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What is Temperature Cross in Shell and Tube Heat Exchanger and how to avoid it? 5

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Hari1207

Hari1207

Mechanical
Mar 9, 2020
14
Hello All,

I'm currently learning S&T Thermal design, which is very new to me.

In this I come to know about the Temperature cross. I do not find enough explanations in google and methods to resolve it.

Can anyone please explain what is Temperature Cross and why it is happening?

Also, please explain how to avoid it in S&T Thermal design. May be shells in series?

I would like to know whether there is correlation between this temperature cross and LMTD correction factor.

Is Temperature Cross really a inevitable and to be avoided for good design?

Thanks in Advance.
 
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Hello IRstuff,

I read that Cividac website but it doesn't answer my question completely.

Thats why I approached here.

Can you please explain here in detail?

Thank you.
 
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Temperature cross is something that happens in counter-current flow exchangers when the cold side exits at a higher temperature than the hot side exits. It isn't a bad thing, but instead just something that can happen in certain conditions.

It is not an inevitable outcome from an exchanger, so if it is undesirable for your process, you can prevent it by adjusting the setup:

- A faster flowrate of either the cold or hot side could reduce the heat exchange enough to prevent it.
- Adjusting the inlet temperature of either side, so the differential between your hot and cold sides is smaller, could prevent it.

The LMTD correction factor is a calculation tool for modelling/design, whereas the temperature cross is something that physically happens in the exchanger. So they aren't really related beyond that the correction factor would be part of the calculations for predicting when you might get a temperature cross.

Andrew H.
 
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Hi Andrew!

Thank you so much for the explanation.

I'm doing now thermal design of Shell and Tube heat exchanger.

Here I have provided with temperatures & flowrates as well as heat duty to be removed from my client process team.

Hence I don't have controls over the temperatures, flowrates etc.

What I can do is adjusting my exchanger geometry in such a way that I can overcome this problem.

For example, shell side fluid is Cooling water with temperatures - 35 & 55 deg. C and flowrate is 444724 kg/h, whereas on the tube side Gas is flowing at 144458 kg/h and temperatures are 123 & 40.

While design this exchanger in HTRI, I got the following message.

image_zx3oeq.png


Please suggest me the steps to be taken care in selecting the proper geometry for heat exchanger. Example making shells in series or using 'F' type shell. I'm not sure about series & F types one of my friends suggested this. But I would like to know the exact solution.

Thank you.
 
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I honestly don't know much about exchanger design. I understand the basic concepts, but design details are not something I am well versed in at all. Hopefully another member here can assist you with the specifics.

Andrew H.
 
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If you are meeting your design requirements for cooling the gas flow, then there's really no issue, per se, with the crossover.

The simplest solution to that would be to run more water through the exchanger.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Temperature cross flips your hot and cold sides partway into the heat exchanger, essentially robbing performance. This should not be done intentionally as you're creating a heat exchanger that is purposefully wasteful, but it shouldn't cause damage or anything beyond design efficiency.

With the cold side no longer colder, how is the hot side being cooled through that section? Plot your temperatures as a function of HX length and you should see it a lot better.

If you can't adjust mass/volume flowrates or approach temperatures, your only other option is to control via geometry to decrease performance. Limit surface area, decrease reynolds number, adjust tube size and density, decrease baffling on the shell to limit dwell time, add series shells, etc.

If I had to guess I'd say you should reduce the length by about 25.875% and see how short of the design goal you are, and adjust from there.
 
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A temperature-cross does not "rob performance". It is what makes a counter-current heat exchanger perform better than a co-current one. There is only a problem when the heat exchanger is not purely a counter-flow type, such as a shell and tube exchanger with a U-tube bundle rather than straight tubes.
 
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Compositepro said:
A temperature-cross does not "rob performance". It is what makes a counter-current heat exchanger perform better than a co-current one. There is only a problem when the heat exchanger is not purely a counter-flow type, such as a shell and tube exchanger with a U-tube bundle rather than straight tubes.
Click to expand...

OP did not specify what arrangement he's working with aside from Shell & Tube.

My choice of words is probably not the best since my time to write a response is rather limited, but there are many instances where temperature cross is to be avoided. And if your software is warning you about decreased performance due to temperature cross it's probably either sophisticated enough to know when it's an issue, or the user should know when to ignore the error. I'm assuming based on the OP coming here wanting to know how to avoid it he is being told to eliminate it, which wouldn't apply to a single pass counterflow unit.
 
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Rputvin, I apologize for my insensitive wording. I, too, was pressed for time and should have chosen my wording more carefully. The U-tube Shell and tube heat exchanger will always be less efficient than a pure counter-flow exchanger was kind of my point, so why worry about temperature-cross? There are plenty of reasons that U-tube exchangers are so popular, though. So if your are designing a new U-tube heat exchanger, a temperature cross means that the exchanger is over-sized for the application, I guess.
 
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Lots of confusion in this thread. Traditionally a "temperature cross" in a heat exchanger happens when the outlet temperature of cold fluid exceeds the outlet temperature of the hot fluid. This can easily be done if you have pure countercurrent flow in your heat exchanger, but shell and tube exchangers rarely have pure countercurrent flow mainly for reasons involving the mechanical construction (typically requires the use of longitudinal baffles, fixed tubesheet designs, or single-pass floating head designs with internal bellows expansion joints).

What the OP is talking about is what HTRI calls an "internal temperature cross", i.e. where the heat transfer reverses direction and actually flows from the "cold fluid" to the "hot fluid". Typically this happens in multi-pass shell and tube exchanger where there is an external temperature cross (cold fluid outlet temperature exceeds the hot fluid outlet temperature). I don't think this situation is cause for concern as long as the program you are using to do the thermal design accounts for the heat transfer reversal. The temperature difference where the heat transfer is reversed is typically quite small so it usually doesn't have much effect on the overall performance.

Normally the way I resolve this problem is by running the flow in the first tube pass in the cocurrent direction instead of countercurrent.


-Christine
 
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Not sure if that's accurate; if there's a temperature crossover, it should mean that the shell side has insufficient heat carrying capacity to keep the temperature low, and if it is a U-tube, then part of the shell is actually heating back up the outlet of the hot side , and the remedy should be to crank up the flow of the cool side.

U-tube_heat_exchanger.PNG


TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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It's accurate, and the OP said that the temperatures and flowrates are fixed.


-Christine
 
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In HTRI go to Geometry > Exchanger > Flow Direction > Flow in 1st Tube Pass and select "Cocurrent" from the drop-down list. This will reverse the positions of the shell-side nozzles and make the last tube pass countercurrent, which should eliminate most (if not all) of the internal temperature cross problem.

Switching to an F-shell would also work because it would create pure countercurrent flow (temperature cross but no heat transfer reversal) but it will increase your shell-side pressure drop substantially, plus they have maintenance issues.


-Christine
 
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Christine74,

Thank you for those explanations. I see now what I didn't understand about the OP's question and why it wasn't as straight forward as I originally thought. So when the tube geometry causes that heated 'cold' fluid to pass by the now cooled 'hot' fluid that is when performance suffers. This is an incident caused by where the path of the tubes reside in the flow path of the shell side.

Andrew H.
 
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