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Friday, 1 March 2013
Thursday, 4 October 2012
2 Mark Question and Answers
Ohm
Sakthi
Adhiparasakthi
Engineering College
Department
of Chemical Engineering
CH2304
HEAT
TRANSFER
for
V SEMESTER B.Tech. (Chemical Engg.)
Important 2
Marks Q & A
with
PART-B QUESTION
BANK
Prepared
by:
Ms.G.Saraswathy
B.Tech.,M.E.,
Assistant Professor
Ohm sakthi
UNIT 1
HEAT TRANSFER BY CONDUCTION
1. What is the driving force for heat transfer? Why?
Temperature gradient is the driving
force for heat transfer. Only when there is a temperature difference between
the hot body and the cold body, heat will flow spontaneously from the hot body
to the cold body until thermal equilibrium is reached between the two bodies.
In other words, the temperature difference between the two bodies drives the
heat transfer between them. Hence, it is known as the driving force for heat
transfer.
Q
= Driving Force (Temp.
gradient)
--------------------------------------------
Thermal
Resistance
2. Give some examples of heat
transfer operations?
* Evaporators
* Dryers
* Furnaces
* Reactors
* Refrigerators
* Radiators
* Shell and Tube Heat Exchangers
3. Define Conduction?
Conduction is the flow of heat that
occurs either due to the exchange of energy from one molecule to another
without appreciable motion of the molecules (or) due to the motion of free
electrons if they are present.
Conduction occurs only in solids
and they should be in physical contact with each other.
Eg: Heat flow through brick wall of furnace.
4. Define Convection?
Convection is the mode of heat transfer which occurs in fluids where the
molecules are farther apart. When a fluid flows inside a duct or over a solid
body and the temperatures of the fluid and solid surface are different, heat
transfer between the fluid and the solid surface will take place. This type of
heat transfer is called convection. There are two types of convection.
(i).
Free convection – which occurs
due to buoyancy effects or density difference created by temperature
difference.
(ii). Forced convection – Created by external agencies like fan, blowers etc,
The heat transfer by convection is
always accompanied by conduction.
Eg :
Boiling of water in a vessel.
5. Define Radiation?
The mode of heat transfer called
radiation refers to the transfer of energy through space by electromagnetic
waves. Radiation is an electromagnetic phenomenon and it requires no medium. In
fact, the energy transfer by radiation is maximum when the two bodies
exchanging energy are separated by a perfect vacuum.
Eg: Transfer of heat from the Sun to
the Earth.
6.
Differentiate between forced convection & free convection.
|
Free Convection
|
Forced Convection
|
|
If the fluid motion is setup by buoyancy effects resulting from the
density variation caused by the
temperature difference in the
fluid, the mode of heat transfer is said to be free or natural convection.
It occurs naturally.
The rate of heat transfer is slow when
compared to forced convection.
Eg : Heating of water in a vessel.
|
It the fluid motion is artificially created by means
of external agency such as blower, fan etc, the mode of heat transfer is
termed as forced convection.
It is created artificially.
The rate of heat transfer is high.
Eg : Cooling of a fluid by means of over head fan.
|
7. What is LMTD?
LMTD stands for Logarithmic Mean
Temperature Difference. The usual temperature difference ∆T varies with
position in the heat exchanger along its length. So, an average ∆T has to be
used in the calculation of heat transfer rate Q. In order to estimate the
average or mean ∆T, the individual ∆T values at each and every point along the
length of the heat exchanger has to be known which is highly impractical to
measure. Instead, the logarithmic mean temperature difference which uses only
the two ∆T values at the inlet and outlet is convenient to use and also found
to approximately give the values close to experimental ones.
Rate of heat flow, Q = UA ∆Tm
Here, ∆Tm =
(∆TA - ∆TB)
------------------------ is the
LMTD value.
ln (∆TA
/ ∆TB )
where ∆TA =
Temp difference between hot and cold fluid on side ‘A’ of heat
exchanger.
and ∆TB =
Temp difference between hot and cold fluid on side ‘B’ of heat
exchanger.
The larger the LMTD, the more heat is transfer.
8. Give some examples of one dimensional steady state conduction type
heat transfer.
Heat
flow through: * brick wall of furnace.
* Metal sheet of boiler.
* Metal wall of heat exchanger tube
* Reactor surfaces.
9. State the Fourier’s law of
heat conduction?
|
|
dQ α - A. dT
----
dx
dQ = - K. A. dT
----
dx
This is the mathematical
representation of Fourier’s law of conduction.
10. Define Thermal Resistance?
Thermal resistance in heat
transfer is the ratio of driving force DT for heat transfer and the rate of
heat transfer.
Thermal
Resistance = Driving force for heat transfer (Temp.
difference)
----------------------------------------
Rate of heat transfer
∆
T ∆
T
Q =
-------- R =
---------
R
Q
The unit of thermal resistance is K /W.
The resistance of a plane wall of
thickness x is given by x / kA.
11. Define Thermal conductance?
The reciprocal of resistance
is called conductance which for heat conduction is,
1
Rate of heat
Transfer
C = -------
═ --------------------------------------
R Driving force
for heat transfer (Temp. difference)
∆ T
R =
---------
Q
Q
gives C
= ---------
∆ T
Its unit is W / K. For a plane wall of
thickness x, conductance can be written as kA / x.
12. Define Thermal conductivity?
Thermal conductivity (k) is
the quantity of heat passing through a quantity of material of unit thickness
with unit heat flow area in unit time when unit temperature difference is
maintained across the opposite faces of material.
dQ = -
k. A. dT
----
dx
- dQ .
dx
k =
----------- When A=1m2, dx=1 m, dT =1 K, then
k =dQ
A .
dT
The unit of thermal
conductivity is W / m K
13. What is meant by series of resistances ?
When a wall is formed out of
series of layers of different materials it is called composite wall (or) series
of resistances.
14. Give the equations for one-dimensional (1D) steady state heat
conduction through a plane wall, compsite plane wall, cylinder, composite
cylinder and sphere. [ NOTE: Each equation is a separate 2 mark question. Write
the equation as below and also explain the terms completely. Better to draw the
respective figures also.]
Heat conduction through plane wall:
k A ( T1 – T2
)
Q =
----------------------
x
where k is thermal conductivity, A is
area perpendicular to the direction of heat flow, x is thickness of material
through which heat flows, T1 &
T2 are the hot face and cold face surface temperatures respectively.
Heat conduction through a composite
plane wall:
DToverall
Q
=
----------------------
x1/k1 A +
x2/k2 A + …..
where x1,x2
…are the thicknesses of each successive layer of material and k1, k2 ….are the
respective thermal conductivity values of layer 1, layer 2 and so on.
Heat conduction through a cylinder :
k Am ( T1 –
T2 )
Q =
----------------------
(r2 – r1)
where, Am = 2πrmL is the
logarithmic Mean Area and rm is the logarithmic mean radius which is
=
Heat conduction through a composite
coaxial cylinder ( or ) a series of (cylinder) resistances:
DToverall
Q =
--------------------------
( r2 – r1 ) ( r3 – r2 )
----------- + -----------+….
k1 Am1 k2 Am2
Where, Am1
= 2πrm1
L
Am2 = 2π rm2 L
Heat conduction through a sphere:
Q =
4π k ( T1 – T2 )
----------------------
1 1
---- - ----
r1 r2
15. Draw the equivalent electrical circuit for heat flow through the
following systems? (Each is a separate 2 mark question. Draw the figure and
explain the analogy).
Fig
1 : One dimensional heat flow
through plane wall & its electrical Analogy / Equivalent electric circuit.
Fig 2 : Heat flow through composite
plane wall and its Electrical Equivalent.
Fig 3 :
Heat flow through Hollow cylinder
and its electrical analogy.
Fig 4 : Heat flow through a composite cylinder and
its equivalent electrical circuit / analogy.
Fig 5 : Overall heat transfer
through plane wall and equivalent electric current.
16. What is Newton’s law of
cooling ?
Newton’s law of cooling gives
the heat flow by convection. It states that “the heat flux from a solid surface
to a fluid is proportional to the temperature difference between the surface
and the fluid.”
Thus if fluid mean temp T∞ is in contact with a solid surface at a temp Tw
and if TW > T∞,
then,
Q
------
( TW – T∞ )
A
Q
------ = h ( TW – T∞ )
A
Q = h A ( TW
– T∞)
The constant of
proportionality ‘h’ is
called film heat transfer co-efficient.
17. Write a note on conduction in
fluids?
Conduction is greater only in
solids, where atoms are in constant contact. Conduction is not possible or
negligible in liquids except in liquid metals like gallium, Indium that have
very low melting points and are liquids at room temperature and gases because
the molecules are usually farther apart,
giving a lower chance of molecules colliding and passing on thermal
energy or heat.
18. How does ‘k’ vary with
temperature and how to find average ‘k’ value?
For small temperature ranges, the
thermal conductivity may be taken as constant but for large temperature ranges,
‘k’ varies linearly with temperature and the relationship is given as k = a+bT
where a & b are empirical constants
and T is the temperature.
The average k, k may
be obtained either by using arithmetic average of individual values of k at
surface temperatures T1 , T2 etc.
i.e. k =
k1 + k2
------------- ( or )
2
by calculating the
arithmetic average of temperatures
i.e. (T1 + T2)/
2 and using value of ‘k’ at that temperature.
19. Write the Analogous
parameters in flow of electricity and heat conduction?
|
Analogous Parameter
|
Electricity
|
Heat conduction
|
|
1.
Rate of flow
2.
Driving force
3.
Governing factors
4.
Resistance
|
Current (I ) amperes
Potential Voltage difference (∆ V) volts
Resistivity etc.
Electrical Resistance which is a function of resistivity &
dimensions of the conductor.
|
Heat flow ( Q ) watts
Temperature difference (∆T) Kelvin.
Thermal conductivity etc.
Thermal resistance which is a function of thermal
conductivity and dimensions of the conducting solid.
|
Ohm sakthi
UNIT 2
FILM COEFFICIENTS AND THEIR APPLICATION
1. Give examples of conduction
with heat source?
There
are situations in which heat generation occurs in a conducting medium. A few
common examples are: (i) a nuclear fuel element irradiated by high energy
neutrons to trigger nuclear fission (ii) an electrical conductor in which heat
generation occurs because of a chemical reaction etc.
The
heat generation rate may be a function of its position in the solid.
2. What do you understand by
two-dimensional steady state conduction?
In
many practical situations, real heat conduction may take place along more than
one dimension. This is especially true of systems with irregular boundaries
and/or with non-uniform temperatures along boundaries.
Such
situations may lead to two- or multi- dimensional conduction.
3. Give some common examples of
multi- dimensional conduction?
·
Heat
treatment of metallic parts of different shapes
·
Composite
bodies
·
Cooling
of IC engine blocks, chimneys, air-conditioning ducts etc.
4. Name the various methods of
analysis used in multi- dimensional heat conduction problems?
(i).
Analytical
method
(ii).
Graphical
method
(iii).
Numerical
method
(iv).
Analogical
method
5. What is an isotherm?
An
Isotherm is defined as a curve on a graph that connects points of equal temperature.
It is also called an isothermal line.
6. Define a conduction shape factor?
It is
defined as the ratio of total number of heat flow tubes in the cross section
and the number of temperature increments between T1 and T2
so that T1-T2=MDT.
7. Write the relationship between
the conduction shape factor and thermal resistance?
Conduction
shape factor, S and thermal resistance Rth are related by: Rth = 1/kS where k is
thermal conductivity of the material. Thus , the shape factor for any configuration
may be obtained after evaluating its thermal resistance.
8. Define: Transient Heat
Conduction?
A
solid body is said to be in a steady state if its temperature does not vary
with time. However, if there is an abrupt change in its surface temperature or
environment, it takes some time before the body to attain an equilibrium
temperature or a steady state.
During this
interim period, the temperature varies with time & the body is said to be
in an UNSTEADY STATE or TRANSIENT STATE.
9. Give examples of occurrence of
transient heat conduction.
Boiler
tubes, rocket nozzles, electric irons, automobile engines, cooling of IC
engines, cooling and freezing of food, heat treatment of metals by quenching,
heating and cooling of buildings etc.
10. How is the temperature field
represented in any transient heat problem?
T=f(x,y,z,t).
The solution of an unsteady state problem will be more complex than that of a
steady state one because of the presence of another variable time, t.
11. What are the types of transient heat
conduction problems?
I.
Periodic
heat flow type problems – in which the temperature varies on a regular basis.
Eg: the variation of temperature of the surface of earth during a twenty-four
hour period.
II.
Non-Periodic
heat flow type problems – in which the temperature at any point within the
system varies non-linearly with time.
Ohm sakthi
UNIT 3
CONVECTION
1. What is Dimensional -
Analysis?
Dimensional - Analysis is
a method by which we combine the independent variables of a problem into
dimensionless groups and the experimental data can be very conveniently
expressed in terms of these dimensionless numbers.
2. State the Buckingham’s
theorem?
It states that, “The number of
independent dimensionless groups that can be formed from a set of ‘n’ variables
having ‘r’ basic dimensions is (n-r).”
This theorem is useful in
determining the number of independent dimensionless groups that can be obtained
from a set of variables.
3. Define Hydrodynamic boundary layer (or) velocity boundary layer?
When a fluid moves over a solid
surface, the fluid particles at the surface of the solid have the velocity approximately
zero. The transition from zero velocity at the surface of the solid to the free
stream velocity at some distance away from the solid surface in the direction
normal to the direction of flow takes
place in a very thin layer called “ Velocity ( or) Hydrodynamic boundary layer
”.
It can also be defined as a region in
which the fluid velocity is less than 99% of the bulk fluid velocity or free
stream velocity.
4. Define Thermal boundary layer?
When a heated solid body is placed
in a fluid stream, due to the difference in temperature between the solid
surface and the fluid, heat transfer will occur and a temperature gradient will
be set up. This temperature gradient is considered to exist within a layer
close to the surface. This layer of fluid close to the surface within which the
temperature gradient exists is called as the “Thermal boundary layer”.
5. What is Nusselt Number? What is its significance?
Nusselt Number is a dimensionless number
which is given by:
Nu =
where h = heat transfer coefficient
L
= Characteristic length
k
= Thermal conductivity
Significance: It is the ratio of wall
temperature gradient to the temperature gradient across the fluid in the pipe.
6. What is Reynolds Number & what is its significance?
Reynolds Number is a dimensionless
number which is given by:
Re =
Where D = Diameter of pipe
u
= Velocity of fluid
ρ
= Density of fluid
μ = Viscosity of fluid
Significance: It can be described on the ratio of inertial
force to viscous force.
7. What is Prandtl Number & write its significance?
Prandtl Number is a dimensionless
number which is given by:
Pr =
where Cp = heat capacity of fluid, μ is
the viscosity of the fluid and k is thermal conductivity of fluid .
Significance: It is the ratio of momentum diffusivity (
) to thermal diffusivity (
) i.e.
8. Write the equation for Stanton Number & write its significance?
Stanton Number is a dimensionless
number which is given by:
Nu h
St = ----------- = ----------
Re. Pr ρ u Cp
where h = convective heat transfer coefficient,
ρ = density of fluid, u is fluid
velocity, Cp = specific heat capacity of fluid.
Stanton Number is the ratio of
Nusselt Number to the product of Reynolds Number and Prandtl Number.
Significance: It gives the ratio of
rate of wall heat transfer by convection to the rate of heat transfer of bulk
flow.
9. Define Peclet Number and give its significance?
Peclet Number (Pe) is a dimensionless number which is given by:
ρ u L Cp
Pe = Re. Pr = ------------
K
where ρ = density
of fluid , u = velocity, L = Characteristic length, Cp = specific heat
capacity of fluid, k =thermal conductivity.
Significance: It gives the ratio of rate
of heat transfer by Bulk flow to the rate of heat transfer by conduction.
10. What is Graetz Number & write its significance?
Graetz Number (Gz) is a dimensionless
number which is given by:
m Cp Pe . d =
Pe . d
Gz =
-------- = -------- --------
K L L L
where m is mass flow rate, Cp and K
are respectively heat capacity and thermal conductivity, L = characteristic linear dimensions for flow.
Significance: It is similar to Peclet
number but used in connection with analysis of heat transfer in laminar flow of
fluids in pipes.
11. What is Grashoff Number and what is its significance?
Grashoff Number (Gr) is a dimensionless number which is given by:
g
L3
( Ts – To)
Gr = -------------------------
2
where L – Characteristic length,
g - Acceleration due to gravity,
- coefficient
of volumetric expansion, Ts - wall temperature, To
– ambient fluid temperature,
– viscous force.
Significance:
Buoyancy force
It the ratio of --------------------------
Viscous force
Grashof number plays the same role in natural convection as the Reynolds
number does in forced convection.
12. What is Biot Number? What is
its Significance?
Biot Number (Bi) is a dimensionless
number which is given by:
where, Lc
– Characteristic length, k – thermal conductivity of solid & h - heat
transfer coefficient.
Significance: It gives the ratio of
internal thermal resistance of a solid body to its surface (external) thermal resistance.
It is used in analysis of unready state conduction problems.
13. Write Sieder – Tate equation
for laminar flow.
Seider and Tate equation for laminar flow:
Nu = 1.86
The above correlation is applicable
when,
(a) 0.48 < Pr < 16700
(b) the viscosity ratio is within the range 0.0044 <
<
9.75
(c)
> 108
14. Write Sieder and Tate
equation for Turbulent flow?
Sieder and Tate equation for
Turbulent flow
Nu = 0.027 Re 0.8 Pr 0.33
The condition of applicability of
this equation is
(a) 0.7
Pr
16700
(b)
Re
10000
(c)
D /L
0.1
15 . Write the Dittus – Boelter Equation?
Dittus – Boelter Equation for
turbulent flow through a circular pipe:
Nu = 0.023 Re0.8 Prn
where n = 0.4 for heating ( Tw
> T )
n = 0.3 for cooling ( Tw >
T )
The condition for applicability of this equation is
(a)
0.7
Pr
160
(b) D /L
0.1
(c)
Re
10000
16. State the Reynolds Analogy?
Reynolds was the first person who observe that “ There
exists a similarity between the exchange of momentum and exchange of heat
energy in laminar motion and for that reason it has been termed as “Reynolds Analogy “
Nu h
------ = St
= ------- = f/2
Re.Pr Pu Cp
This can be used to determine the heat transfer coefficient ‘h’ if the
function factor ‘f’ is known.
17. State Prandtl Analogy?
St = -----------------------
1+5
(Pr -1)
Here ‘f ‘is fanning friction factor. It provides a more realistic
equation of turbulent flow. It reduces to Reynolds analogy if Pr = 1
18. Chilton - Colburn Analogy –
Define?
Nu
------------ = jH = = f/2
Re.Pr1/3
Where jH is colburn j factor
Using the well known
correlation for the friction factor f = 0.046 Re¯0.2 for pipe flow, j
-factor is given by jH = 0.023 Re-0.2
19. What is Rayleigh Number?
Rayleigh Number is the product of
Grashof number and Prandtl number.
Ra =
Gr x Pr
L3 p2 g
T Cp μ
= ---------------------- *
----------
2 K
L3 p2 g
T μ
Ra =
---------------------
2 K
20. What are the types of boiling?
Types of Boiling
Pool Boiling (or)
Flow Boiling (or )
Natural convection Boiling
Forced convection Boiling
21. Differentiate pool Boiling from flow boiling?
If heat is added to a liquid
from a submerged solid surface, the boiling proves is called pool boiling. In
this process, the vapours produced may form bubbles which grow and subsequently
detach themselves from the surface, rising to the free surface due to buoyancy
effects.
E.g. Boiling of water in a kettle.
In contrast, “flow boiling
“occurs in a flowing stream & the boiling surface itself be a portion of
the flow passage.
E.g.: Heating of fluid flowing
through heat exchanger tubes.
22. What is the classification of pool boiling with respect to temperature?
Based on the liquid temperature, the types of
pool boiling are:
(1) Sub-cooled or local Boiling where
Liquid temperature < Saturation temperature
(2) Saturated or bulk Boiling where Liquid
temperature
Saturation
temperature
23. Name the six regimes of pool boiling curve?
There are six regimes in pool
Boiling curve which are,
Zone - I : Free Convection
Zone - II :
Bubbles condense in super heated liquid
Zone - III :
Bubbles rise to surface
Zone - IV :
Unstable film boiling
Zone -V :
Stable Film boiling
Zone - VI :
Radiation coming into play
24. Define Critical heat flux
point or peak heat flux point?
In the pool boiling of a saturated
liquid, for excess temperatures
Te) beyond 50°C,
nucleate boiling regime ends and film boiling starts. The maximum heat flux
point occurs at this transition which is of the order of 1 MW / m2. This maximum heat flux point is called
CRITICAL HEAT FLUX POINT OR PEAK HEAT FLUX POINT.
The aim is to operate an equipment
close to this point by never beyond it. If the heating of metallic surfaces is
not limited to this point, the metal may be damaged or it may even melt. That
is why it is also known as BURN OUT POINT.
25. Define Burn out Point?
In the pool boiling of a saturated
liquid, for excess temperatures
Te) beyond 50°C,
nucleate boiling regime ends and film boiling starts. The maximum heat flux
point occurs at this transition which is of the order of 1 MW / m2.
If the heating of metallic surfaces
is not limited to this critical heat flux point, the metal may be damaged or it
may even melt. That is why this peak heat flux point is also known as BURN OUT
POINT.
26. What are the types of condensation?
Film
wise condensation Drop wise condensation
In this type, the condensate wets In this type, the vapor
condenses
the surface forming a continuous
into small
liquid droplets of
film which covers the entire surface. vapors various sizes which
fall down the surface in a
random fashion .
Generally occurs on a clean Occurs on surface which is uncontaminated
surface contaminated
or coated with certain
additives
27. Which condensation gives high
heat transfer rate? Why?
The dropwise condensation gives
a much higher rate of heat transfer than filmwise condensation.
This is because in dropwise
condensation, a large portion of the area of the surface is directly exposed to
the vapour facilitating higher heat transfer rates.
But in filmwise condensation, the
condensate film covers the entire surface and further grows in thickness as it
moves down by gravity. There exists a temperature gradient in the film yielding
lesser heat transfer rates.
28. How will you find Thermal
layer thickness?
If δt is
thermal boundary layer thickness and δ is the velocity boundary layer thickness,
then
δ
δt = -------------
Pr 1/3
Using Pr
0.01, we get
δ
--------
0.16
δt
This relationship gives the flat plate analysis and slug flow model for
heat transfer in liquid metals.
29. What are the factors that
influence the heat transfer coefficient during nucleate Boiling?
The factors which affect the nucleate
boiling are:
(a) Pressure: It controls the rate of bubble
growth and therefore affects the temperature difference causing heat energy to
flow.
(b) Heating surface characteristics: The material of heating element has a
significant effect on boiling heat transfer coefficient.
(c) Thermo – mechanical properties of liquids: High
‘k’ value causes high heat transfer rate.
(d)
Mechanical Agitation: The rate of heat transfer will increase with the increase
in mechanical agitation of the fluid.
30. How will you find boundary layer thickness?
The boundary layer thickness (δ) can
be found by using the following equation,
=
Where δ
= boundary layer thickness
x = distance
from leading edge of plate
Rex = local Reynolds Number =
Ohm sakthi
UNIT 4
HEAT EXCHANGERS
1.
What is a heat exchanger? Mention some of its
applications?
A heat exchanger is any device used for effecting the process
of heat exchange between two fluids that are at different temperatures.
Uses: Heat exchangers are useful in many engineering
processes like those in:-
a) Refrigerating and
air-conditioning systems
b) Power systems
c) Food processing systems
d) Chemical reactors
e) Space or aeronautical
applications
2.
What are the types of heat
exchangers?
(i). Direct contact heat
exchangers
(ii). Recuperators or surface
heat exchangers
(iii).
Regenerators
3.
What is a direct contact
heat exchanger? Give eg.
A heat exchanger in which two fluids exchange heat by coming
into direct contact is called a direct contact heat exchanger. Eg: open feed
water heaters, desuperheaters, jet condensers etc.
4.
What are recuperators?
Give eg.
Recuperators are surface heat exchangers in which the fluids
are separated by a wall. The wall may be a simple plane wall or a tube or a
complex configuration involving fins, baffles and multiples of tubes. For eg:
Double pipe heat exchangers, shell-and-tube heat exchangers, condensers, evaporators
etc.
5.
What are regenerators?
Give eg.
A periodic flow type of heat exchanger is called a
regenerator. In this type of heat exchanger, the same space is alternatively
occupied by the hot and cold gases between which heat is exchanged. Eg: They are
used in preheaters for steam power plants, blast furnaces, oxygen producers
etc.
6.
How will you classify heat
exchangers based on fluid flow arrangement?
(i)
If both the hot and cold fluids flow in the same direction,
the arrangement is called PARALLEL FLOW heat exchanger
(ii)
If both the fluids move in opposite directions, the
arrangement is called COUNTER FLOW heat exchanger
(iii)
If both the fluids
move at right angles to each other through the heat exchanger, the arrangement
is called CROSS FLOW heat exchanger
7.
What is the role of
baffles in a heat exchanger?
ü To create a turbulence in
the shell-side fluid
ü To enhance the cross flow
velocity of shell-side fluid relative to the tubes, baffles are generally
provided.
8.
What are compact heat
exchangers? Give eg.
Compact heat exchangers are special types of heat exchanger s
that are compact in size. However their relative heat transfer area is
increased by the usage of fins, pins or spiral grooves on their outer surface.
Normally a liquid flows through the tubes and a gas with a low heat transfer
coefficient flows over the extended surfaces.
9.
What is overall heat
transfer coefficient?
For a plane wall, U =
where i and o represent inside and outside
surfaces of wall respectively.
For a cylindrical wall, Uo =
and Ui =
Overall heat transfer coefficient takes into account the inside
and outside surface heat transfer coefficients of convection hi and ho as well
as the thermal conductivity k of convection.
10. Define : Fouling Factor
The surfaces of a heat exchanger do not remain clean after it has been in use
for some time. The surfaces become fouled with scaling or deposits which are
formed due to impurities in the fluid, chemical reaction between the fluid and
the wall material, rust formation etc.
The effects of these deposits is felt in terms of greatly
increased surface resistance affecting the value of U. This effect is taken
care by introducing an additional thermal resistance called the FOULING
RESISTANCE or FOULING FACTOR, Rf.
The unit of Rf is m2 K / W.
11. Which heat exchanger requires lesser area – parallel flow or
counter flow? Why?
The area requirement of a counter flow heat exchanger is lesser
compared to that for a parallel flow heat exchanger. This is because for the
same terminal temperatures of fluids and for the same heat transfer rate, the
LMTD for a counter flow type is MORE than that for a parallel flow type.
12. Draw the temperature distribution of fluids in (a) condenser
and (b) evaporator?
In the case of a condenser, the hot fluid will remain at
constant temperature since it undergoes a phase change, while the temperature
of the cold fluid increases.
In the case of an evaporator, the cold fluid will remain at
constant temperature since it undergoes a phase change, while the temperature
of the hot fluid decreases.
13. What is the role of correction factor F in heat exchanger
calculations?
The flow conditions in multiple-pass and cross-flow heat
exchangers are much more complicated than those in double-pipe and single-pass
heat exchangers. For such complex cases, the determination of the mean
temperature difference is so difficult that the usual practice is to modify the
general equation Q = U A DTm, by including the correction
factor F, as follows:
Q = U A F DTm where DTm is the same value as
for a counter flow double-pipe heat exchanger with the same hot and cold fluid
temperatures as in the more complex design.
14. Define P and R in heat exchanger design?
The two temperature ratios P and R used in the design of
multipass and cross flow heat exchanger are defined as follows:
P =
R =
Where to & ti refer to outlet and inlet temperatures of
tubeside fluid and To & Ti refer to outlet and inlet temperatures of
shellside fluid.
A good design should involve the selection of parameters P and R
such that the value of F is always greater than 0.75
15. What is Effectiveness-NTU Method? When it is used?
This method is used for heat exchanger analysis if the terminal
temperatures of the fluids are not known. This type of situation is encountered
in the selection of a heat exchanger or when the exchanger is to be run at
off-design conditions.
This method is based on effectiveness of a heat exchanger in
transferring a given amount of heat.
Effectiveness, ε =
=
ε = ε
The group
is called the NTU –Number of Transfer Units.
16. Define NTU? What is its significance?
The group
is called the NTU –Number of Transfer Units.
ü NTU is a dimensionless
parameter.
ü It is a measure of the
heat transfer size of the exchanger.
ü The larger the value of
NTU, the closer the heat exchanger reaches its thermodynamic limit operation.
Ohm sakthi
UNIT 5
RADIATION AND EVAPORATION
1. Define Radiation and give two examples?
The mode of heat transfer called
Radiation refers to the transfer of energy through space by electromagnetic
waves. Radiation is an electromagnetic phenomenon and it requires no medium.
Examples: The tube stills in petroleum refineries, Operation
of a furnace etc.
2. Define Absorptivity,
Reflectivity and Transmissivity ?
Thermal radiation incident on a body
tends to increase its temperature. However, depending upon the nature of the
material constituting the body and its surface characteristics, the incident
radiation may be absorbed, reflected or transmitted, partly or fully.
The fraction of incident radiation
absorbed by a body is called absorptivity (α). The fraction reflected is called
reflectivity ( p) and the fraction transmitted through the body is the
Transmissivity (τ). These fractions should add up to unity:
α+
ρ + τ = 1
3. Define Black body.
A surface which absorbs light of all
wavelengths in the visible range is called Black body. Thus for an opaque black surface, ρ =
o ;
τ = o ; α = 1
·
A black body completely absorbs
incident radiation irrespective of their wavelength i.e. α=
1.
·
A black body is a perfect emitter, ε
= 1 . A black body is a perfectly
“diffuse emitter ”
4. Define White Body ?
A surface that reflects light of all
wavelengths in the visible range and does not absorb any light preferentially
is called a “ White Body ”
Thus for opaque white surface, α= o ; τ =
o ; ρ =
1
5. Define Gray body?
A gray body is defined as a
substance whose emissivity and absorptivity are independent of wavelength. Thus
a gray body is also an ideal body, but its ε
and α
values are both less than unity.
6. Define Emissivity?
The ratio of the total emissive
power of a body (E) to that of a black body (Eb) is called
emissivity (ε).
E
ε = -------
Eb
7. Differentiate between Specular
and Diffuse surfaces?
|
Specular surface
|
Diffuse surface
|
|
If
angle of incidence of radiation is
equal to the angle of reflection then the reflection is called
specular.
There
is only one reflected radiation.
|
If
the angle of incidence is not equal to the angle of reflection in diffuse
surfaces.
The
incident radiation is reflected uniformly in all directions.
|
8. What are the characteristics of
a black body?
A
blackbody is a surface that has the following characteristics:
·
A black body completely absorbs
incident radiation irrespective of their wavelength i.e. α=
1.
·
A black body is a perfect emitter, ε
= 1
·
A black body is a perfectly “diffuse
emitter ”
9.
Write the Planck’s equation?
2π h
C2 λ-5 K1 λ-5
Ebλ =
-------------------------------
= ------------------
exp (h
C / λ KB T ) -1 exp (K2 / λ
T )-1
Where,
Ebλ
= Emissive power of monochromatic black body.
T
= Absolute Temp of black body.
h =
Planck’s constant
KB =
Boltzmann constant
C
= Velocity of light
K1 =
2π C2 h and K2 = h
C / kB
The above
equation is known as “ Planck’s law ” or “ Planck’s distribution.”
10. State Planck’s law.
By Planck’s
law, monochromatic emissive power Ebλ
can be defined as the amount of radiant energy emitted by a
surface per unit area per unit time and per unit wavelength.
(Also write the Planck’s equation here as in Q.No:9)
11. State the wein’s displacement law ?
Wein’s
Displacement law can be deduced from Planck’s law that the maximum wavelength λmax
corresponding to the peak of the λ
Vs Ebλ
plot is inversely proportional to the temperature of black
body.
In other
words, λ
max T = constant = 2898 µm
K
The above
eqn is called “Wein Displacement law ”
12. State the Stefan Boltzmann law ?
A
basic Relationship for blackbody radiation is the Stefan Boltzmann law which
states that the total emission power of a black body is directly proportional
to fourth power of Absolute temperature.
Eb
=
T4
Where,
is universal constant = Stefan Boltzmann constant = 5.729x10-8 W / m2 K4
13. What is meant by
monochromatic emission?
The
term monochromatic emission refers to emission of radiation consisting of
electromagnetic waves of a single wavelength. The monochromatic emission power
(or) spectral emission power Ebλ
of a black body as function of wavelength of radiation and is
derived in Planck’s law.
14. State Kirchhoff’s
law ?
Kirchhoff’s
law states that the emissivity of a body which is in temperature equilibrium
with its surroundings is equal to its absorptivity.
ε = α (or )
(
ε / α)
= a constant
15. Write the factors
which affect (or) determine the rate of radiant heat exchange between two
bodies?
The factors
are,
* The temperature of the individual surfaces
* The emissivities of the individual surfaces
* How well one surface can view/see the other
* The absorptivity of the intervening medium
16. Define view factor
(or) shape factor (or) Configuration factor?
The fraction
of total radiant energy that is emitted by the surface “ i “ and is received or
intercepted by the surface “ j “ is called the view factor or Configuration factor or shape factor.
The fraction of total
radiation emitted by i &intercepted by j
View factor, Fij = ------------------------------------------------------------------------------------
Total
radiation emitted by “ i “
17. What is called overall
Interchange factor?
For heat exchange by radiation of infinitely two parallel
plates, the overall Interchange factor can be expressed as
1
F12 = ---------------------------
1 1
----
+ ----
- 1
e1 e2
Here, F12
is called overall Interchange factor which is a function of e1 and e2 i.e.
emissivities of two plates.
18. Define Radiation
shield?
In order to
reduce the transfer by radiation between two surfaces, a third surface is
introduced in between them. This surface is known as Radiation shield.
For the simple
case, when ‘n’ shields are employed each having the same
emissivities as the
initial planes,
1
Qn
= ---------- Q
n + 1
where Qn = Q
with ‘n’ shield s
Q = Exchange of energy is the initial planes were not separated
by
radiation
shield.
Q before shield
(
T14 – T34 )
----
=
--------------------
A 1
1
----- + ---- - 1
e1 e3
Q after shield
( T14
– T24 )
( T24
– T34 )
---- = -------------------- = -----------------------
A 1 1 1 1
-----
+ ----
- 1 ----- + ---- - 1
e1 e2 e2 e3
1. What is Evaporation?
Evaporation is the
process of vapourisation of mainly aqueous solution in order to get a
concentrated solution.
Concentration of dilute solutions in chemical industries is common. For example, juice of sugarcane has to be
concentrated in order to obtain sugar. The concentration is performed by
evaporation of water from juice. In evaporation normally the thick liquor is valuable product and the
vapours are discarded.
2. Define BPR
or BPE ?
BPR- Boillng point rise
; BPE- Boiling point elevation
BPE=Boiling point of solution - Boiling
point of pure solvent
BPE is the difference between boiling
point of solution and boiling point of pure solvent. BPE is caused due to the
increased boiling point when a solute is dissolved in a solvent.
3. State Duhring rule.
Duhring rule is
useful for determining BPR or BPE. It states that boiling point of a given
solution at a given pressure is linear function of boiing point of water at the
same pressure. Thus by plotting boiling point of solution Vs
boiling point of water a straight line prevails at same pressure.
4. What is driving force in
evaporation?
The difference in
temperature, ∆T is the
driving force in evaporation. In evaporation, ∆T is the
difference between the temperature of the steam (being used for
evaporation) and the temperature of solution at which it boils.
5. What are the types of evaporators?
6. What is the mechanism involved in the
natural circulation evaporators?
Natural circulation evaporators are those in which the solution
circulates due to density difference or buoyancy effect caused by heating the
solution in contact with the heating surface.
7. What is the mechanism involved in
the forced circulation evaporators?
In
forced circulation evaporators, the centrifugal pump forces solution through
the tubes at the velocity of 1-6 m/s. the boiling does not start in the tubes
due to sufficient static heat. Due to this static heat the solution becomes
superheated. Since the velocity of the flashing mixture is high, the separation
of vapours and concentrated solution takesplace due to inertial force at
impingement baffles. Impingement baffles minimize the entrainment also.
8. Define Capacity of evaporator
Capacity of evaporator is defined as amount of water evaporated per
hour.
9. Define Economy of evaporator
Economy of evaporator is defined as number of kgs of water evaporated
per kg of steam used.
10. What is the use of multiple
effect evaporator?
Multiple effect evaporator is used to enhance the steam economy by utilizing the enthalpies of water
vapours obtained from the previous
evaporator.
11. How will you determine steam
consumption ratio?
The steam consumption ratio can be determined by dividing capacity with
economy. It is given by number of kgs of steam consumed per hour.
12. Overall material balance for
evaporator?
Kg/h of feed = Kg/h water evaporated + Kg/h of thick liquor.
13. Types of multiple effect
evaporator?
Generally, four types of multiple
effect evaporators are commonly used,
Ø Forward feed multiple effect
evaporator.
Ø Backward feed multiple effect
evaporator.
Ø Parallel feed multiple effect
evaporator.
Ø Mixed feed multiple effect
evaporator.
Ohm Sakthi
PART-B QUESTION BANK
UNIT -1
1.
Discuss in detail the three modes of heat transfer with
illustrations.
2.
Derive the equation for one-dimensional steady state heat
conduction equation for a plane wall /flat plate.
3.
Problems based on the above derivation in Q.No:2
4.
What is thermal conductivity? Write its significance.
5.
Derive the equation for one-dimensional steady state heat
conduction equation for a composite plane wall or composite flat plate or
through a series of resistances.
6.
Problems based on the above derivation in Q.No:5
7.
Derive the equation for one-dimensional steady state heat
conduction equation for a hollow cylinder.
8.
Problems based on the above derivation in Q.No:7
9.
Derive the equation for one-dimensional steady state heat
conduction equation for a composite coaxial cylinder or through a series of
cylindrical resistances.
10.
Problems based on the above derivation in Q.No:9
11.
Derive the equation for one-dimensional steady state heat
conduction equation for a hollow sphere.
12.
Problems based on the above derivation in Q.No:11
13.
Discuss the analogy between heat flow and electricity flow in
detail.
UNIT -2
1.
Derive the equation for conduction with heat source.
2.
Derive the equation for two-dimensional steady state
conduction using analytical method.
3.
Describe the graphical analysis of two-dimensional systems.
4.
Discuss the transient heat conduction problem with examples.
5.
Problem based on transient heat conduction.
UNIT -3
1. Derive an expression for
forced convection heat transfer under turbulent flow conditions using
dimensional analysis.
2. Derive an expression for
free convection heat transfer under turbulent flow conditions using dimensional
analysis.
3. Discuss the influence of
boundary layer on heat transfer?
4. Write the significance of
the following dimensionless numbers: (i) Nu (ii) Re (iii) Pr (iv) Gr (v) Gz
(vi) St (vii) Pe (viii) Bi
5. Write short notes on the
heat transfer in packed and fluidized beds.
6. Write short notes on the
heat transfer in molten metals and liquid metals.
7. Discuss the different
regimes of pool boiling curve with a neat explanatory diagram. Also write its
significance.
(or) Explain the various stages involved in the boiling of
saturated liquid.
8. Problem based on equations
for forced convection.
9. Problem based on equations
for free convection.
10. Problems based on boundary
layer thickness.
UNIT-4
1. Give a neat sketch of
typical heat exchange equipment indicating its components and functions. (or)
Describe the parts and functions of a 1-2 heat exchanger with a neat sketch.
2. Problems based on
calculating Overall Heat Transfer Coefficient U with and without fouling
resistance.
3. Derive an expression for Logarithmic
Mean Temperature Difference (LMTD) for rate of heat transfer per unit area of parallel/counter
flow heat exchanger.
4. Problems based on LMTD to
calculate area or length of parallel/counter flow heat exchanger.
5. Problems to calculate area
of condenser or evaporator.
6. Problems to calculate P,R
in multipass or cross flow exchanger and finally to calculate area.
7. Derive an expression for
ε-NTU for parallel flow heat exchanger. Also give graphical representation.
8. Derive an expression for
calculating the effectiveness of a counter-current flow heat exchanger
UNIT -5
1. Explain the concept of
black body. What are its characteristics?
2. Describe the following
radiation laws in detail: (i) Planck’s Law (ii) Wein’s Displacement Law (iii)
Stefan-Boltzman’s Law (iv) Kirchoff’s Law
3. Explain the grey body
concept and derive the Kirchoff’s Law.
4. Derive an expression for
radiation heat exchange between two parallel surfaces or planes.
5. Problems based on above
derivation in Q.No:4
6. Explain the effect of
radiation shield kept between two radiating surfaces.
7. Problem based on
calculation of heat transfer rate before and after placing the radiation
shield.
8. With a neat sketch,
explain the construction and working of an evaporator. (short tube calendria
type standard vertical evaporator is preferred). What are its merits and
limitations?
9. Discuss the different
feeding arrangements in multiple effect evaporator with neat sketches.
10. Problem based on
calculating area and capacity of a single effect evaporator.
11. Problem to calculate steam
economy.
12. Problem to calculate the
boiling points of solutions in a triple effect evaporator.
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