Designing a Heat Exchanger Network

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Designing a Heat Exchanger Network

Designing a Heat Exchanger Network


In this screencast, we’re going to design
a heat exchanger network based on information we collected in a previous screencast with
the transfer of energy between two hot streams and two cold streams, and utilities that we
designated also in the last screencast. So a little background first on defining this
pinch point, and how we would go about starting to design our heat exchanger network. So if
we look at a heat exchanger between a hot stream and a cold stream, we’ve designated
our cold stream as C, so we have our inlet cold temperature, and our cold outlet, we
have our hot inlet, and our hot outlet. We have some heat transfer, Q, and a temperature
difference between the hot inlet and cold outlet as delta T2, the same on the other
side, we designate it delta T1, and then the values Ch and Cc are the specific heat flow
rates, and the units on this is given as an energy per temperature. So if we write out
our equation for our heat transfer, it’s just going to be for the hot stream our Ch times
the temperature difference between the outlet and the inlet, and we can do the same for
the other side. So we can rearrange these equations to get the following. Now hopefully
what you see here is that if we subtract the two equations, these two groups form our delta
T2 and these two groups form our delta T1. This means that we get the following result,
where delta T2 minus delta T1 is equal to the heat transfer, Q, times Cc minus Ch over
Cc times Ch. So what does this mean? This means that when we’re designing our two heat
exchanger networks, one on the hot side and one on the cold side of the pinch point, we’re
going to examine them in the following manner. First, we use this equation to say that if
we’re designing on the hot side, then delta T1, this is for the hot side of the pinch,
delta T1 is going to be our designated minimum approach temperature. This leaves us with
a delta T2 equaling delta Tmin plus the value on the right side. Now, to ensure that our
delta T2 here is going to be greater than our approach temperature, this value on the
right side has to be positive. Now Q is positive and our heat capacity flow rates are positive,
so that means Cc has to be greater than or equal to Ch. Now the following is going to
be true if we’re looking at the cold side: We set delta T2 equal to delta Tmin, we rework
this equation, this time there’s going to be a negative sign. Now for this to be true
and to make our delta T1 is going to be greater than delta Tmin, our Ch has to be greater
than or equal to our Cc. So we use these two guidelines when we’re designing our heat exchanger
networks to match streams and determine whether or not it’s a feasible match. So let’s practice
this on our diagram here where we have our four streams, our pinch point, our pinch temperature
for both the hot and cold stream, and our source and target temperatures. So if we start
with the hot side, which is arbitrary but something that’s typically done is to start
with the hot side of the pinch, then we want to match two streams such that Cc is greater
than or equal to Ch. So in that case, if we matched C2, we could match it with either
H2 or we could match it with H1, since in both cases, our value of 5 is greater than
the values for the hot streams. However, if we match C2 with H2 and try to match C1 with
H1, you would see that H1 has a higher specific heat flow rate value than the cold stream,
and that would mean that the stream match is an infeasible match. So we wouldn’t want
to do that. That leaves us with let’s start with C2 to H1, and then also draw C1 to H2.
So we want to do this to maximize the potential load on the heat exchanger, so we would calculate
what the heat transfer would be for that connection. So if we look at this node right here, and
we say that Q is equal to 3 times the difference of 180 minus 90, then we would get 270 kilowatts.
We do the same thing for the other node, and we would see that 5 times 140 minus 80 is
equal to 300 kilowatts. As you can see, at this node, we could decrease the temperature
from 180 all the way to 90, but we cannot increase the cold stream from 80 to 140. So
we can check mark that node knowing that we can go from 180 degrees and cool it to 90
degrees, but for the other node, we know that this load is 270 kilowatts, which means that
we could raise the temperature from 80 degrees, and we get that using the same calculation,
just showing that Q is equal to 270 times 5 and the temperature difference, which we
could say is x, some unknown temperature, minus 80. We could calculate the load from
our stream, our connection between H2 and C1, again you can see with the hot stream
we have a lower load than that with the cold stream. So we can decrease from 150 to 90,
but we cannot increase the cold stream 1 from 80 to 135. So again we write the load under
the connection, and we calculate what the new temperature would be, which in this case
would be a 30 degree difference and a maximum of 110 degrees. So we’ve already reduced our
hot streams from the inlet to our pinch temperature, so we would need to add some kind of hot utility
to raise it from their temperatures to their final target temperatures, and we can write
again what the duty on these utilities would be. So to go from 110 to 135, change of 25
degrees, times our capacity ratio of 2 would give us 50 kilowatts. So our temperature difference
of 6 degrees times our heat capacity flowrate gives us a load of 30 kilowatts for this exchanger.
So as you can see, adding up the exchangers on the left side, we have 80 kilowatts, which
is what we had calculated before as our minimum energy target requirement for the hot side.
Now moving to the cold side, we do the same thing and we follow the rule that our hot
side loads have to be greater than our cold side. So we don’t have a C2, we just have
the C1 to work with. So let’s match that, following the rule that it has to be higher,
we’ll match it with H1 and we’ll calculate the Q associated with this. If it were to
go from 90 to 60, that would be a temperature change of 30 times 3, which gives us 90 kilowatts.
On the bottom, to go from 30 to 80 gives us 100 kilowatts. So again our limit is 90 kilowatts,
so we’ll put that under the exchange, calculate what our temperature difference would be,
so we can completely cool our hot stream 1 from 90 to 60, but we can only heat up our
cold side stream from 80, and this is going to be 35 degrees, and we calculate that again
by 90 equaling 2 times 80 minus x, and you’ll see that x is equal to 35. So now we need
to cool our H2, and we could either do that trying to use our cold stream or a cold utility.
Since we’re at 30 degrees as our inlet and 35 as our outlet, we’re not going to worry
about crossing over our minimum approach temperature. So we could go ahead and try to match these
up, and the load that we would have would be the 5 degrees times 10, so it would be
10 kilowatts, so we can effectively cool down H2 10 degrees to our 80 degrees Celsius. However,
our final temperature that we are looking for for our H2 stream is 30 degrees. So at
this point, we’ve completely heated our C1 stream, we’ve completely cooled our H1 stream,
and we have to further cool H2, so we have to add some kind of cold utility that would
take us from 80 to 30, and that would be equivalent to using 50 kilowatts of cooling. And if you
recall, that’s what we had determined in the previous screencast for our MER target for
the cold side of the pinch point. So our redrawn final heat exchanger network would look like
the following, making sure we have appropriate connections between the streams. We would
label the duties under each connection, and then our utilities. You can see we have our
cold utility here, and our two hot utilities on the hot side. So this is a network designed
to minimize the energy requirements from our utilities. Now, another possibility is to
identify heat loops and minimize the amount of heat exchangers, and that’s something we’ll
discuss in further screencasts. So hopefully this gives you a good idea on how to design
a heat exchanger network using the temperature interval method and stream matching at the
pinch.

49 thoughts on Designing a Heat Exchanger Network

  1. This is an excellent video, however you make a slight error in your final exchanger. You can't have a heat exchanger going from a hot stream with a lower specific heat to a cold stream with a higher specific heat on the cold side of the pinch.
    Keep up the good work and I can't wait for the heat loop breaking video!

  2. Actually, that rule only applies at the pinch. Heat exchange between H2 and C1 do break the convention of Ch>Cc on the cold side but this is okay since the other exchanger is internal to this at the pinch. The minimum approach temperature dTmin is not broken.

  3. Thanks for the educative screen cast. i think the estimated temperature of C1 in the cold side of the pinch should be 90/2 + 30 = 75. Pls check

  4. This is very helpful. As mentioned by another viewer, I would really appreciate a video on the heat loops. Thanks!

  5. I made the same mistake, you need to use the 1kW/deg C not the 2 kW/deg C. Then you can get 80 deg C. The 2 kW/deg C is used to obtain the excess 10 kW.

  6. Hi there.. Was wondering how do you calculate the estimated temperatures at the pinch points; i.e. 90 deg for H1 and H2 as well as 80 deg for C1 and C2?

  7. Quick Question. On the hot side, can you have a heat exchanger where Ch>Cc but part of the Ch stream has already been cooled by another Cc stream?

  8. Dear Prof,

    I have the following discrepancies in your presentations

    1.In Step 1 procedure, you actually swap the values of H1 and H2 streams.
    1.In HEN Network Formation, You would not be able to connect H2 with C1 because, C of H2 is less than C of C2

  9. How do you say that the C1 to H2 is 10 kw, in Cold section. Is the pinch temperature of C2 is changed is from 80 to 35 deg C in Cold Station?…… Then It make sense.

    Kindly share your feedback.

  10. In cold side networking you have to remove heat from H1 and add heat to C2 . 90kW Adding to C1 heats it from 30 degrees to 75 degrees. How can you get 35 deg , you are not cooling down the cold stream i suppose, and the remaining 10kW is supplied by H2 making the cold utility 50kW.

  11. While evaluating in the cold side, the connection is made between H2 and C1. but the rule specifically says that Ch>=C1. In this case 1!>2. How can one connect these two streams? and what did you exactly mean by crossing over at that point?[8:12]

  12. Hello! I've a question over here. We know that AspenTech Plus has this utility to execute pinch analysis. Does Aspen works exactly like what u've expained? It's just for making sure of what Aspen does exactly. Thank you 🙂

  13. i already watched this video and this helped me alot… but i asked you for the Network Optimization

  14. Question at around 5:00. Why can't we increase the cold stream from 80 C to 140 C? (@C2) I didn't understand the reasoning behind that 🙂

  15. Hi you mentioned you would post a video of identifying heat loops and minimizing the amount of heat exchangers. Do you know where can I find the video?

  16. Is that possible to have multiple pinch point? And how to solve the problem if it has two pinch?

  17. Where is the next video about the minimisation of heat ex and identify heat loop? I can't find it..
    Thanks for video, it helps me a lot though the exam 🙂

  18. Hello, I'm a Chemical Engineering student. I have a question, we are required to design a heat exchanger network based on an already reported process, what I don't really know is if we can only apply this method to the flows of the heat exchangers that we already have, or if we can also include the flows coming from other equipments (like reactors o flash separation devices)

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