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How Plate Heat Exchangers Work (Industrial Engineering)

WEBVTT
Kind: captions
Language: en

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- [Instructor] Welcome to the lesson,
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and this lesson we're gonna look at
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how plate heat exchangers work.
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As mentioned in the previous lesson,
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we have two inlets and two discharges,
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and they're installed on the fixed plate.
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Notice the hot fluid
goes in the top left side
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and comes out the bottom left side.
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The cold fluid goes in the bottom right
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and comes out the top right.
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Let me just spin it around
so we can see the arrows.
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So hot goes in at the top
and out of the bottom,
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and cold goes in at the
bottom and out of the top.
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The reason we feed the fluids
into the heat exchanger
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like that is actually quite simple.
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As the cold fluid goes
in at the bottom here,
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it's going to pass
through the heat exchanger
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and become warmer, and as it
does so, it becomes less dense,
00:00:57.500 --> 00:01:00.200
and then it's gonna come out of the top
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through this hole here.
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When it becomes less dense,
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it's gonna have a tendency
to try and rise above
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any of the fluid that is more dense.
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So that means it's gonna try and
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rise upwards out of the heat exchanger.
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So if we're pumping fluid
around the heat exchanger,
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it makes more sense to
pump it into the bottom.
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And then that way, as it gets warm,
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it's gonna pass up anyway
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and then out of the heat exchanger.
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If we did it the other way round,
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we'd be pumping the fluid
in cold through here,
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it would get warmer and
it would want to actually
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rise back up as its density decreases,
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and we'd have to put more effort into
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pumping it down and out through this hole.
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So as a general rule, if we're putting
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a cold fluid into a heat exchanger
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and it's gonna be heated up slightly,
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then it's gonna go in at the
bottom and out at the top.
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Reversely, if we take a hot fluid
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and we put it into the heat exchanger,
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like seeing here on the upper left side,
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then we're gonna put
the fluid in at the top
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because as the fluid cools,
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its density increases,
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and then we can take it out of the bottom.
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So there's no point trying
to fight the laws of physics,
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because if we do that, then
we're just gonna waste energy.
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And that's something that
we definitely don't want,
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because energy usually costs money.
00:02:22.140 --> 00:02:25.020
In order to learn how the
plate heat exchanger works,
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let's just imagine for a
moment, on the hot fluid side,
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we've got the fluid
going in at 50 degrees.
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I'm not gonna say Celsius or Fahrenheit,
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because you can work with
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whatever units you're comfortable with.
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But it goes in at 50 and comes out at 40.
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Correspondingly, if we've got
a 10 degree temperature drop,
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we're going to have roughly a 10 degree
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temperature gain on the cold fluid side.
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So if we go in at 50 and come
out at 40 on the hot side,
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let's just imagine that on the cold side,
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we go in at 30 and we come out at 40.
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Now these are rough approximations,
00:03:04.270 --> 00:03:05.880
but we'll stick with them for now.
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So 10 degrees lost on the hot fluid side
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equals 10 degrees gained
on the cold fluid side.
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If we're looking at thermodynamics,
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the example we're using
is not strictly correct,
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but for us, it's a good
enough approximation.
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Let's have a look what
happens to the fluids
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when they go in to the heat exchanger.
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We can see that on the first
plate, this plate here,
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none of the fluid flows
between the area of the plate,
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that is to say the start
plate, and the fixed frame.
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The reason we don't have
any flow in this area
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is because the frame itself
is a very poor heat exchanger.
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We don't really wanna heat up the frame
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and try and get rid of
that heat to the air.
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As you can see, the frame is quite thick,
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and so it's not gonna
transfer the heat very well.
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So the start plate has
gaskets that surround
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both of the inlets and discharge holes.
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Can see here it's completely surrounded,
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and on the next one as well.
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And if we go up here, you
can also see that the gasket
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is completely round here,
and also round there.
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And that means that
when we press the plate
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against the fixed cover, so we're pressing
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the start plate against the fixed cover,
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no flow is going to be able
to pass through the gaskets
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and onto the side of this plate here.
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So keep that in mind, the gaskets
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are sealing certain areas of each plate.
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For the start plate,
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we're sealing this entire
section of the plate
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so that no flow is allowed
between the fixed plate
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and the start plate,
at least on this side.
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However, on the next side,
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we can see that our gasket
has changed slightly,
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and it is the gasket that
is controlling the flow
00:05:01.180 --> 00:05:03.630
to every single plate.
00:05:03.630 --> 00:05:06.740
So now imagine this
plate is pressed up tight
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against the one on the
left where my mouse is now.
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We're gonna allow the fluid to come in,
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and it's gonna flow downwards,
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and it's gonna flow all the way down here,
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down here, down, down, down,
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this plate's quite long.
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And if we get down to the
bottom and I zoom out slightly,
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you can actually see the
flow is gonna come down
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and it's gonna pass to the bottom section
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where my mouse is now.
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When we get to this section,
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the flow is gonna flow to the left,
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and it's gonna be carried
out of the heat exchanger.
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So let's go up to the top again.
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Can see it comes
00:05:52.020 --> 00:05:56.203
in over here, try and align
this a little bit better.
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Comes in here, flows
downwards, zoom out slightly.
00:06:05.420 --> 00:06:08.710
In there, flows downwards,
downwards, downwards, downwards,
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and then goes out on this side here,
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and that is all that is
occurring for the hot fluid.
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If we have a look at some other plates,
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we can see the same thing happening again.
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If I angle this correctly,
you might actually see
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exactly how the flow goes
through the heat exchanger.
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Hot, cold, hot, cold, hot, cold, hot.
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Every hot plate, or every
hot side of a plate,
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has the same gasket.
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Can see here, this is a hot plate,
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look at the gasket in this area.
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Go across, same again, same again.
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So we know that we're
taking flow in at the top,
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it's coming down, it's flowing downwards,
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and it's coming out at the bottom left.
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Once we get all the way to the end,
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you can actually see that
we've got a hot plate
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on this side, or this is
the hot side of the plate.
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And on the opposite side,
'cause it's an end plate,
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we actually have a gasket,
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which was the same as
what we used at the start.
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And this prevents any flow
going between the plate
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and the movable cover,
or the movable frame.
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Now we do this for the
same reason as before.
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The movable frame itself is
not a good heat exchanger,
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so we don't use it as part of
the heat exchanging process.
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But let's now have a look at
what happens to the cold fluid.
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We've seen that the hot
fluid goes in the top left
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and comes out the bottom left,
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and it passes through
each of these plates.
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The cold fluid comes in the bottom right
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and it goes out of the top right.
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Let's pull up one of the cold plates.
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We'll start with this one here.
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The cold fluid comes in here,
00:08:04.200 --> 00:08:06.490
it flows upwards, upwards,
00:08:06.490 --> 00:08:10.420
and then reaches the top
right section of the plate
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and is pushed outwards by the flow
00:08:12.920 --> 00:08:15.523
from all the other
plates and from the pump.
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So the cold fluid is
entering the bottom right
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and flowing out of the top right,
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and the hot fluid is entering the top left
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and flowing out of the bottom left.
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And we're circulating the two fluids
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within the heat exchanger.
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Because we always have
this hot, cold, hot, cold,
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hot, cold pattern throughout
the heat exchanger,
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and because the plates
themselves are quite thin,
00:08:42.350 --> 00:08:44.140
we're gonna exchange heat
00:08:44.140 --> 00:08:46.600
from one side of the heat exchanger,
00:08:46.600 --> 00:08:49.510
so here where it's blue and cold,
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compared to the other
side which is red and hot.
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So they're coming not into
direct contact with each other,
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but they are coming into thermal contact.
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The hot fluid is going to be cooled down
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by the cold fluid, and the cold fluid
00:09:05.340 --> 00:09:07.773
is going to be heated up by the hot fluid.
00:09:08.760 --> 00:09:13.120
And that is essentially how
a plate heat exchanger works.
00:09:13.120 --> 00:09:17.370
The plate heat exchanger
allows us to bring two fluids
00:09:17.370 --> 00:09:21.370
into close contact with
each other, not direct,
00:09:21.370 --> 00:09:23.600
they're actually indirect contact,
00:09:23.600 --> 00:09:26.560
and allows us to exchange heat,
00:09:26.560 --> 00:09:29.450
so they're in thermal
contact with each other,
00:09:29.450 --> 00:09:31.180
and that allows us to exchange heat
00:09:31.180 --> 00:09:33.000
between the two flow mediums
00:09:33.000 --> 00:09:36.880
without them coming into
direct contact with each other.
00:09:36.880 --> 00:09:40.220
The process is incredibly efficient.
00:09:40.220 --> 00:09:41.590
The reason it's so efficient
00:09:41.590 --> 00:09:44.810
is because of the design of the plates.
00:09:44.810 --> 00:09:46.250
So let's now go and have a look
00:09:46.250 --> 00:09:48.933
at these plates in a lot more detail.
00:09:54.100 --> 00:09:55.260
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