Both hollow cone and solid cone spray nozzles have been used successfully. The hollow cone nozzles are less likely to plug as they do not have an insert in them. The solid cone nozzles provide a more thorough wash to the cross section of the tower but they have an insert that reduces the maximum size sphere that can pass through the nozzle.
My preference is to use the solid cone nozzles as that is better from a fractionation and contact standpoint and then use the proper sized strainers in the wash oil stream to prevent the nozzles from plugging. The pressure drop through the wash oil header should be monitored to confirm the condition of the spray nozzles. If the actual pressure drop through the wash oil header is higher than the calculated pressure drop at the wash oil flow rate, the spray nozzles are restricted indicating poor coverage by the wash oil which will lead to poor quality HCGO.
A typical wash zone in a Coker fractionator has shed decks to provide some contact between the wash oil and the vapors from the coke drums. If those are dislodged or non-functional the CCR and asphaltene content of the HCGO will increase.