Heat Transfer Flashcards
Heat Transfer
transmission of energy from one region to another region as a result of a temperature difference between them.
Differentiate Heat Transfer and Thermodynamics
Heat Transfer
*Deals with transient condition
*Predict temperature distribution and rate of energy transfer
Thermodynamics
*Deals with equilibrium
*Help quantify work and heat interactions. How much heat is transferred.
Conduction
transfer of thermal energy within a solid medium due to temperature difference
Convection
Heat transfer through the movement of fluids
Radiation
Stefan-Boltzmann Law
Fourier Equation
Thermal Conductivity, k
Rate of heat transfer through a cross-section area, when a temperature gradient exits perpendicular to the area.
Thermal Conductivity:
Gas, Liquid, Solid
Solid > Liquid > Gas
Thermal Conductivity and Temperature:
Gas, Liquid, Solid, Metals, Alloys
Increase with T:
Gas, Non-Metals
Decrease with T:
Liquid, Solid, Metals
Isotropic Material
Thermal conductivity does not vary with change in direction.
Anisotropic Material
Thermal conductivity of the material depends on the direction of the heat flow.
Insulators
*Material used to minimize the heat flow to or from the system.
*Low thermal conductivity
Heat Capacity, Cp
Material’s ability to store thermal energy
Thermal Diffusivity, a
Ability of a material to conduct thermal energy relative to its ability to store thermal energy.
Critical radius
*Radius of insulation at which heat flow = 0
*r,cr = k/h
Heat Transfer and Resistance:
Series and Parallel
Series
*Heat transfer: Constant
*Resistance: Additive
Parallel
*Heat transfer: Additive
*Resistance: Constant
Free or Natural Convection
Fluid motion by buoyancy, density and temperature differences
Forced Convection
Fluid motion by artificially induced mechanical device
Heat Transfer Coefficient
(Rate of heat transfer per unit area) per temperature difference between bulk fluid temperature and surface temperature
Heat Exchanger
Used for exchange of heat between the two fluids that are at different temperatures
Baffles
- used on the shell to create turbulence
- to keep the uniform spacing between the tubes
Baffle pitch
distance between baffles
Fouling
Deposition of fluid impurities, rust formation and other reactions between the fluids and surfaces.
Number of Transfer Units
NTU
*measure of the physical size (heat transfer area) of the heat exchanger.
Radiation
Blackbody Radiation
- Reflected = 0
Absorbed = 1
Emission = 1
*ideal radiator and emittor
Kirchoff’s Law
*Total emissivity = Absorptivity
Stefan-Boltzmann Law
total emissive power of a black body is directly proportional to the fourth power of the absolute temperature.
Stefan-Boltzmann Constant
5.670410^−8 W/m^2K^4
Planck’s Law
*Relates monochromatic emissive power at a temperature T and a wavelength
*Used to calculate total emissive power leaving a surface with temperature T over all wavelengths.
Wein’s Displacement Law
Used to determine wavelength of black body emissive power at given temperature
(wavelength) * T = 2.89810^-3 mK
VESSEL JACKETS:
Dimple Jacket
*High pressure
*Circulating steam and hot oil
VESSEL JACKETS:
Plain Jacket
*Covers all around the vessel
*Small capacity vessels
*For hot fluids only
VESSEL JACKETS:
Coil Jacket
*Provide high turbulence and high velocity
*High temperature operation
*Reduces vessel costs
When vaporisation takes place directly at the heating surface, it is called
film boiling
nucleate boiling
vapour binding
none of these
nucleate boiling
With increase in temperature, the thermal conductivity of non-metallic amorphous solids ___
increases
Fourier’s law applies to the heat transfer by ____
conduction
With increase in porosity, the thermal conductivity of a solid substance ____
decreases
For an ideal black body ….
absorptivity = 1
emissivity = 1
Fouling factor ….
accounts for additional resistances to heat flow.
The steam ejector is used to ___
create vacuum
Leidenfrost point is a term concerned with the ___
boiling of a liquid on a hot surface.
Kirchoff s law applies to __________ radiation.
total and monochromatic
The unit of heat transfer co-efficient in SI unit is ___
W/m^2*K
Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.
the largest
Calculated the monochromatic emissive power for black body radiation
Planck’s law
Temperature profile in steady state heat transfer is ___
linear
Heat transfer by __________ may not necessarily require the presence of a medium.
radiation
Stefan-Boltzman law applies to __________ body.
black
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by ____
conduction
The sum of reflectivity and absorptivity for an opaque body is equal to __
1.0
A hot body will radiate heat most rapidly, if its surface is ___
black & rough
The critical radius ‘r’ of insulation on a pipe is given by ___
r = k/h
Steam traps are provided in steam carrying pipelines to
____
remove condensate and inert gases.
Which of the following is correct ?
Rate = Driving force x Resistance
Driving force = Rate x Resistance
Resistance = Driving force x Rate
Rate = Resistance/Driving force
Driving force = Rate x Resistance
Film condensation is promoted on a/an __________ surface.
clean & smooth
Trap is used to remove __________ from steam pipe lines.
condensate
A body is called grey if the monochromatic emissivity of the body is ____
same for all wavelengths
For a perfectly transparent surface (like gases), the transmissivity is ___
1
An insulator should have ____
low thermal conductivity.
Air is best heated with steam in a heat exchanger of ___
plate type.
double pipe type with fin on steam side.
double pipe type with fin on air side.
shell and tube type.
double pipe type with fin on air side.
Which of the following is generally considered as opaque surface towards radiations?
Gases
Solids
Liquids
Both (b) and (c)
Both (b) and (c)
With increase in temperature, the thermal conductivity of non-metallic amorphous solids ___
decreases
increases
remains constant
first decreases upto certain temperature and then increases
increases
Shell side pressure drop in a shell and tube heat exchanger does not depend upon the ____
baffle spacing & shell diameter.
tube diameter & pitch.
viscosity, density & mass velocity of shell side fluid.
none of these.
none of these.
With increase in porosity, the thermal conductivity of a solid substance ___
increases
decreases
remains unchanged
may increase or decrease; depends on the solid
decreases
Fouling factor ___
is a dimensionless quantity.
does not provide a safety factor for design.
accounts for additional resistances to heat flow.
none of these.
accounts for additional resistances to heat flow.
The steam ejector is used to ____
remove condensate from the steam pipelines.
create vacuum.
superheat the steam.
none of these.
create vacuum.
In a heat exchanger, floating head is provided to ____
facilitate cleaning of the exchanger.
increase the heat transfer area.
relieve stresses caused by thermal expansion.
increase log mean temperature gradient.
relieve stresses caused by thermal expansion.
Leidenfrost point is a term concerned with the ___
condensation of the saturated vapor on a cold surface.
concentration of a corrosive solution by evaporation.
heat transfer between two highly viscous liquids.
boiling of a liquid on a hot surface.
boiling of a liquid on a hot surface.
Heat produced when a steady state current, I passes through an electrical conductor having resistance, ‘R’ is ___
IR
I2R
IR2
I2R2
I2R
Convective heat transfer co-efficient in case of fluid flowing in tubes is not affected by the tube length/diameter ratio, if the flow is in the __________ zone.
laminar
transition
both ‘a’ & ‘b’
highly turbulent
highly turbulent
Air is to be heated by condensing steam. Two heat exchangers are available (i) a shell and tube heat exchanger and (ii) a finned tube heat exchanger. Tube side heat transfer area are equal in both the cases. The recommended arrangement is ___
finned tube heat exchanger with air inside and steam outside.
finned tube heat exchanger with air outside and steam inside.
shell and tube heat exchanger with air inside tubes and steam on shell side.
shell and tube heat exchanger with air on shell side and steam inside tubes.
finned tube heat exchanger with air outside and steam inside.
Increasing the liquor level in the evaporator results in the ___
decreased capacity.
increase in liquor film co-efficient.
decreased effect of hydrostatic head.
increased true temperature drop.
decreased capacity.
A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides ____
higher economy
lower capacity
both (a) & (b)
lower economy
higher economy
The absorptivity of a grey body at a given temperature __________ with increasing wavelength of radiation.
increases
decreases
remains constant
may increase or decrease ; depends on the material
may increase or decrease ; depends on the material
In a shell and tube heat exchanger, ___
the temperature drops in the two fluids and the wall are proportional to individual resistances.
the temperature drop is inversely proportional to the resistance across which the drop occurs.
there is no relationship between temperature drop and resistance.
the relationship is not generalised.
the temperature drops in the two fluids and the wall are proportional to individual resistances.
Temperature profile in steady state heat transfer is ___
asymptotic
hyperbolic
parabolic
linear
linear
It is not preferable to use superheated steam in evaporators, because of its very ____
high temperature
high pressure
low film co-efficient
none of these
low film co-efficient
Use of transverse baffles in a shell and tube heat exchanger is done to increase the ___
rate of heat transfer.
flow velocity.
turbulence of shell side fluid.
all (a), (b) and (c).
all (a), (b) and (c).
The actual temperature drop across the heating surface of an evaporator depends on the ___
liquid depth over the heating surface.
solution being evaporated.
pressure difference between the steam chest and the vapor space above the boiling liquid.
all (a), (b) and (c)
all (a), (b) and (c)
With increase in temperature, the thermal conductivity of most liquids ___
increases
decreases
remains same
first increases upto a certain temperature and then becomes constant
decreases
In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used ___
in low range of temperature differences.
in high range of temperature differences.
because of its low cost.
to prevent corrosion of the tube bundles.
in high range of temperature differences.
Stefan-Boltzman law applies to __________ body.
black
white
grey
any colour
black
A perfect black body is a perfect __________ of radiation.
absorber
emitter
both (a) & (b)
neither (a) nor (b)
both (a) & (b)
A 2-4 heat exchanger involves ___
only counter-flow of fluids.
only parallel-flow of fluids.
both counter and parallel-flow of the fluids.
smaller pressure drop compared to 1-2 exchanger.
both counter and parallel-flow of the fluids.
With increase in temperature, the thermal conductivity of steel ___
increases
decreases
remains unchanged
increases exponentially
decreases
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids ?
Dittus-Boeltier equation
Sieder-Tate equation
Nusselt equation
None of these
Sieder-Tate equation
The main function of baffles provided in a shell and tube heat exchanger is to ___
facilitate the cleaning of outer tube surface.
enhance turbulence.
hold the tubes in position.
all ‘a’, ‘b’ & ‘c’.
enhance turbulence.
In counter flow compared to parallel flow, ___
LMTD is greater.
less surface area is required for a given heat transfer rate.
both (a) and (b).
more surface area is required for a given heat transfer rate.
both (a) and (b).
Crystallisation of solids from a homogeneous solution is a/an __________ process.
exothermic
mildly endothermic
highly endothermic
none of these
exothermic
A diathermaneous substance __________ the thermal radiation completely.
absorbs
reflects
transmits
none of these
transmits
A __________ surface has the maximum thermal emissivity out of the following.
black & smooth
black & rough
white & smooth
white & rough
black & rough
Heat transfer rate per unit area is called ____
thermal conductivity
heat flux
heat transfer co-efficient
thermal diffusivity
heat flux
In a forward feed multiple effect, the pressure build up will be ___
least at the inlet of the first effect.
least at the outlet of the last effect.
highest at the inlet of the last effect.
highest at the outlet of the last effect.
least at the outlet of the last effect.
Which of the following is unimportant in forced convection ?
Reynolds number
Prandtl number
Grashhoff number
none of these
Grashhoff number
A body is called grey if the monochromatic emissivity of the body is ___
zero
unity
same for all wavelengths
different for all wavelengths
same for all wavelengths
In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is ____
linear
hyperbolic
parabolic
none of these
parabolic
Which of the following has the lowest Prandtl number ?
Molten sodium (a coolant used in fast breeder reactor)
Water
Transformer oil
Dilute H2SO4
Molten sodium (a coolant used in fast breeder reactor)
A black body does not __________ radiation.
absorb or emit
refract
reflect
both(b) & (c)
both(b) & (c)
Fourier’s law of heat conduction applies to __________ surfaces.
isothermal
non-isothermal
both (a) and (b)
neither (a) and (b)
both (a) and (b)
Pick out the wrong statement:
The capacity of an evaporator is reduced by the boiling point elevation.
Corrosive liquid is normally passed through the tubes in a shell and tube heat exchanger.
Steam jet ejector is used for vapor compression in a thermal recompression evaporator.
Heat sensitive materials should be concentrated in high pressure evaporators.
Heat sensitive materials should be concentrated in high pressure evaporators.
