Heat Transfer (Set 5) 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.
In a heat exchanger, the rate of heat transfer from the hot fluid to the cold fluid
varies directly as the area and the LMTD.
With increase in temperature, the total emissivity of conductors
increases
Loss of heat from untagged steam pipe to the ambient air is by
all (a), (b) & (c)
Conduction, Convection & Radiation
Fruit juice (a heat sensitive material) can be concentrated in a ____ evaporator.
falling film
Tubes are held between top and bottom tube sheets in Calenderia type evaporator by keeping
both the tube sheets fixed.
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
2^0.8
Heat sensitive materials can be concentrated in an evaporator employing
vacuum
A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires
more space.
In a shell and tube heat exchanger, floating head is used for
large temperature differentials.
Steam trap is used to
remove water resulting from partial condensation of steam.
The average heat transfer co-efficient over the entire length of the plate (ha) and the local heat transfer co-efficient (hL), in case of heat transfer over a flat plate in lminar zone is related as
ha = 2hL
What is the logarithmic mean of r1 and r2 ?
(r1-r2/ln(r1/r2))
Maximum heat transfer rate is achieved in ____ flow.
turbulent
Wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. This is ____ law.
Wien’s
Three material A, B and C of equal thick-nes and of thermal conductivity of 20, 40 & 60 kcal/hr. m. °C respectively are joined together. The temperature outside of A and C are 30°C and 100°C respectively. The interface between B and C will be at a temperature of ____ °C.
70
Which of the following parameters of the fluid is not very important, while deciding its route in a shell and tube heat exchanger?
density
Corrosiveness of steam condensate is due to the presence of
a. CO2
b. dissolved O2
both (a) & (b)
The equation, (NSt x N2/3Pr) = f/2, is the ____ analogy.
Colburn
The driving potential for the crystal growth during crystallisation is the ____ of the solution.
super-saturation
jH factor for heat transfer depends upon the ____ number.
Reynolds
Kg of liquid evaporated per hour in an evaporator is defined as its
capacity
Pick out the correct statement.
In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity.
Open pan evaporators are preferred to be used, when the solution to be concentrated is
highly viscous
Cp.μ/K is termed as the ____ number.
Prandtl
The maximum heat transfer co-efficient from steam heating will be attained when the steam is
saturated
Kirchoff’s law is applicable to
a. monochromatic radiation only
b. total radiation only
both (a) and (b)
In a forward feed multiple effect evaporator, the pressure is
lowest in last effect
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
remain unchanged
Which of the following situations can be approximated to a steady state heat transfer system?
10 kg of dry saturated steam at 8 kgf/cm2 flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C.
Sensible heat absorbed by 1 lb of water when it is heated from 32 to 212°F may be around ____ BTU.
180
The unit of conductance in SI unit is
W/°K
In a shell and tube heat exchanger, square pitch compared to triangular pitch
gives a higher shell side pressure drop.
Erratum: should be lower
Which of the following has maximum thermal conductivity?
Iron
A wall has two layers of materials A and B; each made of a different material. Both the layers have the same thickness. The thermal conductivity of materialA is twice that of B. Under the equilibrium, the temperature difference across the wall is 36°C. The temperature difference across the layer A is ____ °C.
12
The film co-efficient between condensing vapour and metal wall increases with
increasing temperature of the vapour.
Critical value of the ____ number governs the transition from laminar to turbulent flow in free convection heat transfer.
Prandtl & Grashoff
The thermal boundary layer at NPr > 1
is thinner than hydrodynamic boundary layer.
In the free convection regime of pool boiling, the heat flux is proportional to
Δt^5/4
With increase in temperature, the thermal conductivity of fresh lubricating oil
decreases
Fresh orange juice contains 12% (by weight) solids and the rest water 90% of the fresh juice is sent to an evaporator to remove water and subsequently mixed with the remaining 10% of fresh juice. The resultant product contains 40% solids. The kg of water removed from 1 kg fresh juice is
0.7
Nusselt number is the ratio of the temperature gradient at the wall to
that across the entire pipe
The Stefan-Boltzman constant depends on the
none of these
The units of resistance to heat transfer is
W-1m2K
Thermal conductivity of a gas at low density, ____ with increase in temperature.
increases
Nusselt number is a function of Prandtl number and ____ number of fluid in natural convection heat transfer.
Grashoff
____ heat exchanger is used for chilling oil to be dewaxed.
Double pipe
It is not recommended to use a 1-2 shell and tube heat exchanger for a particular heat duty, whenever the LMTD correction factor is
< 0.75
Pick out the wrong statement.
Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature.
In sub-cooled boiling,
temperature of the heating surface is more than the boiling point of the liquid.
With increase in temperature, the thermal conductivity of a gas
increases.
A ____ evaporator employs an annular downtake.
basket type
Radiation heat losses from satisfactorily insulated high pressure boilar may be about ____ percent.
7
In natural convection heat transfer, the correlating parameter is the
Grashoff number
Boiling of milk in an open vessel is an example of ____ boiling.
film
Pick out the wrong statement.
The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1.
Mode of heat transfer involved in the cooling of air cooled internal combustion engine is
natural convection
Steady state one dimensional heat flow by conduction as given by Fourier’s low does not assume that
material is anisotropic.
The left face of a one dimensional slab of thickness 0.2 m is maintained at 80°C and the right face is exposed to air at 30°C. The thermal conductivity of the slab is 1.2 W/m.K and the heat transfer co-efficient from the right face is 10 W/m2.K. At steady state, the temperature of the right face in °C is
48.7
In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the
tube diameter
To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is
1-1 heat exchanger
Heat transfer co-efficient (h) for a fluid flowing inside a clean pipe is given by
This is valid for the value of NRe equal to
> 10000
Steam side heat transfer co-efficient in an evaporator is in the range of ____ kcal/hr.m2°C.
5000-15000
Correction is applied to LMTD for ____ flow.
cross
If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is
always between h1 and h2
One kilogram of water at 0°C is changed to superheated steam of one atm pressure and 300° C. The major heat consumption in the process will be to
evaporate the water.
For ____ Prandtl number values, the heat conduction will be negligible in the buffer zone.
high
The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is 8 kW/m2. The flux, when the element temperature is increased to 1000°C for the same furnace temperature is
16.5 kW/m2
Heat exchangers operating, when the asymptotic range is reached,
results in making part of the heating surface inactive.
In case of a shell and tube heat exchanger, the minimum and maximum baffle spacing is respectively (where, D = inside diameter of the shell)
D/5 and D
In case of heat transfer by conduction in a hollow cylinder, ____ mean area is used to calculate the heat transfer rate.
logarithmic
In a gas-liquid shell and tube heat exchanger, the
a. presence of a non-condensible gas decreases the condensing film co-efficient.
b. gases under high pressure are routed through the tube side, because high pressure gases are corrosive in nature.
c. gases to be heated/cooled is normally routed through the shell side, because the corrosion caused by the cooling water or steam condensate remain localised to the tubes.
all ‘a’, ‘b’ & ‘c’
Which is the most suitable for the concentration of foamy & frothy liquors?
Long tube vertical evaporator
Boiling point elevation of an ideal solution
increases rapidly with temperature rise.
The rate of heat transfer is a product of overall heat transfer co-efficient, the difference in temperature and the
heat transfer area
Which of the following has the lowest overall heat transfer co-efficient?
Air
Which of the following has the minimum absorptivity?
Aluminum foil
A black body when hot, emits heat radiation of ____ wavelengths.
all
A composite wall consists of two plates A and B placed in series normal to the flow of heat. The thermal conductivities are kA and kB and the specific heat capacities are CPA and CPB for plates A and B respectively. Plate B has twice the thickness of plate A. At steady state, the temperature difference across plate A is greater than that across plate B, when
kA < 0.5kB
Vibrations in the tubes of a shell and tube heat exchanger is induced due to the
a. flow of fluid on the tube and shell sides.
b. oscillations in the flow of shell/tube sides fluid.
c. vibrations transmitted through piping and/or supports due to external reasons.
all (a), (b) and (c)
Heat transfer by conduction in the turbulent core of a fluid flowing through a heated pipe is negligible, if the value of Prandtl number is
0.6
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
3.66
Sensible heat of hot industrial flue gases can not be recovered by a/an
a. economiser
b. regenerator
c. ceramic recuperator
none of these
The critical radius of insulation for a spherical shell is (where, K = thermal conductivity of insulating material h0 = heat transfer coefficient at the outer surface)
2K/h0
Radiator of an automobile engine is a ____ type of heat exchanger.
direct contact
In pipe flow, heat is transferred from hot wall to the liquid by
forced convection and conduction
The value of Stefan-Boltazman constant in SI unit is
5.6697 x 10-8W/m2.°K4
The unit of heat transfer co-efficient is
BTU/hr. ft2°F
For turbulent flow in a tube, the heat transfer co-efficient is obtained from the Dittus-Boelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer co-efficient will change by a factor of
6.1
The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 W.m-1.°C-1 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-2is
165.4
Value of Nusselt number [Nu = (hD/k)] for the heat transfer by conduction from a droplet or a spherical particle to a surrounding stagnant film is
2
Low thermal conductivity of heat insulating materials is due to its
high proportion of air space
The most conducive surface for dropwise condensation to occur is the ____ surface.
oily
In an interphase heat transfer process, the equilibrium state corresponds to equality of temperature in the two phases, while the condition for equilibrium in an interphase mass transfer process is equality of
concentrations
Calburn analogy is applicable for the value of Prandtl number from
0.6 to 120
Pick out the correct equation.
jH = (St)(Pr)^2/3 = f/2
Multiple effect evaporators ar used to
increase the steam economy & the capacity.
Prandtl number is given by
(Cp · μ)/k
Small scale evaporation is done in a
steam jacketed kettle
Heat exchanger tubes are never made of
lead
At constant temperature, the thermal conductivities of gases ____ with rise in pressure.
decrease
