A refinery near New Orleans has a coker with vertical tubes (converted visbreaker heater) and it worked well. When I was there we modified the burners to even out the heat flux. The heater was arranged with a mixture of single fired and dounle fired tube. The heater worked well because of the low total heat flux and the wide spacing between the burners and the tubes. There were some issues with the return bends but that is somewhat common with a vertical tube heater – at least I seen this problem before.