Heat Transfer (2) Flashcards
Black liquor generated during paper manufacture is concentrated in a
multiple effect evaporator.
Reynold’s analogy states that
Nst α f
When vaporisation takes place directly at the heating surface, it is called
nucleate boiling
Shell side pressure drop in a shell and tube heat exchanger does not depend upon the
none of these.
With increase in porosity, the thermal conductivity of a solid substance
decreases
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
34.4
Fouling factor
accounts for additional resistances to heat flow.
The steam ejector is used to
create vacuum.
For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area
passes through a maxima
Convective heat transfer co-efficient in case of fluid flowing in tubes is not affected by the tube length/diameter ratio, if the flow is in the __________ zone.
highly turbulent
Presence of a non-condensing gas in a condensing vapour
none of these.
In case of evaporators, liquid entrainment results primarily due to
foaming of the solution.
Baffles in the shell side of a shell and tube heat exchanger
increase the shell side heat transfer co-efficient.
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 outside and steam inside.
Economy of a multiple effect evaporator is not influenced much by the
boiling point elevations
Pick out the wrong statement.
The equivalent diameter of heat transfer for a duct of square cross-section (having each side as ‘x’) is equal to 4x.
Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.
the largest
Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is
mainly confined to a thin film of fluid near the surface.
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
less than that of the uninsulated steam pipe.
The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is
Schmidt number.
The absorptivity of a grey body at a given temperature __________ with increasing wavelength of radiation.
may increase or decrease ; depends on the material
Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid
for fluids in tubulent flow.
Out of the following four assumptions used in the derivation of the equation for LMTD , which one is subject to the largest deviation in practice ?
Constant rate of fluid flow.
Heat flux increases with temperature drop beyond the Leiden frost point in the plot of heat flux vs. temperature drop for a boiling liquid, because
radiation becomes important
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
increases the overall heat transfer coefficient.
Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)
0.2 D
It is not preferable to use superheated steam in evaporators, because of its very
low film co-efficient
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).
The range of electromagnetic spectrum important in heat transfer by radiation is __________ microns.
0.5-50
Economy of a multiple effect evaporator depends upon the
heat balance consideration
The Graetz number is concerned with the
heat transfer in laminar flow.
Solid angle subtended by the finite surface at the radiating element is
called the angle of vision.
Pick out the wrong statement.
The emissivity of a surface decreases, if it gets corroded by atmospheric environment.
With increase in temperature, the thermal conductivity of most liquids
decreases
Heat transfer by __________ may not necessarily require the presence of a medium.
radiation
In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used
in high range of temperature differences.
The film co-efficient is decreased due to the presence of non-condensing gases in the vapors. The film co-efficient of superheated vapor as compared to that of saturated vapor is
some
An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a
heat exchanger
Planck’s distribution law is valid for __________ bodies.
coloured
For hot feed, forward feeding as compared to backward feeding results in __________ economy.
increased
Baffles are provided in heat exchangers to increase the
heat transfer co-efficient
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids ?
Sieder-Tate equation
Which of the following is not concerned with the heat transfer?
Schmidt number
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
<1
Among liquids, water has a comparatively high value of thermal conductivity, due to its
partial ionisation
The film thickness for laminar film condensation on vertical surface __________ from top to bottom.
cummulatively increases
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
∝ 1/t
Heat transfer co-efficient (h1) for liquids increases with
increasing temperature.
A diathermaneous substance __________ the thermal radiation completely.
transmits
Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.)
d»2
Mechanical recompression evaporation is used in the production of
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.
Brinkman
The purpose of providing a ‘catchall’ in the vapor line of an evaporator is to
arrest the entrained liquid
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»-0.2
Heat transfer occurs by natural convection because change in temperature causes difference in
density
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
straight line
Convective heat transfer, in which heat is transferred by movement of warmed matter is described by
Newton’s law of cooling
Which of the following is correct ?
Driving force = Rate x Resistance
Log mean temperature difference (LMTD) cannot be used, if
none of these.
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
goes through a minimum.
In a forward feed multiple effect, the pressure build up will be
least at the outlet of the last effect.
For what value of Prandtl number, the Col-burn analogy is valid ?
0.6 to 120
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
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
first increase and then decrease
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
1
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
Prandtl number
Multiple effect evaporators are commonly used in the manufacture of
P. Paper Q. Superphosphate
R. Sugar S. Fats
P and S
Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid ?
Catchall
Which of the following is unimportant in forced convection ?
Grashhoff number
In a multiple effect evaporator, the effect of boiling point elevation is to
reduce the capacity
Dropwise condensation occurs on __________ surfaces.
contaminated cooling
For specified tube outside diameter, higher BWG means higher
cross-sectional area
Electromagnetic radiations propagate in vacuum with a velocity of __________ metre/second.
3 x l0^8
Film boiling occurs at __________ pressure.
very high
Pick out the wrong statement.
Heat transfer by radiation can not occur across an absolute volume.
The Grashoff number is defined as the ratio of the
buoyancy to viscous forces.
The absorptivity of a body is equal to its emissivity
under thermal equilibrium.
Pick out the wrong statement.
S.I. unit of fouling factor is Watt/m2.°K.
A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300°C and allowed to cool in natural air. Which one will cool the slowest ?
Sphere
Vent pipes are provided in a condenser to
remove non-condensable gases.
In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
pool
Trap is used to remove __________ from steam pipe lines.
condensate
Tube pitch is the __________ of tube diameters and the clearances.
sum
Natural convection is characterised by
Grashhoff number
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
< 1
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe )
K/h0
The critical radius ‘r’ of insulation on a pipe is given by
r = k/h
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
(ΔT)^3
A body is called grey if the monochromatic emissivity of the body is
same for all wavelengths
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.
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 temperature follows __________ law.
linear
__________ equation relates the thermal conductivity of a solid to its temperature.
Lee’s
For small temperature difference, the heat transfer rate as per Newton’s law of cooling is proportional to (where, Δt = excess temperature)
Δt
Pick out the wrong statement.
Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.
