Heat Exchanger & LMTD Video Lecture | Heat Transfer - Mechanical Engineering

[Music]
hello students today we will learn that
what is an heat exchanger and what are
the basic attributes in regards of heat
exchangers what other types of heat
exchangers and we'll also learn that
what is the concept of LMT D that is
logarithmic mean temperature difference
okay first of all remember this thing
that heat exchanger is a device in which
heat is transferred between two flowing
fluids without mixing of each other
means there are two fruits one is hot
and other is cold okay and heat is
transferred from the hot fruit to cold
flu however they are not mixed with each
other and heat exchangers are the
devices which are widely used in
industries for corresponding purpose the
purpose is the main purpose in
industries in the regards of this heat
exchanger is that when we want one cold
fluid to receive heat from some hot food
for example suppose there is a dairy
industry in which pasteurization process
is happening so what happens generally
the common practice of pasteurization of
milk in dairy industry is there is a
steam hot steam okay so by the help of
hot steam the heat is transferred to the
cold milk okay so heat of hot steam is
transferring to the cold mnek the
temperature of milk is rising and we
noticing that pasteurization process is
what that we have to increase the
temperature of milk to 80 degree
centigrade and by this way the bacterias
and the other harmful germs contained in
the milk can be killed
so the common practice I told you that
by the help of steam high temperature
steam by the help of an heat exchanger
the heat is supplied to the cold milk so
the cold milk the temperature of Coleman
Rises and by this way the pasteurization
process completes and for an example
real-life example making tea over the
stove is also a sort of heat exchanging
process over here I have surely
the small diagram this is the utensil in
which this is the liquid the T is liquid
we know this thing okay so below it
there is a stove so what is happening
the flame consists of flue gases flue
gases are fluids okay so these high
temperature gases when touches this
bottom of potential transfers its heat
to this utensil outer surface ultimately
heat conducts toward this T means T is
acting as a cold fluid and hot flue
gases as a hot food so heat exchanger is
a device in which heat is transferred
from a high temperature fuel to a load
of the food without mixing with each
other okay now let us talk that how many
types of heat engines are there see
these are some commonly used heat
exchanger in Hinda cities first is
double pipe or tube in tube heat
exchanger if this type of heat exchange
what happens there is a thin tube and it
is enveloped inside a big diameter tube
okay both are Co centric okay what
happens from this inner tube one fuel
flows freely hot food okay so from this
inner
fluid I am showing by this red colored
food it is hot food and from this outer
fuel cold fluid is flowing so what
happens when these two fruits are
flowing then the heat is transferred
from hot fluid to the cold fruit through
the walls of the tube means heat is
first of all transferred to inner
surface of the wall then it conducts
through the thickness of wall and
ultimately goes to the cold fluid it is
like this it is sealed with the either
end so that flute cannot skip out or
leak out from the system so this type of
heat exchanger is called as tube in tube
or double pipe it is now in this type of
utensil also there are two types one is
we are using tube in tube heat exchanger
in parallel flow mode other is we are
using in counter flow mode now what is
parallel flow see what happens in case
both the fluids hot and cold are
entering to this cube into petitioners
from the same side okay in that case the
direction of flow remains the same means
hot fluid is moving
for this it is in case I'm download then
hot food is moving from left to right
and cold is also moving from left to
right so this type of arrangement this
type of flow is called as parallel flow
heat a senior the make is double five or
even two but this arrangement in which
the hot and cold foods are entering from
the same side and ultimately comes out
from the other opposite side then such
type of flow is called external flow and
in case one fluid is entering from one
side suppose in digital I am swinging by
arrow that cold fluid is entering to
this outer peripheral tube from one side
while hot fluid is entering from the
opposite end to this inner central do so
in this case what is happening both the
fluids are moving opposite to each other
so such type of flow is called as
counter flow heat Arsenal means double
pipe counter flow Here I am showing the
magnified view of this to be witticism
you can see this is the center tube
through which hot fluid is flowing and
this is the outer peripheral tube
through which code through this flow and
heat from this central tube from this
fluid which is flowing through the
central tube is transferred through the
walls of this inner tube to the cold
fluid you can see this the direction of
flow of heat it is like this other type
of make is plate type it is here by the
help of arrow I am showing it plate type
it is here it is consisting of one plate
or another okay this is a magnified view
okay suppose it is consisting of five
states 1 2 3 4 5 ok so you can see
between first 2 plates cold fluid is
flowing and in the immediate next 2
plate hot fluid is flowing then again
cold food and again hot food so
arrangement is of such kind that between
two plates some cold fluid is flowing
and immediate next the two plates hot
fluid is flowing so the arrangement is
of such time that a hot fluid layer is
sandwiched between two cold food layer
and one cold food layer is sandwich
between two what fruit layers by the
help of arrangement of plates so such
type of it as singer is called as plate
type eternal it is widely used type of
attention in industries okay so this
blue colored food is cold food and this
red colored food is hot food blue
again code hard so between two hot fuel
strings there is one cold fluid stream
and between two co2 stream there is one
hot photostream
again this type of heater cell can also
be parallel so in case fuels are
entering from same end and coming out
from the same opposite end then well
also in case the fluid flow is of such
kind that a hot fluid is entering from
one end while cold food is entering from
other opposite end then it is plate type
counter flow in case one fluid is
flowing suppose I am taking this heaters
inner as shown in the diagram from left
to right other fluid is flowing inside
the screen okay in such case the two
fluids are moving making an angle 90
degree to each other first type of
arrangement is called as cross flow type
of heater snare now this is the shell
and you with a fringe inshalla tube heat
exchanger what happens that there is a
big shell okay through which one fluid
is passing for example by the help of
arrow I am showing a type that is single
shell 2 to power shell and tube type of
addiction here suppose cold fluid is
entering from this end and ultimately
coming out of the other end okay and
there is a tube pass inside the shell
you can see through this tube
hot fluid is flowing okay so actually
what is happening that cold fluid is
flowing through a big shell while hot
fluid is flowing through a tube
so this is the reason it is called a
shell and tube heat exchanger okay so it
also consists of its various types for
example this is the case of single shell
and to to pass the reason it the reason
is that this is the shell and cube is
going in and then coming out so it is to
tube pass okay once going in and then
coming out one pass to pass slowly this
type is to shell for to pass because it
consists of two shell cold food is
flowing through this shell and hot food
is passing ones going in and then coming
out and then from this other shell going
in and then coming out so it is two
shell because shells are two
however the hot fluid is passing four
times that is in out and then in out so
two shell for to pass and this third one
is single shell for to pass because it
consists of
one shell only old fluid is passing
through this shell and then what fruit
is going and then turning and then again
turning and then again Turing and then
going out so it is called as single
shell for tube pass this is also shell
and I would beautiful in which old food
is flowing through the shell suppose
from left hand side to right hand side
while the hot fluid is passing in a
cross flow arrangement to this flow of
cold food because see these are the hot
flue tubes so these tubes are running
inside the screen of this computer okay
so cold fluid is flowing from left to
right while hot fluid is flowing at an
angle 90 degree to the cold fluid such
type of eternal is called a shell and
tube cross flow type of heat exchanger
however these diagrams are just
indicating just to make you understand
the concept of these different types of
heat exchangers now next case you will
talk about the heat transfer rate you
know what is heat afraid the amount of
heat transferred per unit time is called
as heat transfer rate okay remember in
case there are two bodies which are at
temperatures t1 and t2 and t1 is greater
than t2 okay then what happens heat
transfers always from high temperature
body to low down visibility
okay this temperature this body is at
higher temperature and this body is at
lower temperature t2 t1 is higher then
t2 ok so heat will is transfer from
items it will double now the heat
transfer rate formula the general
formula for heat transfer rate in case
heat is transferred between two bodies
is given by Q Q stands for heat transfer
it is equals to you a t1 minus t2 here U
is called as overall heat transfer
coefficient which is a constant in
regards of those heat transferring
bodies a is the area through which heat
is transferring okay a is the area
through which heat is transferring and
t1 minus c2 is the temperature
difference okay so U is overall heat
transfer coefficient a is the area of
exposure through which heat is
transferring and TN minus between the
temperature difference this is what the
heat transfer force formula but let us
take an example of parallel flow heat
exchanger and cube and cube type okay
so hot flow is entering inside this
center
suppose from left hand side and cold
fluid is entering from the same left
hand side in this outer periphery tube
and both are moving parallel to each
other
now when hot fluid is entering just
entering this heat exchanger at that
time the hot foods temperature is
highest suppose it is thi the hot to
temperatures inland similarly when cold
dude is entering to this outer
peripherals to do at that time it is
having its lowest temperature because no
heat it has gained it is just entering
inside the system now when hot fluid
will move inside this tube by and bias
since it is giving its heat to cold fuel
so hot fluids temperature will by an by
degrees and ultimately the hot fluid
will come out this temperature th oh
okay similarly since this cold fluid is
gaining heat receiving heat from this
hot food so continuously by and by its
temperature will increase and it will
come out ultimately from this opposite
end with temperature TC you okay so this
is the graph the temperature gradient of
the two fluids for the case of parallel
flow heat exchanger okay now heat
transfer rate formula Q equals to u
eight t1 minus t2 where t1 - t2 is the
temperature difference okay but in this
case what we see that from beginning to
the end the temperature difference
between the two fluids is continuously
changing okay so what we can keep in
this place in place of T 1 minus C 2
what temperature difference we have to
keep this is the question initially
there is greatest temporal difference by
and by the hot fluid is cooling down and
cold food is heating up so the
temperature difference is decreasing so
in place of t1 minus t2 what we can keep
the concept is that since the
temperature difference is continuously
decreasing so we have to take the
average of all those temperature
differences called as LMT
the full form is logarithmic mean
temperature difference the significance
of logarithmic mean temperature
difference is that however the
temperature difference of the two fluids
is continuously decreasing but we can
assume that from beginning to end of
heat exchanger
the temperature difference is constant
that is the logarithmic mean temperature
difference which is the hypothetical
temporal difference means it is mean of
all those LC temper the difference is
continually decreasing but the mean of
all those separate events we can assume
that both the fluids are transferring
their heat with this constant
temperature differences hypothetical
assumption which is called as LM TD
logarithmic means of the difference and
the formula of logarithm in term of the
difference is LM TD equals to theta 2
minus theta 1 upon log theta 2 beta 1
here what is Theta 2 theta 2 is the
finite temperature difference of the two
fruits you know that fluid is hot food
is coming out with temperature th oh and
cold food is coming out with a media TCO
so this final temperature difference is
thi minus TC oh it is what theta 2 is
and theta 1 is the initial temperature
difference thi minus TCI okay so in
place of theta 2 and theta 1 we can keep
these corresponding values to get the
logarithmic mean temperature difference
once again what is the significance of
logarithm interval difference however
the temperature difference of the two
fluid is continuously changing
but this is an hypothetical constant
temperature difference that we can
assume that heat is transferring some
hot fruit to cold food with this
constant temperature difference through
the entire run of this heat exchanger
now we have a fixed temperature
difference at a lambda T so here Q we go
so you ATO not see too in case of T nos
- I can keep Q equals to UA
lmpd which is the average constant
temperature difference of the two foods
which are running through the heat
exchanger okay now we will take the case
of counterfeit exchanger suppose this is
a tube in tube double pipe counterfeit
Effinger so hot fluid is entering from
left hand side to this inner central
tube it is flowing toward right hand
side and cold fluid is entering from
right hand side to this outer
peripheral tube and ultimately coming
out from this left hand side okay so
when hot fluid is entering inside this
inner center tube it is super hot
it is having with this highest
temperature okay and by and by when it
will move inside the tube it will lose
its heat so it will come out from the
opposite end with temperature Thu which
is lower than thi okay
similarly cold fluid is entering from
right-hand side so it has temperature TC
a lowest temperature since by-and-by it
is gaining heat from this high
temperature so ultimately it comes out
from the opposite end is temperature TC
OH
okay so these are the temperature
profiles with respect to the advance of
food flowing inside this double pipe
fitter fringes okay now in this case
also the heat transfer rate formula
remains the same that is equal to UA LM
TD LM TD corresponds to the mean
temperature difference logarithmic be
interpretations because over here also
the case is same that from one end to
other okay the temperature difference is
continuously changing however in this
diagram it might be appearing the
temporal difference is same but
practically at every point of this heat
exchanger the temperature difference can
be different okay so again we have to
take the concept of logarithmic mean
temperature difference which is the
hypothetical constant temperature
difference which we can assume for the
flowing fruits in the heat exchanger so
the formula of LM Katti remains the same
as like that of parallel flow
temperature that is equals to theta 2
minus theta 1 identity equal to Z 2
minus theta 1 upon log 1002 Waseda 1 but
the definition of theta 1 and theta 2
changes over here what you have to take
that theta 1 is the temperature
difference over the left hand side of
this heat exchanger means over this and
the leftmost end the temperature of hot
food is thi while the cold food is TCO
so theta 1 would be thi minus TCI OH
similarly theta 2 would be what it is
the temperature difference of the
rightmost side of this heater when this
code food is just entering and hot food
is just a fitting so that is equal to
thi minus TCI okay this theta 2 so the
only change is you will use the same
formula for heat transfer it gives us 2
LM trading birds up in formula of
elaborate e the definition of theta 2
and theta 1 is this theta 1 equals to
thi minus TC oh and theta 2 is th over
Seca so you can keep remember is this
thing by this way that we have to take
the temperature difference of the either
n for parallel flow heat exchanger theta
1 equals to thi minus 3 say for counter
flow it is equals to thi minus EC you
the definition of theta 1 and for
parallel flow theta 2 equals to 1 th o
minus T Co for counter flow it is equals
to thi minus TCI so we have to take the
difference temperate differences of the
either ends for the case of theta 1 and
theta 2 so that will be the definition
of LMT D so the value of those theta 1
theta 2 can be kept in this for another
lambda T to get variability and to where
the heat transfer rate Q goes to you al
n Creedy here you is what you is the
overall heat transfer coefficient of
this particular type of it official and
a is the area of exposure through which
it is transferring for example for this
case for this double pipe heat exchanger
case the area of exposure is the outer
peripheral area of this inner tube outer
peripheral area that would be what if L
is the length of this tube then 2 pi RL
is the area of exposure for this double
pipe it is ginger in which counter flow
in which counter flow motion is
happening with the flute same for this
case also the area of exposure is what
for parallel for additional the area of
exposure reward the outer peripheral
area of this central tube through which
ultimately heat is transferring from hot
fluid food so hope you would have
understood the concept of exchanges and
the types of attacks injure and you
would have understood the concept of the
LM TT by this lecture thank you
you