Field Reference / Fundamentals / Heat Transfer
Module 01
01 · Fundamentals

Heat Transfer in Fired Heaters

Understanding how heat moves from the flame to the process fluid explains most of what operators observe day to day — and most of what goes wrong. This page covers the radiant and convection sections, heat flux, pass balance, and bridgewall temperature.

The Two Heat Transfer Zones

A fired heater transfers heat in two distinct sections. The radiant section surrounds the burner flame and absorbs heat primarily by radiation. The convection section sits above the radiant zone, where hot flue gases pass over tube banks and transfer heat by convection. Each section contributes differently to the total absorbed duty.

Zone 1
Radiant Section (Firebox)
The lower zone surrounding the burners and their flames. Tubes lining the firebox walls or floor absorb heat primarily through thermal radiation from the flame and hot combustion gases. Heat fluxes here are high and non-uniform — the tubes nearest the flame receive the most heat.
  • Dominant heat transfer: radiation (flame and hot gas)
  • High and variable flux — hot spots possible
  • TMT is most at risk in this zone
  • Bridgewall temperature marks the top of this section
60–70%
of total absorbed duty (indicative)
Zone 2
Convection Section (Flue Gas Path)
The upper zone where flue gases leaving the firebox pass over banks of process tubes (and sometimes steam generation coils) before exiting to stack. Heat transfer here occurs by convection from flue gas flow across the tube surface, augmented by the fin tubes used in many designs.
  • Dominant heat transfer: forced convection from flue gas
  • More uniform flux — lower tube temperature risk
  • Fin tubes or bare tubes depending on service
  • Stack temperature reflects efficiency of this section
30–40%
of total absorbed duty (indicative)

Heat Transfer Mechanisms

Three mechanisms are active in a fired heater, in different proportions across the two zones. Operators can't directly control the mechanisms, but understanding them explains why changes in firing, excess air, and flue gas flow affect tube temperatures as they do.

Radiation
Thermal Radiation
Heat transferred by electromagnetic emission from the flame, hot combustion gases (CO₂ and H₂O are active emitters), and incandescent refractory surfaces. Radiation intensity increases rapidly with temperature — proportional to T⁴. A small increase in firebox temperature produces a large increase in radiant heat transfer.
Primary in: Radiant section — typically 90%+ of firebox heat transfer
Convection
Forced Convection
Heat transferred by flue gas flowing across the outer surface of process tubes. Effectiveness depends on flue gas velocity, temperature, and tube geometry. Fin tubes in the convection section increase effective surface area to enhance this mechanism. Reduced draft or fouled fins reduce convective efficiency.
Primary in: Convection section — nearly all heat transfer here
Conduction
Conduction Through Tube Wall
Heat conducted through the tube wall thickness from the outer surface (heated by radiation/convection) to the inner surface (in contact with process fluid). For clean tubes this resistance is relatively small. Coke deposits or external scale increase this resistance and raise tube metal temperatures.
Active in: All tubes — increases significantly with fouling
ℹ
Why Excess Air Affects Heat Transfer
Higher excess air dilutes the flue gas and reduces its temperature. A cooler flame radiates less heat (radiation ∝ T⁴), reducing radiant section duty. Simultaneously, higher gas volume increases convective heat transfer slightly — but the net effect is usually a less efficient heater. This is why excess air must be managed, not simply maximised.

Radiant Heat Flux

Radiant heat flux is the rate of heat absorbed per unit of tube surface area in the firebox (kW/m²). It is not uniform — tubes closest to the flame or burner centres receive the highest flux. The peak flux at any hot spot, not the average, determines the critical tube metal temperature.

Radiant Heat Flux — Typical Operating Ranges
Service / Heater Type Typical Average Flux (kW/m²) Peak Flux Allowance Notes
Crude preheat / atmospheric distillation 25 – 45 Up to 1.5× average Moderate flux, lighter service
Vacuum heater 30 – 55 Peak controlled carefully Vaporisation in tubes — hot spots critical
Reformer / high-severity duty 60 – 90+ Peak management essential High-alloy tubes required
Visbreaker / thermal cracking 45 – 65 Coke formation risk Decoking frequency linked to flux levels
⚠
Peak Flux and Hot Spots
Average flux values mask the risk. Localised hot spots — at burner centrelines, near refractory irregularities, or on passes with reduced flow — can exceed the average by 50% or more. Tube failures typically originate at these peak-flux locations, not at the average-flux point.

Tube Absorption Profile

Heat flux is not constant along the length of a radiant tube. The profile varies based on proximity to the burner flame and the tube's position in the firebox. Understanding the profile shape helps operators interpret TMT readings from different tube positions.

Illustrative Radiant Tube Heat Flux Profile (Single Pass)
Inlet ↑ Burner zone Outlet
Peak flux — burner centreline
Elevated flux — near-burner zone
Lower flux — tube ends

Indicative shape only. Actual profile depends on burner spacing, firebox geometry, and firing rate.

Pass Balance

Most fired heaters have two or more parallel passes — separate flow paths through the radiant section. All passes receive approximately the same heat input from the firebox, so equal flow distribution is essential. An imbalanced pass receives less cooling per unit of heat input, raising its tube metal temperatures relative to the others.

Below is an illustrative example: four-pass heater, balanced vs. imbalanced flow.

Balanced — Equal flow, equal TMT across passes

Pass A
25%
of total flow
Pass B
25%
of total flow
Pass C
25%
of total flow
Pass D
25%
of total flow

Imbalanced — Pass C restricted, elevated TMT risk

Pass A
28%
of total flow
Pass B
28%
of total flow
Pass C
9%
of total flow
Pass D
35%
of total flow
⚠
Pass Imbalance Is Not Always Visible in Total Flow
Total feed flow can appear normal while one pass is starved. Always monitor individual pass flows and compare pass outlet temperatures. A significant COT spread between passes (e.g., >15°C) is a sign of imbalance requiring investigation.

Common Causes of Pass Imbalance

Pass Imbalance — Cause and Indication
Cause What You See Operator Action
Pass control valve fault or sticking One pass low flow, valve position doesn't match signal Check valve position, attempt manual override, notify maintenance
Partial coke or fouling blockage Gradual pass ΔP increase, COT spread developing slowly Monitor trend, schedule decoking — do not force additional flow
Restriction at inlet manifold Low ΔP across the affected pass compared to others Check inlet isolation valves, inspect strainers
Vapour locking / two-phase in one pass Erratic flow, COT swings, ΔP fluctuation Increase feed flow if possible, consult process engineer

Bridgewall Temperature

Bridgewall temperature (BWT) is the flue gas temperature measured at the exit of the radiant section — effectively at the transition between the firebox and convection section. It is one of the most operationally useful single-point measurements in a fired heater.

Bridgewall Temperature — Operational Interpretation
BWT Range (°C) Interpretation Typical Operator Response
750 – 900 Normal — balanced radiant absorption Continue monitoring at normal frequency
900 – 1000 Elevated — possible excess firing, low feed, flame impingement Review firing rate, check pass flows, check excess air
> 1000 High — investigate immediately Reduce firing, identify cause — risk of refractory damage at sustained levels
Below design minimum Under-firing or high excess air diluting firebox temperature Review excess air, check burner operation, assess heater efficiency
ℹ
BWT as a Diagnostic Tool
BWT is related to firing rate, excess air, and radiant absorption. If BWT rises with no change in firing, it may indicate reduced feed flow (less heat absorbed in radiant tubes) or flame characteristics changing (e.g., after burner maintenance or fuel change). Trending BWT over time provides an early indication of heater performance degradation.

Related Pages

01 · Fundamentals
Combustion Theory
06 · Inspection & Maintenance
Tube Assessment
03 · Abnormal Operations
Tube Failure / Leak
03 · Abnormal Operations
Loss of Feed
05 · Performance
Performance Monitoring
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