1 reply to this topic

[ChemLee]

Hello,

I am designing a firetube for a TEG glycol reboiler (direct fired heater), burning natural gas with a MW of 24. Required process duty is 900 kW (nett heat to the process). I have decided to fit in the one reboiler vessel two U-tube firetube heaters each fitted with a burner and to size the firetube diameter and length based on a flux rate of 22 kW/m2 within the recommended range of 19 - 25 kW/m2 in the GPSA handbook.I need 900/2/22 = 20.5 m2 surface for each firetube. My questions are:

1) What is better, a 24" firetube with a tube length of 10.7 m (2 x 5.34m) or a 20" firetube with a length of 12.8 m (2 x 6.4m)? This matters for allowed space considerations of the plant module for installation and shipment limits. I am assuming the flame I get from the burner vendor is a long stable flame 2 meters long and both 20" and 24" are acceptable so that the flame will not touch the tube. Is there a general recommendation for firetube size to use based on duty?

2) Why are all the firetube heaters in amine and glycol reboilers I have seen the same diameter across the tube length? Or put another way: why is the recommended flux rate applied as an average across the tube? Would it be better to have a larger diameter where the flame is because this is where radiation heat transfer takes place and a smaller diameter where heat transfer is only due to convection with hot flue gases?

Best regards,
ChemLee

[sanaullah]

Dear Chem Lee,
As per my understanding with reboiler design, heat flux and heat density per in2 is the main area of selection.
In GPSA and API 12K, heat flux for glycol is 10,000 and heat release density is about 15000 Btu/hr.in2
So formula says lenth is in inverse relationship with Heat Density so maintaining long lenth and large diameter will help to maintain heat flux as well as heat density.
Remember, Firetube diameter and lenth are key components for reboiler design.

Regards

Sana Ullah-Process Engineer