Temperature Cross - better understanding 7

[Hari1207]

Hello all!

I'm new to Shell & Tube Heat Exchanger Thermal design and I have come across the term "Temperature Cross" in Counter flow Shell & Tube Heat Exchanger.

What is Temperature cross in Counter flow Shell & Tube Heat Exchanger?

I read in Heat Exchanger Design Handbook by Kuppan Thulukkanam (2nd Ed), it is written as follows in the screenshot.

I have also posted my question in the same image.

Please explain me to understand this term in a better way.

Thanks in Advance.

 

[MintJulep]

The x-axis of the graph is length.

At every point along the length Thot > Tcold.

 

[Hari1207]

Hello MintJulep,

I don't understand your answer. Your answer is just the explanation of the counter flow heat exchanger.

Also your statement "At every point along the length Thot > Tcold" is contradicting from your picture. Please have a look again.

Thanks for any response related to Temperature cross.

 

[MintJulep]

Look at the arrows.

 

[Hari1207]

Please explain it in detail if you know the answer.

Thank you.

 

[MintJulep]

At the left side:
The hottest hot fluid goes in and the hottest cold fluid goes out.
The hot fluid is hotter than the cold fluid, so heat is transferred from hot to cold.

At the right side:
The coldest hot fluid goes out and the coldest cold fluid goes in.
The hot fluid is hotter than the cold fluid, so heat is transferred from hot to cold.

In the middle:
Middle temperature hot fluid flows left to right.
Middle temperature cold fluid flows trigger to left.
The hot fluid is hotter than the cold fluid, so heat is transferred from hot to cold.

At any other point along the length the same thing is happening.

The hot fluid is always hotter than the cold fluid at the same point, so heat is transferred from hot to cold.

 

[Hari1207]

Thank you. This is my understanding of Counterflow heat exchanger working same as yours. However my question is about the Temperature cross. Also about the extract from the book.

Please share your insights if you know anything about it.

Thanks again for your time.

 

[Gr8blu]

Hari What Mint is telling you is that if the design (or at least an assumption) is wrong it is possible that we get a TEMPERATURE CROSS which is a point somewhere along the path where Thot is LESS than Tcold. In that instance the heat transfer is from Tcold to Thot, which for the purposes of the supposed design is a NEGATIVE transfer - which means you have a problem in your design and the only way to fix it is to DRASTICALLY increase the surface area between the two fluids OR significantly increase the flow rate of the cold fluid.

Converting energy to motion for more than half a century

 

[MintJulep]

Need to see what's earlier in the book.

Perhaps the author of trying to explain why some simplified calculation doesn't work when there is temperature cross.

In the context of a simple counterblow heat exchanger, "temperature cross" simply means that the leaving cold fluid temperature is higher than the leaving hot fluid temperature. That's all.

 

[GBTorpenhow]

There is some good discussion in your previous thread a couple years back:

https://www.eng-tips.com/viewthread.cfm?qid=483916

I think "thermodynamic violation" is too strong, temperature cross is just a region of the heat exchanger where the transfer of heat is going the opposite way from what is desired. Might be easier to see with a 1-2 heat exchanger:

 

[Hari1207]

GBTorpenhow, thanks for referring to my previous threads. In fact I was trying to reopen it, but I couldn't. So, I started the new again. And also I started working in other equipment after my earlier discussion.

Coming back to discussion, I agree with your image explanation, In 1-2 exchanger, this is clearly explained & I could visualize it very clearly. My doubt is in 1-1 heat exchanger. How that Author is saying it is thermodynamic violation. That is not correct according to me. That's why I approached this forum to understand it in a better way.

Gr8blu, your explanation and the book explanation is same. But your reply is not the exact answer to my question I believe.

Please note that "No heat will transfer from Cold fluid to Hot fluid at any point of time & heat exchanger length for 1-1 heat exchanger". Because there exists always a positive temperature difference between hot & cold fluids along the length of the heat exchanger.

In that case why one should go and check the differences of Hot fluid outlet temperature and Cold fluid outlet temperature and make a statement/ comment that Heat Exchanger will not work, need more area or increased flow rate. What is the correlation of the Heat Exchanger area and flowrate between Temperature cross.

 

[Christine74]

I can understand why the OP is confused, Thulukkanam's book is just flat out wrong. It absolutely is not a thermodynamic violation to have the exit temperature of the cold fluid exceed the exit temperature of the hot fluid in a countercurrent heat exchanger. In fact it's often desirable because reduces the quantity of heat exchangers required. A temperature cross is pretty typical for countercurrent exchangers. It can also sometimes be accomplished in a multi-pass shell & tube exchanger but that's not nearly as common.

The author also seems to be using the same "temperature cross" term to describe the phenomenon in a multi-pass heat exchanger where there is a reversal inside the exchanger and the cold fluid actually heats up the hot fluid as shown in sketch (b) in the post above. These are completely different things (one is desirable, the other clearly is not) so he should have used different terminology. HTRI calls the undesirable cross an "internal temperature cross" to avoid confusion.

I also noticed that the author uses different nomenclature in the text (t c,o & t h,o) than he uses in the figures (t2, T1), making it even more confusing. The book basically looks like CopyPasta to me.


-Christine

 

[Hari1207]

Christine74, thanks for your great post.

 

[IRstuff]

It absolutely is not a thermodynamic violation to have the exit temperature of the cold fluid exceed the exit temperature of the hot fluid in a countercurrent heat exchanger.

Likely, it was a grad student doing the graph. The author's text says only:

The temperature cross is undesirable, particularly for shell and tube exchangers, because the tube surface area is not utilized effectively and hence there is wastage of capital cost.

The issue is the integrated delta-T, which is small, so the efficiency is poor, compared to the author's "normal" temperature distribution.

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[georgeverghese]

Better to read DQ Kern's "Process Heat Transfer" (now out of print in the Western world, but still being printed in India I think) or Perry Chem Engg Handbook on this topic.

 

[Christine74]

Even the small quoted portion of the book above is full of errors:

"(3) if tc.o is greater than t h.o, the difference (tc.o − t h.o) is referred to as the temperature cross or temperature pinch (Wrong. Temperature pinch refers to the minimum temperature difference between the hot fluid and the cold fluid at any point in the exchanger or exchanger network). In this case, the temperature approach (t h.o − tc.o) (Wrong. This formula only provides the temperature approach for a co-current flow exchanger which are very rare) is negative and loses its meaning (Huh? Negative numbers still carry meaning). Temperature cross indicates a negative driving force for heat transfer between the fluids (here he switches to discussing an Internal Temperature Cross which is an entirely different thing. He does not seem to be aware of this.). It requires either a large area for heat transfer or the fluid velocity to increase overall heat transfer coefficient (Sort of/not really. The portion of the exchanger that crosses typically has a small temperature difference so not a lot of the heat flow actually gets reversed). The underlying meanings of these three cases are brought out in Table 2.3 and the same are shown in Figure 2.5a. The temperature cross is undesirable, particularly for shell and tube exchangers (Not really. There's nothing special about shell and tube heat exchangers that makes the Internal Temperature Cross problem worse than any other type of heat exchanger)."

I agree that Kern's Process Heat Transfer is probably a better choice.


-Christine

 

[MintJulep]

Perhaps the author of trying to explain why some simplified calculation doesn't work when there is temperature cross.

I was trying to be kind.

Cristine74 has provided a more detailed review of the passage from Heat Exchanger Design Handbook by Kuppan Thulukkanam (2nd Ed),