S2 Flashcards
Which area is used in case of heat flow by conduction through a cylinder?
logarithmic mean area
Heat transfer by conduction results duet to the transfer of free electrons, kinetic energy, and vibrational energy from one molecule to another. Conduction heat transfer can not take place
between two bodies not in physical contact with each other
The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is
Schmidt number
Finned tube heat exchangers
**give larger area per tube*&
The absorptivity of a grey body at a given temperature _______ with increasing wavelength of radiation.
may increase or decrease; depends on the material
Which on gives the monochromatic emissive power for black body radiation?
Planck’s law
For what value of Prandtl number, St= f/2?
1
In a shell and tube heat exchanger,
the temperature drops in the two fluids and the wall are proportional to individual resistances
Dietus-Boelter equation used for the determination of heat transfer coefficient is valid
for fluids in turbulent flow
The equation, Nst, corresponds to ______ analogy
Prandtl
For large heat transfer area requirement, shell and tube heat exchanger is preferred, because it
All a,b,c
A. Occupies smaller space
B. Is more economical
C. Is easy to operate and maintain
Out of the following four assumptions used in the derivation of the equation for LMTD, which one is subject to the largest deviation in practice?
constant rate of fluid flow
Heat flux increases with temper drop beyond the Leiden frost point on the plot of heat flux vs. temperature drop for a boiling liquid, because
radiation becomes important
Resistance to heat flow by conduction is proportional to
all a,b, and c
A. t
B. 1/density
C. 1/A
Temperature profile in steady state heat transfer is
Linear
For the same heat load and mass flow rate in the tube side of a shell and tube heat exchanger, one may use multipass on the tube side, because it
increase the overall heat transfer coefficient
When vaporization takes place through a blanketing film of gas, the phenomenon is termed as _____ boiling.
film
Minimum recommended baffle spacing in a shell and tube heat exchanger is about
0.2D
Stefan-Boltzmann law which describes the radiation heat transfer states that, it is proportional to
T^4
The Nusselt number for fully developed laminar flow through a circular pipe, where the wall heat flux is constant, is
4.36
What is the unit of thermal conductivity?
Kcal/hr.m.degC
The thermal radiation emitted by a body is proportional to T^n, where T is its absolute temperature. The value of ‘n’ is exactly 4 for
all bodies
It is not preferable to use superheated steam in evaporators, because of its very
low film coefficient
It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of solution) to a 30% salt solution in an evaporator. Consider a feed of 300 kg/min at 30°C. The boiling point of the solution is 110°C, the latent heat of vaporisation is 2100 kJ/kg and the specific heat of the solution is 4 kJ/kg.K. The rate at which the heat has to be supplied in (kJ/min) to the evaporator is
3.06x10^5 kJ/min
LMTD for counterflow and parallel flow heat exchanger will be the same, when the
cold fluid is heated to a certain temperature by condensing steam (isothermal fluid)
For flow over a flat plate, the ratio of thermal boundary layer thickness, ‘xt’ and hydrodynamic boundary layer thickness ‘x’ is equal to (where, NPr = Prandtl number)
NPr^1/3
The range of electromagnetic spectrum important in heat transfer by radiation is _____ microns
0.5-50
Use of transverse baffles in a shell and tube heat exchanger is done to increase the
All a,b,c
A. Rate of heat transfer
B. Flow velocity
C. Turbulence of shell side fluid
The actual temperature drop across the heating surface of an evaporator depends on the
All a,b,c
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
Economy of multiple effect evaporator depends upon the
heat balance consideration
The Graetz number is concerned with the
heat transfer in laminar flow
Solid angle subtended by the finite surface at the radiating element is
called the angle of vision
Pick out the wrong statement.
The emissivity of a surface decreases, if it gets corroded by atmospheric environment
In forced convection, the heat transfer depends on
Re, Pr
With increase in temperature, the thermal conductivity of most liquids
decreases
Heat transfer by _______ may not necessarily require the presence of a medium.
radiation
Pick out the CORRECT statement
Logarithmic mean area is used for calculating the heat flow rte through a thick walled cylinder
In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used
in high range of temperature differences
Stefan-Boltzmann law applies to _____ body
black
The film co-efficient is decreased due to the presence of non-condensing gases in the vapors. The film co-efficient of superheated vapor as compared to that of saturated vapor is
some
A perfect black body is a perfect _____ of radiation
both a and b
A. Absorber
B. Emitter
In case of parallel flow heat exchanger, the lowest temperature theoretically attainable by the hot fluid is _____ the outlet temperature of the cold fluid.
equal to
An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporization of another, is called a
heat exchanger
Which of the following is directly concerned with the heat transfer?
Grashof number
Pick out the WRONG statement
With change in temperature, the radiant energy emitted by a black body remains unchanged.
Planck’s distribution law is valid for ____ bodies.
colored
For hot feed, forward feeding as compared to backward feeding results in ______ economy.
increased
Radiation energy is emitted by all the substances, which are above
0 Kelvin
