Lecture - 25 Heat Exchangers - 1

Lecture Series on Heat and Mass Transfer by Prof. S.P.Sukhatme and Prof. U.N.Gaitonde, Department of Mechanical Engineering, IIT Bombay. For more details on NPTEL visit http://nptel.iitm.ac.in

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So far we have studied the modes of heat transfer;
we have studied heat conduction in solids,
we have studied thermal radiation and we have
also studied convection. In convection, we
have looked at forced convection and natural
convection. Now today we start with the topic
of heat exchangers. The first thing we ask
ourselves is, the first thing we will do rather
is to describe various types of heat exchangers.
Before that, let me just restate again the
textbooks and the reference books which we
are following.
We are following my book on a textbook on
heat transfer by Universities Press; we are
also following Incropera and Dewitt - the
fundamentals and heat and of heat and mass
transfer. These are 2 books which we are closely
following so if you miss some part or you
are not able to take down some notes, you
are not able to take down a figure generally,
you will find a figure which is very similar
to what we are showing in the, in one of these
2 books. Now, the first thing we ask ourselves
is what are heat exchangers?
Well, heat exchangers heat exchangers are
devices in which heat is transferred between
2 fluids at different temperatures without
any mixing of the fluids. There are 2 fluids,
one is hotter than the other, they are at
different temperatures. Heat is transferred
from the hot fluid to the cold fluid; a device
in which this happens is called a heat exchanger.
And this occurs without any mixing of the
2 fluids that is the hot fluid doesn't mix
with the cold fluid; the heat transfer takes
place without any mixing of the fluids with
each other - that is called a heat exchanger.
Now broadly speaking there are 3 types of
heat exchangers of the various, hundreds of
heat exchangers which are built and hundreds
of geometries, etcetera which are built. There
are 3 broad classification now, 3 types of
heat exchangers.
The 3 types are: number one what is the called
as the direct transfer type, number 2 what
is called as the storage type and 3 what is
called as the direct contact type. These are
3 broad types of heat exchangers; let us look
at each of these for a moment.
First, the direct transfer type -
a direct type of heat exchanger, a direct
transfer type of heat exchanger is one in
which the cold and the hot fluids flow simultaneously
through the device and heat is transferred
through a wall separating the 2 fluids. I
repeat a direct type of heat exchanger is
one in which the cold and the hot fluids flow
simultaneously through the device and heat
is transferred through a wall separating the
2 fluids. Let us look at an example of a direct
transfer type heat exchanger; the example
we are going to look at is a very simple one.
The particular one, the particular direct
type of heat exchanger we are loing at is
what is called as a concentric tube heat exchanger.
So we say it is a direct transfer type concentric
tube heat exchanger; the words 'concentric
tube' describe the geometry of the heat exchanger
that is there is one tube inside another tube.
This is the one tube that is inside, this
is the outer tube here so and they are concentric
to each other; that is why it is called a
concentric tube heat exchanger.
The 2 fluids, the one fluid in this case,
the hot fluid flows through the inner tube.
It is shown here, it is entering the inner
tube on the left hand side, flowing along
the inner tube and getting out here and the
hot fluid is leaving the heat exchanger here.
The cold is fluid is entering the outer tube
flowing through the annulus and then exiting
out here; flows around the whole length and
then exiting out here. So one fluid flows
inside the inner, in this case it is the hot
fluid; the other fluid - the cold fluid - flows
inside the annulus and that is why use the
word concentric tube heat exchanger. It is
also referred to sometimes as a tube in tube
heat exchanger.
Now, the heat transfer in this heat exchanger
takes place across the wall of the inner tube.
So the hot fluid is inside the inner tube
here, the cold fluid is in the annulus here,
so the heat transfer is taking place like
this from the hot fluid to the cold fluid
across the wall of the inner tube. This is
where the heat transfer is taking place and
as the cold fluid flows in the annulus it
is picking up heat, its temperature is rising
and as the hot fluid flows along length, it
is giving up heat and therefore it is dropping
in temperature. So this is the heat transfer
surface, this is the heat transfer surface
where heat transfer is taking place - the
wall of the inner tube of the heat exchanger.
So, this is a direct transfer type heat exchanger;
both the fluids are flowing simultaneously
through the heat exchanger and heat is flowing
across the wall of the inner tube from the
hot fluid to the cold fluid. There is of course
no mixing of the fluids. This is one type
and this is the type that we really be studying
in detail. Now, we come to the second type
of broad classification
The second type of heat exchanger is what
we have called a storage type heat exchanger.
Now, let us look at the description of it
- a storage type heat exchanger is one in
which the heat transfer from the hot fluid
to the cold fluid occurs through a coupling
medium in the form of a porous solid matrix.
I repeat a storage type heat exchanger is
one in which the heat transfer from the hot
fluid to the cold fluid occurs through a coupling
medium in the form of a porous solid matrix.
The hot and cold fluids flow alternately through
the matrix, the hot fluid storing heat in
it and the cold fluid extracting heat from
it. The hot and cold fluids flow alternately
through the matrix, the hot fluid storing
heat in it and the cold fluid extracting heat
from it. Not let us look at a sketch so that
we describe this heat exchanger a little better.
What I have shown in the sketch is a single
matrix storage type heat exchanger. The cross
hatching shows the porous matrix, the porous
matrix for instance maybe pebbles with passages
through them or it may be some brick work
with holes in it - porous brick work through
holes in it, could be loosely kept, some finer
material loosely kept, etcetera. Basically
something through which fluid can flow; a
gas can flow or air can flow and something
which will absorb heat or give up heat when
the hot fluid or the cold fluid flows through
it as maybe the case. So, this is the porous
matrix and what we have here - the circles
with crosses on them - are valves which can
be opened or closed. So let us just - what
you call - give them a nomenclature; let us
say this is valve A, this is valve B, this
is valve C and this is valve D. Let us say
first of all that during the operation of
this heat exchanger, valves A and D are open
and B and C are closed. So, obviously, the
hot fluid can enter here if this valve is
open, go through this porous matrix and come
out through D. If it does this, it is going
to give up its energy, it is hot, it is heat
to the material of this porous matrix and
this matrix is going to heat up; we do this
for sometime, then we and during this period,
of course, B and C are closed.
Now after doing this for some time, we close
A and D and open B and C so that the cold
fluid starts to flow through this matrix and
the cold fluid picks up heat from this matrix,
gets heated and then flows out. So one by
one, the hot fluid flows through, stores heat
in the matrix, the cold fluid flows through
and picks up heat from the matrix; so that
is why this is called a storage type of heat
exchanger. This is, there are many versions,
of course, of the storage type; this is the
simplest type. We just want to show one here.
And then the third type of heat exchanger
- third broad classification - is what we
called as a direct contact type heat exchanger.
What is this? A direct contact type heat exchanger
is one in which the 2 fluids are not separated.
If heat is to be transferred between a gas
and a liquid, the gas is bubbled through the
liquid or the liquid is spread in the form
of droplets into the gas and once it is spread
in the form of droplets, heat gets transferred
in the direction between hot fluid and the
cold fluid or the gas is bubbled through the
liquid in which case also heat gets transferred
between the hot fluid and the cold fluid.
Let us again illustrate this with a sketch
to show a direct or a contact type for heat
exchanger.
Here is a direct contact type heat exchanger
now. This is a fluid A; let us say it is a
liquid which is entering here so this is like
a shower head, it is entering through the
shower head here and bubbles, sorry, drops
of this liquid come out of the shower head.
These drops flow through like this and ultimately
come down and collect here and go out of the
heat exchanger. The other fluid - let us say
it is a gas - flows in here and flows upwards
through this heat exchanger so let us say
it is water and air for instance. So the water
is being spread in the form of drops and the
air is flowing upwards through this heat exchanger.
If for instance the water is hotter than the
air, obviously heat is going to flow from
the water to the air or if the air is hotter
than the water the reverse will happen. So
this is what we call as a direct contact type
heat exchanger in which one of the fluids
is deformed. Deformed means put into the shape
of say droplets or bubbles and then the heat
transfer takes place; after heat transfer
takes place, it is all collected and flows
out so the droplets all get collected here
and flow out. So that is why we call it a
direct contact type heat exchanger because
the process of heat transfer here occurs in
this case at each drop.
If for instance this is a drop - one drop;
then the heat transfer that is taking place
assuming that there is, let us say air is
hotter, the heat transfer is taking place
from the air to the water like this across
each drop that we are seeing here. The red
arrow shows the direction of the heat transfer;
if the air is hot flowing upwards and the
water is cold and it is flowing downwards
in the form of drops. So there is no metal
wall separating the 2 as we had in a direct
transfer type of heat exchanger, so we call
it a direct contact type of heat exchanger.
So these are 3 broad types of heat exchanger.
Now during these few lectures that we have,
we shall be concerned, we shall be concerned
only with the first type that is the direct
transfer type of heat exchanger. I mean we
could have looked at others but generally
for an elementary undergraduate course it
is adequate to look at the direct transfer
type of heat exchanger. Now let us talk a
little more about the direct transfer type
of heat exchanger.
Direct transfer type heat exchangers can be
broadly classified as belonging to one of
3 categories. What are these? We call them
as - one category is what you call as tubular
heat exchangers, second category what we call
as plate heat exchangers and third as extended
surface heat exchangers. Tubular heat exchangers,
plate heat exchangers, extended surface heat
exchangers - they are all direct transfer
type heat exchangers. That means they follow
the basic definition that we gave earlier:
both fluids are flowing simultaneously through
the device, there is some wall separating
the fluids, heat transfer is taking place
across the separating wall or walls; so that
definition is satisfied. But depending on
the shapes that we use, we give these names;
we call direct transfer types as tubular heat
exchangers, plate heat exchangers or extended
surface heat exchangers.
Now let us look at each of these; first - tubular
heat exchangers. And in fact, we have already
looked at one tubular heat exchanger. What
was that? That was the concentric tube or
the double pipe heat exchanger. We have already
looked at that - one tube inside another tube,
the hot fluid, one of the fluids flowing through
the inner tube, the other fluid flowing in
the annulus around the inner tube. We have
already seen that. Now let us look at one
more tubular heat exchanger and that is called
the shell and tube heat exchanger. So we are
now going at another type of a tubular heat
exchanger which is called as shell and tube
heat exchanger. Now let me first show you
a sketch of shell and tube heat exchanger
and describe it.
A shell and tube heat exchanger typically
consists of a cylindrical shell; that is what
we have here and a bundle of tubes inside
the shell, I have shown here 4 tubes - 1,
2, 3 and 4. There are actually many, I will
be just showing, it is a sketch so we are
showing 4. It is a bundle of tubes inside
a shell, the tubes, the axis of the tubes
is parallel to the axis of the shell. One
fluid flows inside the tubes and that is the
fluid that I am showing here entering here.
It is breaking the flow, flow is breaking
up into 4 parts, going through each of the
tubes that I am showing, flowing through the
tubes, coming out here and then going out.
So this is one fluid going inside, flowing
inside the tubes and coming out. The other
fluid flows on the shell side that is outside
the tubes; it enters the shell here, flows
outside the tubes in the shell space outside
the tubes. And in this case, you can see it
is flowing on the outside of the tubes and
then it is exiting through an outlet at this
point in the shell, so this is the second
fluid.
Now there are some more parts inside this
which I want to describe. First of all, let
me again repeat. The 2 main elements of the
shell and tube heat exchanger are number 1
- a cylindrical shell, number 2 - a bundle
of tubes. I have shown 4 here; it will actually
be a bundle whose axis is parallel to the
axis of the shell so that is 2. Then the tubes
are fixed in sheets at the ends here and these
are usually called as tube header sheet so
what we are saying here; at the ends here
these are called tube header sheets in which
the tubes are fixed. Then we have the front
and the rear ends of the heat exchanger; this
is one end of the heat exchanger, this is
another cap at the end of the heat exchanger.
So these are called the front and, front and
rear end heads of the heat exchanger.
The fluids enter through inlets; as you can
see this is the tube inlet for the tube side,
this is the outlet for the tube side, this
is the inlet for the shell side fluid, this
is the outlet for the shell side fluid. These
are typically called nozzles, these 4 are
called nozzles. And finally in order that
the shell side fluid should flow over all
the tubes properly usually sheets are placed
across the flow and which make the shell side
fluid flow in the jig jag fashion as you are
seeing here, which ensure a jig jag flow on
the shell side and these sheets are called
as baffles. So, these are the main elements
of a shell and tube heat exchanger. I repeat
- tube, the tubes which are enclosed, a bundle
of tubes enclosed within a shell - the tube
header sheets, the front, the rear end heads
- the nozzles, 4 of them and baffles will
ensure a good flow on the shell side of the
shell side fluid.
One of the characteristics to describe a heat
exchanger is the amount of heat transfer area
it provides per unit volume of the heat exchanger.
Typically, a shell and tube heat exchanger
will provide anything from 100 to 500 meter
square meters per cubic meter of volume of
the heat exchanger. So keep this in mind whenever
you are thinking of a shell and tube heat
exchanger. Now let me again show another transparency
just to repeat the same things.
The major components of a shell and tube heat
exchanger are a bundle of tubes, the shell,
the tube header sheets, the front and the
read end heads, nozzles and baffles. The heat
transfer area available in a shell and tube
heat exchanger per unit volume - it can range
from 100 to 500 square meters per meter cube.
Now I am going to show a typical tube bundle
in a shell and tube heat exchanger; there
you see it on the screen.
Now what you have got on the screen is a typical
tube bundle; I told you there is a big bundle
of tubes. In this case, as you can see you
have 3369 and 16 and 24 and 731 something
like 50 tubes or a little more than 50 tubes
making up this heat exchanger. This plate
at the end is the header sheet, the tube header
sheet which I described in which the tubes
are fitted. This is the other tube header
sheet at the bottom and you can see it is
on a flange there because it has to be fixed
to the rear and the front or the front end
head of the heat exchanger. And here you can
see the baffle; you see this is a baffle,
this is a baffle plate, this is a baffle plate
to ensure that the shell side fluid flows
in a jig jag fashion on the shell side and
flows across all the tubes of the heat exchanger.
So, I have taken out a tube bundle to show
you so that you get a pictorial - a photographic
view - of a typical shell and tube heat exchanger
so this is a typical shell and heat exchanger.
Now let us move on; we have described the
tubular heat exchanger and what did I say
first? We looked of course at concentric tube
heat exchanger, then at the shell and tube
heat exchanger. Now. we want to look at the
next one and that is a plate heat exchanger
A plate heat exchanger as its very, as its
name says is nothing but a series in a plate
heat exchanger; we have nothing but a series
of large
rectangular thin metal plates
which are clamped together to form. Now we
look at the next type of direct transfer type
of heat exchanger that is the plate heat exchanger.
A plate heat exchanger in very simple terms
is nothing but a large number of thin rectangular
metal plates; a series of large rectangular
thin metal plates which are clamped together
to form narrow parallel plate channels - that
is what its plate heat exchanger. And in order
to illustrate this let us look at the photograph
which I have here of a parallel plate heat
exchanger. Here is a parallel plate heat exchanger.
Now in the parallel plate heat exchanger these
are plates which I am showing here with the
cursor 1, 2, 3, 4, 5 - these are the plates.
These are the end plates which are helping
to clamp all those thin metal rectangular
plates together. So we have a series of passages
which are in the form of parallel plate channel
- like this is number 1, number 2, number
3, number 4, number 5, number 6 - 6 parallel
plate channels. One fluid let us say the one
with dots here is entering here and flowing
through these channels number 1, number 3
and number 5. It is going down through number
1, going down through number 3, going down
through number 5, then it is being, there
are ways in which they all join together and
coming out at this point.
The other fluid you can see is entering here
and there is an arrangement inside so that
it automatically gets directed into passages
number 2, 4 and 6; you see that there and
it goes up through each of this passage. This
is passage number 2, this is passage number
4, this is passage number 6 and then they
again link up; the flow is all linked up together
and it comes out. So basically here we have
a 6 parallel plate channel heat exchanger
with one fluid flowing through channels 1,
3 and 5, the other flowing through 2, 4 and
6 and heat being transferred from one fluid
which is hotter to the fluid which is colder
at the end. So, this is what we call as a
parallel as a plate heat exchanger.
Typically for such a heat exchanger, for such
a heat exchanger the heat transfer that you
would get in such a heat exchanger is the
heat transfer area per unit volume. For a
plate, heat exchanger would typically be from
about 100 to 200 square meters heat transfer
area per unit volume of the heat exchanger.
So this is the second type of heat exchanger
- the plate heat exchanger, a series of narrow
parallel plate channels. Now the third the
third is what we have called as the extended
surface heat exchanger.
Let me first describe - its fins are attached
on the primary heat transfer surface with
the object of increasing the heat transfer
area and as a result, because of the fins,
one provides much more heat transfer area
per unit volume than a tubular or plate heat
exchanger. Typically in an extended surface
heat exchanger because of the fins one has
more than 700 square meters of heat transfer
area per unit volume of the heat exchanger.
Compare this with the numbers that I gave
you earlier. So, you put fins on the primary
heat transfer surface in order to enhance
the heat transfer area per unit volume of
the heat exchanger. Now again, let us look
at some sketches to illustrate ideas.
Here is what we call as the plate fin heat
exchanger - so it is a plate heat exchanger
but with the fins added on, it becomes really
an extended surface heat exchanger. So here
are the channels now; as you can see, here
are the channels - this is, there are here
1, 2, 3, 4, 5, 6, 7, 8 and 9 plates. And you
can see the, if I start numbering the channels
from here, channels 1, 3, 5 and 7 are one
set of channels and 2, 4, 6 and 8 constitute
the other set of channel - 9 plates giving
us 8 channels, 4 for the hot side, 4 for the
cold side. And it is not just channels that
we are providing but you can see that we are
providing fins which are interconnecting the
one side to the other. That means they are
long thin strips which are inside these channels
which are connecting one side to the other.
I am providing this much more heat transfer
area so the fluid flowing through has to flow
through narrow passages, narrow in this direction,
narrow in this direction as well. And the
same is also true on the other side; we have
got fins also on the cold side here and so
there are narrow passages, narrow long rectangular
passages on the cold side as well so as a
result, you provide a lot more heat transfer
area per unit volume.
We are showing here is only the core of the
heat exchanger; in reality, there would be
some manifold at the end, here a manifold
at the end, here a manifold on this side.
So the hot fluid enters, gets distributed
into this channels, comes out at this end,
gets collected and then flows out so it is
not just the, it is not the whole heat exchanger.
What we are showing you is just the core.
Now I want you to show you a typical core
of a real physical model and here we go.
This is a core of plate for fin heat exchanger
or what is broadly called as an extended surface
heat exchanger and here you again see we have
got in this case 1, 2, 3, 4, 5 channels here.
And you can see in between the plates we have
these fins which are attached; they are not
all equal in size but these are the fins which
have been attached in between this is one
side. And then, if I go to the other side
here we have the other side of the heat exchanger
where you can see I have the other fluid flowing
through these channels which are slightly
narrower in width and again we have fins here.
So, it is an extended surface heat exchanger
or to describe it in terms of its geometry,
it is a plate heat exchanger with fins so
we call it a plate fin heat exchanger and
again this is only the core of the heat exchanger.
One gas typically would flow through this
side through these channels - these 4 channels
- the other would flow these 5 channels. One
would be hot, the other would be cold; heat
would be transferred from the hot fluid to
the cold fluid.
Now let us look at some more examples of extended
surface heat exchangers. These are, what we
showed just now was a plate heat exchanger
with fins - a plate fin heat exchanger.
You could also put fins and tubes in which
case we get what are called as tube fin heat
exchanger and here we have 2 types of tube
fin heat exchanger cores. Now here is a bundle
of tubes; look on the left side, these are
3 tubes like this and one fluid is flowing
on the outside of the tubes like this, the
3 vertical arrows, the other fluid is flowing
inside the tubes. Typically, this is, this
type of heat exchanger is used for exchanging
heat between a gas and a liquid. So the liquid
will flow inside the tube because the liquid
you already have a high transfer coefficient,
on the gas side you don't have a high transfer
coefficient. So, in order to enhance or reduce
the thermal resistance on that side, we put
fins on the gas side so this is called a tube
fin heat exchanger.
Now a typical, either the fins can be on individual
tubes as you are seeing here, these are round
fins on each tube and they will be closely
stacked together or you have got a continuous
fin sheet like you are seeing and the bundle
of tubes is like this through which the liquid
is flowing. So this is also possible in which
case you have continuous fin sheets fixed
on the array of tubes or here individual fins
on each tube making up the tube array. This
is typically what we have in an automotive
car radiator; we have the tubes through which
the hot water flows and these are the continuous
fin sheets through which air is pulled and
that helps to cool that hot water which flows
back to the engine of the car. So, an automotive
radiator is typical of this type; a continuous
fin sheet on an array of tubes is what constitutes
- what it is - a tube fin heat exchanger.
And it is a compact, it is compact - the heat
exchanger - in the sense it provides a lot
more heat transfer area per unit volume.
We can also classify a direct transfer type
heat exchangers by the flow arrangement; we
can use that also as a description of a direct
transfer type heat exchanger like we have
described them in terms of the geometry when
we say they are tubular, say that they are
plate heat exchangers or extended surface
heat exchangers. Similarly, the way in which
the 2 fluids flow relative to each other is
another way of classifying direct transfer
type heat exchangers.
So let us classify them now by their flow
arrangement. There are 3 basic flow arrangements
and what are these? The 3 basic flow arrangements
are parallel flow, counter flow and cross
flow. We want to describe each of these carefully;
let us look at each of these carefully. Now
first let us look at parallel flow.
First let us look at parallel flow. I am going
to describe parallel flow arrangement with
respect to the concentric tube heat exchanger,
concentric tube heat exchanger - this is one
tube inside another tube. Now let us say this
is the hot fluid flowing through the inner
tube; it is flowing in at this point so we
say hot fluid flowing in at this point and
it is flowing out at this point so let us
say this is hot fluid flowing out at this
point
out and in. Now the cold fluid comes in at
this point; let us say this is cold fluid
in and goes out at this point.
Now, notice the 2 fluids - the hot fluid and
the cold fluid - are both flowing parallel
to each to other in the same direction along
the length of the heat exchanger. The hot
fluid is flowing through the inner tube like
this and the cold fluid is flowing through
the annulus like this. Since they are both
flowing in the same direction we call this
a parallel, when they are both flowing in
the same direction we call it a parallel flow
arrangement. And of course as mentioned earlier,
the heat transfer is taking place through
the
surface of the inner tube. This is the heat
where the heat transfer is taking place from
the hot fluid to the cold fluid across the
surface of the inner tube, so this is why
we call it a parallel flow arrangement.
On the other hand, if I reverse the flow of
one of these fluids then it will get a counter
flow arrangement. So, all I have to do now
for a counter flow arrangement is to reverse
the flow of one of the fluids. So let us say
now, this is still the hot fluid in, we will
call this still hot fluid in and going out
here hot fluid out. But now, the cold fluid
enters at this point so we will say this is
cold fluid in
and leaves at this point. So now, notice the
hot fluid flowing through the inner tube is
flowing axially in this direction, the cold
fluid flowing through the annulus is flowing
in this direction. They are flowing in opposite
directions, both parallel to the axis of the
heat exchanger but in opposite directions
and that is what we call as a counter flow
arrangement. The heat exchanger is still taking
place in the same way from across the surface
of the inner tube; there is no change there
but the direction of the 2 fluids relative
to each other is different. So this we call
as a counter flow arrangement when the 2 fluids
flow in opposed directions.
The third type of basic arrangement is what
we call as a cross flow arrangement and let
me describe that for you now next.
In a cross flow arrangement, the 2 fluids
basically flow at right angles to each other
like this. Let us say this rectangle which
I am showing is part of some heat exchanger;
it is a plate which is part of some heat transfer
surface. And the hot fluid let us say is flowing
on the backside of this plate so we will say
this is hot fluid in and flowing out here;
so let us say the hot fluid is flowing on
the backside of the this plate and the cold
fluid is flowing on the front side of this
plate here like this. So we will say cold
fluid in and flowing out
like this, so we say this is cold fluid out.
Now this is a cross flow arrangement in which
the 2 fluids are flowing at right angles to
each other. One, in this case this is the
heat transfer surface which is the plane of
this paper, one fluid is flowing maybe in
front of this paper, one fluid is flowing
at the back of this paper. Now let us go back
again to the plate fin heat exchanger which
I showed you earlier; here is the plate fin
heat exchanger which I showed you earlier
and let us look at it again for a moment.
Now look at it - this is one fluid which would
be flowing like this and this is the other
fluid which will be flowing through like this
so this is a cross flow arrangement that you
have got here.
Now there is something more which I want to
also say - this is a cross flow arrangement
but in addition what is to be noted is we
have got fins here. Now when you have fins
which are in this heat exchanger on both sides,
both on the hot side and the cold side, a
fluid particle which enters here say in this
passage will go through the heat exchanger
and come out at this end here. It has no choice
- a fluid particle which enters here has to
be come out at this point. A particle which
goes in here has to come out at this point
because the fins make passages right through
along the length of the heat exchanger.
Now when this happens, when the fluid is,
let us say this is the hot fluid enters like
this with fins on it. It is forced to move
in not only between these plates but also
in the particular passage which is created
by the fins which are right here so this flow,
hot fluid, cannot move in a transverse direction
inside these channels. Similarly, this is
the cold fluid; once it enters here, some
cold fluid enters here, it has to stay in
that passage and move along the length of
the heat exchanger. It cannot move crosswise
in a transverse direction inside this parallel
plate channel. So the presence of the fins
in the parallel plate channels prevents any
transverse motion of the fluids - either the
hot fluid in its own channel or the cold fluid
in its channel. Now when this happens we say
this is cross flow, both fluids unmixed; unmixed
means not mixed with each other but cannot
mix in a transverse direction, no fluid in
a direction transfer type, the cold fluid
doesn't mix with the hot fluid. So, when we
say unmixed, we mean mixed with each other;
it cannot, it cannot mix with each other.
So both fluids unmixed is the situation that
you will get when you have fins on both sides
like this.
Now suppose I don't have fins both the sides
but I just have parallel plates, then in that
case a hot fluid entering in this channel
can - while flowing through - can also move
in this direction and mix with itself. In
which case we will say this is case of single
pass cross flow, both fluids mixed because
there are fins on either side, the fluid can
mix with itself. And if I have fins on one
side and no fins on the other side, then it
will be a case of single pass cross flow mixing
on one side, no mixing on the other side.
So in single pass cross flow, it is possible
to have flow arrangements in which the fluids
can either mix with each other or not mix
with themselves. Now, therefore, we say there
are 3 possibilities in single pass cross flow
and 3 possibilities are cross flow unmixed
on both sides, cross flow mixed on one side
unmixed on the other side, cross flow mixed
on both sides. When there are fins it is unmixed
on both sides, when there are no fins it is
mixed on both sides; keep that in mind. So,
in cross flow also there are 3 categorizations
possible.
Now there are 3, I mean one point which is
worth noting is that generally if we calculate
the heat transfer area for these 3 arrangements
- that is parallel flow, counter flow and
cross flow - if we calculate the heat transfer
area, typically we will find that the heat
for a given situation and we want to transfer
a certain amount of heat. Typically, we will
find that in parallel flow you need the maximum
area - heat transfer area, in counter flow
you require the minimum area. And cross flow
whether it is unmixed on both sides or mixed
on both sides or mixed on one side, unmixed
on the other, will require an area in between
these 2 extremes. So keep that in mind as
something which is a rough idea of what will
be the sizing that we get of heat exchangers
in a given situation.
Now let us talk a little about the overall
heat transfer coefficient and fouling factor.
In a heat exchanger, in a direct transfer
type heat exchanger suppose there is a plane
wall. From your study of conduction, you know
that in a plane wall there are heat transfer
coefficients on the two sides - h1 and h2
- and if b is the thickness of the wall and
k is the thermal conductivity of the wall,
then the overall heat transfer coefficient
is given by 1 upon U is equal to 1 upon h1
plus b by k1 plus upon h2 and if it is a tube
like in a shell and tube heat exchanger or
a concentric tube heat exchanger, the overall
heat transfer coefficient 1 upon Uri will
be given by1 upon hi plus ri by k log to the
base e ro by ri plus ri by ro 1 upon ho.
Now these equations for calculating the overall
heat transfer coefficient where h1 and h2
will come from our knowledge of convection
or hi and ho will come from our knowledge
of convection. These equations for calculating
U are valid for clean surfaces. In practice,
with the passage of time, solid deposit is
usually tend to accumulate in the form of
thin layers on the surfaces on the heat transfer
surface and these deposits present an additional
thermal resistance to the flow of heat.
Now next time, we will talk about how to take
account of this additional thermal resistance
that comes in when because of the thin layers
which form on the heat transfer surface. This
formation of thin layers is called fouling
and we account for it by what is known as
introducing fouling factors in our expression
for the heat transfer coefficient; next time,
we will look at that aspect.