Heat Transfer (Set 4) Flashcards
As one of the transport phenomena, the concepts in heat transfer are crucial for design, operation, and maintenance of heat transfer equipment such as heat exchangers and insulation.
Intermittant tube cleaning is possible to be done in case of a ____ evaporator.
basket type
Harmonic mean temperature difference is given by
(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.
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
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
50 -100 BTU/hr. ft.2°F
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity?
coke breeze
Thermal diffusivity is given by
k/p.Cp
At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical?
1
Fourier’s law of heat conduction applies to ____ surfaces.
a. isothermal
b. non-isothermal
both (a) and (b)
____ chart is known as transient heat conduction chart.
Dirhing’s
Choose the most important factor on which the heat conducted through a wall in a unit time will depend on?
Temperature difference between the two surfaces of the wall.
Which of the following has the minimum thermal conductivity?
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.
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
Pick out the wrong statement:
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.
vacuum
Circulation pump is located below the evaporater to
create more suction head.
The statement that “maximum wavelength of radiation is inversly proportional to the temperature” is ____ law.
Wien’s displacement
Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low
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.
Duhring rule is important in solving problems on
evaporation
In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)
0.75 D
Multiple effect evaporation is generally recommended, when the
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.
4^0.8
____ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
U-tube
In which mode of heat transfer, the Biot number is important?
Transient heat conduction
_________ paint has the minimum absorption co-efficient.
White lead
Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube
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
400
Which is the most suitable for the con-certration of highly concentrated solution?
Open pan evaporation
Unsteady state heat conduction occurs, when
temperature distribution is dependent on time.
What is the absorptivity of a black body?
1
Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called
forced convection
Baffle spacing
should be less than the inside diameter of the shell.
Boiling point elevation of a solution of NaOH
a. increases rapidly with temperature rise.
b. is almost independent of temperature.
c. is almost independent of pressure.
both (b) and (c).
The advantage of backward feed multiple effect evaporators over forward feed units is that
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?
Interfacial tension
Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger?
None of these
Thermal conductivities of most of the liquids ____ with rise in temperature.
decreases
A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m3 and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m2.K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is
0.15
The purpose of floating head in a heat exchanger is to
avoid buckling of tubes.
1000 Kg of liquid at 30°C in a well stirred vessel has to be heated to 120°C, using immersed coils carrying condensing steam at 150°C. The area of the steam coils is 1.2 m2 and the overall heat transfer co-efficient to the liquid is 1500 W/m2.°C. Assuming negligible heat loss to the surrounding and specific heat capacity of the liquid to be 4 kJ/kg.°C, the time taken for the liquid to reach desired temperature will be
51 min
Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the
sum of all resistances.
Double pipe heat exchangers are preferably useful, when
requirement of heat transfer area is low.
Removal of ____ heat is involved in the condensation of a vapor under saturated conditions.
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)^3/4
If air (a non-condensing gas) is present in a condensing vapor stream, it will ____ the condensation rate of vapor.
decrease
Prandtl and Reynold’s analogies are identical for Prandtl number value of
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
excessive corrosion
In evaporators, lowering the feed temperature
both (a) and (b).
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
In case of surface condensers, a straight line is obtained on plotting ____ on an ordinary graph paper.
1/ ¯v^-0.8
Thermal diffusivity is the most important in heat transfer by
conduction
(NGr x NPr) is called the ____ number.
Reyleigh
A dephlegmator is a
partial condenser
A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun?
Black paint
The separation of liquid droplets from the vapor is done by a/an ____ , in the evaporators.
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)
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 2^16
A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions ? Assume that the conductive heat transfer co-efficient is infinitely large.
5 minutes
Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the
a. existence of thermal boundary layer.
b. temperature gradient produced due to density difference
c. buoyancy of the bubbles produced at active nucleation site.
none of these.
Evaporator tubes are generally
vertical
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
In a boiling curve, the peak heat flux is called the ____ point.
burnout
Pick out the wrong statement.
Filmwise condensation takes place on non-wettable surfaces.
h.D/K is called the ____ number.
Nusselt
Prandtl number is the ratio of
momentum diffusivity to thermal diffusivity
Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.)
KA/x
Heat flux, as defined in heat flow is analogous to ____ in electricity flow.
current
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)
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.
twice
When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is
mixed convection (both free and forced)
The purpose of providing bleed points in the evaporator is to
facilitate removal of non-condensable gases
Water always boils when its
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
higher steam economy
Which is the most suitable for cold viscous feed?
Backward feed
Dropwise condensation is promoted on a/an ____ surface.
oily
At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the
viscous sub-layer
In a single evaporator system, the steam economy ____ by creating vacuum in the evaporator.
increases
(NRe.NPr)(D/L)is called the ____ number
Graetz
Nucleate boiling is promoted
on rough surfaces
In forced circulation, the heating element is injected
internally
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
2
The equation, Nst = f/2, is the ____ analogy.
Reynolds
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
Rate of crystallisation does not depend upon the
a. extent of supersaturation.
b. turbulence within the solution.
c. number and active surface area of the crystals.
none of these.
Stefen’s block body radiation law can also be derived from ____ law.
Planck’s
Multipass heat exchangers are used
to obtain higher heat transfer co-efficient and shorter tube.
A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,
a. the cooling rate will be the same for the two spheres and hence the two spheres will have equal temperatures at any instant.
b. both the spheres will emit equal amount of radiation per unit time in the beginning.
c. both will absorb equal amount of radiation from the surrounding in the beginning.
both(b) & (c).
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
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
The Sieder-Tate correlation for heat transfer in turbulent flow in pipe gives Nu α Re^0.8, 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)^-0.2
A long iron rod initially at a temperature of 20°C has one end dipped in boiling water (100°C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40°C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time
t = 800 s
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.
a. parallel
b. mixed
c. counterflow
same in either ‘a’, b’ or ‘c’
Thermal diffusivity of a material
all (a), (b) and (c).
Viscosity of gases ____ with increase in temperature.
increase slowly
What is the emissivity of a black body?
1
Steam is routed through the tube in case of a ____ evaporator.
horizontal tube
Dropwise condensation of steam on cooling surface is promoted
an oily surface.
Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is
0.1
