Heater Types & Configurations
Fired heaters are built in several configurations to suit different heat duties, plot space, and process requirements. Knowing your heater type tells you where to look for problems, how air and flue gas flow, and what inspection access you have.
The two sections every heater shares
Regardless of shape or size, every fired heater has the same two functional sections. All type-specific differences are variations on this common theme.
The radiant section (firebox) is where fuel burns and heat transfers to the process coil primarily by radiation — from the flame itself and from the hot refractory walls. This is the high-intensity zone. Tube metal temperatures are highest here, and it is where overheating and coking problems originate.
The convection section sits above the radiant section. Flue gas leaving the firebox passes through banks of closely-spaced tubes, giving up heat primarily by convection. Temperatures are lower here, but the convection section recovers significant heat that would otherwise be lost to stack. Some heaters also generate steam or preheat boiler feedwater in the convection section.
Box and cabin heaters
The most common configuration in refinery service. A rectangular steel shell lined with refractory forms the firebox. Process tubes run horizontally or vertically along the walls and roof. The convection section sits directly above the radiant section, separated by the bridgewall.
A single rectangular firebox with burners on the floor (upward-firing) or on the side walls. Process tubes are arranged in one or two rows along the side walls and sometimes the roof (shield section). The convection section sits directly above.
Burner arrangement: Floor-fired box heaters have burners spaced along the centreline of the floor, firing upward between the tube rows. Wall-fired configurations place burners on one or both end walls, firing horizontally across the firebox.
Advantages: Simple layout, good access for inspection and tube replacement, predictable flame and flue gas patterns, well understood by most operators.
Watch for: Uneven firing between burners causing hot spots on tubes nearest the most heavily-fired burners. End-of-row tubes adjacent to the bridgewall can be vulnerable to higher-than-average heat flux.
Two radiant cells arranged symmetrically, sharing a common convection section above. Process passes are split equally between the two cells, allowing very large total heat duties while keeping each cell manageable in size.
Operational implication: The two cells must be fired as evenly as possible. If one cell runs significantly hotter than the other, the process passes through that cell will be overheated relative to the other side. Operators monitor coil outlet temperatures (COTs) on each pass and balance firing accordingly. COT spread between passes is a primary operating parameter on cabin heaters.
Watch for: Imbalanced pass flows caused by fouling, partial blockage, or control valve issues — which then drive COT imbalance. Also watch for refractory damage on the interior dividing wall between cells.
Vertical cylindrical heaters
A vertical steel cylinder with a central burner (or small cluster of burners) firing upward from the floor. Process tubes are arranged vertically in a circle around the inner wall of the cylinder, surrounding the flame. The convection section sits above the radiant cylinder, sometimes as a separate smaller-diameter section.
Heat distribution: The circular tube arrangement means all radiant tubes are equidistant from the central flame — which gives a more even heat distribution across passes than in a rectangular box heater. This makes the cylindrical heater well-suited to services requiring tight COT control.
Limitations: The compact geometry limits the number of burners that can be installed, capping maximum heat duty. Access for tube inspection is more restricted than in a box heater — peepholes are typically located at the base of the cylinder looking up. Tube replacement is more involved.
Watch for: Flame impingement on radiant tubes if the central burner flame is too long or if burner alignment drifts. Also watch for uneven draft distribution across the tube circle if the stack is not centred.
Arbor and wicket heaters
Process tubes are bent into a U-shape — the "wicket" — standing vertically in the radiant section. Multiple wickets stand side by side, with burners firing in the lanes between them. This arrangement allows a large number of parallel passes in a compact radiant footprint.
Where you'll find them: Most commonly as the inter-heaters in catalytic reformer service, where the process stream must be reheated between each reactor bed. The arbor configuration handles the multiple short-residence-time passes required efficiently.
Watch for: Uneven firing in the lanes between wickets causing differential heating across adjacent passes. Tube inspection is challenging due to the closely-spaced U-bends — infrared surveys are particularly valuable on these heaters.
Type comparison
The table below summarises the key practical differences between heater types from an operator's perspective.
| Type | Typical heat duty | Pass balance critical? | Tube inspection access | Common refinery service |
|---|---|---|---|---|
| Box (single cell) | Small to large | Moderate | Good | CDU, VDU, hydrotreater |
| Cabin (twin cell) | Large to very large | Critical | Good (outer walls) | Large CDU, VDU, coker |
| Vertical cylindrical | Small to medium | Low | Limited | Smaller services, older units |
| Arbor / wicket | Small to medium | Moderate | Difficult | Reformer inter-heaters |
Process coil arrangements
Within any heater type, the process fluid flows through the heater via a coil — a series of tubes connected by return bends at each end. The coil is arranged in passes: parallel flow paths from inlet to outlet.
Single pass vs. multi-pass
A single-pass heater has one flow path from inlet to outlet. All the process fluid travels through the same tubes in sequence. This is simpler to operate but offers no ability to balance heat input across the coil.
A multi-pass heater splits the process flow into two or more parallel paths, each flowing through a portion of the radiant and convection tubes. Multi-pass designs reduce pressure drop, allow higher throughput, and — critically — allow the operator to balance flow between passes to equalise tube metal temperatures. On large heaters, pass flow controllers are a primary operating tool.
Shock tubes
The first rows of tubes at the top of the radiant section — just below the bridgewall — are called shock tubes or shield tubes. They are exposed to both radiant heat from below and convective heat from the hot flue gas. This double exposure makes them the highest-duty tubes in the heater. They are typically thicker-walled or made from higher-grade alloy than the main radiant bank, and they deserve close attention during inspections.