6 replies to this topic

[Steve Hall]

There have been several queries and examples about temperature crosses in shell-and-tube heat exchangers. With this post and spreadsheet I show how this works.

Consider an exchanger with cold fluid t and hot fluid T. By definition t-in < T-in. Usually t-out is also < T-out. But in some cases, t-out > T-out. This is a temperature cross.

Example: water enters an exchanger at 85 degF and leaves at 95 degF. The heating medium is tempered water at 100 degF and it leaves at 90 degF. There is a temperature cross.

If this occurs in a perfect counter-current heat exchanger there is no problem. But what if there are two or more tube passes? The Rule of Thumb is that you cannot have a temperature cross in such an exchanger.

However, temperature crosses are found in multi-pass exchangers. It can happen because the shell fluid leaves (or enters) the exchanger in the middle of the tube bundle.

I created a simple Excel spreadsheet to illustrate the concept and compute a hypothetical temperature profile through the exchanger. You'll see that the section of the exchanger closest to the tube in/out nozzles does most of the work (corresponding to the two ends of the tube path). The workbook includes a macro to perform the calculations in the algorithm that is described.

Steve

Attached Files

•  exchanger zones.xls   105KB   1223 downloads

[sheiko]

Thanks Steve.

Do you know other causes that could explain temperature cross in, let's say, a E-type shell heat exchanger for example?

Edited by sheiko, 19 July 2012 - 09:22 PM.

[Steve Hall]

My example shows an E type shell. I didn't mean to give a "cause" for temperature cross, just to illustrate that it is possible. There have been conflicting posts on this topic.

A temperature cross is indicated when the inlet temperatures and flow rates of the shell and tube fluids are close and there is sufficient heat transfer area to result in the cross. With a true countercurrent exchange, it is theoretically possible for the outlet temperature of one fluid to essentially equal the inlet temperature of the other.

[srfish]

I think everyone agrees that there is no temperature cross problem in a counter-current heat exchanger. Sometimes a temperature cross is desired in an E-type shell if you need to use as few of shells as possible. The problem is that in a multiple tube pass heat exchanger usually a temperature cross of only approximately 5 F. is acceptable.

[Santoshp9]

Dear All,

In Conical Head E type Air Preheater temp. cross is possible.

We have air preheater from M/s Doright,China having Design Inlet Smoke gas temp: 1070 degC & Outlet : 720degc tube side .

Whereas Shell side Air Inlet 80degc & outlet 950degC.

 

Regards,

Santosh

[meetyourmaker]

Santosh:

 

This is a significant temperature cross. This exchanger must be a 1-1 exchanger.

[Santoshp9]

meetyourmaker,

Yes you are right.