7 replies to this topic

[NBD3000]

Dear All,

              I wanted to know what is the preferred location in a tray column and a packed column(random and structured both) for thermowell insertion.

 

Thanks,

 

ND.

[fseipel]

I prefer to first have head and pot temperature.  Temperature profile across column should be examined to determine locations -- especially in a continuous column, where feed is injected into middle of column, there may be much change above but little below or vice versa, but for specialty chemical batch stills I typically space them evenly and include at least 3.  I would suggest considering RTD's and calibrating the sensors so they all read identically when idle since you will likely want to read small differences.  RTD's are less subject to ground loops and other issues than thermocouples so you may wish to consider them.  Taking a handheld lab surface RTD or thermocouple and inserting into insulation seams may give a good indication of existing temperature profile.  In a pinch surface temperature can be used until couplings can be installed (insulate over sensor contact point); internal with wells is preferred!  In a packed or tray column the temperature profile is a good indication of equilibration, though I prefer to control batch columns on DP where DP controls heating.

[curious_cat]

What's your purpose? If it's for a control loop typically those trays where maximum temp. change takes place. 

[NBD3000]

I preferably wanted know the location, whether in the liquid hold up for saturated liquid temperature or the vapor temperature??

[fallah]

Hi,

 

As far as i know, in a tray column it is better the thermowell to be inserted in liquid hold up inside the downcommer...

[Sharma Varun]

I agree with fallah temperature probe is to be inserted in liquid hold up. Remember that on a tray the vapor & liquid are assumed to be in equillibrium & thus at same temperature. However as far as my experience goes, remember a thumb rule that whereever it is possible temperature probe is to be inserted in liquid pool. 

Edited by Sharma Varun, 03 September 2013 - 03:07 PM.

[Art Montemayor]

When given a choice, always select the LIQUID phase to insert a temperature probe.  Basic heat transfer tells us that the heat transfer through a liquid film is much, much more efficient than through a gas phase film.  Common sense also points us to the fact that gases are used both by Nature and Industry as INSULATORS due to the poor heat transfer coefficients they offer.  Therefore, based on this you will find that the response time of a temperature sensor will be much faster (& accurate) when the probe is immersed in liquid than when it is immersed in gas.

 

Don't forget:  when dealing with heat transfer, gases are the worse heat transmitters of the 3 states of matter.  Just check out the thermal conductivity coefficients between gases and liquids and that should convince you.

[sheiko]

Art,
Thank you again for reminding us the fundamentals!