3 replies to this topic

[Mia]

Hi,

I have done an experiment using a U-tube heat exchanger, heating water using steam. Steam is on the tube-side and water on shell-side. For a fixed steam pressure, the flow rate of water is varied.

For steam at 8 psi, the overall heat transfer coefficient increases as the flowrate of water increases.

However, using steam at 5 psi, the overall heat transfer coefficient increased from 140 to 160 W/m2.C (at water flow rates of 3 and 5 L/min, respectively) but then fell to 86 and 92 W/m2.C (7 and 9 L/min).

Why does this happen?

Regards,
Mia

[Art Montemayor]

Mia:

We need to know more details and data about your experiment before giving you a firm response on your resulting Overall Heat Transfer Coefficient going bad.

Please refer to the attached Workbook and read my comments. We don't know if the 8 & 5 psig saturated steam (and here I have to criticize you for not specifying that the pressure cited is in gauge units and the steam in question is saturated. These are important details for all engineers. We can't all be "guessing") is sufficient to overcome the pressure drop through the tube bundle and the condensing rate. Why such a low saturated steam pressure? How are you extracting the condensate? How are you maintaining the steam pressure constant - and where are you measuring the steam pressure? How do you know that the tubes are 100% drained and devoid of any liquid condensate that would affect the conductive film coefficient? I once had a very similar problem with an MEA reboiler steam bundle in Jamaica, W.I. We resolved the drainage problem by fabricating our tube bundle in a "V" shaped profile rather than a "U". The overall heat transfer increased dramatically with the "V" shaped bundle. Since then, I have always made it point to install that design rather than a "U" - which tends to sag if the bundle is relatively long. Efficient and consistant drainage is the main key factor.

I hope this experience and effort helps you out.
 BEU_Steam_Condenser.xls   49KB   99 downloads

[fallah]

We resolved the drainage problem by fabricating our tube bundle in a "V" shaped profile rather than a "U". The overall heat transfer increased dramatically with the "V" shaped bundle. Since then, I have always made it point to install that design rather than a "U" - which tends to sag if the bundle is relatively long.

Dear Art,

How would be affected the tubesheet(s) and shell by "V" shaped profile?What would be the shape and jeometry of the shell?

Regards

[Art Montemayor]

Fallah:

As I recall, we only lost about 1% of the available tube bundle surface area. However, the heat transfer increase within the revised, "V" shaped tubes was such that our heat transfer rate increased with the "V"-shaped bundle. We proved this fact when we started up the reboiler; the time to heat up the reboiler from cold was halved from what it took with the old, conventional "U" tube bundle.

We required no rework or modifications on the conventional reboiler shell or tube bundle tubesheet. The tube portion entering the tubesheet was formed into a straight, horizontal position - exactly as in a "U" tube hair pin arrangement.

When one considers what is actually, physically happening inside the tubes of a steam-heated tube bundle and one also has first-hand experience with the ACTUAL shape and position of the tube bundle inside the reboiler, one can appreciate the fact that it is inevitable that SOME CONDENSATE will form in pools inside some of the "U" tubes. It is practically impossible to ensure that every tube in the tube bundle is perfectly straight, level, and true in its hairpin shape. Some of the tubes will be clogged with condensate some of the time - it is something that can't be avoided due to the frailties and imperfections in the design, fabrication, and installation. If you have ever had to pull out a reboiler tube bundle and re-install it again - PERSONALLY - you can appreciate what I am stating. I have spent many a week end doing just that very thing when I was a young, up-and-coming engineering graduate.