Deck 5: Heat Transfer and Evaporator

Question 1

The actual temperature drop across the heating surface of an evaporator depends on the

A)Liquid depth over the heating surface
B)Solution being evaporated
C)Pressure difference between the steam chest and the vapor space above the boiling liquid
D)All (A), (B) and (C)

Answer 1

All (A), (B) and (C)

Question 2

In SI units, thermal conductivity is expressed in

A)Watt/m.°K
B)Watt/m². °K
C)Watt/m². °K.
D)Watt/m4. °K.

Answer 2

Watt/m.°K

Question 3

jH factor for heat transfer depends upon the __________ number.

A)Biot
B)Nusselt
C)Reynolds
D)Prandtl

Answer 3

Reynolds

Question 4

In pipe flow, heat is transferred from hot wall to the liquid by

A)Conduction only
B)Forced convection only
C)Forced convection and conduction
D)Free and forced convection

Question 5

A perfect black body is a perfect __________ of radiation.

A)Absorber
B)Emitter
C)Both (A) & (B)
D)Neither (A) nor (B)

Question 6

Heat transfer rate described by Fourier's law will decrease, if the __________ increases.

A)Thermal conductivity
B)Thickness
C)Temperature difference
D)Heat transfer area

Question 7

The absorptivity of a body is equal to its emissivity

A)At a particular temperature
B)For circular bodies
C)Under thermal equilibrium
D)None of these

Question 8

In a co-current double pipe heat exchanger used for condensing saturated steam over the inner tube, if the entrance and exit conditions of the coolant are interchanged, then the rate of condensation will

A)Increase
B)Decrease
C)Remain unchanged
D)Either increase or decrease; depends on the coolant flow rate

Question 9

The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and ² 0 cm thick with thermal conductivities of 0.05 and 0.1 W.m^-1 .°C1 respectively. The ambient air is at 30°C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m ^-2 .°C^-1 . The rate of heat loss from the outer surface in W.m ^-2 is

A)165.4
B)167.5
C)172.5
D)175

Question 10

Which one gives the monochromatic emissive power for black body radiation?

A)Planck's law
B)Kirchhoff's law
C)Wien's law
D)Stefan-Boltzmann law

Question 11

If heat transfer rate varies with the time, it is termed as

A)Forced convection
B)Steady state conduction
C)Monochromatic radiation
D)None of these

Question 12

Absorptivity and reflectivity of a perfect black body are respectively

A)1 and 0
B)0 and 1
C)1 and ?
D)0 and 0.5

Question 13

__________ heat exchanger is used for chilling oil to be dewaxed.

A)U-tube
B)Double pipe
C)Fixed tube
D)Floating head

Question 14

Pick out the correct equation.

A)jH = (St)(Pr) ^2/3 = f/2
B)jH = (St)(Pr) ^1/3 = f/2
C)jH = (St) ^2/3(Pr) = f/2
D)jH = (St) ^1/3(Pr) = f/2

Question 15

It is not preferable to use superheated steam in evaporators, because of its very

A)High temperature
B)High pressure
C)Low film co-efficient
D)None of these

Question 16

Water is normally used as a coolant in the heat exchange equipments mainly because of its

A)Abundance & high heat capacity
B)Low density
C)Low viscosity
D)High fluidity

Question 17

A single pass air heater is connected to a two pass unit. For the air flow rate and other conditions remaining the same, the film heat transfer co-efficient for air will vary in the ratio of

A)2
B)2 0.8
C)2 0.2
D)2 0.5

Question 18

At constant temperature, the thermal conductivities of gases __________ with rise in pressure.

A)Decrease
B)Increase
C)Remain unchanged
D)May increase or decrease; depends on the pressure

Question 19

Stefan-Boltzmann law applies to __________ body.

A)Black
B)White
C)Grey
D)Any colour

Question 20

When warm and cold liquids are mixed, the heat transfer is mainly by

A)Conduction
B)Convection
C)Radiation
D)Both (A) & (C)

Question 21

A BTU/hr.ft.2 °F is equal to

A)1 kcal/hr. m² °C
B)4.88 kcal/hr. m.² °C
C)1 kcal/hr. m² .°K
D)None of these

Question 22

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

A)Scaling
B)Highly viscous
C)Corrosive
D)Salty

Question 23

A __________ evaporator employs an annular downtake.

A)Basket type
B)Horizontal
C)Long tube vertical
D)None of these

Question 24

The equation, Nst = (f/2)/[1 + 5 (N_Pr - 1) ?(f/2)], corresponds to __________ analogy.

A)Von-Karman
B)Reynolds
C)Colburn
D)Prandtl

Question 25

The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is

A)Lower tube side pressure drop
B)Lower shell side pressure drop
C)Higher tube side heat transfer co-efficient
D)Higher shell side heat transfer co-efficient