Heat Exchanger Puzzler: Make the Best of a Blunder

Readers suggest ways to get an incorrect exchanger to do the job.

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If I had mistakenly assumed an adiabatic process, the temperature would be: (388 + 460)×[(14.7+180)/(14.7+200)](1.3-1)/1.3 = 829°R (369°F). This is an error of only 3%. The pressure drop is small, but the temperature difference is significant: 12°F.

The original LMTD was: (253 – 153)/Ln (253/153) = 198.8°F; ∆T1 = 353°F – 200°F = 153°F; ∆T2 = 353°F – 100°F = 253°F, where 353°F is the temperature of saturated steam at 125 psig. The new LMTD with 180-psig steam would be: (281-181)/Ln (281/181) = 227°F. Raising the steam pressure increases the LMTD by 14%.

One serious problem with increasing the temperature difference is its effect on thermal expansion inside and outside the heat exchanger. You will want a structural engineer to review the piping; the exchanger manufacturer should look at the effect a higher temperature will have on the tubesheet, internals and the gaskets. Also, check the maximum allowable operating temperature (MAOP); usually 500°F is a convenient limit for carbon steel, but verify.

I considered switching to an oil but the heat transfer coefficient would be in the 100–300 BTU/hr-ft2-°F range, well below the 1,000 expected with steam. With 500°F as a maximum, thermal fluid is out. Electric heating might be a possibility but this would involve a serious overhaul and electric heaters cost about 170% more to operate compared to steam generated by methane.

Now, let's consider some operational options. Recycling the process fluid through for a second pass might make sense if piping allows. This will require new temperature controls to avoid overheating and allowances for thermal expansion of oil trapped in isolated piping.

Another option would be to increase the process flow velocity as high as possible. One of the risks with this choice is tubesheet vibration, which can cause a number of corrosion problems.
Dirk Willard, senior process engineer
Middough Consultants, Holland, Ohio

JULY'S PUZZLER
In our gas plant we pump natural gas liquids (NGL) with a double-suction high-speed centrifugal pump. It runs with a discharge pressure of 60 Barg at 6,700 rpm, and is designed for a flow rate of 670 m3/hr. A booster pump discharges at 20 Barg to the NGL pump suction. The NGL, which has a specific gravity of 0.52, then travels about 400 km to our refinery for fractionation. About 35–40% of the pumped fluid recycles into the surge bullet via a recycle valve, wasting energy. There are other problems: vibration trips in low flow due to shaft deflections, seal leaks, etc. So, we're planning to buy a new pump, preferably one that can save energy and avoid such problems. What type of pump do you consider best for this application?

Send us your comments, suggestions or solutions for this question by June 10, 2011. We'll include as many of them as possible in the July 2011 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you'd like to pose to our readers, send it along and we'll be pleased to consider it for publication.

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