6 replies to this topic

[theleftcoast]

Hello Everyone,

I found a resource for sizing heat exchangers that recommends a tubeside fluid temperature difference of no more than 50F (28C) citing that the "high temperature gradients can cause undesirable thermal stress on the tubesheet". I only have a limited mechanical background, but this seems to be a very small allowable tubeside temperature change. Does this seem reasonable? If not, what is the most appropriate maximum temperature change for the tubeside fluid?

Thanks,
JT

[Art Montemayor]

Lefty:

Please: ALWAYS cite your reference and quote directly from the same reference. We can't evaluate the merit or worthiness of your source of information without knowing what, who, where, why. Don't keep it a secret to yourself. The quality of a response is totally dependent on the quality of the query and the information, data, and background offered.

While you are correct, it is also correct that the tubesheet MATERIAL will also play a significant role in determining any limitations to its service. I have used (& fabricated) Brass tubesheets - and it should be obvious that this material is more limited than Stainless Steel Duplex.

We can't answer a general questions like, "what is the most appropriate maximum temperature change for the tubeside fluid?" without knowing what conditions, material, fluids, etc. are involved. That is just common sense.

[theleftcoast]

Thank you for your reply Mr. Montemayor.

The book I found this maximum temperature recommendation in was "Heat Transfer in Process Engineering" By Eduardo Cao. Unfortunately, I looked through this resource at the library and can't reference the exact passage right now because I don't have the book with me at my apartment. The maximum temperature difference of 50F for the tubeside fluid was suggested for any exchanger where multiple tube passes were used. The couple of lines that suggested that maximum temperature difference were kind of thrown out there without much supporting evidence other than citing the thermal stresses that would result over the entire tubesheet area. For this temperature difference recommendation, there were no guidelines specified for different materials of construction and no indication of what range of tubesheet dimensions (i.e. sheet diameter, sheet thickness, tube spacing or pitch) the rule applied to.

I am interested in the maximum temperature changes allowable in multi-pass exchangers because I am required to design a heat exchanger network as part of my senior project. The fluids I am heating and cooling in my process are light naphtha liquids and vapors ranging from 115F to 750F at pressures ranging from 300-400 psi. I know that many types of heat exchanger will be required for the heating requirements and wanted to research the limitations of common exchanger designs before I do fluid allocation and equipment sizing. Also, based on the temperature and pressures the fluids are at, steel construction will be the most practical.

This allowable temperature difference is one of the limitations uncovered by my research that I was unsure about and I posted my question here because none of my textbooks really cover details beyond basic transfer coefficient determination use of LMTD corrections. Practical aspects of fluid allocation and mechanical limitations were absent so I looked for other sources that would help.

Anyway, that is a little more background information to help frame my question. I will post the full excerpt when I get a chance to look through the book later today.

[wojtar]

Regarding your question: I also heard from people dealing with heat exchanger design that ~30C temperature difference between tubes & shell is a limit when exchanger may work without compensation...(I believe it's something like rule of thumb - i would check anyway during mechanical design).... never heard about such general limitation that exchanger may work only in range 28C in tubes...it would be quite narrow range...everything depends on material of construction & proper mechanical design as previous person said.

[theleftcoast]

Hello everyone. The recommendation is from pages 127-8 in "Heat Transfer in Process Engineering" By Eduardo Cao.

"Figure 6-15 shows possible pass-partition configurations for different numbers of passes. Since in each pass the temperature of the fluid changes, different sectors in each head will contain fluid at different temperature. Since high temperature gradients can cause undesirable thermal stress on the tubesheet, it is recommended that the pass partition configuration be such that the fluid temperature difference between the adjacent sectors is less than 28C (50F)"

Somebody recommended the sixth volume of Coulson and Richardson's "Chemical Engineering Design" as it goes into the topic of mechanical design in a little more depth. The book has a chapter entitled "Mechanical Design of Process Equipment" that was very clear on some design basics. There was even a small section section dedicated to just tubesheet mechanical design considerations. I could't find a way to calculate the limiting temperature gradient before thermal stress became too large so I'll just use the 50F delta T rule of thumb for my project.

Thanks again.

[latexman]

For that low of a delta T, I think they are referring to differential thermal expansion of the tubes compared to the shell. This will put stress on the tubesheets, but the most likely damage is bent/broke tubes. I notice the title of the post says "U-tube heat exchanger", but thats all that is said about it. U-tube heat exchangers are inherently immune to this problem compared to fixed tubesheet exchangers because the individual tubes are free to expand towards the U end. They are not anchored by a second tubesheet.

[theleftcoast]

I finally found an answer to my question at www.gulleyassociates.com/news.htm.

Too Large of Temperature Change in 2 Tube Passes

Warning! Large tube side temperature change. A big difference between the inlet and outlet temperature of the tube side causes leakage and bypass problems. The worst case is a shell and tube exchanger with two (2) tube passes where a gasket is used to seal between the passes. A careful analysis should be made if the temperature difference across the pass plate is more than 300 F. For a channel type that has a welded in pass plate, make an analysis if the temperature difference is more than 450 F. If this temperature difference causes an over stressed condition, possible cures are:

• Add a unit in series so each unit has a smaller temperature difference.
• Use one tube pass if the penalty isn't too great.
• For aircoolers, use a split headers design.

No specific material of construction is mentioned so I am assuming that this rule of thumb only applies to certain high strength alloys. Just thought I would share.

Thanks,
JT