 2

For what value of Prandtl number, St = f/2?

A)1.5
B)1
C)> 1
D)< 1

Answer 2

1

Question 3

The rate of emission of radiation by a body does not depend upon the

A)Wavelength of radiation
B)Surface temperature of the body
C)Nature of the surface
D)Shape and porosity of the body

Answer 3

Shape and porosity of the body

Question 4

Heat sensitive materials can be concentrated in an evaporator employing

A)Vacuum
B)High pressure
C)High residence time
D)None of these

Question 5

In a cooling tower, water becomes cool by

A)Loosing sensible heat
B)Heat transfer to surroundings
C)Vaporisation due to heat loss to air
D)Loosing latent heat

Question 6

Conductance is given by (where, x = thickness, A = heat flow area, K = thermal conductivity.)

A)x/KA
B)KA/x
C)K/Ax
D)A/Kx

Question 7

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 ?

A)Constant overall heat transfer co-efficient.
B)Constant rate of fluid flow
C)Constant specific heat
D)No partial phase change in the system

Question 8

LMTD can't be used as such without a correction factor for the

A)Multipass heat exchanger
B)Baffled heat exchanger
C)Condensation of mixed vapour in a condenser
D)All (A) (B) and (C)

Question 9

What is the logarithmic mean of r1 and R² ?

A)(r1 - R² )/ln(r1/R² )
B)(r1 - R² )/ln(R² /r1)
C)(R² - r1)/ln(r1/R² )
D)(r1 - R² )/-ln(r1/R² )

Question 10

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

Question 11

The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe, where the wall heat flux is constant, is

A)2.36
B)4.36
C)120.36
D)Dependent on NRe only

Question 12

Heat flux, as defined in heat flow is analogous to __________ in electricity flow.

A)Current
B)Voltage
C)Resistance
D)None of these

Question 13

The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances.

A)Average
B)Geometric mean
C)Product
D)Sum

Question 14

The driving potential for the crystal growth during crystallisation is the __________ of the solution.

A)Concentration
B)Viscosity
C)Super-saturation
D)Density

Question 15

Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re 0.8. Prn, is not valid, if the value of

A)n = 0.4 is used for heating
B)n = 0.3 is used for cooling
C)Reynolds number for the flow involved is > 10000
D)Reynolds number for the flow involved is < 2100

Question 16

For flow over a flat plate, the ratio of thermal boundary layer thickness, 'xt' and hydrodynamic boundary layer thickness 'x' is equal to (where, N_Pr = Prandtl number)

A)N_Pr
B)N_Pr ^1/3
C)N_Pr ^-1
D)N_Pr ^-1/3

Question 17

In a shell and tube heat exchanger, putting a longitudinal baffle across the shell, forces the shell side fluid to pass __________ through the heat exchanger.

A)Once
B)Twice
C)Thrice
D)Four times

Question 18

An ejector is used to

A)Increase pressure
B)Increase temperature
C)Remove condensate
D)None of these

Question 19

In a forward feed multiple effect evaporator, the pressure is

A)Highest in last effect
B)Lowest in last effect
C)Same in all effects
D)Dependent on the number of effects

Question 20

In a laboratory test run, the rate of drying was found to be 0.5 x 10-3 kg/m² .s, when the moisture content reduced from 0.4 to 0.1 on dry basis. The critical moisture content of the material is 0.08 on a dry basis. A tray dryer is used to dry 100 kg (dry basis) of the same material under identical conditions. The surface area of the material is 0.04 m² /kg of dry solid. The time required (in seconds) to reduce the moisture content of the solids from 0.3 to 0.2 (dry basis) is

A)2000
B)4000
C)5000
D)6000

Question 21

With increase in temperature, the thermal conductivity of most liquids

A)Increases
B)Decreases
C)Remain same
D)First increases upto a certain temperature and then becomes constant

Question 22

Peclet number (Pe) is given by

A)Pe = Re.Pr
B)Pe = Re/Pr
C)Pe = Pr/Re
D)Pe = Nu.Re

Question 23

The film co-efficient between condensing vapour and metal wall increases with

A)Increasing temperature of the vapour
B)Decreasing temperature of the vapour
C)Increasing viscosity of the film of condensate
D)Increasing temperature drop

Question 24

In Fourier's law, the proportionality constant is called the

A)Heat transfer co-efficient
B)Thermal diffusivity
C)Thermal conductivity
D)Stefan-Boltzmann constant

Question 25

The non-dimensional temperature gradient in a liquid at the wall of a pipe is the

A)Heat flux
B)Nusselt number
C)Prandtl number
D)Schmidt number