S&Tube Hex -Overall Heat Transfer Coefficient

[Angsi]

Shell side is cooling water and tube side (hot side) has booster compressor discharge gas (dry hydrocarbon gas). The tubes are in stainless steel material, 316L, bare/plain tubes.

Can someone please advise me a typical overall heat transfer coefficient that can be used for this purpose? I have used 220 W/m2 K but was commented upon by a vendor saying that this figure was overly conservative and a figure of between 620 to 650 W/m2K was more appropriate?

Any comments please?

 

[MortenA]

If the data is for a specification of the equipment (your datasheet) then you could either leave the field "empty" and write: Vendor to provide, or use the range provided by the vendor and write: "vendor to confirm".

You should however consider that the overall heat transfer coefficient most likely will be "controlled" by the gas side. This means that if you e.g. change the gas flow rate (low flow scenario) then the overall heat transfer coefficient will change too!

E.g. the gas is really hot during start up (this is just an example) but at the same time the flow rate is low. This could cause the overall heat transfer coefficient to go down and mean that the HX has insufficient capacity to cool the gas to the required temperature - even though the flow rate is lower!

Overall heat transfer coefficient is a bit like selling rubberband by the meter.

Best regards

Morten

 

[25362]

From my sources, for cooling, not condensing, the hot hydrocarbon gas, your estimated 220 W/(m².K) seems to be in the ballpark, and not conservative at all.
A figure of 600 W/(m².K) appears to be erroneous and quite overstated.

 

[Angsi]

Thanks Morten A and 25362. 25362, the texts that I have been refering to quote the same too.

Because of the discrepancies in the U values used, the surface area required by the vendor's calc is like 1/3 of mine and that is a huge difference.

 

[Latexman]

h for a gas is a fairly strong function of pressure drop. Could the vendor be assuming much more dP than you?

Good luck,
Latexman

 

[Angsi]

Latexman, I just checked Vendor's datasheet. He has met the allowable 0.34 bar at the tubeside which is the gas (hot) side.

 

[Latexman]

At that dP, my preliminary design reference says use about 150 W/(m^2 K). The vendor's U is way out of line for plain tubes.

Good luck,
Latexman

 

[Angsi]

I have been checking various texts and articles for the last few days now and non of them commit any U values above 350 W/m2.K for the above service.

What I did then was to refer to some past projects we had done previously and I noticed that the U values used there (by a different Vendor to the one I am discussing with currently) was also approx 650 W/m2.k for the hot gas/cooling water exchanger. The software used was HTRI.

Does anyone know if the U values can be software (HTRI/HTFS) calculated or if has to be manual input to the software? How could this value be so different from the ones suggested by texts?

Comments will be appreciated.

 

[Latexman]

If you have identical (or similar) units in operation, just calculate the actual U from the process data.

Good luck,
Latexman

 

[pmover]

Angsi,

fyi, the HTRI software is widely used by seasoned and experienced heat transfer specialists in the business of designing heat exchangers. while i do not know exactly how the HTRI software operates, i do know that based on the data i've witnessed, the U is initially assumed and then computed, considering the TEMA type of exchanger and process conditions.

definitely trust your instincts and Latexman suggestions.

good luck!

-pmover