A fired heater, or process heater, raises the temperature of a process stream by burning fuel, and it does that heating in two distinct zones. The radiant section is the firebox itself, where burner flames and glowing refractory transfer heat mostly by thermal radiation to the tubes lining the walls. The convection section sits above the firebox, in the path of the hot flue gas headed for the stack, and it scavenges the leftover heat by convection before the gas escapes. Understanding how these two sections divide the work, and how the process coil threads through both, is the key to running a heater hard enough to meet duty without cooking its own tubes.
Fired Heater Radiant & Convection Sections in one line: A fired heater transfers heat in two zones: the radiant section is the firebox, where the process tubes absorb heat by radiation from the flames and hot walls, and the convection section above it recovers additional heat from the flue gas by convection. The process fluid usually flows through the convection tubes first, then the radiant tubes, picking up heat in both.
The process fluid does not enter the hottest part of the heater first. It usually enters the convection section at the top, where the flue gas has already given up much of its heat, and flows downward through the convection tubes, warming as it goes. Only after that preheat does it drop into the radiant section, where it passes through the tubes lining the firebox and absorbs the intense radiant heat close to the flames. Routing the coil this way lets the coolest flue gas meet the coolest process fluid at the top and the hottest gas meet the fluid that has already been preheated, which is efficient and protects the tubes.
In the radiant section, the dominant mode of heat transfer is thermal radiation - the fourth-power relationship between temperature and radiated energy means the flames and incandescent refractory throw enormous heat flux at the wall tubes. This is where most of the heater's duty is delivered, and it is also where the tubes run closest to their metallurgical limits. The tubes are arranged around the firebox walls, sometimes with an extra row across a bridge or roof, to see as much of the fire as possible without any single tube taking a disproportionate blast of flux.
The convection section is a bank of tubes, often finned or studded to boost their effective area, sitting in the flue-gas duct above the firebox. Here heat transfer is by convection from the moving gas rather than by radiation, which is a gentler, lower-flux process. Its whole purpose is to claw back heat that would otherwise vanish up the stack, raising the heater's thermal efficiency. The first row or two of convection tubes, called the shield or shock tubes, straddle both worlds - they still see significant radiation from the firebox below and are often built from more robust material for that reason.
The boundary between the two sections has a name and a temperature that operators live by. The bridgewall, or arch, is the top of the radiant firebox where flue gas turns to enter the convection section, and the bridgewall temperature is the flue-gas temperature at that point. It is a direct readout of how hot the firebox is running and how much heat is left in the gas heading into the convection bank. A rising bridgewall temperature means more heat is escaping the radiant section unabsorbed, which points to fouled or coked tubes, over-firing, or a coil that is not picking up its share of the load.
The real constraint on a fired heater is not total duty but local heat flux - the rate of heat crossing a unit of tube surface at the hottest spot in the firebox. Push firing too hard and the radiant flux at the worst tube climbs until the tube metal overheats, accelerating coking on the inside and creep damage on the outside. A heater can be well within its total duty rating and still be destroying a tube in one high-flux corner. This is why the radiant section, not the convection section, sets the safe operating envelope, and why operators cannot simply fire harder to make more product.
Balancing the burners is a large part of keeping flux under control. Uneven firing - one burner running rich, a flame licking a tube, a plugged tip - concentrates flux where it should not be and creates hot spots that the average firebox temperature hides. Operators use bridgewall temperature, individual pass temperatures, and visual flame patterns together to keep the heat distributed, because a uniform firebox is the difference between a heater that runs for years between decoking and one that fails a tube early.
A fired heater is one of the most instrumented and most safety-critical items in a plant, and a cloud SCADA platform like Merobix historizes the handful of readings that define its condition. The pass outlet temperatures show whether each parallel coil path is picking up its share of heat, since an under-flowing pass overheats while the others coast. Bridgewall temperature reports how hard the firebox is working and how much heat is slipping into the convection section. Stack oxygen, read at the top, reveals how much excess combustion air is present and therefore how efficiently the fuel is burning.
Watching these together is what keeps the heater inside its heat-flux limit from a remote location. A pass temperature climbing away from its siblings warns of a maldistributed or partially blocked coil before the tube itself is damaged. A bridgewall temperature drifting up over weeks signals fouling or coking that is pushing unabsorbed heat downstream. Stack oxygen trending too low warns of incomplete combustion and a soot or safety risk, while too high wastes fuel by heating excess air up the stack. None of these numbers means much alone, but as a trended set they tell the whole story of how the heater is aging and firing.
Because heaters at remote gas plants and pipeline sites often run with no operator standing in front of them, the historized trends and alarms carry the safety and reliability load. Alarming on high pass temperature, high bridgewall temperature, and out-of-range stack oxygen lets a monitoring team catch a coking coil or a combustion problem while there is still time to cut firing or plan a decoke, rather than after a tube ruptures. The same stored data, reviewed over months, tells the reliability engineer exactly how each pass and the firebox are trending toward the next turnaround.
The radiant section is the firebox, where the process tubes absorb heat mainly by thermal radiation from the burner flames and hot refractory, and where most of the heater's duty and highest tube temperatures occur. The convection section sits above it in the flue-gas path and recovers additional heat by convection from the moving gas before it leaves the stack. The process fluid usually flows through the convection tubes first to preheat, then through the radiant tubes.
Bridgewall temperature is the flue-gas temperature at the arch, where the gas leaves the radiant firebox and enters the convection section. It indicates how hot the firebox is running and how much heat remains in the gas headed downstream. A rising bridgewall temperature can signal coked or fouled tubes, over-firing, or a coil not absorbing its share of heat, so operators watch it closely.
Heat flux is the rate of heat crossing a unit of tube surface at the hottest point in the firebox, and it is what actually damages tubes. Firing too hard drives the radiant flux at the worst tube up until the metal overheats, accelerating internal coking and external creep even if total duty is within rating. Because the local flux limit is reached before the overall duty limit, it sets the heater's safe operating envelope.
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