Convection Section in Fired Heaters: Design, Efficiency, and Maintenance

Content

1 What the Convection Section Does in a Fired Heater
2 How the Convection Section Recovers Heat
3 Convection Section Design: Pressure Drop, Velocity, and Materials
4 Why the Convection Section Controls Heater Efficiency
5 Fouling, Cleaning, and Cold-End Corrosion
6 Convection Sections in Refining and Petrochemical Applications
7 What to Verify When Buying a Convection Section
8 Frequently Asked Questions about Convection Sections

After three years on a delayed coking unit, a fired heater's flue gas outlet temperature had drifted from 180 °C to 246 °C. The burners were tuned and the radiant tubes looked sound, yet fuel gas consumption had climbed by nearly four percent. The problem was found where operators often look last: the convection section. Finned tubes were covered with a hard layer of salt and ash, and a damaged liner behind the bundle was letting hot flue gas bypass the tubes entirely.

The convection section normally delivers 25 to 35 percent of a fired heater's total process duty. When its performance degrades, efficiency drops, fuel costs rise, and the next turnaround becomes more urgent. This article explains what the convection section does, how it is designed, why it controls heater efficiency, and what to check before buying a new bundle.

What the Convection Section Does in a Fired Heater

In a vertical cylindrical heater, the convection section sits directly above the radiant section. In a cabin-type heater, it sits at one end of the radiant box. Both layouts pass hot flue gas across a bank of horizontal tubes, and the process fluid inside those tubes absorbs heat through the tube wall. Because the flue gas entering the convection section is already far below flame temperature, the dominant heat transfer mode changes from radiation to forced convection.

Extended surfaces are normally used to compensate for the lower gas-side heat transfer coefficient. The table below compares the working conditions in the two sections of a typical refinery heater.

Comparison of typical operating conditions in the radiant and convection sections of a refinery fired heater.
Parameter	Radiant section	Convection section
Flue gas temperature	950–1100 °C at the flame zone	650–800 °C inlet; 150–250 °C outlet
Dominant heat transfer mode	Thermal radiation	Convection with some radiation
Typical tube surface	Bare, smooth tube	Finned, studded, or bare
Share of total duty	60–70 percent	25–35 percent
Main operational risk	Overheating and creep	Fouling and cold-end corrosion

How the Convection Section Recovers Heat

The gas-side heat transfer coefficient for a bare tube in the convection section is low, roughly 15 to 30 W/(m²·K), while the process-side coefficient is usually several times higher, especially when the fluid is boiling. The limiting resistance is therefore almost always on the flue gas side of the tube.

Finned and studded tubes solve this problem by multiplying the external area. Serrated fins at a density of 4 to 5 fins per inch, or about 157 to 197 fins per metre, increase the external surface by five to ten times relative to a bare tube of the same diameter. The chart below shows a typical flue gas temperature profile through a three-pass convection section.

Figure 1. Typical flue gas temperature reduction across a three-pass convection section.

The hottest pass, closest to the radiant section, handles the process stream that needs the highest temperature. The cooler passes recover heat for lower-temperature services such as steam generation, dilution steam superheating, and feed preheating. This countercurrent arrangement keeps a useful temperature difference in every pass.

Convection Section Design: Pressure Drop, Velocity, and Materials

API 560 provides the design framework used for most fired heaters in refinery and petrochemical service. The convection section cannot be sized in isolation: bundle depth, fin geometry, and required draft must be settled together, because each one affects the others.

The critical checks are flue gas pressure drop, gas velocity, tube metal temperature, and fin material. A typical heater keeps the convection section pressure drop between 3 and 10 mbar, or 30 to 100 mm H₂O. A deeper bundle recovers more heat but demands more stack or induced-draft fan capacity. Gas velocity at the lowest firing rate should stay above about 4.5 m/s to prevent ash settling, and below 15 to 20 m/s to avoid erosion and flow-induced vibration. Tube metal temperature at the cold end must remain above the sulfuric acid dew point when sulfur is present; otherwise the lower rows and the casing corrode quickly. Fin material follows local gas temperature: alloy fins at the hot end, carbon steel fins at the cold end, and sometimes bare tubes where erosion or cleaning access is critical.

A practical engineering checklist looks like this:

Confirm flue gas mass flow, composition, and sulfur content.
Select the tube layout and pitch for the target velocity and pressure drop.
Calculate fin efficiency and choose the fin type, density, and material pass by pass.
Check tube metal temperature against the acid dew point and oxidation limits.
Provide access lanes, sootblower ports, and removable inspection panels.

Why the Convection Section Controls Heater Efficiency

The rule of thumb in fired heater design is that a 22 °C reduction in flue gas outlet temperature adds about one percentage point of efficiency. Lowering the outlet from 250 °C to 170 °C is therefore worth roughly 3.5 percent. On a 100 MW heater, that is 3.5 MW of fuel, a meaningful saving over a year of continuous operation.

Extended surfaces make such a low outlet temperature possible without a huge enclosure, as the bar chart below shows.

Figure 2. Typical external area of a 114 mm OD tube with bare, studded, and serrated finned surfaces.

Finned tubes add surface area but also lengthen the gas path and raise the pressure drop. Choosing between bare, studded, and finned tubes in each pass is an economic trade-off among surface area, draft, cleaning access, and corrosion allowance.

Fouling, Cleaning, and Cold-End Corrosion

Fouling is the convection section's most persistent operating problem. Deposits act as an insulation layer, so the flue gas outlet temperature rises and the process receives less heat. At the same time, the deposit layer reduces the open flow area and increases draft loss. In severe cases, the heater cannot reach its design firing rate.

The main causes of convection section fouling are shown below. Fuel quality, burner performance, and process carryover all contribute, and the distribution varies from unit to unit.

Salt/ash deposits (35%)
Coke/carbon deposits (30%)
Corrosion products (25%)
Other deposits (10%)

Figure 3. Typical distribution of fouling causes in convection sections.

Fouling control starts with combustion tuning and fuel management, but regular cleaning is still required. The table below summarizes the methods most commonly used in operating plants.

Typical convection section cleaning methods and their application.
Cleaning method	Best suited for	Typical frequency
Online steam sootblowing	Light ash or salt deposits	Weekly to monthly
High-pressure air sootblowing	Soot from liquid or solid fuels	Daily or continuous cycle
Offline hydroblasting	Hard, thick deposits	Every turnaround
Chemical cleaning	Specialized deposits such as salts or coke	As needed

Convection Sections in Refining and Petrochemical Applications

Every fired heater on a refinery or petrochemical site has a convection section, but each service imposes a different design. In a crude and vacuum distillation heater, the convection section preheats crude oil and sometimes raises steam. In an ethylene cracking furnace, it typically contains a hydrocarbon feed preheat coil, dilution steam superheating coils, and a high-pressure steam bank. In hydrogen and DRI reformers, the convection section determines the overall energy consumption of the unit by recovering heat for steam and feed preheating.

For crude unit service, the choice of fin type and the decision to include steam coils dominate the engineering discussion. A supplier with experience in these trade-offs can reduce project risk and improve the operating result.

Fired Heater for Atmospheric and Vacuum DistillationThis heater addresses fin-type and steam coil trade-offs in crude service, offering uniform multi-zone heating and high thermal efficiency to support stable ADU/VDU separation while lowering operational risk.View Product →

Petrochemical heaters add another layer of complexity because several coils must share the same flue gas path. The principles behind fired heater selection and engineering applications in the petrochemical industry explain how operators balance coil temperatures, steam demands, and furnace economy in these services.

Ethylene cracking furnaces are the most demanding example. Their convection sections handle cracked gas, dilution steam, and steam generation coils in one compact arrangement, and the alloy selection is driven by severe operating temperatures.

Ethylene Cracking Furnace for PyrolysisGiven the severe alloy demands in ethylene convection sections, this furnace's precise temperature control and optimized radiant/convection design help maintain cracking consistency and yield under continuous high-temperature operation.View Product →

What to Verify When Buying a Convection Section

Whether you are replacing a damaged bundle or adding steam generation capacity, the same checks apply.

Confirm that the new bundle matches the existing radiant section's flue gas mass flow and available draft.
Review the design code basis: API 560 for the heater, ASME B31.3 for the tube-side piping, and the client's site specification.
Evaluate the weld-quality plan around the fin-to-tube connection and the tube-to-header joints, since these are the most common failure locations.
Check the insulation and liner system for the expected gas inlet temperature, including erosion protection where the gas enters the bundle.
Confirm the manufacturer's capability: welding procedure qualifications, hydrostatic testing, dimensional control, and third-party inspection.

Most convection sections are delivered as shop-fabricated modules, including tubes, headers, insulation, and casing. This approach shortens site erection time and improves weld quality, because the heavy assembly work is done under controlled conditions. When the module is lifted into place, the connections to the radiant section and the stack of the existing heater must accommodate thermal expansion and remain gas-tight.

Shop-Fabricated Convection Section ModuleThis modular convection section is built for controlled shop fabrication and easy site lift, with durable alloys and customizable design to accommodate thermal expansion and maintain gas-tight connections during installation.View Product →

Logistics are just as important. A large convection section module is an over-width, heavy cargo, so the transport route, lifting method, and site handling plan deserve the same scrutiny as the thermal design. A manufacturer that can manage both fabrication and heavy transport reduces the risk of schedule delays. For a project-specific evaluation or quotation, you can contact the Tuopu engineering team with your heater data sheet.

Frequently Asked Questions about Convection Sections

Why do convection section tubes have fins?

Fins multiply the external heat transfer area by five to ten times, which compensates for the low heat transfer coefficient on the flue gas side. Without fins, the convection section would need to be several times larger to recover the same amount of heat.

What is the normal flue gas temperature at the convection section outlet?

For an efficient, well-maintained heater, the outlet temperature is typically 150 to 200 °C. When the fuel is high in sulfur, the outlet is kept above the acid dew point, often around 250 to 300 °C, or the cold end is made of corrosion-resistant material.

How much efficiency is lost when the convection section is fouled?

Fouling raises the flue gas outlet temperature. Because every 22 °C of temperature rise costs about one percentage point of efficiency, a 50 °C rise reduces efficiency by roughly 2.3 percent, which shows up directly as additional fuel consumption.

Can a convection section be replaced without touching the radiant section?

Yes. Existing fired heaters often receive a new convection section module while keeping the radiant box, burners, and stack. This is a common revamp route, provided the structure and foundations have sufficient remaining life and the new bundle's pressure drop fits within the available draft.