Shell Guideline on Fired Heater Tube Skin Thermocouples

[Patassa]

I'm hoping someone knows the correct interpretation of this line in the context of where to place tube skin TI's.

"In mixed flow vertical tubes, a down-flow tube is selected"

I'm unclear as to what "mixed flow" means in this context. This is a naphtha tower reboiler.

Thank you

 

[Compositepro]

That refers where the flow in some tubes is up and in others it is down. Select a tube where the flow is down.

 

[chicopee]

Terms used in references to these tubes are downcomers and risers.