Posted 29 April - 01:50 PM
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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.
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Stainless steels also do not transfer heat very well through them, so making your tubes out of stainless would likely make your boiler not steam as well. this is also why you wouldn't make a heat exchanger or feedwater heater out of stainless.
I would echo what someone said above... There is a reason why 90% or more of large scale locomotives have welded mild steel boilers with copper tubes in them... It the most cost-effective way to build a reliable boiler for your locomotive that will perform well. With some care and regular clean-out it should last you 20+ years. I've seen some fail after only 7 or 8 years, and I've seen some that were still OK after 40 years. Depends on your water, how clean you keep it, how you lay it up, how often you run it, how often you blow down while running, boiler treatment used, etc... lots of different factors.
I've seen several boilers fail, and even had one fail on me. Like was mentioned before, the boiler always developed a leak which was not dangerous but that made the boiler not able to perform. In my case, a tube started leaking around the rear tube sheet and would put out the right hand half of the fire.
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