For the liquid-flow section (single-phase) of a furnace, questions of pressure drop, velocity and heat transfer have straightforward solutions. Vaporization on the other hand, greatly complicates the flow and heat transfer variables. Nevertheless, the many interdependent variables can be controlled by proper tube sizing. The results of enlarging or reducing vaporization tubes may be surprising. More information on this subject is given in Section 210. This is a complex area, and consulting a specialist may be appropriate.