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Topic: Where should the thermowell be located?

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I am working on a depropanizer column bottom tray temp (greatest DeltaT per stage is the bottom tray). The control loop will manipulate hot oil to a kettle reboiler. This is a 7' diameter column. The process folks prefer that the thermowell be located in the downcomer to achieve composition averaging. I recommend an 18" thermowell in the tray vapor space, which I feel would provide a representative temperature and much quicker temperature response to BTU input changes. Unfortunately, we need to add this flange soon. What do you suggest?
  • Avatar Greg McMillan Forum Moderator 36 Posts

    Re: Where should the thermowell be located?

    Here is the reply from my associate Terry Tolliver (retired Solutia Senior Fellow and Process Automation Hall of Fame Inductee):

    The 6 inches above the tray has a froth that would keep the thermowell wet.  In the liquid on the tray or in the downcomer you need to make sure you don't disrupt the flow.  Either should work, although I agree the space above the tray is more responsive. For an ideal stage the vapor and liquid would be at the same temperature.  For an actual tray with typical efficiency there may be a difference, but that would just mean that a different setpoint would be used.  As long as the tray efficiency stayed about the same I don't think a load transient would matter.  (Remember the temperature is a function of the composition which changes slowly.)

  • Avatar Ron Lutz Community Member 1 Post

    Re: Where should the thermowell be located?

    Consider putting them both in. I would normally put it in the downcomer, but if your system is such that things can change quickly I can see the merit of measuring the temperature on the first tray. I think you will find the downcomer reading is more stable. You don't want your kettle reboiler responding to noise. I also worry that the temperature on the tray might be influenced by the heat load on the reboiler. If the gas/liquid flow for the tray changes, will it change the temperature even though the composition has not changed. If you have both you can do step changes to see if there is any influence.
  • Avatar Frank Fox Community Member 1 Post

    Re: Where should the thermowell be located?

    Last edited: 7 February 2013 at 1:36pm
    I would think that the process folks are on to it. It seems to me the tower operation would be more reliable and not be prone to swings if their is a tower upset, or you get fouling in your bubble caps on the trays. On your bottoms recycle do you have it flowing through a exchanger? Also if costs are not to prohibitive put both thermowells in and follow them both, then you can have your cake and eat it too. Good luck, Success comes before Work only in the dictionary.