Hi Claudio and Evan! I’m new to this forum, so I’m still trying to find my way around the site, but sharing experiences with friends around the world is a great way to start!
In our 6-drum DCU in Venezuela we typically have a 13°C (23°F) DT between the heater outlet and the drum inlet.
To me the reason of the drop is simply due to the endothermic nature of the coking process. Temperature decreases as reactions start to take place. Of course, having poor insulation or high pressure drop can increase the DT, but if you consider that those items are ok, I’d say that the difference between in coker and another is related to the characteristics of the feed (it would be interesting to try to quantify or to correlate that effect).