It is typical to use 1% by weight velocity media, steam or water. If you use the Bechtel distillate recycle technology, a lower velocity media can be used. As you say, adding velocity steam, condensate or BFW increases the pressure drop through the furnace. At higher than design flow rates, the velocity media can be reduced to manage the pressure drop through the furnace. If you have the ability to shift the location of the velocity media injection to further down the coil, that can help manage the pressure drop through the furnace. The last 5 rows of tubes normally have the highest exposure to coking and have the highest need for higher velocities. High pressure drop through the furnace is an indication of coke deposition and that the heater should be decoked.
A mass velocity of 350 lb/ft2-sec is a typical minimum value and provides a cold oil velocity of approximately 6 ft/sec for a feed that has a feed specific gravity of 0.935, more dense feeds will have a higher mass flux. Values of as high as 450 lb/ft2-sec have been used for the design flow rate.
Coke deposition for a particular feed is dependent on the film temperature of the hydrocarbon in the tube at the tube wall and the residence time at coking temperatures or higher. Heater fouling rate increases exponentially with temperature and linearly with residence time. High velocities minimize the film thickness and the temperature of the film, which minimizes heater fouling rates.
Velocity media is also needed to keep the velocity in the heater transfer line high enough to minimize the coke deposition between the heater and the coke drums. At high enough velocities, coke forms fairly uniformly around the circumference of the transfer line piping. At low velocities, coke forms in the stagnant zones and is non-uniform. This can cause high back pressure on the heater which will increase the heater fouling rate. When reducing velocity media, check the velocity in the transfer line as well so that velocity stays above the minimum.