Heat Transfer Flashcards
Crystal size in a continuous crystallizer depends upon the
- rate of heat transfer.
- degree of turbulence.
- degree of supersaturation.
- all (a), (b) and (c).
all (a), (b) and (c).
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.
Black liquor generated during paper manufacture is concentrated in a
- single effect evaporator.
- single effect evaporator followed by a crystalliser.
- multiple effect evaporator.
- multiple effect evaporators followed by a crystalliser.
multiple effect evaporator.
When vaporisation takes place directly at the heating surface, it is called
- film boiling
- nucleate boiling
- vapour binding
- none of these
nucleate boiling
Steam consumption in kg/hr in case of an evaporator is given by (where, C & E are capacity and the economy of the evaporator respectively)
- C/E
- E/C
- CE
- 1/CE
C/E
Reynold’s analogy states that
- Nst α f
- Nst α NRe
- N/Nu α f
- NRe α f
Nst α f
Nusselt number concerns about heat transfer characteristics
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
true ans: decreases
Fourier’s law applies to the heat transfer by
- convection
- radiation
- conduction
- all (a), (b) & (c)
conduction
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.
In the equation Q = UAΔt; Δt is
- geometric mean temperature difference.
- arithmetic mean temperature difference.
- logarithmic mean temperature difference.
- the difference of average bulk temperatures of hot and cold fluids.
logarithmic mean temperature difference.
With increase in porosity, the thermal conductivity of a solid substance
- increases
- decreases
- remains unchanged
- may increase or decrease; depends on the solid
decreases
For an ideal black body
- absorptivity = 1
- reflectivity = 1
- emissivity = 0
- transmissivity = 1
absorptivity = 1
In Joule’s experiment, an insulated container contains 20 kg of water initially at 25°C. It is stirred by an agitator, which is made to turn by a slowly falling body weighing 40 kg through a height of 4 m. The process is repeated 500 times. The acceleration due to gravity is 9.8 ms^-2. Neglecting the heat capacity of agitator, the temperature of water (in °C) is
- 40.5
- 34.4
- 26.8
- 25
34.4
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.
In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient
- increases
- decreases
- is not affected
- may increase or decrease; depends on the feed
decreases
The steam ejector is used to
- remove condensate from the - steam pipelines.
- create vacuum.
- superheat the steam.
- none of these.
create vacuum.
Steam ejectors are widely used to create vacuum by using high-pressure steam. They operate on the Venturi effect, where the steam expands through a nozzle, creating a low-pressure zone. This suction effect pulls gases or vapors from a system, making them useful in vacuum distillation, condenser evacuation in power plants, and other industrial processes.
For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area
- increases
- decreases
- remains constant
- passes through a maxima
passes through a maxima
true ans: decreases
The thermal efficiency of a reversible heat engine operating between two given thermal reservoirs is 0.4. The device is used either as a refrigerator or as a heat pump between the same reservoirs. Then the coefficient of performance as a refrigerator (COP)R and the co-efficient of performance as a heat pump (COP)HP are
- (COP)R = (COP)HP = 0.6
- (COP)R = 2.5; (COP)HP = 1.5
- (COP)R = 1.5; (COP)HP = 2.5
- (COP)R = (COP)HP = 2.5
(COP)R = 1.5; (COP)HP = 2.5
Hot water (0.01 m^3 /min) enters the tube side of a counter current shell and tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m^3/min) of density 800 kg/m^3 and specific heat of 2 kJ/kg.K enters at 20°C. The log mean temperature difference in °C is approximately
- 32
- 37
- 45
- 50
32
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.
- The Leidenfrost point occurs when a liquid comes into contact with a surface that is significantly hotter than its boiling point.
- At this point, the liquid forms a vapor layer between itself and the surface, which reduces direct contact and slows down heat transfer. As a result, the liquid appears to “levitate” on the surface, gliding on the vapor film.
- This phenomenon is often observed with water droplets on a very hot pan.
A process stream of dilute aqueous solution flowing at the rate of 10 Kg.s^-1 is to be heated. Steam condensate at 95°C is available for heating purpose, also at a rate of 10 Kg.s^-1. A 1 - 1 shell and tube heat exchanger is available. The best arrangement is
- counter flow with process stream on shell side.
- counter flow with process stream on tube side.
- parallel flow with process stream on shell side.
- parallel flow with process stream on tube side.
counter flow with process stream on shell side.
Heat produced when a steady state current, I passes through an electrical conductor having resistance, ‘R’ is
- IR
- I^2 R
- IR^2
- I^2 R^2
I^2 R
Pick out the wrong statement.
- Orifice baffles are never used in a shell and tube heat exchanger.
- Pressure drop on the shell side of a heat exchanger depends upon tube pitch also.
- In a horizontal tube evaporator, surface blanketing by air is avoided.
- Split ring type and pull through type floating heads are two commonly used floating heads is heat exchangers.
Orifice baffles are never used in a shell and tube heat exchanger.
The actual temperature drop across the heating surface in an evaporator depends on the
- feed.
- depth of liquid over heating surface.
- pressure difference between steam chest and vapour space.
- all (a), (b) and (c).
all (a), (b) and (c).
Which of the following is not used as a medium for high temperature heating ?
- Dowtherm
- mercury
- liquid metal (e.g. molten sodium)
- fused salts (e.g., an eutectic mixture of 53% KNO3, 40% NaNO2and 7% NaNO3)
mercury
- None of the listed options are entirely unsuitable for high-temperature heating, but among them, mercury might be considered less favorable due to its toxicity and environmental issues. However, all listed options can serve as high-temperature heating mediums in specific applications.
Presence of a non-condensing gas in a condensing vapour
- increases the rate of condensation.
- decreases thermal resistance.
- is desirable to increase the film co-efficient.
- none of these.
none of these.
- The presence of a non-condensing gas in a condensing vapor leads to decreased condensation rates, increased thermal resistance, and a lower film heat transfer coefficient.
With the increase of temperature, the Col-burn jH factor
- increases.
- decreases.
- remains unchanged.
- may increase or decrease ; depending on temperature.
increases
- While the Colburn jH factor can behave differently depending on the specific fluid and conditions, in many cases, increasing temperature tends to enhance the jH factor due to improved flow characteristics and thermal properties of the fluid. This results in increased convective heat transfer performance, leading to better efficiency in heating or cooling applications.
In case of evaporators, liquid entrainment results primarily due to
- high vacuum in the evaporator.
- high evaporation rate.
- foaming of the solution.
- high heat transfer rate.
foaming of the solution.
- foaming of the solution plays a crucial role in liquid entrainment during the evaporation process. The formation and stability of foam lead to the creation of larger vapor bubbles that can carry liquid droplets into the vapor phase, resulting in entrainment. Managing foam formation is essential for optimizing evaporator performance and minimizing unwanted liquid carryover.
Baffles in the shell side of a shell and tube heat exchanger
- increase the cross-section of the shell side liquid.
- force the liquid to flow parallel to the bank.
- increase the shell side heat transfer co-efficient.
- decrease the shell side heat transfer co-efficient.
increase the shell side heat transfer co-efficient.
- Baffles enhance heat transfer by promoting turbulent flow and increasing the interaction between the shell side fluid and the tube surfaces, leading to a more efficient heat exchange process.
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.
The unit of heat transfer co-efficient in SI unit is
- J/M^2°K
- W/m^2°K
- W/m°K
- J/m°K
W/m^2°K
- This unit indicates that heat transfer occurs in watts (energy per unit time) per square meter of area for each degree Kelvin (or Celsius) temperature difference. This is the standard unit for the heat transfer coefficient in SI units.
The number of kg vaporised per kg of steam fed to the evaporator is defined as
- capacity
- rate of evaporation
- economy
- rate of vaporisation
economy
- In the context of evaporators, economy is defined as the mass of liquid that is vaporized (or evaporated) per unit mass of steam fed into the evaporator. This is an important performance indicator, as it reflects the efficiency of the evaporator in utilizing the steam to vaporize liquid.
Pick out the wrong statement.
- In case of heat transfer by purely forced convection, GR/Re2 ≤ 1.
- The equivalent diameter of heat transfer for a duct of square cross-section (having each side as ‘x’) is equal to 4x.
- Distillation process is not the same as evaporation.
- The effectiveness of nucleate boiling depends basically on the ease with which the bubbles are formed and detached from the heating surface.
The equivalent diameter of heat transfer for a duct of square cross-section (having each side as ‘x’) is equal to 4x.
Economy of a multiple effect evaporator is not influenced much by the
- boiling point elevations
- temperature of the feed
- rate of heat transfer
- ratio of the weight of the thin liquor to thick liquor
boiling point elevations
- while boiling point elevation can affect the operation of a multiple effect evaporator by requiring more energy to achieve the boiling point, its overall impact on the economy is mitigated by the efficient design of multiple effect systems. The ability of the evaporator to reuse vapor for heating in subsequent effects allows it to maintain a favorable economy, making boiling point elevation less critical in comparison to other factors influencing evaporator performance.
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.
- Increasing the liquor level in an evaporator can lead to decreased capacity. A higher liquor level means more liquid is present, which can increase the hydrostatic pressure at the evaporator’s bottom. This added pressure can reduce the effective driving force for heat transfer (the temperature difference between the heating medium and the boiling liquid), resulting in decreased evaporation rates and capacity.
The equivalent diameter for pressure drop is ___ that for heat transfer.
- smaller than
- greater than
- equal to
- not related with
smaller than
Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.
- no
- the least
- the largest
- lower
the largest
- the statement that a controlling heat transfer film coefficient offers the largest resistance to heat transfer is false. Instead, an effective (or controlling) film coefficient results in the least resistance to heat transfer, allowing for optimal thermal performance in heat exchangers and other systems involving heat transfer.
The heat flux (from outside to inside) across an insulating wall with thermal conductivity, K= 0.04 W/m.°K and thickness 0.16m is 10 W/m^2. The temperature of the inside wall is - 5°C. The outside wall temperature is
- 25°C
- 30°C
- 35°C
- 40°C
35°C
Pick out the wrong statement.
- The controlling resistance in case of heating of air by condensing steam is in the air film.
- The log mean temperature difference (LMTD) for counter flow and parallel flow can be theoretically same when any one of the fluids (hot or cold fluid) passes through the heat exchanger at constant temperature.
- In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.
- Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice-versa occurs at saturation temperature.
In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.
- This statement is misleading. In a 1-2 shell and tube heat exchanger, a temperature cross occurs when the outlet temperature of the cold fluid exceeds the outlet temperature of the hot fluid. This scenario usually leads to a reduction in the LMTD correction factor because it indicates that the heat exchanger is not operating efficiently, and the actual temperature differences are compromised. The correction factor does not increase sharply; rather, it can significantly decrease or become ineffective.
The average heat transfer co-efficient for laminar film condensation on vertical surface is inversely proportional to (where, ΔT = Temperature drop across condensate film )
- (ΔT)^2
- (ΔT)^1/2
- (ΔT)^1/4
- (ΔT)^3/2
(ΔT)^1/4
Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is
- in the central core of the fluid.
- uniformly distributed throughout the fluid.
- mainly confined to a thin film of fluid near the surface.
- none of these.
mainly confined to a thin film of fluid near the surface.
- The majority of the convective heat transfer resistance occurs in a thin film of fluid close to the hot surface. This is because the thermal boundary layer, which develops due to the no-slip condition at the surface, has a higher temperature gradient, leading to increased resistance. As the distance from the surface increases, the fluid temperature approaches that of the bulk fluid more quickly, reducing resistance.
Which of the following is concerned with both heat and mass transfer ?
- Lewis relationship
- Nusselt number
- Kutateladze number
- Froude number
Lewis relationship
A 10 cm dia steam pipe, carrying steam at 180°C, is covered with an insulation (conductivity = 0.6 W/m.°C). It losses heat to the surroundings at 30°C. Assume a heat transfer co-efficient of 0.8 W/m2.°C for heat transfer from surface to the surroundings. Neglect wall resistance of the pipe and film resistance of steam. If the insulation thickness is 2 cms, the rate of heat loss from this insulated pipe will be
- greater than that for uninsulated steam pipe.
- less than that of the uninsulated steam pipe.
- equal to that of the uninsulated steam pipe.
- less than the steam pipe with 5 cms insulation.
less than that of the uninsulated steam pipe.
Which area is used in case of heat flow by conduction through a cylinder?
- Logarithmic mean area
- Arithmetic mean area
- Geometric mean area
- None of these
Logarithmic mean area
Heat transfer by conduction results due to the transfer of free electrons, kinetic energy & vibrational energy from one molecule to another. Conduction heat transfer can not take place
- between two bodies in physical contact with each other.
- between two bodies not in physical contact with each other.
- from one part of a body to the another part of the same body.
- both ‘b’ & ‘c’
between two bodies not in physical contact with each other.
- Heat transfer by conduction requires physical contact between the bodies. When two bodies are in contact, energy is transferred through free electrons and molecular interactions.
The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is
- Nusselt number.
- Sherwood number.
- Schmidt number.
- Stanton number.
Schmidt number.
Finned tube heat exchangers
- give larger area per tube.
- use metal fins of low thermal conductivity.
- facilitate very large temperature drop through tube wall.
- are used for smaller heat load.
give larger area per tube.
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
Which one gives the monochromatic emissive power for black body radiation ?
- Planck’s law
- Kirchhoffs law
- Wien’s law
- Stefan-Boltzman law
Planck’s law
For what value of Prandtl number, St = f/2?
- 1.5
- 1
- > 1
- < 1
1
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.
Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid
- for fluids in laminar flow.
- for fluids in tubulent flow.
- when Grashhoff number is very important.
- for liquid metals.
for fluids in tubulent flow.
The equation, Nst = (f/2) / [1+5 (NPr - 1) (f/2)^1/2], corrosponds to __________ analogy.
- Von-Karmann
- Reynolds
- Colburn
- Prandtl
Prandtl
For large heat transfer area requirement, shell and tube heat exchanger is preferred, because it
- occupies smaller space.
- is more economical.
- is easy to operate and maintain.
- all (a), (b) and (c).
all (a), (b) and (c).
Out of the following four assumptions used in the derivation of the equation for LMTD [LMTD = (Δt1 - Δt2) / ln(Δt1/Δt2)] , which one is subject to the largest deviation in practice ?
- Constant overall heat transfer co-efficient.
- Constant rate of fluid flow.
- Constant specific heat.
- No partial phase change in the system.
Constant rate of fluid flow.
true ans: Constant overall heat transfer co-efficient.
Heat flux increases with temperature drop beyond the Leiden frost point in the plot of heat flux vs. temperature drop for a boiling liquid, because
- convection becomes important
- conduction becomes important
- radiation becomes important
- sub-cooled boiling occurs
radiation becomes important
- While radiation is a fundamental mode of heat transfer, it does not significantly influence the increase in heat flux beyond the Leidenfrost point in boiling liquids. The increase is primarily due to the transition to sub-cooled boiling, enhanced convection, and the collapse of the vapor layer, which allows for improved thermal contact between the liquid and the surface. Thus, the correct explanation for the increase in heat flux is not related to radiation becoming important.
Resistance to heat flow by conduction is proportional to (where, t & ρ are thickness & density of the material respectively and A = area normal to the direction of heat flow. )
- t
- 1/ρ
- 1/A
- all (a), (b) & (c)
all (a), (b) & (c)
Temperature profile in steady state heat transfer is
- asymptotic
- hyperbolic
- parabolic
- linear
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
- decreases the pressure drop.
- decreases the outlet temperature of cooling medium.
- increases the overall heat transfer coefficient.
- none of these.
increases the overall heat transfer coefficient.
When vaporisation takes place through a blanketting film of gas, the phenomenon is termed as __________ boiling.
- pool
- nucleate
- transition
- film
film
Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)
- 0.2 D
- 0.5 D
- 0.66 D
- 0.80 D
0.2 D
Stefan-Boltzman law which describes the radiation heat transfer states that, it is proportional to (where, t = temperature in °C; T = absolute temperature in ° K )
- t^4
- T^4
- 1/t^4
- 1/T^4
T^4
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe, where the wall heat flux is constant, is
- 2.36
- 4.36
- 120.36
- dependent on NRe only
4.36
What is the unit of thermal conductivity ?
- Kcal/hr. m^2 °C
- Kcal/hr.m.°C
- Kcal/hr.m
- Kcal/hr. °C
Kcal/hr.m.°C
The thermal radiation emitted by a body is proportional to Tn, where T is its absolute temperature. The value of ‘n’ is exactly 4 for
- black painted bodies only
- all bodies
- polished bodies only
- a black body
all bodies
true ans: a black body
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
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
- 3.06 x 10^5
- 6.12 x 10^5
- 7.24 x 10^5
- 9.08 x 10^5
3.06 x 10^5
LMTD for counterflow and prallel flow heat exchanger will be the same, when the
- cold fluid is heated to a certain temperature by condensing steam (isothermal fluid).
- outlet temperature of both the hot and cold fluid are same.
- outlet temperature of hot fluid is less than the outlet temperature of the cold fluid.
- none of these.
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
- NPr^(1/3)
- NPr^(-1)
- NPr^(-1/3)
NPr^(1/3)
The range of electromagnetic spectrum important in heat transfer by radiation is __________ microns.
- 0.38-0.78
- 0.5-50
- 100-1000
- 5-50
0.5-50
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)
Economy of a multiple effect evaporator depends upon the
- heat balance consideration
- rate of heat transfer
- both (a) and (b)
- neither (a) nor (b)
heat balance consideration
The Graetz number is concerned with the
- mass transfer between a gas and a liquid.
- absorption with chemical reaction.
- heat transfer in turbulent flow.
- heat transfer in laminar flow.
heat transfer in laminar flow.
Solid angle subtended by the finite surface at the radiating element is
- called the view factor.
- called the angle of vision.
- proportional to the square of the distance between surfaces.
- expressed in terms of radians.
called the angle of vision.
true ans: called the view factor
Pick out the wrong statement.
- The emissivity of a surface decreases, if it gets corroded by atmospheric environment.
- The emissivity of a surface increases with increase in surface roughness.
- The emissivity of a polished surface is quite low.
- The emissivity of a non-metallic surface decreases with increase in the temperature.
The emissivity of a surface decreases, if it gets corroded by atmospheric environment.
In forced convection, the heat transfer depends on
- Re, Pr
- Re, Gr
- mainly Gr
- Re only
Re, Pr
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
true ans: increases
Heat transfer by __________ may not necessarily require the presence of a medium.
- conduction
- natural convection
- forced convection
- radiation
radiation
Pick out the correct statement.
- Higher is the temperature of the radiating body, higher is the wavelength of radiation.
- Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder.
- The wavelength corresponding to maximum mono-chromatic emissive power increases with rise in temperature.
- Solid angle subtended by the finite surface at the radiating element is called the angle of incidence.
Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder.
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
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
- more
- less
- some
- either more or less ; depends on the nature of vapor
some
true ans: less
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
- more than
- less than
- either more or less than (depending upon the fluid)
equal to
true ans: more than
An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a
- boiler
- heat exchanger
- recuperator
- regenerator
heat exchanger
Which of the following is directly concerned with the heat transfer ?
- Strouhal number
- Sherwood number
- Euler number
- Grashoff number
Grashoff number
Pick out the wrong statement:
With change in temperature, the radiant energy emitted by a black body remains unchanged.
Absorptivity of a body approaches unity in case of diffuse reflection.
Absorptivity of a perfectly black body is unity.
Value of Stefan-Boltzman constant is 4.876 x 10^-8 KCal/m^2.hr.°K^4.
With change in temperature, the radiant energy emitted by a black body remains unchanged.
Planck’s distribution law is valid for __________ bodies.
- black
- white
- coloured
- all (a), (b) & (c)
coloured
true ans: black
For hot feed, forward feeding as compared to backward feeding results in __________ economy.
- increased
- decreased
- no effect on
- none of these
increased
Radiation energy is emitted by all the substances, which are above
- 0K
- 0°C
- 100°C
- room temperature
0K
The interchange factor for radiation heat transfer from surface ‘x’ to surface ‘y’ in case of an infinite parallel planes with emis-sivities εx & εy is given by
- εx + εy
- εx . εy
- 1/εx + 1/εy
- εx+εy / εx+εy - εx * εy
εx+εy / εx+εy - εx * εy
What is the steam economy in case of a single effect evaporator system ?
- 1
- > 1
- < 1
- 0.1
< 1
If average heat transfer co-efficient is ha and the local coefficient at the end of the plate is hl then in case of heat transfer to a fluid flowing over a flat plate, heated over its entire length
- ha = hl
- ha = 2hl
- ha = 0.5 hl
- ha = 0.75 hl
ha = 2hl
The Fourier number (defined as a.t/L^2) is used in the analysis of problem involving heat transfer by
- forced convection
- natural convection
- transient conduction
- steady state conduction
transient conduction
The energy radiated from a surface Q at absolute temperature T is related as
- Q ∝ T^2
- Q ∝ T^4
- Q ∝ T^3
- none of these
Q ∝ T^4
Baffles are provided in heat exchangers to increase the
- fouling factor
- heat transfer area
- heat transfer co-efficient
- heat transfer rate
heat transfer co-efficient
200 kg of solids (on dry basis) is subjected to a drying process for a period of 5000 seconds. The drying occurs in the constant rate period with the drying rate as, Nc = 0.5 x 10^-3 kg/m^2.s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid. The interfacial area available for drying is 4 m^2/1000 kg of dry solid. The moisture content at the end of the drying period is (in kg moisture/kg dry solid)
- 0.5
- 0.05
- 0.1
- 0.15
0.1
In case of a vertical tube evaporator, with increase in the liquor level, the
- capacity of the evaporator is decreased.
- capacity of the evaporator is increased.
- true temperature drop increases.
- both (b) and (c).
capacity of the evaporator is decreased.
true ans: both (b) and (c).
The ratio of total radiating power to the absorptivity of the body depends upon the __________ as per Kirchoffs law.
- wavelength of radiation
- nature of the body
- temperature of the body
- none of these
temperature of the body
For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally
- decreases
- increases
- remains unchanged
- increases exponentially
decreases
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.
In a liquid-liquid heat exchanger, for the same process temperature, the ratio of the LMTD in parallel flow to the LMTD in counter flow is always
- < 1
- > 1
- 1
- ∞
< 1
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 a single effect evaporator, the economy is
- 1
- < 1
- > 1
- none of these
< 1
LMTD correction factor which is to be applied for a cross-flow heat exchanger increases with increase in the number of shell passes. Its value for a single pass cross flow heat exchanger is
- 0
- 1
- > 1
- <1
<1
Which of the following is not concerned with the heat transfer?
- Brinkman number
- Stanton number
- Schmidt number
- Peclet number
Schmidt number
Latent heat absorbed by 1 lb of water at 212°F, when it is changed to steam at 212°F, may be around __________ BTU.
- 180
- 970
- 3.97
- none of these
180
true ans: 970
The thermal radiative flux from a surface of emissivity = 0.4 is 22.68 kW/m^2. The approximate surface temperature (K) is
(Stefan-Boltzman constant = 5.67x10^-8 W/m^2.K^4)
- 1000
- 727
- 800
- 1200
1000
Crystallisation of solids from a homogeneous solution is a/an __________ process.
- exothermic
- mildly endothermic
- highly endothermic
- none of these
exothermic
A multiple effect evaporator has a capacity to process 4000 kg of solid caustic soda per day, when it is concentrating from 10% to 25% solids. The water evaporated in kg per day is
- 6000
- 24000
- 60000
- 48000
24000
Value of Prandtl number for water ranges from
- 1 to 2
- 5 to 10
- 100 to 500
- 1000 to 2000
5 to 10
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by
- eddies current
- conduction
- convection
- none of these
conduction
Among liquids, water has a comparatively high value of thermal conductivity, due to its
- low density
- high viscosity
- partial ionisation
- dense structure
partial ionisation
true ans: dense structure
Reynold’s analogy states that (where, St = Stanton number f = friction factor)
- St = f/2
- St = f/4
- St = 4f
- St = f^1/2
St = f/2
For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is
- forward
- backward
- parallel
- none of these
backward
The sum of reflectivity and absorptivity for an opaque body is equal to
- 0.5
- 1
- 0
- 2
1
The film thickness for laminar film condensation on vertical surface __________ from top to bottom.
- cumulatively increases
- cumulatively decreases
- remains constant
- and the surface conductance increase
cumulatively increases
In case of a multipass shell and tube heat exchanger, providing a baffle on the shell side __________ the heat transfer rate.
- increases
- decreases
- does not affect
- may increase or decrease, depends on the type of baffle
increases
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
- ∝ t
- ∝ 1/t
- ∝ (t)^1/2
- independent of ‘t’
∝ 1/t
Heat transfer co-efficient (h1) for liquids increases with
- increasing temperature.
- decreasing temperature.
- decreasing Reynolds number.
- none of these.
increasing temperature.
A diathermaneous substance __________ the thermal radiation completely.
- absorbs
- reflects
- transmits
- none of these
transmits
A hot body will radiate heat most rapidly, if its surface is
- white & rough
- black & rough
- white & polished
- black & polished
black & rough
Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.
- 0.4 - 0.6
- 1-1.3
- 1.8-2
- 2 - 2.4
1-1.3
A metal wire of 0.01 m dia and thermal conductivity 200 W/m.K is exposed to a fluid stream with a convective heat transfer coefficient of 100 W/m^2.K. The Biot number is
- 5.6
- 0.025
- 3.5
- 0.0035
0.025
A __________ surface has the maximum thermal emissivity out of the following.
- black & smooth
- black & rough
- white & smooth
- white & rough
black & rough
Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.)
- d^1/2
- d
- d^2
- d^1.5
d^2
Mechanical recompression evaporation is used in the production of
- alcohol
- distilled water
- salt
- fruits jam
distilled water
A measure of the extent to which viscous heating is important relative to the heat flow resulting from the impressed temperature difference is represented by the __________ number.
- Condensation
- Grashoff
- Stantan
- Brinkman
Brinkman
Out of 100 kcal/second of incident radiant energy on the surface of a thermally transparent body, 300 kcal/second is reflected back. If the transmissivity of the body is 0.25, the emissivity of the surface will be
- 0.35
- 0.45
- 0.55
- 0.85
0.45
The purpose of providing a ‘catchall’ in the vapor line of an evaporator is to
- create vacuum
- regulate the vapor flow
- vent the non-condensible gases
- arrest the entrained liquid
arrest the entrained liquid
What is the thermal conductivity of a perfect heat insulator ?
- zero
- one
- ∞
- between 0 and ∞
zero
‘Fouling factor’ used in the design of a multipass shell and tube heat exchanger is a
- non-dimensional factor.
- factor of safety.
- conversion factor for individual film heat transfer co-efficient to overall heat transfer co-efficient.
- none of these.
factor of safety.
A fluid is flowing inside the inner tube of a double pipe heat exchanger with diameter ‘d’. For a fixed mass flow rate, the tube side heat transfer co-efficient for turbulent flow conditions is proportional to
- d^0.8
- d^-0.2
- d^-1
- d^-1.8
d^-0.2
Prandtl and Reynolds analogy are same, when Prandtl number is
- 0.5
- 1
- > 2
- 1.5
1
Heat transfer occurs by natural convection because change in temperature causes difference in
- viscosity
- density
- thermal conductivity
- heat capacity
density
The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.
- Peclet
- Prandtl
- Stanton
- Nusselt
Prandtl
The critical radius ‘r’ of insulation on a pipe is given by
- r = 2k/h
- r = k/h
- r = k/2h
- r = h/k
r = k/h
As per Kirchoffs law, the ratio of the total radiating power to the absorptivity of a body depends on the
- temperature of the body only.
- wavelength of monochromatic radiation.
- both (a) and (b).
- nature of material of body.
temperature of the body only.
true ans: both (a) and (b).
If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid
- remains unchanged.
- increases.
- increases or decreases depending on number of shell passes.
- decreases.
decreases.
The heat transfer by radiation from a mild steel surface is to be reduced by reducing the emissivity of the surface. This can be best achieved by
- painting the surface black.
- painting the surface white (with aluminium paint).
- giving the surface a mirror finish.
- roughening the surface.
painting the surface white (with aluminium paint).
__________ paint has the maximum absorption coefficient.
- Black
- Yellow
- White
- Grey
Black
What is Nusselt number ?
- Cp*μ / k
- hD / k
- h*CP / μ
- Cp*μ / h
hD / k
Steam traps are provided in steam carrying pipelines to
- condense steam.
- release excess steam pressure by bleeding steam.
- remove condensate and inert gases.
- none of these.
remove condensate and inert gases.
The Dietus-Boelter equation for convective heat transfer [(i.e.) h=0.023 (K/D)(Re)^0.8 (Pr)^0.4]
cannot be used for
- low Reynold’s number
- very low Grashoff number
- molten metals
- all (a), (b) and (c)
all (a), (b) and (c)
true ans: molten metals
A hot liquid is kept in a big room. The logorithm of the numerical value of the temperature difference between the liquid and the room is plotted against time. The plot will be very nearly a/an
- ellipse
- straight line
- parabola
- circular arc
straight line
Radiation heat transfer rates does not depend upon the
- type of absorbing surface.
- distance between the heat source and the object receiving the heat.
- surface area and temperature of the heat source.
- none of these.
none of these.
true ans: distance between the heat source and the object receiving the heat.
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
Convective heat transfer, in which heat is transferred by movement of warmed matter is described by
- Fourier’s law
- Newton’s law of cooling
- Fick’s law
- none of these
Newton’s law of cooling
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
- continues to increase.
- continues to decrease.
- goes through a minimum.
- goes through a maximum.
goes through a minimum.
Heat transfer rate per unit area is called
- thermal conductivity
- heat flux
- heat transfer co-efficient
- thermal diffusivity
heat flux
Log mean temperature difference (LMTD) cannot be used, if
- heat transfer co-efficient over the entire heat exchanger is not constant.
- there exists an unsteady state.
- the heat capacity is not constant and there is a phase change.
- none of these.
none of these.
true ans: Log mean temperature difference (LMTD) cannot be used, if
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
- twice
- four times
- square root of
- square of
four times
For a given ambient air temperature with increase in the thickness of insulation of a hot cylinderical pipe, the rate of heat loss from the surface would
- decrease
- increase
- first decrease and then increase
- first increase and then decrease
first increase and then decrease
true ans: first decrease and then increase
In a parallel flow heat exchanger, if the outlet temperature of hot and cold fluids are the same, then the log mean temperature difference (LMTD) is
- minimum
- maximum
- zero
- infinity
zero
For what value of Prandtl number, the Col-burn analogy is valid ?
- 0.06 to 120
- 0.6 to 120
- 1 to 103
- l to 50
0.6 to 120
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.
Film boiling is usually not desired in commercial equipments, because
- the heat transfer rate is low in view of the large temperature drop.
- it is difficult to maintain.
- it is not economic.
- none of these.
the heat transfer rate is low in view of the large temperature drop.
For the same heat transfer area and the terminal conditions, the ratio of the capacities of a single effect evaporator to a triple effect evaporator is
- 3
- 0.33
- 1
- 1.33
1
The variation of thermal conductivity of a metal with temperature is often correlated using an expression of the form K = K0 + at. where, K is the thermal conductivity and T is the temperature (in °K). The units of ‘a’ in SI system will be
- W/m.k
- W/m
- W/m.k^2
- none, ‘a’ is just a number
W/m.k^2
Film condensation is promoted on a/an __________ surface.
- oily
- coated
- clean & smooth
- dirty
clean & smooth
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
- Prandtl number
- Grashoff number
- thermal conductivity
- viscosity
Prandtl number
true ans: viscosity
If Prandtl number is greater than the Schmidt number, then the
- thermal boundary layer lies inside the concentration boundary layer.
- concentration boundary layer lies inside the thermal boundary layer.
- thermal & concentration boundary layers are of equal thickness.
- hydrodynamic(i.e., momentum)boundary layer is thicker than the other two.
thermal boundary layer lies inside the concentration boundary layer.
Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid ?
- Bleed point
- Vent
- Catchall
- Baffle
Catchall
Rate of heat transfer by vaporisation from pools of water is affected by the
- nature of heating surface and distribution of bubbles.
- surface tension of water.
- viscosity of water.
- all (a), (b) and (c).
all (a), (b) and (c).
Multiple effect evaporators are commonly used in the manufacture of
P. Paper
Q. Superphosphate
R. Sugar
S. Fats
- P and Q
- P and R
- P and S
- R and S
P and S
Which of the following has the highest thermal conductivity ?
- Brick
- Air
- Water
- Silver
Silver
Which of the following is unimportant in forced convection ?
- Reynolds number
- Prandtl number
- Grashhoff number
- none of these
Grashhoff number
Steam is to be condensed in a shell and tube heat exchanger, 5 m long with a shell diameter of 1 m. Cooling water is to be used for removing the heat. Heat transfer co-efficient for the cooling water, whether on shell side or tube side is the same. The best arrangement is
- vertical heat exchanger with steam on tube side.
- vertical heat exchanger with steam on shell side.
- horizontal heat exchanger with steam on tube side.
- horizontal heat exchanger with steam on shell side.
vertical heat exchanger with steam on shell side.
In a multiple effect evaporator, the effect of boiling point elevation is to
- reduce the capacity
- reduce the economy
- increase the economy
- none of these
reduce the capacity
- The rate of heat transfer through a pipe wall is given by, q= [2nK(Ti - T0)] / [ln(ri/r0)] . For cylinder of very thin wall, q can be approximated by
Dropwise condensation occurs on __________ surfaces.
- clean and dirt free
- smooth clean
- contaminated cooling
- polished
contaminated cooling
true ans: smooth clean
For specified tube outside diameter, higher BWG means higher
- tube thickness
- cross-sectional area
- weight per unit length
- none of these
cross-sectional area
true ans: none of these
Pick out the wrong statement.
- Swenson-Walker crystalliser is a batch crystalliser.
- Super saturation of the solution is the driving potential for a crystal growth.
- The liquor left after the removal of crystals from a solution is called mother liquor.
- The first stage of crystal formation is called nucleation.
Swenson-Walker crystalliser is a batch crystalliser.
- note: all choices are correct
Electromagnetic radiations propagate in vacuum with a velocity of __________ metre/second.
- 3 x 10^5
- 3 x 10^8
- 3 x 10^10
- 3 x 10^12
3 x 10^8
Heat flux is the time rate of heat transfer per unit
- length
- area
- volume
- none of these
area
Film boiling occurs at __________ pressure.
- atmospheric
- sub-atmospheric
- negative
- very high
very high
Heat flux through several resistances in series in analogous to the current flowing through several
- resistances in parallel.
- capacitors in series.
- resistances in series.
- none of these.
resistances in series.
Extended heat transfer surface like fins are used to increase the heat transfer rate. Fin efficiency is defined as the ratio of heat transferred across the fin surface to the theoretical heat transfer across an equal area held at the
- surrounding temperature.
- average temperature of the fin.
- temperature of the fin end.
- constant temperature equal to that of the base
constant temperature equal to that of the base
Vacuum is generally maintained in the vapour space of an evaporator mainly to
- get economical temperature difference by using moderate pressure steam.
- facilitate forward feeding in multiple effect evaporation.
- concentrate heat sensitive materials.
- achieve very high concentration of the final product.
get economical temperature difference by using moderate pressure steam.
true ans: concentrate heat sensitive materials.
Double pipe heat exchangers are used
- when heat transfer area required is very high.
- when heat transfer area required is very low, i.e (100-200 ft^2).
- because it occupies less floor area.
- because it is less costly.
when heat transfer area required is very low, i.e (100-200 ft^2).
Pick out the wrong statement.
- Heat transfer by radiation can not occur across an absolute volume.
- In case of a shell and tube heat exchanger, the pressure drop through the shell is proportional to the number of times the fluid crosses the bundle between baffles.
- Propagation velocity for travel of heat radiation through vacuum is equal to the velocity of the light.
- The amount of heat involved in the condensation or vaporisation of 1 kg of a fluid is the same.
Heat transfer by radiation can not occur across an absolute volume.
Which type of heat exchanger is preferred for heavy heat loads ?
- Double pipe
- Plate fine
- Series and parallel set of shell and tube
- None of these
Series and parallel set of shell and tube
Pick out the wrong statement.
- Reciprocal of the resistance to heat flow is called thermal conductance.
- Unit of thermal conductance is W/°K.
- Thermal conductance of a wall of thickness ‘L’, thermal conductivity ‘k’ and heat flow area ‘A’ is kL/A.
- none of these.
Thermal conductance of a wall of thickness ‘L’, thermal conductivity ‘k’ and heat flow area ‘A’ is kL/A.
While the total emissivity of a perfect black body is unity, the same for a real body is
- 0
- 1
- > 1
- between 0 and 1
between 0 and 1
The Grashoff number is defined as the ratio of the
- buoyancy to inertial forces.
- buoyancy to viscous forces.
- inertial to viscous forces.
- buoyancy to surface tension forces.
buoyancy to viscous forces.
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
- 2f
- f
- f/2
- f/4
f/2
The absorptivity of a body is equal to its emissivity
- at a particular temperature.
- for circular bodies.
- under thermal equilibrium.
- none of these.
under thermal equilibrium.
Absorptivity and refletivity of a perfect black body are respectively
- 1 and 0
- 0 and 1
- 1 and ∞
- 0 and 0.5
1 and 0
In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
- α = 1; ε ≠ 1
- α ≠ 1, ε = 1
- α ≠ 1, ε ≠ 1
- α = 1; ε = 1
α = 1; ε = 1
Pick out the wrong statement.
- Superheated steam is preferably not used for process heating because of its low heat transfer film co-efficient.
- In a shell and tube heat exchanger, the shell pressure drop is maximum for orifice baffles.
- S.I. unit of fouling factor is Watt/m2.°K.
- Longitudinal fins are used in extended surface heat exchangers, when the direction of fluid flow is parallel to the axis of the tube.
S.I. unit of fouling factor is Watt/m2.°K.
When warm and cold liquids are mixed, the heat transfer is mainly by
- conduction
- convection
- radiation
- both(a) & (c)
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 ?
- Cube
- Plate
- Sphere
- All will cool at the same rate
Sphere
Vent pipes are provided in a condenser to
- remove non-condensable gases.
- purge the condenser.
- facilitate easy cleaning of tubes.
- none of these.
remove non-condensable gases.
Heat transfer by natural convection is enhanced in system with
- high viscosity.
- high co-efficient of thermal expansion.
- low temperature gradients.
- low density change with temperature.
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.
- nucleate
- pool
- low pressure
- none of these
pool
true ans: none of these
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
- > 10
- <10
- either (b) or (c), depends on the mass of the body
> 10
In forced convection, the Nusselt number is a function of
- Re and Pr
- Re and Gr
- Pr and Gr
- Re and Sc
Re and Pr
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical ?
- 0.5
- 1
- 1.5
- 80
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 heat transfer area of all the effects is -less than that in a single effect evaporator system.
- total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
- boiling point elevation in a single effect system is much higher than that in any effect in a multieffect system.
- heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system.
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.
- enhancing the fluid pumping rate.
- increasing the tube length.
- none of these.
keeping the heat transfer surface clean.
Trap is used to remove __________ from steam pipe lines.
- steam
- condensate
- non-condensables
- none of these
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
- 125
- 133
- 150
- 160
160
Tube pitch is the __________ of tube diameters and the clearances.
- sum
- difference
- ratio
- none of these
sum
With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
- decreases
- increases
- increases exponentially
- remains unaffected
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.
- more
- less
- same
- unpredictable
less
Natural convection is characterised by
- Grashhoff number
- Peclet number
- Reynolds number
- Prandtl number
Grashhoff number
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
- 0.72
- 4.364
- 18
- 83
4.364
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
- 1
- < 1
- > 1
- > 5
< 1
true ans: >1
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe )
- K/h0
- 2K/h0
- hi/K
- 2hi/K
K/h0
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
- (ΔT)^2
- (ΔT)^4
- (ΔT)^3
- (ΔT)
(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
- equal to the difference between hot and cold fluids inlet temperature.
- equal to the difference between hot fluid inlet temperature and cold fluid outlet temperature.
0
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 an extended surface heat exchanger, fluid having lower co-efficient
- flows through the tube.
- flows outside the tubes.
- can flow either inside or outside the tubes.
- should not be used as it gives very high pressure drop.
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.
- zero.
- same as average heat transfer co-efficient for tube side.
- none of these.
infinity.
true ans: none of these
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.
- viscous
- elastic
- inertial
- none of these
viscous
Pick out the wrong statement.
- By increasing the number of shell passes, the temperature cross in a shell and tube heat exchanger can be prevented.
- An underdesigned steam trap will back up the condensate instead of discharging it out.
- Steam condensate is corrosive because of the presence of dissolved oxygen in it.
- Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.
Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.
Graetz number is given by
- mCp / kL
- kL/mCp
- mCp/kμ
- kL/mCp
mCp / kL
For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in
- increased economy
- lower capacity
- decreased economy
- no effect on economy
increased economy
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.
- hyperbolic
- parabolic
- linear
- logarithmic
linear
In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred
- through a metallic wall.
- by direct contact of hot flue gas with air.
- by heating an intermediate material (like chequor bricks) and then heating the air from this hot material.
- none of these.
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.
Antonie
Kopp’s
Lee’s
Kistyakowski
Lee’s
true ans: Kopp’s
For small temperature difference, the heat transfer rate as per Newton’s law of cooling is proportional to (where, Δt = excess temperature)
- Δt
- Δt^2
- Δt^3
- (Δt)^1/2
Δt
Pick out the wrong statement.
- The condensing film co-efficient is about 3 times lower for vertical condenser as compared to the equivalent horizontal condenser for identical situation.
- Film co-efficient for vaporisation decreases as a result of vapor binding.
- In industrial practice, sub-cooling of condensate is required, when the condensate is a volatile liquid and is to be transferred for storage.
- Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.
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.
- open pan
- long tube
- agitated film
- none of these
agitated film
1 BTU/hr.ft.°F is equal to __________ kcal/hr. m.°C.
- 1.49
- 1
- 4.88
- none of these
1.49
Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on
- T1 - T2
- T1 - Ta
- T2 - Ta
- none of these
none of these
The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
- Prandtl
- Nusselt
- Stanton
- Grashoff
Prandtl
For a perfectly transparent surface (like gases), the
- absorptivity = 0
- transmissivity = 1
- reflectivity = 0
- all (a), (b) & (c)
transmissivity = 1
true ans:
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
- same
- different
- never different
- linearly related
different
During crystallisation, formation of crystal can occur in __________ solution only.
- saturated
- supersaturated
- undersaturated
- all (a), (b) and (c)
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
- 9000
- 4600
- 3000
4000
An insulator should have
- low thermal conductivity.
- high thermal conductivity.
- less resistance to heat flow.
- a porous structure.
low thermal conductivity.
In a backward feed multiple effect evaporator
- feed is introduced in the first effect.
- feed flows from low pressure to high pressure.
- no pumps are required between successive effects.
- none of these.
feed flows from low pressure to high pressure.
Choose the correct equation.
- Nu = (Re) (Pr) (Gz)
- Nu = (Re) (Pr) (St)
- Nu = (Re)(Pr)
- Nu = (Pr)(St)
Nu = (Re) (Pr) (St)
In a forward feed multiple effect evaporator unit
- viscosity of liquid is highest in first effect.
- transfer from effect to effect is done by pumps.
- no pump is required to withdraw the product from the last effect.
- none of these.
none of these.
true ans: In a forward feed multiple effect evaporator unit
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
For heat flow through very thick walled cylinder, use____mean radius.
- arithmetic
- logarithmic
- geometric
- either (a) or (c)
logarithmic
Indirect contact heat exchangers are preferred over direct contact heat exchangers, because
- heat transfer co-efficient are high.
- there is no risk of contamination.
- there is no mist formation.
- cost of equipment is lower.
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
- low density
- low viscosity
- high fluidity
abundance & high heat capacity
The rate of emission of radiation by a body does not depend upon the
- wavelength of radiation.
- surface temperature of the body.
- nature of the surface.
- shape and porosity of the body.
shape and porosity of the body.
Which of the following has maximum thermal conductivity at the same temperature?
- Steel
- Petrol
- Air
- All have the same conductivity
Steel
Maximum heat transfer rate is obtained in __________ flow.
- laminar
- turbulent
- creeping
- transition region
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.
- nucleate
- local
- pool
- saturated
local
The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is
- 2
- 1/2
- 4
- 1/4
1/4
Terminal point temperature differences between fluids in case of a heat exchanger is termed as
- approach.
- log mean temperature difference.
- arithmetic mean temperature difference.
- geometric mean temperature difference.
approach.
LMTD can’t be used as such without a correction factor for the
- multipass heat exchanger.
- baffled heat exchanger.
- condensation of mixed vapour in a condenser.
- all (a) (b) and (c).
all (a) (b) and (c).
For cold viscous feed, backward feed gives __________ than forward feed.
- a higher capacity
- a lower capacity
- lower economy
- none of these
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 ?
- 35.5
- 43.75
- 81.25
- 48.25
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
- U2 > U1
- U1 > U2
- U1 = dirt factor - U2
U1 = U2
In SI units, thermal conductivity is expressed in
- Watt/m.°K
- Watt/m^2. °K
- Watt/m^3. °K
- Watt/m^4. °K
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)
- viscosity
- density
- thermal conductivity
boiling point (at the same pressure)
The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances.
- average
- geometric mean
- product
- sum
sum
At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe ?
- 0.5
- < 0.5
- > 0.6
- < 0.1
> 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
- 0
- R2/(R1+R2)
- 1
- (R1/R2)^2
R2/(R1+R2)
true ans: 1
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
- 1.26 h1
- 0.794 h1
- 0.574 h1
- 1.741 h1
0.794 h1
The main purpose of providing fins on heat transfer surface is to increase the
- temperature gradient.
- mechanical strength of the equipment.
- heat transfer area.
- heat transfer co-efficient.
heat transfer area.
A BTU/hr.ft.2 °F is equal to
- 1 kcal/hr. m^2°C.
- 4.88 kcal/hr. m^2°C.
- 1 kcal/hr. m^2.°K.
- none of these.
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)
- Water
- Transformer oil
- Dilute H2SO4
Molten sodium (a coolant used in fast breeder reactor)
In SI units, fouling factor is expressed in
- m^2°K/W
- W/m^2°K
- m2°K
- m°K/W
m^2°K/W
An ejector is used to
- increase pressure
- increase temperature
- remove condensate
- none of these
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
- less corrosion problems.
- flexibility possible in the baffle arrangement.
- low pressure drop.
- high heat transfer co-efficient.
flexibility possible in the baffle arrangement.
All analogy equations connecting friction factor and heat transfer co-efficient apply only to
- wall or skin friction
- form friction
- both (a) and (b)
- turbulent flow
wall or skin friction
Electro-magnetic spectrum range, which is important for radiation varies from __________ microns.
- 1 to 100
- 0.5 to 50
- 10 to 100
- 100 to 1000
0.5 to 50
The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are
- Re, Gz
- Nu, Pr
- Nu, Pr, Re
- Nu, Gz
Nu, Gz
Heat transfer rate described by Fourier’s law will decrease, if the __________ increases.
- thermal conductivity
- thickness
- temperature difference
- heat transfer area
thickness
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.
- evaporators operate under vacuum.
- evaporators operate at atmospheric pressure.
- none of these.
solution has an elevation of boiling point.
Peclet number (Pe) is given by
- Pe = Re.Pr
- Pe = Re/Pr
- Pe = Pr/Re
- Pe = Nu.Re
Pe = Re.Pr
Nusselt number is the ratio of the
- temperature gradient of the wall to that across the entire pipe.
- temperature difference to the temperature gradient at the wall.
- heat flux at the wall to that across the entire pipe.
- none of these
heat flux at the wall to that across the entire pipe.
Prandtl number is the ratio of
- momentum diffusivity to mass diffusivity.
- momentum diffusivity to thermal diffusivity.
- thermal diffusivity to mass diffusivity.
- thermal diffusivity to momentum diffusivity.
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 ?
- Tube side pressure drop and the heat transfer rate.
- Convective heat transfer co-efficient.
- Effective tube surface area for convective heat transfer.
- All (a) (b) and (c).
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?
- Long vertical evaporator
- Horizontal tube evaporator
- Agitated film evaporator
- Calenderia vertical tube evaporator
Agitated film evaporator
“The ratio of the total emissive power to the absorptivity for all bodies is same at thermal equilibrium”. This is __________ law.
- Kirchoffs
- Planck’s
- Wien’s displacement
- Stefan-Boltzman
Kirchoffs
View factor is important in heat transfer by
- steady state conduction
- natural convection
- forced convection
- radiation
radiation
The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is
- lower tube side pressure drop.
- lower shell side pressure drop.
- higher tube side heat transfer co-efficient.
- higher shell side heat transfer co-efficient.
higher tube side heat transfer co-efficient.
For a multipass shell and tube heat exchanger, the LMTD correction factor is always
- 1
- > 1
- < 1
- between 1 & 2
< 1
If heat transfer rate varies with the time, it is termed as
- forced convection
- steady state conduction
- monochromatic radiation
- none of these
none of these
Pick out the wrong statement.
- Fluid movement under the influence of buoyant forces resulting from change in density takes place in case of natural convection.
- The ratio NNu/NRe * Npr is called the Stanton number.
- The Peclet number is a measure of the ratio of energy transport by convection to that by conduction.
- 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.
- multipass fixed tube sheet
- U-tube
- single pass fixed tube sheet
- none of these
U-tube
Pick out the wrong statement.
- In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2.
- Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number.
- Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.
- Hot gases at moderate pressure are usually in the shell side of shell and tube heat exchangers. At higher pressure, however, it is customary to put gas in the tube side.
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.
- foaming
- viscous
- very thin
- corrosive
viscous
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
- hyperbolic
- linear
- logarithmic
linear
Intermittant tube cleaning is possible to be done in case of a __________ evaporator.
- basket type
- horizontal tube
- calendria
- none of these
basket type
Harmonic mean temperature difference is given by
- (ΔT1* ΔT2)^1/2
- (2 * ΔT1* ΔT2) / (ΔT1+ ΔT2)
- (2 * ΔT1* ΔT2) / (ΔT1- ΔT2)
- (ΔT1- ΔT2) / (ΔT1* ΔT2)
(2 * ΔT1* ΔT2) / (ΔT1+ ΔT2)
Arithmetic mean area can be used in heat transfer problem to calculate the heat flow by conduction through a cylinder which is
- thin walled having the value of Ao Ai/< 2.
- thick walled.
- having the value of Ao/Ai > 2.
- both (b) and (c).
thin walled having the value of Ao Ai/< 2.
Three solid objects of the same material and of equal mass-a sphere, a cylinder (length = diameter) and a cube are at 500°C initially. These are dropped in a quenching bath containing a large volume of cooling oil each attaining the bath temperature eventually. The time required for 90% change in temperature is the smallest for
- cube
- cylinder
- sphere
- equal for all the three
cube
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
50 -100 Kcal/hr.m^2.°C
50 -100 W/m^2 . °K
50 -100 BTU/hr. ft.^2°F
1000 - 1500 BTU/hr. ft.^2°F
50 -100 BTU/hr. ft.^2°F
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity ?
- aluminium foil
- refractory bricks
- iron plate
- coke breeze
coke breeze
Thermal diffusivity is given by
- k/ρCp
- ρCp/k
- Cpμ/a
- μ/hCp
k/ρCp
At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical ?
- 1
- < 1
- > 1
- none of these
1
__________ chart is known as transient heat conduction chart.
- Dirhing’s
- Heisler’s
- Mollier’s
- Cox
Dirhing’s
true ans: Heisler’s
Choose the most important factor on which the heat conducted through a wall in a unit time will depend on ?
- Thickness of the wall.
- Area of the wall perpendicular to heat flow.
- Material of the wall.
- Temperature difference between the two surfaces of the wall.
Temperature difference between the two surfaces of the wall.
Which of the following has the minimum thermal conductivity ?
- Nitrogen
- Steel
- Carbon black
- Tar
Nitrogen
In a heat exchanger, one transfer unit means
- a section of the exchanger in which change in temperature of one stream equals the average driving force in the section.
- the size of the exchanger in which heat transfer rate is 1 kcal/hr.
- both (a) and (b)
- none of these
a section of the exchanger in which change in temperature of one stream equals the average driving force in the section.
For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then
- hf = hd
- hf > hd
- hf < hd
- hf could be greater or smaller than hd
hf < hd
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.
For concentrating an aqueous solution of a material like anhydrous Na2SO4, whose solubility decreases with rise in temperature, the most suitable evaporator is a __________ evaporator.
high pressure
vacuum
backward feed
none of these
vacuum
Circulation pump is located below the evaporater to
- avoid cavitation.
- avoid frequent priming.
- create more suction head.
- none of these.
create more suction head.
The statement that “maximum wavelength of radiation is inversly proportional to the temperature” is __________ law.
- Stefan-Boltzman’s
- Planck’s
- Wien’s displacement
- none of these
Wien’s displacement
Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low
- hardness
- dissolved solids content
- suspended solids content
- all (a), (b) and(c)
all (a), (b) and(c)
At steady state the temperature variation in a plane wall, made of two different solids I & II is shown below :
The thermal conductivity of material I
is smaller than that of II.
is greater than that of II.
is equal to that of II.
can be greater than or smaller than that of II.
is smaller than that of II.
Duhring rule is important in solving problems on
- distillation
- crystallisation
- evaporation
- humidification
evaporation
In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)
- 0.25 D
- 0.75 D
- 0.50 D
- none of these
0.75 D
Multiple effect evaporation is generally recommended, when the
- large scale evaporation of liquor is needed.
- corrosive liquids are to be concentrated.
- fuel is cheaply available.
- evaporation on small scale is to be done.
large scale evaporation of liquor is needed.
The inside heat transfer co-efficient in case of turbulent flow of liquid in the tube side in a 1-2 shell and tube heat exchanger is increased by __________ times, when the number of tube passes is increased to 8.
- 2^0.8
- 4^0.8
- 4^0.4
- 2^0.4
4^0.8
__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
- Single pass, fixed tube sheet
- U-tube
- Three pass, fixed tube sheet
- none of these
U-tube
In which mode of heat transfer, the Biot number is important ?
- Transient heat conduction
- Natural convection
- Forced convection
- Radiation
Transient heat conduction
__________ paint has the minimum absorption co-efficient.
- Black
- White lead
- Grey
- Light cream
White lead
Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube
- exit-fluid temperature > wall temperature.
- exit fluid temperature < wall temperature.
- exit fluid temperature = wall temperature.
- Graetz number > 100.
exit fluid temperature = wall temperature.
1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is
- 520
- 200
- 400
- 500
400
Which is the most suitable for the con-certration of highly concentrated solution?
- Open pan evaporation
- Long tube vertical evaporator
- Agitated film evaporator
- None of these
Open pan evaporation
true ans: Which is the most suitable for the con-certration of highly concentrated solution?
Unsteady state heat conduction occurs, when
- temperature distribution is independent of time.
- temperature distribution is dependent on time.
- heat flows in one direction only.
- three dimensional heat flow is concerned.
temperature distribution is dependent on time.
What is the absorptivity of a black body ?
1
0
0.78
0.95
1
Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called
- radiation
- natural convection
- forced convection
- conduction
forced convection
Baffle spacing
- is not the same as baffle pitch.
- should be less than one fifth the diameter of the shell.
- should be less than the inside diameter of the shell.
- none of these.
should be less than one fifth the diameter of the shell.should be less than the inside diameter of the shell.
true ans: should be less than one fifth the diameter of the shell.
The advantage of backward feed multiple effect evaporators over forward feed units is that
- heat sensitive material can be handled.
- there is no additional cost of pumping.
- most concentrated liquid is at highest temperature.
- equal heat transfer co-efficients exist in various effects.
most concentrated liquid is at highest temperature.
Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface ?
- Liquid density
- Liquid viscosity
- Hot surface temperature
- Interfacial tension
Interfacial tension
Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger ?
- Corrosiveness
- Fouling characteristic
- Viscosity
- None of these
None of these
Thermal conductivities of most of the liquids __________ with rise in temperature.
- increases
- decreases
- remains unchanged
- may increase or decrease; depends on the liquid
decreases
The purpose of floating head in a heat exchanger is to
- avoid buckling of tubes.
- provide support for tubes.
- decrease the pressure drop.
- facilitate its lengthening, if needed.
avoid buckling of tubes.
true ans: facilitate its lengthening, if needed.
Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the
- maximum resistance in the series.
- sum of all resistances.
- average of all resistances.
- minimum resistance presents in the series.
sum of all resistances.
Double pipe heat exchangers are preferably useful, when
- high viscosity liquid is to be cooled.
- requirement of heat transfer area is low.
- overall heat transfer co-efficient is very high.
- a corrosive liquid is to be heated.
requirement of heat transfer area is low.
Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.
- super
- sensible
- latent
- both(b) & (c)
latent
The rate of heat transfer from a vertical plate by natural convection depends upon the temperature differences (ΔT) between wall and outside bulk. The proportionality is given as
- (ΔT)^1/4
- ΔT^1/2
- (ΔT)^5/4
- (ΔT)^3/4
(ΔT)^3/4
true ans: (ΔT)^1/4
If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor.
- increase
- decrease
- not affect
- increase the condensing film co-efficient as well as
decrease
Prandtl and Reynold’s analogies are identical for Prandtl number value of
- 0
- 0.5
- 1
- 5
1
The outlet temperature of cooling water in a heat exchanger is generally not allowed to exceed above 50°C in industrial practice mainly to avoid
- its evaporation loss
- excessive corrosion
- uneconomic LMTD
- decrease in heat exchanger efficiency
excessive corrosion
The type of liquor circulation system to be ‘ employed in evaporators (viz. natural or forced circulation) is determined mainly by the __________ of the liquid.
- viscosity
- density
- thermal conductivity
- corrosive nature
viscosity
In case of surface condensers, a straight line is obtained on plotting __________ on an ordinary graph paper.
- 1/V^(-0.8)
- V^(-0.8)
- V^(-2)
- 1/V^(-2)
1/V^(-0.8)
Thermal diffusivity is the most important in heat transfer by
- conduction
- radiation
- condensation
- natural convection
conduction
(NGr x NPr) is called the __________ number.
- Graetz
- Reyleigh
- Nusselt
- Stanton
Reyleigh
A dephlegmator is a
- total condenser
- vacuum evaporator
- partial condenser
- double pipe heat exchanger
partial condenser
A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun ?
- Yellow paint
- White paint
- Black paint
- Grey paint
Black paint
The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators.
- steam ejector
- entrainment separator
- compressor
- vacuum pump
entrainment separator
Fouling factor for a heat exchanger is given by (where, U1 = heat transfer co-efficient of dirty surface U2 = heat transfer co-efficient of clean surface)
- U1 - U2
- 1/U1 - 1/U2
- 1/U2 - 1/U1
- U2 - U1
1/U1 - 1/U2
If a single tube pass heat exchanger is converted to two pass, then for the same flow rate, the pressure drop per unit length in tube side will
- increase by 1.8 times
- decrease by 2^2
- increase by 2^1.6
- remain unchanged
increase by 2^1.6
Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the
- existence of thermal boundary layer.
- temperature gradient produced due to density difference
- buoyancy of the bubbles produced at active nucleation site.
- none of these.
none of these.
Evaporator tubes are generally
- horizontal
- vertical
- inclined
- random
vertical
true ans: horizontal
For a counter current heat exchanger with Tih = 80°C, T°c = 60°C, T°h = 50°C and Tic = 30°C, and the temperature difference between the two streams being the same everywhere along Z, the direction of flow of hot fluid. The temperature profile should satisfy
- d^2T / dZ^2 > 0
- d^2T / dZ^2 = 0
- d^2T / dZ^2 < 0
- dT / dZ =0
d^2T / dZ^2 = 0
In a boiling curve, the peak heat flux is called the __________ point.
- Nusselt
- Leidenfrost
- boiling
- burnout
burnout
Pick out the wrong statement.
- Heat transfer from a hot body to cold body by the emission of heat waves is called radiation.
- Filmwise condensation takes place on non-wettable surfaces.
- The boiling point of a solution is affected by liquid head as well as boiling point elevation.
- none of these.
Filmwise condensation takes place on non-wettable surfaces.
h.D/K is called the __________ number.
- Nusselt
- Peclet
- Rayleigh
- Grashoff
Nusselt
Prandtl number is the ratio of
- mass diffusivity to thermal diffusivity.
- momentum diffusivity to thermal diffusivity.
- thermal diffusivity to mass diffusivity.
- thermal diffusivity to momentum diffusivity.
momentum diffusivity to thermal diffusivity.
Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.)
- x/KA
- KA/x
- K/Ax
- A/Kx
KA/x
Heat flux, as defined in heat flow is analogous to __________ in electricity flow.
- current
- voltage
- resistance
- none of these
current
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)
- αT
- α 1/T
- α T^4
- independent of T
α 1/T
In a shell and tube heat exchanger, putting a longitudinal baffle across the shell, forces the shell side fluid to pass __________ through the heat exchanger.
- once
- twice
- thrice
- four times
twice
When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is
- free convection.
- entry length problem in laminar forced conduction (developing thermal boundary layer).
- mixed convection (both free and forced).
- forced convection.
mixed convection (both free and forced)
The purpose of providing bleed points in the evaporator is to
- admit the feed
- remove the product
- facilitate removal of non-condensable gases
- create vacuum
facilitate removal of non-condensable gases
Water always boils when its
- temperature reaches 100° C.
- vapour pressure equals 76 cm of Hg.
- saturated vapour pressure equals the external pressure on its surface.
- saturated vapour pressure is less than the atmospheric pressure.
saturated vapour pressure equals the external pressure on its surface.
A multiple effect evaporator as compared to a single effect evaporator of the same capacity has
- lower heat transfer area.
- lower steam economy.
- higher steam economy.
- higher solute concentration in the product.
higher steam economy.
Which is the most suitable for cold viscous feed?
- Forward feed
- Backward feed
- Mixed feed
- Parallel feed
Backward feed
Dropwise condensation is promoted on a/an __________ surface.
- glazed
- oily
- coated
- smooth
oily
At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the
- buffer zone
- turbulent core
- both (a) and (b)
- viscous sub-layer
viscous sub-layer
In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator.
- increases
- decreases
- remains constant
- may increase or decrease, depends on the vacuum
increases
(NRe * NPr) is called the __________ number
- Peclet
- Stanton
- Graetz
- none of these
Graetz
If all the conditions and dimensions are same, then the ratio of velocity through the tubes of a double pass heat exchanger to that through the single pass heat exchanger is
- 1
- 2
- 1/2
- 4
2
Boiling point elevation for a strong and concentrated solution is found by Duhring’s rule, which states that at the same pressure, the boiling point of a solution is a linear function of the __________ of pure water.
- boiling point
- dynamic viscosity
- kinematic viscosity
- density
boiling point
Rate of crystallisation does not depend upon the
- extent of supersaturation.
- turbulence within the solution.
- number and active surface area of the crystals.
- none of these.
none of these.
Stefen’s block body radiation law can also be derived from __________ law.
- Kirchoffs
- Planck’s
- Fourier’s
- none of these
Planck’s
Multipass heat exchangers are used
- because of simplicity of fabrication.
- for low heat load.
- to obtain higher heat transfer co-efficient and shorter tube.
- to reduce the pressure drop.
to obtain higher heat transfer co-efficient and shorter tube.
Condensing film co-efficient for steam on horizontal tubes ranges from 5000 to 15000 Kcal/hr.m2 .°C. Condensation of vapor is carried out inside the tube in a shell and tube heat exchanger, when the
- higher condensing film co-efficient is desired.
- condensate is corrosive in nature.
- lower pressure drop through the exchanger is desired.
- temperature of the incoming vapor is very high.
condensate is corrosive in nature.
When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids ?
- At high fluid velocity
- At low velocity
- When fluid flows past a smooth surface
- None of these
At high fluid velocity
The Sieder-Tate correlation for heat transfer in turbulent flow in pipe gives Nu α Re0.8, where, Nu is the Nusselt number and Re is the Reynolds number for the flow. Assuming that this relation is valid, the heat transfer co-efficient varies with the pipe diameter (D) as
- (D)^-1.8
- (D)^-0.2
- (D)^0.2
- (D)^1.8
(D)^-0.2
LMTD for evaporators & condensers for a given terminal parameters & set of conditions for counterflow is equal to that for parallel flow. In such heat exchangers, with one of the fluids condensing or evaporating, the surface area required is the least in the __________ flow.
- parallel
- mixed
- counterflow
- same in either ‘a’, b’ or ‘c’
same in either ‘a’, b’ or ‘c’
true ans: counterflow
Thermal diffusivity of a material
- has the unit m2 /sec.
- is defined as K/ρ . Cp.
- is the ratio of thermal conductivity to thermal capacity.
- all (a), (b) and (c).
all (a), (b) and (c).
Viscosity of gases __________ with increase in temperature.
- increase very rapidly
- increase slowly
- decrease slowly
- remain unaffected
increase slowly
What is the emissivity of a black body?
- 1
- 0
- 0.90
- 0.5
1
Steam is routed through the tube in case of a __________ evaporator.
- basket type
- horizontal tube
- short tube vertical
- long tube vertical
horizontal tube
Dropwise condensation of steam on cooling surface is promoted
- an oily surface.
- when both the steam and the tube are clean.
- only in presence of air.
- none of these.
an oily surface.
Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is
- 0.1
- > 1
- < 1
- 1
0.1
true ans: >1
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.
- directly proportional to LMTD and inversely proportional to the area.
- varies as square of the area.
- none of these.
varies directly as the area and the LMTD.
With increase in temperature, the total emissivity of conductors
- increases
- decreases
- remains same
- decreases linearly
increases
Loss of heat from untagged steam pipe to the ambient air is by
- conduction
- convection
- radiation
- all (a), (b) & (c)
all (a), (b) & (c)
Fruit juice (a heat sensitive material) can be concentrated in a __________ evaporator.
- long tube
- falling film
- high pressure
- none of these
falling film
Tubes are held between top and bottom tube sheets in Calenderia type evaporator by keeping
- both the tube sheets fixed.
- both the tube sheets floating.
- the top tube sheet floating and bottom tube sheet fixed.
- the top tube sheet fixed and the bottom tube-sheet floating.
both the tube sheets fixed.
true ans: the top tube sheet fixed and the bottom tube-sheet floating.
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
- 2^0.8
- 2^0.2
- 2^0.5
2^0.8
Heat sensitive materials can be concentrated in an evaporator employing
- vacuum
- high pressure
- high residence time
- none of these
vacuum
A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires
- less heating surface.
- more space.
- lower maintenance cost.
- none of these.
more space.
In a shell and tube heat exchanger, floating head is used for
- large temperature differentials.
- high heat transfer co-efficient.
- low pressure drop.
- less corrosion of tubes.
large temperature differentials.
Steam trap is used to
- condense the steam flowing in the pipeline.
- remove water resulting from partial condensation of steam.
- stop the supply of steam.
- none of these.
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 = 0.8hL
- ha = 2hL
- ha = hL
- ha = 5hL
ha = 2hL
true ans: 0.8hL
What is the logarithmic mean of r1 and r2 ?
- r1- r2 / ln(r1/r2)
- r1- r2 / ln(r2/r1)
- r2- r1 / ln(r1/r2)
- r1- r2 / -ln(r1/r2)
r1- r2 / ln(r1/r2)
Maximum heat transfer rate is achieved in __________ flow.
- co-current
- counter-current
- turbulent
- laminar
turbulent
Wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. This is __________ law.
- Stefan’s
- Dalton’s
- Wien’s
- Kirchoffs
Wien’s
Which of the following parameters of the fluid is not very important, while deciding its route in a shell and tube heat exchanger?
- Corrosiveness & fouling characteristics
- pressure
- viscosity
- density
density
The equation, (NSt x (NPr)^2/3) = f/2, is the __________ analogy.
- Colburn
- Reynolds
- Prandtl
- none of these.
Colburn
The driving potential for the crystal growth during crystallisation is the __________ of the solution.
- concentration
- viscosity
- super-saturation
- density
super-saturation
jH factor for heat transfer depends upon the __________ number.
Biot
Nusselt
Reynolds
Prandtl
Reynolds
Kg of liquid evaporated per hour in an evaporator is defined as its
- capacity
- economy
- steam load
- none of these
capacity
Pick out the correct statement.
- 1 kcal/hr.m.°C is equal to 1 BTU/hr. ft.°F.
- In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity.
- In unsteady state heat conduction, heat flows in the direction of temperature rise.
- In heat transfer by forced convection, Grashoff number is very important.
In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity.
true ans: 1 kcal/hr.m.°C is equal to 1 BTU/hr. ft.°F.
Open pan evaporators are preferred to be used, when the solution to be concentrated is
- scaling
- highly viscous
- corrosive
- salty
highly viscous
true ans: salty
Cp*μ / K is termed as the __________ number.
- Grashoff
- Nusselt
- Prandtl
- Stanton
Prandtl
The maximum heat transfer co-efficient from steam heating will be attained when the steam is
- supersaturated
- saturated
- wet
- none of these
saturated
In a forward feed multiple effect evaporator, the pressure is
- highest in last effect
- lowest in last effect
- same in all effects
- dependent on the number of effects
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
- increase
- decrease
- remain unchanged
- either increase or decrease; depends on the coolant flow rate
remain unchanged
true ans: decrease
Which of the following situations can be approximated to a steady state heat transfer system ?
- A red hot steel slab (having outside surface temperature as 1300°C) exposed to the atmosheric air at 35°C.
- 10 kg of dry saturated steam at 8 kgf/cm^2 flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C.
- Boiling brine kept in open vessel when the bottom surface temperature of the vessel is maintained constant at 180°C.
- A sub-cooled refrigerant liquid at 8°C flowing at the rate of 6 Kg/minute through a copper pipe exposed to atmospheric air at 35°C.
10 kg of dry saturated steam at 8 kgf/cm^2 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
- 970
- 3.97
- data insufficient, can’t be predicted
180
The unit of conductance in SI unit is
W/m
W/m2
W/°K
W/m°K
W/°K
In a shell and tube heat exchanger, square pitch compared to triangular pitch
- gives a higher shell side pressure drop.
- gives a lower shell side pressure drop.
- can pack more surface area into a shell of given diameter.
- none of these.
gives a lower shell side pressure drop.
true ans: gives a higher shell side pressure drop.
Which of the following has maximum thermal conductivity ?
- Iron
- Coal
- Nitrogen
- Tar
Iron
The film co-efficient between condensing vapour and metal wall increases with
- increasing temperature of the vapour.
- decreasing temperature of the vapour.
- increasing viscosity of the film of condensate.
- increasing temperature drop.
increasing temperature of the vapour.
Critical value of the __________ number governs the transition from laminar to turbulent flow in free convection heat transfer.
- Grashoff
- Reynolds
- both ‘a’ & ‘b’
- Prandtl & Grashoff
Prandtl & Grashoff
true ans: Grashoff
The thermal boundary layer at NPr > 1
- is thicker than hydrodynamic boundary layer.
- is thinner than hydrodynamic boundary layer.
- and the hydrodynamic boundary layer are identical.
- disappears.
is thinner than hydrodynamic boundary layer.
true ans: The thermal boundary layer at NPr > 1
In the free convection regime of pool boiling, the heat flux is proportional to
- Δt^1/2
- Δt^2
- Δt^5/4
- Δt
Δt^5/4
true ans: Δt^1/2
With increase in temperature, the thermal conductivity of fresh lubricating oil
- increases
- decreases
- remains unchanged
- may increase or decrease; depends on its composition
decreases
true ans: increases
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
- 0.4
- 0.5
- 0.6
- 0.7
0.7
Nusselt number is the ratio of the temperature gradient at the wall to
- temperature difference
- heat flux
- that across the entire pipe
- none of these
that across the entire pipe
true ans: none of these
The Stefan-Boltzman constant depends on the
- medium
- temperature
- surface
- none of these
none of these
The units of resistance to heat transfer is
- J.m^-2.K^-1
- J.m^-1.K^-1
- W.m^-2.K^-1
- W^-1m^2K
W^-1m^2K
Thermal conductivity of a gas at low density, __________ with increase in temperature.
- decreases
- increases
- remains unchanged
- may increase or decrease; depends on the gas
increases
Nusselt number is a function of Prandtl number and __________ number of fluid in natural convection heat transfer.
- Grashoff
- Biot
- Stantan
- Reynolds
Grashoff
__________ heat exchanger is used for chilling oil to be dewaxed.
U-tube
Double pipe
Fixed tube
Floating head
Double pipe
true ans: U-tube
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
< 0.75
< 0.50
< 0.25
< 0.75
Pick out the wrong statement.
- Economy of a multiple effect evaporator is not influenced by the boiling point elevation.
- Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature.
- Double pipe heat exchangers are mostly used in the field of refrigeration.
- Finned tube heat exchangers are suitable for heating air by steam.
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 less than the boiling point of the liquid.
- temperature of the heating surface is more than the boiling point of the liquid.
- bubbles from heating surface are absorbed by the mass of the liquid.
- very large vapour space is necessary.
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.
- decreases.
- remains same.
- may increase or decrease depending on the type of gas.
increases.
A __________ evaporator employs an annular downtake.
- basket type
- horizontal
- long tube vertical
- none of these
basket type
true ans: long tube vertical
Radiation heat losses from satisfactorily insulated high pressure boilar may be about __________ percent.
- 1
- 7
- 18
- 26
7
true ans: 1
In natural convection heat transfer, the correlating parameter is the
- Graetz number
- Eckert number
- Grashoff number
- Bond number
Grashoff number
Boiling of milk in an open vessel is an example of __________ boiling.
- film
- sub-cooled
- saturated nucleate
- none of these
film
true ans: saturated nucleate
Pick out the wrong statement.
- In process heat exchangers, saturated steam is preferred over the superheated steam.
- 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.
- When a vertical plate is heated in infinite air environmental under natural convection conditions, the velocity profile in air, normal to the plate, exhibits a maximum.
- A body at 925 K emits an energy of 1.42 x 10^11σW/m^2 (σ is the Stefan-Boltzman constant) in the wavelength band between 3 μm to 4 μm. The fraction of this energy in the total energy emitted over the entire wavelength range is equal to emissivity.
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
- conduction
- natural convection
- forced convection
- none of these
natural convection
true ans: forced convection
Steady state one dimensional heat flow by conduction as given by Fourier’s low does not assume that
- there is no internal heat generation.
- boundary surfaces are isothermal.
- material is anisotropic.
- constant temperature gradient exists.
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
- 77.2
- 71.2
- 63.8
- 48.7
48.7
In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the
- tube layout
- tube diameter
- tube pitch
- no. of baffles
tube diameter
To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is
- 1-2 heat exchanger
- 1-1 heat exchanger
- 3-2 heat exchanger
- 2-4 heat exchanger
1-1 heat exchanger
Heat transfer co-efficient (h) for a fluid flowing inside a clean pipe is given by
h=0.023K/D (DVρ/μ)^0.8 (Cp.μ/k)^0.4.
This is valid for the value of NRe equal to
- < 2100
- 2100-4000
- > 4000
- > 10000
- > 10000
true ans: > 4000
Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m^2°C.
- 10-50
- 100-500
- 1000-1500
- 5000-15000
5000-15000
true ans: 1000-1500
Correction is applied to LMTD for __________ flow.
- parallel
- counter
- cross
- none of these
cross
true ans: parallel
If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is
- always less than h1
- always between h1 and h2
- always higher than h2
- dependent on metal resistance
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
- heat the water from 0°C to 100°C.
- evaporate the water.
- to superheat the steam.
- data insufficient, can’t be predicted
evaporate the water.
For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.
- extremely low
- low
- high
- no
high
true ans: extremely low
The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is 8 kW/m^2. The flux, when the element temperature is increased to 1000°C for the same furnace temperature is
- 11.2 kW/m^2
- 12.0 kW/m^2
- 14.6 kW/m^2
- 16.5 kW/m^2
16.5 kW/m^2
Heat exchangers operating, when the asymptotic range is reached,
- provide very large heat transfer co-efficient.
- results in making part of the heating surface inactive.
- results in abruptly increased velocity.
- none of these.
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
- D/2 and 2 D
- D/4 and 2 D
- D and 2 D
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.
- geometric
- arithmetic
- logarithmic
- either (a), (b) or (c)
logarithmic
In a gas-liquid shell and tube heat exchanger, the
- presence of a non-condensible gas decreases the condensing film co-efficient.
- gases under high pressure are routed through the tube side, because high pressure gases are corrosive in nature.
- 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’
all ‘a’, ‘b’ & ‘c’
Which is the most suitable for the concentration of foamy & frothy liquors ?
- Agitated film evaporator
- Long tube vertical evaporator
- Open pan evaporator
- None of these
Long tube vertical evaporator
true ans: Agitated film evaporator
Boiling point elevation of an ideal solution
- increases rapidly with temperature rise.
- decreases rapidly with temperature rise
- in independent of pressure.
- both (b) and (c).
increases rapidly with temperature rise.
true ans: both (b) and (c).
The rate of heat transfer is a product of overall heat transfer co-efficient, the difference in temperature and the
- heating volume
- heat transfer area
- Nusselt number
- none of these
heat transfer area
Which of the following has the lowest overall heat transfer co-efficient ?
- Dowtherm
- Molten sodium
- Water
- Air
Air
Which of the following has the minimum absorptivity ?
- Aluminium foil
- Coal dust
- Refractory bricks
- Iron plates
Aluminium foil
A black body when hot, emits heat radiation of __________ wavelengths.
- small
- large
- all
- one fixed
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
- CPA > CPB
- CPA < CPB
- kA < 0.5kB
- kA>2 kB
kA < 0.5kB
Vibrations in the tubes of a shell and tube heat exchanger is induced due to the
- flow of fluid on the tube and shell sides.
- oscillations in the flow of shell/tube sides fluid.
- vibrations transmitted through piping and/or supports due to external reasons.
- all (a), (b) and (c)
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
- 0.2
- 0.4
- 0.6
- 0.8
0.6
true ans: 0.2
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
- 1.66
- 88.66
- 3.66
- dependent on NRe only
3.66
Sensible heat of hot industrial flue gases can not be recovered by a/an
- economiser
- regenerator
- ceramic recuperator
- none of these
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)
- K/h0
- 2K/h0
- h0/K
- h0/2K
2K/h0
true ans: K/h0
Radiator of an automobile engine is a __________ type of heat exchanger.
- co-current
- cross-current
- counter-current
- direct contact
direct contact
true ans: cross-current
Pick out the wrong statement.
- ‘Solvates’ are chemical compounds formed by solute with their solvents. When water is the solvent, then it is called a ‘hydrate’.
- In heat exchanger calculations (Δt)weighted is used in place of Δt, when it involves more than one sequence of heating or cooling i.e., desuperheating & condensation or condensation & sub-cooling.
- Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.
- In case of short tube vertical evaporators, area of central downtake is about 50 to 100% of the total tube cross-sectional area.
Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.
What is the geometric mean of two heat transfer areas A1 and A2 ?
- (A1 . A2)^1/2
- (A1 + A2)^1/2
- (1/2)(A1 . A2)^1/2
- 2(A1 . A2)^1/2
(A1 . A2)^1/2
Steam economy in case of a triple effect evaporator will be
- 1
- < 1
- > 1
- between 0 and 1
- > 1
