Heat Transfer (Set 3) Flashcards
As one of the transport phenomena, the concepts in heat transfer are crucial for design, operation, and maintenance of heat transfer equipment such as heat exchangers and insulation.
The Grashoff number is defined as the ratio of the
buoyancy to viscous forces.
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
f/2
The absorptivity of a body is equal to its emissivity
under thermal equilibrium.
Absorptivity and refletivity of a perfect black body are respectively
1 and 0
In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
α = 1; ε = 1
Pick out the wrong statement.
S.I. unit of fouling factor is Watt/m2.°K.
When warm and cold liquids are mixed, the heat transfer is mainly by
convection
A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300°C and allowed to cool in natural air. Which one will cool the slowest?
Sphere
Vent pipes are provided in a condenser to
remove non-condensable gases.
Heat transfer by natural convection is enhanced in system with
high co-efficient of thermal expansion.
In case of ____ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
pool
A body cools down from 75°C to 70°C in 10 minutes. It will cool down from 70° C to 65° C in ____ minutes.
> 10
In forced convection, the Nusselt number is a function of
Re and Pr
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical?
1
A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because
total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by
keeping the heat transfer surface clean.
Trap is used to remove ____ from steam pipe lines.
condensate
A composite flat wall of a furnace is made of two materials ‘A’ and ‘B’. The thermal conductivity of ‘A’ is twice of that of material ‘B’, while the thickness of layer of ‘A’ is half that of B. If the temperature at the two sides of the wall are 400 and 1200°K, then the temperature drop (in °K) across the layer of material ‘A’ is
160
Tube pitch is the ____ of tube diameters and the clearances.
sum
With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
decreases
The equivalent diameter for the annulus of a double pipe heat exchanger, whose inner pipe has fins on the outside is ____ compared to the same size pipes without fins.
less
Natural convection is characterised by
Grashoff number
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
4.364
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
< 1
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe)
K/h0
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
(ΔT)^3
In a 1-1 cocurrent heat exchanger, if the tube side fluid outlet temperature is equal to the shell side fluid outlet temperature, then the LMTD is
0
A body is called grey if the monochromatic emissivity of the body is
same for all wavelengths
In an extended surface heat exchanger, fluid having lower co-efficient
flows outside the tubes.
In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is
infinity.
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to ____ force.
viscous
Pick out the wrong statement.
Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.
Graetz number is given by
mCp/kL
For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in
increased economy
Economy of an evaporator is influenced by the
a. steam pressure
b. feed temperature
c. number of effect
both(b) & (c)
If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows ____ law.
linear
In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred
by heating an intermediate material (like chequor bricks) and then heating the air from this hot material.
____ equation relates the thermal conductivity of a solid to its temperature.
Lee’s
For small temperature difference, the heat transfer rate as per Newton’s law of cooling is proportional to (where, Δt = excess temperature)
Δt
Pick out the wrong statement.
Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.
Viscous & heat sensitive liquids are concentrated in ____ evaporators.
agitated film
1 BTU/hr.ft.°F is equal to ____ kcal/hr. m.°C.
1.49
Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on
none of these
The ratio of momentum diffusivity to thermal diffusivity is the ____ number.
Prandtl
For a perfectly transparent surface (like gases), the
transmissivity = 1
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
different
During crystallisation, formation of crystal can occur in ____ solution only.
supersaturated
An evaporator while concentrating an aqueous solution from 10 to 40% solids evaporates 30000 kg of water. The amount of solids handled by the system in kg is
4000
An insulator should have
low thermal conductivity.
In a backward feed multiple effect evaporator
feed flows from low pressure to high pressure.
Choose the correct equation.
Nu = (Re) (Pr) (St)
In a forward feed multiple effect evaporator unit
none of these.
In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is
parabolic
For heat flow through very thick walled cylinder, use____mean radius.
logarithmic
Indirect contact heat exchangers are preferred over direct contact heat exchangers, because
there is no risk of contamination.
Water is normally used as a coolant in the heat exchange equipments mainly because of its
abundance & high heat capacity
The rate of emission of radiation by a body does not depend upon the
shape and porosity of the body.
Which of the following has maximum thermal conductivity at the same temperature?
Steel
Maximum heat transfer rate is obtained in ____ flow.
turbulent
In case of ____ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface.
local
The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is
45295
Terminal point temperature differences between fluids in case of a heat exchanger is termed as
approach
LMTD can’t be used as such without a correction factor for the
all (a) (b) and (c).
Fqra cold viscous feed, backward feed gives ____ than forward feed.
a higher capacity
Walls of a cubical oven are of thickness l, and they are made of material of thermal conductivity k. The temperature inside the oven is 100°C and the inside heat transfer co-efficient is 3k/l. If the wall temperature on the outside is held at 25°C, what is the inside wall temperature in degree centigrade ?
81.25
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
U1 = U2
In SI units, thermal conductivity is expressed in
Watt/m.°K
Boiling point of a solution according to Duhring’s rule is a linear function of the ____ of water.
boiling point (at the same pressure)
The overall resistance for heat transfer through a series of flat resistance, is the ____ of the resistances.
sum
At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe?
> 0.6
A sphere of radius ‘R1’ is enclosed in a sphere of radius ‘R2’. The view (or shape) factor for radiative heat transfer of the outer sphere with respect to the inner sphere is
R2/(R1+R2)
For a laminar flow of fluid in a circular tube, ‘h1’ is the convective heat transfer co-efficient at velocity ‘V1’. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is
0.794 h1
The main purpose of providing fins on heat transfer surface is to increase the
heat transfer area.
A BTU/hr.ft.2 °F is equal to
4.88 kcal/hr. m.2°C.
Which of the following has the lowest Prandtl number?
Molten sodium (a coolant used in fast breeder reactor)
In SI units, fouling factor is expressed in
m2°K/W
An ejector is used to
none of these
Extremely large or small volumes of fluids are generally best routed through the shell side of a shell and tube heat exchanger, because of the
flexibility possible in the baffle arrangement.
All analogy equations connecting friction factor and heat transfer co-efficient apply only to
wall or skin friction
Electro-magnetic spectrum range, which is important for radiation varies from ____ microns.
0.5 to 50
The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are
Nu, Gz
Heat transfer rate described by Fourier’s law will decrease, if the ____ increases.
thickness
A black body does not ____ radiation.
a. absorb or emit
b. refract
c. reflect
d. both (b) & (c)
both(b) & (c)
The capacity of double-effect evaporator is less than half of the capacity of two single effects, each of which is operating over same terminal temperature difference, when the
solution has an elevation of boiling point.
Peclet number (Pe) is given by
Pe = Re.Pr
Nusselt number is the ratio of the
heat flux at the wall to that across the entire pipe.
Prandtl number is the ratio of
momentum diffusivity to thermal diffusivity.
Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger?
a. Tube side pressure drop and the heat transfer rate.
b. Convective heat transfer co-efficient.
c. Effective tube surface area for convective heat transfer.
All (a) (b) and (c).
Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator?
Agitated film evaporator
The ratio of the total emissive power to the absorptivity for all bodies is same at ther-mal equilibrium. This is ____ law.
Kirchoffs
View factor is important in heat transfer by
radiation
The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is
higher tube side heat transfer co-efficient.
For a multipass shell and tube heat exchanger, the LMTD correction factor is always
< 1
If heat transfer rate varies with the time, it is termed as
none of these
Pick out the wrong statement.
a. Fluid movement under the influence of buoyant forces resulting from change in density takes place in case of natural convection.
b. The ratio NNu/NRe . Npr is called the Stanton number.
c. The Peclet number is a measure of the ratio of energy transport by convection to that by conduction.
d. The Colbum jH factor for heat transfer is given by Nst Npr.
The Colbum jH factor for heat transfer is given by Nst Npr.
Tube expansion allowances exist in ____ heat exchanger.
U-tube
Pick out the wrong statement.
Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.
Agitated film evaporator is suitable for concentrating ____ liquids.
viscous
The thickness of condensate layer in filmwise condensation depends on the
a. condensation rate
b. surface configuration
c. liquid flow rate from the surface
all (a), (b) and (c)
If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface tempeature follows ____ law.
parabolic
