Heat Transfer (Set 2) 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.
What is the steam economy in case of a single effect evaporator system?
<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 = 2hl
jH factor for heat transfer is not a function of the ____ number.
both (b) & (c)
Nusselt & Grashoff
The Fourier number (defined as a.t/L2) is used in the analysis of problem involving heat transfer by
transient conduction
The energy radiated from a surface Q at absolute temperature T is related as
Q ∝ T^4
Baffles are provided in heat exchangers to increase the
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/m2.s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid. The interfacial area available for drying is 4 m2/1000 kg of dry solid. The moisture content at the end of the drying period is (in kg moisture/kg dry solid)
0.1
In case of a vertical tube evaporator, with increase in the liquor level, the
capacity of the evaporator is decreased.
The ratio of total radiating power to the absorptivity of the body depends upon the ____ as per Kirchoffs law.
temperature of the body
For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally
decreases
A 2-4 heat exchanger involves
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
With increase in temperature, the thermal conductivity of steel
decreases
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids?
Sieder-Tate equation
The main function of baffles provided in a shell and tube heat exchanger is to
enhance turbulence.
In a single effect evaporator, the economy is
< 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
<1
Which of the following is not concerned with the heat transfer?
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
In counter flow compared to parallel flow,
a. LMTD is greater.
b. less surface area is required for a given heat transfer rate.
both (a) and (b).
The thermal radiative flux from a surface of emissivity = 0.4 is 22.68 kW/m2. The approximate surface temperature (K) is (Stefan-Boltzman constant = 5.67xl0-8 W/m2.K4)
1000
Crystallisation of solids from a homogeneous solution is a/an ____ process.
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
24000
Value of Prandtl number for water ranges from
5 to 10
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by
conduction
Among liquids, water has a comparatively high value of thermal conductivity, due to its
partial ionisation
Reynold’s analogy states that (where, St = Stanton number f = friction factor)
St = f/2
For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is
backward
The sum of reflectivity and absorptivity for an opaque body is equal to
1
The film thickness for laminar film condensation on vertical surface ____ from top to bottom.
cummulatively increases
In case of a multipass shell and tube heat exchanger, providing a baffle on the shell side ____ the heat transfer rate.
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
A hot body will radiate heat most rapidly, if its surface is
black & rough
Evaporation of 1kg of water from a solution in a single effect evaporator requires about ____ kg of steam.
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/m2.K. The Biot number is
0.025
Analogy between mass and heat transfer is not applicable in case of
a. same velocity profile or equal eddy diffusivities.
b. thermal or pressure mass diffusion.
c. viscous heating or chemical reaction.
both (b) and (c).
A ____ surface has the maximum thermal emissivity out of the following.
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^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
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
0.45
The purpose of providing a ‘catchall’ in the vapor line of an evaporator is to
arrest the entrained liquid
What is the thermal conductivity of a perfect heat insulator?
zero
Fouling factor’ used in the design of a multipass shell and tube heat exchanger is a
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^-0.2
Prandtl and Reynolds analogy are same, when Prandtl number is
1
Heat transfer occurs by natural convection because change in temperature causes difference in
density
The ratio of kinematic viscosity to thermal diffusivity is called the ____ number.
Prandtl
The critical radius ‘r’ of insulation on a pipe is given by
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.
If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid
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 white (with aluminium paint).
____ paint has the maximum absorption coefficient.
Black
What is Nusselt number?
hD/k
Steam traps are provided in steam carrying pipelines to
remove condensate and inert gases.
The Dietus-Boelter equation for convective leat transfer
https://www.indiabix.com/_files/images/chemical-engineering/heat-transfer/245-165-1.png
cannot be used for (See picture for equation)
all (a), (b) and (c)
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
Radiation heat transfer rates does not depend upon the
a. type of absorbing surface.
b. distance between the heat source and the object receiving the heat.
c. surface area and temperature of the heat source.
none of these.
Which of the following is correct?
Driving force = Rate x Resistance
Convective heat transfer, in which heat is transferred by movement of warmed matter is described by
Newton’s law of cooling
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
goes through a minimum.
Heat transfer rate per unit area is called
heat flux
Log mean temperature difference (LMTD) cannot be used, if
a. heat transfer co-efficient over the entire heat exchanger is not constant.
b. there exists an unsteady state.
c. the heat capacity is not constant and there is a phase change.
none of these.
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
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
zero
For what value of Prandtl number, the Col-burn analogy is valid?
0.6 to 120
In a forward feed multiple effect, the pressure build up will be
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.
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
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.k2
Film condensation is promoted on a/an ____ surface.
clean & smooth
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
Prandtl number
If Prandtl number is greater than the Schmidt number, then the
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?
Catchall
Rate of heat transfer by vaporisation from pools of water is affected by the
a. nature of heating surface and distribution of bubbles.
b. surface tension of water.
c. viscosity of water.
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 S
Which of the following has the highest thermal conductivity?
Silver
Which of the following is unimportant in forced convection?
Grashoff 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 shell side.
In a multiple effect evaporator, the effect of boiling point elevation is to
reduce the capacity
The rate of heat transfer through a pipe wall is given by, https://www.indiabix.com/_files/images/chemical-engineering/heat-transfer/271-499-1.png . For cylinder of very thin wall, q can be approximated by
https://www.indiabix.com/_files/images/chemical-engineering/heat-transfer/271-499-5.png
Dropwise condensation occurs on ____ surfaces.
contaminated cooling
For specified tube outside diameter, higher BWG means higher
cross-sectional area
Pick out the wrong statement.
Swenson-Walker crystalliser is a batch crystalliser.
In case of a vertical tube evaporator, with increase in the liquor level, the ____ is increased.
neither (a) and (b)
Electromagnetic radiations propagate in vacuum with a velocity of ____ metre/second.
3 x l08
Heat flux is the time rate of heat transfer per unit
area
Film boiling occurs at ____ pressure.
very high
Heat flux through several resistances in series in analogous to the current flowing through several
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
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.
Double pipe heat exchangers are used
when heat transfer area required is very low, i.e (100-200 ft2).
Nusselt number (for forced convection heat transfer) is a function of the ____ number.
both (a) & (b)
Pick out the wrong statement.
Heat transfer by radiation can not occur across an absolute volume.
Which type of heat exchanger is preferred for heavy heat loads ?Which type of heat exchanger is preferred for heavy heat loads?
Series and parallel set of shell and tube
Pick out the wrong statement.
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
between 0 and 1
