Fired Heater Classification According to Fluid Phase Behavior
Liquid phase only. These are mild services, and coking of tubes is seldom of concern. Heaters for circulating hot oil systems and pipeline heaters are in this category. These furnaces are usually single tube pass design.
Vapor phase only. The most common examples are the feed heater and interheaters of a catalytic reformer unit. Minimizing pressure drop is frequently important; this leads to multiple parallel tube passes. Steam superheating is another example.
Vaporizing. Vaporization of the process fluid begins partway through the heater, and as much as 50% vaporization may be reached at the outlet. (Complete vaporization should not be allowed to occur; the concern is deposit of dissolved material in the tubes.) Crude unit, visbreaker, and coker heaters are in the vaporizing category.
Because of the high process temperatures required, proper flow regime throughout the tube coil is important to minimize tube coking. Fired reboilers for distillation columns are another example of vaporizing service. The process fluid in this case is a distillate, a lower-boiling material, and coking is of less concern.
Other two-phase. A reactor preheater in a hydroprocessing plant may handle a hydrogen/hydrocarbon mixture which is two-phase throughout the heater. Relatively little additional vaporization occurs in the heater; the duty is mainly sensible heat. Designing for proper flow regime will be less difficult than in vaporizers; on the other hand, assuring even distribution of two phases to all passes is very important.