13 replies to this topic

[Light]

Hello everyone

 

Although this topic has been discussed many times in the forum, still some of my doubts regarding the temperature cross occuring in heat exchangers aren't clear. I have read that temperature crossover indicates thermodynamic irreversibility and it should be avoided as far as possible. Moreover, it violates the second law of thermodynamics; then how does it occur in the first place? Isn't it advantageous to have our cold fluid heated to a temperature higher than the outlet temperature of the hot fluid? Then why do we consider it to be thermodynamically inefficient and try to avoid it?

Are single shell pass- multiple tube pass exchangers more prone to it or it can also occur in 1-1 type countercurrent HE?

Edited by samkha, 09 July 2014 - 10:14 AM.

[PingPong]

A temperature cross is easy to achieve in a countercurrent heat exchanger.

 

A temperature cross is very difficult to achieve in an exchanger with multiple tube passes, because the value of the LMTD correction factor F would become very low, and therefor it would require much more area just to achieve a little more duty.

[Bobby Strain]

Ping,

      I'm not sure I understand your first statement above. Maybe an example would clarify it.

 

Bobby

[PingPong]

In my 30+ years experience a temperature cross is always defined as: outlet temperture cold stream is higher than outlet temperature hot stream.

 

In a countercurrent exchanger that is easy to achieve (as long as cold outlet temperature is lower than hot inlet temperature, and hot outlet temperature is higher than cold inlet, obviously).

 

In a multiple-tubepass exchanger with segmental baffles F = 0.8 for equal outlet temperatures of hot and cold streams (so no cross). If one would want the cold stream outlet temperature to be higher than the hot stream outlet, thereby creating a temperature cross, then F would rapidly drop towards 0 and the required exchanger area would increase rapidly for only a small increase in duty.

[Bobby Strain]

Ping,

     Thank you. I'm nearing 50 years experience. Now Samkha should fully understand what is meant by the term. All of us with experience, some of it bad, know what the term means. But a lot of definitions we use, and assume everyone understands, are not always obvious. I wonder how a programmer would use words to describe a temperature cross? And you can also express the term as "hot fluid outlet temperature is below cold fluid outlet temperature". And, of course, a Q-T diagram can clearly show what is happening.

 

Bobby

Edited by Bobby Strain, 09 July 2014 - 02:28 PM.

[Light]

Thanks to Ping Pong and Bobby Strain for replying and sharing valuable information. But some questions are still unanswered; Is a temperature cross desired or not? And why is it considered to be thermodynamically inefficient?

[PingPong]

A temperature cross is not thermodynamically inefficient, to the contrary.

 

To achieve a temperature cross in conventional shell and tube exchangers (with multiple tube passes and segmental baffles) merely requires multiple shells in series, as a cross is not achievable in only one shell. This is done all the time in refineries, petrochemical plants, et cetera.

 

In plate and frame exchangers with pure countercurrent flow a big temperature cross is nearly always used.

 

In cryogenic service, using countercurrent flow brazed aluminum exchangers, also a huge temperature cross is achieved, because that is most energy efficient (thermodynamically efficient) as refrigeration requires a lot of power..

 

I don't know what source gave you the wrong impression, but you best ignore it.

[Light]

One more question. It is always tried (I may be wrong) to achieve as low a temperature approach as is economically possible in designing any heat exchanger. Does this have any relation with achieving temperature cross?

[PingPong]

The economically optimal Minimum Temperature Approach (MITA) of a heat exchanger can result in a temperature cross.

No problem, it only means that multiple shells in series are required for that exchanger, unless it can be designed as a countercurrent exchanger.

 

Note however that the number of shells must be included in the economical evaluation for MITA, as two shells of A m² each are more expensive than one shell of 2A m².

[Light]

How does having multiple exchangers in series help in obtaining temperature cross, meaning what difference does it make if we increase the length of the tubes instead of having many shells in series? 

[PingPong]

If each shell has 2 or more tubepasses (which is the case in most shell & tube exchangers that are used) it makes a lot of a difference.

 

You should read about TEMA types and about the LMTD correction factor F.

[srfish]

The more shells in series there are, the larger the temperature cross will be that can be achieved. Refer to a TEMA manual or a  heat transfer text for their charts on LMTD correction factors. In using these charts you can see the effect of the number of shells in series has on the effect of a temperature cross. If you are interested only in the number of shells required in series, there is a graphical method that can determine the number.

[Bobby Strain]

I posted this before. But here again you can download a quick calculator for heat exchanger effectiveness factor.

 

Bobby

 

http://www.mediafire...fectiveness.exe

[xavio]

Hello all,

 

Small temperature cross is achievable using one shell, that is TEMA F shell with two tube passes.

 

Further reading see here: https://www.aiche.or...-heat-exchanger

 

xavio