Heat transfer Flashcards
Air is best heated with steam in a heat exchanger of
double pipe type with fin on air side.
Black liquor generated during paper manufacture is concentrated in a
multiple effect evaporator
Steam consumption in kg/hr in case of an evaporator is given by (where, C & E are capacity the economy of the evaporator respectively
C/E
Reynold’s analogy states that
Nst α f
With increase in temperature, the thermal conductivity of non-metallic amorphous solids
increases
Fourier’s law applies to the heat transfer by
conduction
In the equation Q = UAΔt; Δt is
logarithmic mean temperature difference
With increase in porosity, the thermal conductivity of a solid substance
decreases
For an ideal black body
absorptivity = 1
For every 10% increase in the excess air; the fuel consumption increases by __________ percent.
0.1
Main component of sewage gas produced during anaerobic decomposition of organic waste (by suitable bacteria) during sewage disposal is
CH4
A fuel containing carbon and carbon monoxide (but containing no hydrogen or its compounds) is burnt in pure oxygen at constant pressure. Its gross calorific value as compared to net calorific value will be
Same
Initial pressure of oxygen introduced into the ‘bomb’ of the bomb calorimeter for determination of calorific value of coal/fuel oil may be around __________ atm.
25-30
Pure carbon is completely burnt in oxygen. The flue gas analysis is 70%CO2, 20%CO and 10%O2. The percent excess oxygen used is
0
__________ of the coal is the basis for Seylor’s coal classification.
Ultimate analysis
Main constituents of benzol are
benzene, toluene & xylene.
“Overfire burning” in a furnace is a phenomenon characterized by the
burning of carbon monoxide and other incombustibles in upper zone of furnace by supplying more air.
The calorific value of ‘LPG’ (50% propane + 50% butane) is about __________ kcal/Nm3.
25, 000
petrographic constituents of coal is non-coking
Fussain
__________ has the widest inflammability limit (explosion limit) of all the gases.
Acetylene
In flue gas analysis by Orsat’s apparatus, carbon dioxide is absorbed by
potassium hydroxide.
Caking index of the coal blend used for the manufacture of metallurgical coke should be around
21
Proximate analysis of coal determines its __________ content.
moisture, volatile matter, ash & fixed carbon.
Presence of free moisture in coal is most disadvantageous during
its pulverization (as it requires more power).
The heat of combustion of a fuel
is always negative.
Abrasion index of blast furnace coke should be around __________ percent.
80
The calorific value of producer gas is around __________ kcal/Nm3
1300
The calorific value of L.D. converter gas is about __________ Kcal/Nm3
1800
Higher fuel combustion efficiency can not be achieved by
reducing Sulphur content in the fuel.
Which of the following is the maximum coal producing state in India ?
Jharkhand
Height of coke oven is limited (say maximum up to 7 metres) mainly by the
problem of uniform heating along its height.
Bunsen burner is an example of a/an __________ burner.
inside mixing/premix type
The reaction, C + CO2 2CO, taking place during coal gasification is called the __________ reaction.
Boudouard
The cooling medium used in dry quenching of coke is
nitrogen
Gobar gas is produced by the __________ of ‘gobar’ (cow dung).
fermentation
Shatter index of the coke is a measure of its
strength
When the coal is heated in absence of air, it is called its
carbonization
A carbonaceous fuel (containing no H2 or hydrocarbons) is burnt and the resulting flue gas contains 21% CO2. It means that
complete combustion of fuel has taken place.
Fusion point of coal ash generally varies from 1000 to 1700° C. Ash having fusion point less than __________ °C is liable to form clinker.
1100
Light oil whose major component is benzol, is obtained by the distillation of crude tar in the temperature range of __________ °C.
80-170
Pick the odd man out.
Wood
Which of the following has the highest heat of combustion?
C2H6
Calorific value of wood gas is about __________ kcal/Nm3.
3300
Which adsorbant is used for removing sulphur compounds (S Q, H2S, RSH etc.) removal from coke oven gas in by-products recovery plant ?
Bog iron (i.e., moist ferric hydroxide)
Temperature of coke oven gas just before entering into saturator (for recovery of NH3) is about __________ °C.
65
A sample of natural gas containing 80% methane (CH4) and rest nitrogen (N2) is burnt with 20% excess air. With 80% of the combustibles producing CO2 and the reminder going to CO, the Orsat analysis in volume percent is
CO2 : 7.42, CO : 1.86, O2 : 4.64, N2:86.02
During combustion of a gaseous fuel, the presence of a non-luminous flame is an indication of the
complete combustion
Calorific value of coke even gas produced by low temperature carbonization of coal is about __________ Kcal/Nm3.
6500
By-products recovery process from coal carbonization is termed as direct, indirect and semi direct process depending upon the method of recovery of
ammonia
Naphthalene is removed from coke oven gas by
scrubbing with wash oil (a petroleum product).
Hardgrove grindability index of four coal samples are given below. Which of them is the easiest to grind ?
100
For every 10% increase in the excess air; the fuel consumption increases by __________ percent.
0.1
Main component of sewage gas produced during anaerobic decomposition of organic waste (by suitable bacteria) during sewage disposal is
CH4
Softening temperature of coal ash is a measure of the __________ of coal.
clinkering tendency
A fuel containing carbon and carbon monoxide (but containing no hydrogen or its compounds) is burnt in pure oxygen at constant pressure. Its gross calorific value as compared to net calorific value will be
same
Presence of phosphorous in metallurgical coal
Badly affects the quality of steel.
Initial pressure of oxygen introduced into the ‘bomb’ of the bomb calorimeter for determination of calorific value of coal/fuel oil may be around __________ atm.
25-30
Pure carbon is completely burnt in oxygen. The flue gas analysis is 70%CO2, 20%CO and 10%O2. The percent excess oxygen used is
0
__________ of the coal is the basis for Seylor’s coal classification.
Ultimate analysis
If the specific heat of gaseous products of combustion of a fuel is high, the abiabatic flame temperature will be
low
The calorific value of ‘LPG’ (50% propane + 50% butane) is about __________ kcal/Nm3.
25, 000
Which of the following petrographic constituents of coal is non-coking ?
Fussain
__________ has the widest inflammability limit (explosion limit) of all the gases.
Acetylene
In flue gas analysis by Orsat’s apparatus, carbon dioxide is absorbed by
potassium hydroxide.
Caking index of the coal blend used for the manufacture of metallurgical coke should be around
21
Proximate analysis of coal determines its __________ content.
moisture, volatile matter, ash & fixed carbon.
Abrasion index of blast furnace coke should be around __________ percent.
80
Tar yield in the low temperature and high temperature carbonization of dry coal may be respectively __________ percent.
10 & 3
Cannel coal and boghead coal are the examples of
sapropelic coals
Higher fuel combustion efficiency can not be achieved by
reducing Sulphur content in the fuel.
Height of coke oven is limited (say maximum upto 7 metres) mainly by the
problem of uniform heating along its height.
Bunsen burner is an example of a/an __________ burner.
inside mixing/premix type
When steam is passed over coal resulting in the endothermic reaction represent by, C + H2O = CO + H2, it is called the __________ of coal.
gasification
Dry air requirement for burning 1 Nm3 of CO to CO2 may be around __________ Nm3.
2.4
Coal is heated in absence of air for 7 minutes at a temperature of about __________ °C, during its volatile matter determination for proximate analysis.
950
The calorific value of a gas of composition CO2 = 20%, H2 = 50%, CH4 = 30%, is y. If the composition of gas is changed to H2 = 50%, CH4 = 30%, CO2 = 10%, N2 = 10%, then the calorifice value will be
y
For which pair of the fuel gases, calorific value (C.V.) of one fuel is almost double that of the other on volume basis (i.e., kcal/Nm3 ), while the C.V. is same on weight basis (i.e., kcal/kg) ?
Propane and acetylene
If CO2 is not fully absorbed by the KOH solution meant for its absorption in the orsat apparatus, it will appear as
SO2
By-products are __________ recovered in ‘Beehive ovens’.
not at all
The sequence of absorption in flue gas analysis by Orsat’s apparatus is respectively
CO2, O2, CO
Which of the following is the most important deterrents to an extended use of pulverised coal in boiler firing ?
Ash disposal problem
Calorific value of both the solid & liquid fuels can be determined by using __________ calorimeter.
Bomb
Which of the following fertilisers is produced in the by-products recovery (from coke oven gas) plant attached to an integrated steel plant ?
Ammonium sulphate
Coals used for the generation of producer gas should have
high volatile matter content.
A coal having higher volatile matter content will necessarily have lower
ignition temperature.
Of the total tar present in raw coke oven gas, the tar recovered in primary cooler is about __________ percent.
25
Purity of coke means that, it is high in carbon and low in
volatiles
Out of the following, which is the most important parameter for the blast furnace grade coke ?
CSR & CRI
The gasification reaction represented by, C + H2O = CO + H2, is
endothermic
Insitu theory and drift theory are related to the
origin of coal.
Which of the following will generate maximum volume of product of complete combustion (Nm3/Nm3 of fuel) ?
Natural gas
Which of the following will be unsuitable for dust cleaning from flue gas at 400°C from a pulverized coal fired boiler ?
Bag filter
Emission of dense white smoke out of the chimney of a thermal power plant is an indication of the use of
too much air for combustion.
A good metallurgical coke should have very low
Sulphur & phosphorous content
Presence of force moisture in coal is most disadvantageous during its
pulverization
Main constituent of natural gas is
CH4 (upto 90%)
For the case of a fuel gas undergoing combustion with air, if the air/fuel ratio is increased, the adiabatic flame temperature will
decrease
Correct viscosity of furnace oil at the burner tip for proper atomisation is about 25 centistokes. To reduce the viscosity of high viscosity furnace oil (250 centistokes) to the correct atomisation viscosity (i.e. 25 cst), it should be preheated to about____°C.
105
Use of preheated air for combustion of fuel in the furnace, increases the
flame temperature.
Which of the following has the highest flame velocity ?
H2
Lurgi gasifier (high pressure gasifier) as compared to Kopper Totzek gasifier (atmospheric pressure gasifier) produces
both (a) and (b).
Calorific value of a typical dry anthracite coal may be around __________ Kcal/kg.
8000
The most matured coal out of the following is
semi-anthracite
A liquid/gaseous fuel containing hydrocarbons and high amount of sulphur is burnt with 40% excess air. The flue gas still contains large amount of carbon monoxide. This may be due to the
lack of thorough mixing of fuel with air.
C/H ratio is the minimum in case of
natural gas
A coal having higher volatile matter content, has lower
ignition temperature.
The calorific value of natural gas is about __________ kcal/Nm3.
10, 000
Critical Air Blast (CAB) value of coke is a direct measure of its
reactivity
The ratio of maximum adiabatic flame temperature in air to that in pure oxygen is always
<1
Fixed carbon in coal is defined as
hundred minus the percentage of volatile matter, ash and moisture.
Abel apparatus is used for those oils, whose flash points are___°F.
<120
As the C/H ratio of the fuel increases, the amount of CO2 formed on combustion __________ for the same percentage of excess air.
increases
Which of the following fuel gases has the highest calorific value ?
Liquefied petroleum gas (LPG).
Calorific value as determined by bomb calorimeter is the
higher calorific value at constant volume.
During coal carbonisation process, the conversion of semi-coke to coke is accompanied by an increase in the __________ of the mass.
density
Blast furnace gas burns with a bluish flame, because of the presence of
CO
Lignite is
can be used for gasification and steam generation.
Which of the following is not a by-product fuel?
Producer gas
Shatter index of metallurgical coke on 2 inches and 0.5 inch screen should be respectively around __________ percent.
80 and 97
Method of maintaining fires in furnace during standby periods without undue consumption of fuel is called
banking.
Coal is heated in presence of air to a temperature of about __________ °C, while determining its ash content for proximate analysis.
750
Out of the following fuels, the difference between gross and net calorific value will be minimum in case of
blast furnace gas
Which of the following gaseous fuels is likely to have the highest gross calorific value?
LPG
Which of the following is called “blue gas” ?
water gas
Caking index of a coal is a measure of its
agglutinating (binding) properties
Coking time in a commercial high temperature coal carbonisation plant is about __________ hours.
18
Commercial production of petrol from coal (as practised in a factory at Sasol in South Africa) is done by the __________ of coal.
hydrogenation
The combustion reaction, C + O2 = CO2, is
exothermic
For maximum discharge through a chimney, its height should be
equal to the height of the hot gas column producing draught.
Coal containing large quantity of __________ are difficult to wash.
inherent mineral matter
The maximum adiabatic flame temperature in air is __________ the maximum flame temperature in pure oxygen.
Lower than
Which of the following gaseous fuels has the highest net calorific value (kcal/Nm3)?
Liquified petroleum gas
Undercharging of coal in the by-product coke ovens results in
decrease in the c.v. of coke oven gas.
Which of the following petrological constitutes is responsible for bright and lustrous black band of bituminous coal ?
Vitrain
White flue gas (resembling steam) coming out of the chimney of a thermal power plant indicates that the fuel used in the boiler furnace is
coke oven gas
A gaseous fuel in order to develop luminosity on burning, must contain
hydrocarbons
The lowest temperature, at which a solid fuel produces enough vapors to support continuous combustion, is called
burning temperature
Most of the coking coals are
bituminous
Gobar gas constitutes mainly of
CH4 & CO2
Coal is pulverized before burning in large capacity boiler furnaces mainly to
ensure its complete combustion.
Which of the following is a single stage, fixed bed high pressure coal gasification process ?
Lurgi Process
Kopper-Totzek coal gasifier (installed in a coal based nitrogeneous fertiliser plant) employs a/an __________ bed gasifier.
entrained
Gas yield in the Kopper-Totzek coal gasifier is about __________ Nm3/ton coal (ash = 35%).
1500
Furnace oil consumption in a furnace for a given duty employing preheated combination air (at 300° ) may be reduced by about __________ percent as compared to the use of atmospheric combustion air.
20
The main reason for making the copper calorimeter (used in bomb calorimeter) silvery white and shining/polished is to
avoid radiation heat loss.
Bright coal
contains mainly vitrain & clarain and is generally coking.
In low temperature carbonisation of coal as compared to high temperature carbonisation __________ produced is less.
difference in gross & net calorific value of the coke oven gas
Coal tar (produced by high temperature carbonisation) is the main source of
aromatic compounds
Coke oven gas is stripped of its H2S and mercaptans content by
adsorption on bog iron bed.
When steam is passed over coal resulting in the endothermic reaction represent by, C + H2O = CO + H2, it is called the __________ of coal.
gasification
Degree of carbonization of coal during coke making can be roughly judged by the __________ of the coke produced.
volatile matter
Coal is heated in absence of air for 7 minutes at a temperature of about __________ °C, during its volatile matter determination for proximate analysis.
950
Quantity of coke oven gas produced by high temperature carbonisation of one ton of dry coal may be around __________ Nm3 .
300
The temperature at which plastic layer formation takes place during carbonisation of coal varies from __________ °C.
350 to 450
Producer gas containing least amount of tar is produced by the
partial combustion of large size (< 50 mm) coke.
A __________ evaporator employs an annular downtake.
basket type
Radiation heat losses from satisfactorily insulated high pressure boilar may be about __________ percent.
7%
In natural convection heat transfer, the correlating parameter is the
Grashoff number
Boiling of milk in an open vessel is an example of __________ boiling.
film
Pick out the wrong statement. Heat transfer
The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1.
Mode of heat transfer involved in the cooling of air cooled internal combustion engine is
natural convection
The left face of a one dimensional slab of thickness 0.2 m is maintained at 80°C and the right face is exposed to air at 30°C. The thermal conductivity of the slab is 1.2 W/m.K and the heat transfer co-efficient from the right face is 10 W/m2.K. At steady state, the temperature of the right face in °C is
48.7
In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the
tube diameter
Steady state one dimensional heat flow by conduction as given by Fourier’s low does not assume that
material is anisotropic.
To reduce the tube side pressure drop for the same flow rate, the heat exchanger recommended is
1-1 heat exchanger
Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2°C.
5000-15000
Correction is applied to LMTD for __________ flow.
cross
If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is
always between h1 and h2
For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.
high
One kilogram of water at 0°C is changed to superheated steam of one atm pressure and 300° C. The major heat consumption in the process will be to
evaporate the water.
The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is 8 kW/m2. The flux, when the element temperature is increased to 1000°C for the same furnace temperature is
16.5 kW/m2
Heat exchangers operating, when the asymptotic range is reached,
results in making part of the heating surface inactive.
In case of a shell and tube heat exchanger, the minimum and maximum baffle spacing is respectively (where, D = inside diameter of the shell)
D/5 and D
In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate.
logarithmic
In a gas-liquid shell and tube heat exchanger, the
all ‘a’, ‘b’ & ‘c’
Which is the most suitable for the concentration of foamy & frothy liquors ?
Long tube vertical evaporator
Boiling point elevation of an ideal solution
increases rapidly with temperature rise.
The rate of heat transfer is a product of overall heat transfer co-efficient, the difference in temperature and the
heat transfer area
the lowest overall heat transfer co-efficient ?
Air
has the minimum absorptivity ?
Aluminium foil
A composite wall consists of two plates A and B placed in series normal to the flow of heat. The thermal conductivities are kA and kB and the specific heat capacities are CPA and CPB for plates A and B respectively. Plate B has twice the thickness of plate A. At steady state, the temperature difference across plate A is greater than that across plate B, when
kA < 0.5kB
A black body when hot, emits heat radiation of __________ wavelengths.
all
Vibrations in the tubes of a shell and tube heat exchanger is induced due to the
all (a), (b) and (c)
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
3.66
Sensible heat of hot industrial flue gases can not be recovered by a/an
none of these
Heat transfer by conduction in the turbulent core of a fluid flowing through a heated pipe is negligible, if the value of Prandtl number is
0.6
Radiator of an automobile engine is a __________ type of heat exchanger.
direct contact
The critical radius of insulation for a spherical shell is (where, K = thermal conductivity of insulating material h0 = heat transfer coefficient at the outer surface)
2K/h0
In pipe flow, heat is transferred from hot wall to the liquid by
forced convection and conduction.
The value of Stefan-Boltazman constant in SI unit is
5.6697 x 10-8W/m2.°K4
For turbulent flow in a tube, the heat transfer co-efficient is obtained from the Dittus-Boelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer co-efficient will change by a factor of
6.1
The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 W.m-1.°C-1 respectively. The ambient air is at 30°C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m-2.°C-1. The rate of heat loss from the outer surface in W.m-2is
165.4
Value of Nusselt number [Nu = (hD/k)] for the heat transfer by conduction from a droplet or a spherical particle to a surrounding stagnant film is
2
Low thermal conductivity of heat insulating materials is due to its
high proportion of air space.
In an interphase heat transfer process, the equilibrium state corresponds to equality of temperature in the two phases, while the condition for equilibrium in an interphase mass transfer process is equality of
concentrations
The most conducive surface for dropwise condensation to occur is the __________ surface.
oily
Calburn analogy is applicable for the value of Prandtl number from
0.6 to 120
Pick out the correct equation.
jH = (St)(Pr)2/3 = f/2
Multiple effect evaporators are used to
increase the steam economy & the capacity.
Small scale evaporation is done in a
steam jacketed kettle
At constant temperature, the thermal conductivities of gases __________ with rise in pressure.
decrease
Heat exchanger tubes are never made of
lead
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.
Absorptivity and reflectivity of a perfect black body are respectively
1 and 0
Pick out the wrong statement.
S.I. unit of fouling factor is Watt/m2.°K.
When warm and cold liquids are mixed, the heat transfer is mainly by
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 ?
Sphere
Vent pipes are provided in a condenser to
remove non-condensable gases.
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
different
In a backward feed multiple effect evaporator
feed flows from low pressure to high pressure
An insulator should have
low thermal conductivity.
During crystallization, formation of crystal can occur in __________ solution only.
supersaturated
Viscous & heat sensitive liquids are concentrated in __________ evaporators.
agitated film
The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
Prandtl
When vaporization takes place directly at the heating surface , its called
NUCLEATE BOILING
Fouling factor
Accounts for additional resistance to heat flow
Steam injector is used to
Create vacuum
When vaporization takes place directly at the heating surface, it is called
nucleate boiling
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
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 = 1.5; (COP)HP = 2.5
In a heat exchanger, floating head is provided to
relieve stresses caused by thermal expansion.
Leiden frost point is a term concerned with the
boiling of a liquid on a hot surface.
Heat produced when a steady state current, I passes through an electrical conductor having resistance, ‘R’ is
IR
I2R
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
not used as a medium for high temperature heating ?
mercury
Presence of a non-condensing gas in a condensing vapour
none of these.
