Thermal Efficiency
Thermal efficiency tells you what fraction of the fuel's chemical energy actually reaches the process fluid. For operators, it translates directly into fuel cost, CO₂ emissions, and available firing capacity. This page covers where the losses go, what drives them, and what you can do about them.
What thermal efficiency means
Every unit of fuel burned releases a fixed amount of heat energy — its lower heating value (LHV). Thermal efficiency is the percentage of that energy that ends up in the process stream. The rest is lost, primarily up the stack as hot flue gas.
A well-maintained refinery heater operating at optimum excess air typically achieves 85–92% thermal efficiency. Poorly tuned heaters running with high excess air, fouled convection sections, or air ingress can fall below 80%, wasting significant fuel for zero process gain.
Qabsorbed = heat transferred to process fluid (kW or MMBtu/hr)
Qfuel = total heat released by fuel combustion at LHV (kW or MMBtu/hr)
In practice: η ≈ 100% − stack loss% − radiation loss%
Where the heat goes — the loss categories
Three mechanisms account for virtually all heat loss in a fired heater. Stack loss dominates and is the primary target for efficiency improvement.
Stack loss — the main lever
Stack loss depends on two variables the operator directly controls: stack temperature and excess air. Both must be managed together — reducing one sometimes increases the other.
Stack temperature
Every degree the flue gas leaves the stack above ambient is heat that went up the chimney. A rough rule of thumb: a 20°C rise in stack temperature costs approximately 1% efficiency. The convection section is designed to recover as much of this heat as possible before the flue gas reaches the stack. When the convection section is fouled or bypassed, stack temperature rises and efficiency falls.
Target stack temperatures depend on the fuel and application. With fuel gas, the practical lower limit is set by the acid dewpoint — if stack temperature drops too low, sulphur species in the flue gas condense as sulphuric acid, causing rapid corrosion of the stack and convection section tubes. For typical refinery fuel gas, this lower limit is approximately 120–150°C (250–300°F).
Excess air
More air than stoichiometrically needed means more flue gas mass leaving the stack, carrying more heat with it. Every 1% increase in excess air adds roughly 0.15–0.25% to stack loss, depending on stack temperature. Reducing excess air from 25% down to 10% at a constant stack temperature of 300°C saves approximately 1.5–2% in thermal efficiency.
The constraint on reducing excess air is complete combustion. Below the minimum needed for the specific fuel and burner arrangement, CO appears in the flue gas and unburnt fuel losses begin to exceed the stack loss savings. The optimum is found by monitoring stack O₂ and CO simultaneously.
| Excess Air | Stack O₂ (dry) | Stack Loss (300°C stack temp) | Operating Zone |
|---|---|---|---|
| 5% | ~1% | ~8% | Too lean — CO risk, monitor closely |
| 10% | ~2% | ~9% | Optimum for well-tuned burners |
| 15% | ~2.8% | ~9.5% | Acceptable — typical normal operation |
| 25% | ~4.5% | ~11% | High — review burner setup and air registers |
| 50% | ~7.5% | ~14% | Very high — significant efficiency penalty |
Air ingress — the hidden efficiency thief
Air ingress (also called tramp air or infiltration) occurs when cold atmospheric air is drawn into the firebox or flue gas path through cracks in the refractory, failed casing welds, open inspection ports, or poorly sealed sootblower penetrations. Because the heater operates at negative pressure (natural draft), any gap is a potential ingress point.
The problem is that ingress air raises the O₂ reading at the stack without actually improving combustion. The operator sees what looks like adequate excess air but is actually measuring cold dilution air that entered after the flame zone. The real excess air at the burner may be well below minimum.
How to detect it: compare O₂ measured in the firebox (near the bridge wall) with O₂ at the stack outlet. If stack O₂ is significantly higher — more than 1% above bridge wall O₂ — ingress is present. A heater with known ingress cannot be optimised by trimming burner air alone; the casing must be sealed first.
Convection section fouling
The convection section exists to recover heat from the flue gas before it leaves the stack. When the external fins or bare tubes in the convection section become fouled — typically with coke, soot, or process-side deposits — heat transfer degrades and more heat exits via the stack.
The primary indicator is a rising stack temperature at constant firing rate and excess air. If stack temperature has drifted up 15–20°C from the clean baseline over a run, convection section fouling is the most likely cause. A 20°C increase in stack temperature represents approximately 1% reduction in thermal efficiency — equivalent to burning an extra 1% of fuel for the same heat duty.
Sootblowing — using steam or air lances to dislodge deposits from the outside of convection tubes — is the normal remedy during operation. Frequency depends on fuel quality and operating history. Heaters burning fuel oil or high-sulphur fuel gas require more frequent sootblowing than clean gas-fired units.
| Indicator | Clean Baseline | Monitor | Action Required |
|---|---|---|---|
| Stack temperature drift (°C above baseline) | 0–10°C | 10–20°C | >20°C |
| Process outlet temp drop at same firing rate | Within 5°C of design | 5–10°C below design | >10°C below — sootblow / investigate |
| Bridge wall to stack temp differential | As per design ΔT | ΔT narrowing | ΔT significantly narrowed — fouling confirmed |
Monitoring efficiency in practice
Full calorimetric efficiency calculations require fuel flow measurement, flue gas analysis, and process-side enthalpy data — tools that are available in the DCS but require periodic validation. For day-to-day operation, the following three parameters give a reliable picture without requiring a full calculation.
| Parameter | What it tells you | Target / Normal | Action if abnormal |
|---|---|---|---|
| Stack O₂ (dry) | Excess air level — main efficiency lever | 2–4% O₂ | Adjust air registers. Check for ingress if stack O₂ > bridge wall O₂. |
| Stack temperature | Heat recovery in convection section | 120–200°C above acid dewpoint | If rising trend: check for convection fouling, sootblow, investigate. |
| CO in flue gas | Combustion completeness — lower limit on air reduction | <100 ppm | Increase excess air immediately. Investigate burner condition. |
The efficiency optimisation window
Optimal efficiency sits in a narrow window: enough excess air to keep CO below 100 ppm, but not so much that stack loss climbs. In practice, this window is typically 2–4% O₂ (dry) at the stack, though the exact optimum depends on the burner design and fuel gas composition.
The optimisation is not a one-time set-and-forget. As burners wear, refractory degrades, or fuel gas composition drifts, the window moves. Regular checks — at minimum weekly on a manned unit — are needed to confirm the heater is still operating at the optimum.
Efficiency degradation over a run
Thermal efficiency is not static. It declines gradually over an operating run as tubes foul internally (coke deposition), convection fins foul externally (soot and scale), and refractory cracks, increasing radiation losses and air ingress.
Tracking the trend matters more than any single measurement. A heater showing 88% efficiency today is performing well; the same heater showing a steady decline from 90% to 85% over six months is telling you something is wrong — even if 85% is within an acceptable absolute range.
Maintain a simple efficiency log — stack temperature, stack O₂, and calculated efficiency at least weekly at consistent operating conditions. Plot the trend. Deviations from the trend are the early warning signal, not the absolute value.
Summary — the operator's checklist
Thermal efficiency is a direct measure of fuel cost. The three things an operator can do every shift to maintain it are: keep stack O₂ in the target band (2–4%); confirm CO is below 100 ppm before trimming air further; and watch for a rising stack temperature trend as the early indicator of convection fouling or air ingress. Everything else — refractory sealing, sootblowing, tube cleaning — follows from those three readings.