4 replies to this topic

Hi, this is my firs time in this forum so I do not know if this question was asked before.

First of all, is a pleasure being in this forum.
At this moment I am reviewing a Control Narrative for Duty Control in Stripper Column Reboiler.(MEA+H2S). This narrative indicates that the steam control valve is located in the steam line (steam inlet). However in another control narratives reviewed before, I could observe that the steam control valve is located in the condensate line (steam condensate). I was looking for the best practices for such design, it means where the steam control valve should be located, but I could not find anything related.
In my personal opinion if the control valve is installed in the inlet steam then a delta P will be generated and the temperature of the steam to the reboiler will be affected. But I think that the Process Engineer who designs this system has to considerate such effect and to use the latent heat (hfg) and avoid an undesirable thermodynamic state in the steam to reboiler once leaves the control valve.

I would appreciate your help with this topic.

[Zauberberg]

The basic difference in having (1) steam, or (2) condensate flow control in reboilers is:

1. By having control valve in the steam supply line, the condensation pressure inside the reboiler channel head (and thus the steam condensing temperature) is varied. If you look at the basic heat transfer equation Q = U*A*LMTD, doing this way changes the effective temperature difference in the reboiler and therefore the heat transfer rate as well.

2. By having control valve in the condensate outlet line, the number of tubes in reboiler submerged in condensate is varied by manipulation of the condensate outflow. In such way, the net area available for heat transfer is changed as well (Q = U*A*LMTD).

Both schemes have their advantages and disadvantages, and one of the best places to see pro's and con's of each scheme is Walter Driedger's article attached below.

Attached Files

•  CONTROLLING STEAM HEATERS.pdf   224.81KB   115 downloads

Thank a lot Zauberberg!! Excellent information and article.
Regards!

[Padmakar Katre]

The basic difference in having (1) steam, or (2) condensate flow control in reboilers is:

2. By having control valve in the condensate outlet line, the number of tubes in reboiler submerged in condensate is varied by manipulation of the condensate outflow. In such way, the net area available for heat transfer is changed as well (Q = U*A*LMTD).

Dear Zauberberg,
I have a confusion here, could you tell me how I could vary the heat transfer area in case of a reboiler when the condensate is routed to a condensate pot and it's level is controlled. Now the reboiler elevation and the condensate pot elevation are different i.e. cond pot is at relatively lower elevation (generally on the grade). In this case I am not sure about the submergence of tube in condensate in reboiler.

[Zauberberg]

Hello Padmakar

Look more closely at the P&ID: if level in the reboiler cannot be controlled (either directly or through the pot) then there must be a control valve in the steam supply line. See attached.

Best regards,

Attached Files

•  condensate pot.JPG   57.65KB   64 downloads