Why is 21 used in the O₂ calculation?

In this quick calculation, the oxygen content of air is assumed to be 21. This assumption is consistent with the simplified field formula used for APH leakage calculations.

Why is the correction factor 0.9?

In approximate calculations using the O₂ and CO₂ methods, the correction factor is generally taken as 0.9. The tool uses this value by default, but it can be changed to match the user's own procedure.

Do dry-basis and wet-basis measurements affect the result?

Yes. If the measurement bases differ, the result will not be reliable. Inlet and outlet O₂/CO₂ values must be entered on the same basis; this tool does not automatically convert between dry and wet basis.

Air Preheater (APH) Leakage Rate Calculator

Approximates the APH leakage rate using O₂, CO₂, flow rate ratio, or side A/B O₂ comparison. In O₂ and CO₂ methods, the default correction factor is taken as 0.9; users can change this factor if desired.

APH (air preheater) leakage is usually the leakage of pressurized air side flow into the flue gas side. In regenerative air preheaters, especially Ljungström type rotating element designs, this is not an unknown situation. As the rotor rotates, hot flue gas passes on one side and combustion air on the other. In between are sealing elements, sector plates, radial and axial gaps. These gaps change as operating hours increase. Sometimes the adjustment drifts, sometimes the sealing element wears, and sometimes load conditions make the measurement appear worse than it is.

This calculation tool is used to approximate the leakage rate by looking at O₂ or CO₂ values at the APH inlet and outlet. If the leakage air flow and flue gas flow are directly known, the flow rate method can also be used. In systems with A and B sides (parallel 2 air preheaters), both sides can be compared separately. Thus, rather than the question "Is there leakage?", it approaches the question "Which side seems more problematic?"

Field Implication of APH Leakage

Air leaking from APH sealing may seem like a small detail in boiler performance, but its effects are felt in several places. When additional air mixes with flue gas, O₂ at the APH outlet increases and CO₂ is diluted, i.e., its concentration drops. The ID fan (Induced Draft Fan) has to handle a larger volume. Velocities on the flue gas side change. When performing efficiency calculations, measured oxygen values may be interpreted differently.

If the DCS screen shows that O₂ after the APH increases compared to before the APH, one of the first suspicions is APH air leakage. Of course, making a direct diagnosis is not correct. The sample line might be leaking, the analyzer calibration might have drifted, or the load might have just changed. But if the O₂ increase appears regularly and repeatably, the APH side needs to be examined.

A similar logic exists for the CO₂ side. Since the air mixture dilutes the flue gas, the APH outlet CO₂ value decreases. Some plants use O₂, some use CO₂ for cross-checking. Seeing both together is often more comfortable; if one does not support the other, the measurement is first viewed with suspicion.

Visualizing the Flow Direction

Consider a simple cross-section. On one side, hot flue gas enters the APH, leaves its heat to the rotor elements, and exits. On the other side, cold combustion air comes in, heats up, and goes to the boiler. When the pressure on the air side is higher, it tends to leak to the flue gas side through any gap.

Measurements and Data Entry

First, you must select the method in the calculator. If the O₂ method is selected, APH inlet O₂ and APH outlet O₂ values are entered. If the CO₂ method is selected, the CO₂ values at the same points are used. In the flow rate method, leakage air flow rate and flue gas flow rate are required. In A/B comparison, inlet-outlet O₂ values for side A and side B are entered separately.

The most critical issue here is the measurement basis. If the inlet O₂ value is on a dry basis and the outlet O₂ value is on a wet basis, the result is technically not healthy. The same applies to CO₂. The calculator does not automatically convert dry/wet basis; you must enter both sides on the same basis as the measurement. This warning may seem tedious, but it is one of the things that most often spoils results in the field.

Another issue is measurement timing. APH inlet and outlet values should represent the same operating condition. Making leakage calculations with values taken while the boiler load changes, O₂ value changes, or fan damper moves is not a good idea. Seeing a few minutes of stable values is safer than a single momentary screen capture.

Correction factor

In the O₂ and CO₂ methods, the correction factor defaults to 0.9. This factor can be thought of as a correction multiplier used in practical field calculations. You can change it according to the plant procedure, test notes, or your own acceptance method.

The important thing here is not to "prettify" the result with the factor. If the procedure says 0.9, use 0.9. If another value is defined, enter that value. In a calculation to be reported, the selection basis of the factor should be clearly written.

Calculation Methods

The O₂ method is the most commonly used quick check. Its logic is: if O₂ at the APH outlet has increased relative to the inlet, this increase can be related to the leakage air effect. In the calculation, the oxygen ratio in air is practically assumed to be 21.

The O₂ calculation reads as follows:

Formula

$$\text{leakage rate} = \frac{O_{2,\text{outlet}} - O_{2,\text{inlet}}}{21 - O_{2,\text{outlet}}} \times \text{correction factor} \times 100$$

O₂ outlet = oxygen percentage at APH outletO₂ inlet = oxygen percentage at APH inletcorrection factor = procedure-dependent correction factor (default 0.9)21 = accepted approximate percentage of oxygen in air

For example, let APH inlet O₂ be 2.8%, outlet O₂ be 5.7%, and correction factor 0.9. The O₂ difference is 2.9 percentage points. Calculation: $ \frac{2.9}{21 - 5.7} \times 0.9 \times 100 $. The result is approximately %17.06.

We cannot say that this value alone means "this part is faulty." But it is quite reasonable to say that sealing on the APH side should be checked.

CO₂ method

In the CO₂ method, inlet and outlet CO₂ values are used. Since air mixture dilutes CO₂, the APH outlet CO₂ value decreases. The calculation is made by ratioing this decrease to the outlet CO₂ value.

CO₂ calculation:

Formula

$$\text{leakage rate} = \frac{CO_{2,\text{inlet}} - CO_{2,\text{outlet}}}{CO_{2,\text{outlet}}} \times \text{correction factor} \times 100$$

CO₂ inlet = carbon dioxide percentage at APH inletCO₂ outlet = carbon dioxide percentage at APH outletcorrection factor = procedure-dependent correction factor (default 0.9)

For example, if CO₂ inlet is 14%, CO₂ outlet is 12%, correction factor 0.9, then the calculation is $ \frac{2}{12} \times 0.9 \times 100 $. The result is %15.

If the O₂ method gives around 17% and the CO₂ method around 15%, these two values can be considered close to each other. But if one says 5% and the other 25%, before blaming the calculator, you should check the sampling arrangement. There might be condensation, leakage, calibration drift, or an incorrect sampling point in the analyzer line.

Flow rate method

The flow rate method appears more direct: divide the leakage air flow rate by the flue gas flow rate and convert to a percentage. If leakage air flow is 10 and flue gas flow is 100, the result is %10.

But here the unit issue is important. Both flows must be on the same basis. If both are in Nm³/h, no problem; but if one is kg/h and the other is m³/h, a direct ratio cannot be set. Temperature, pressure, and density corrections come into play. On paper, the flow rate method is the shortest method; in practice, obtaining good flow data is sometimes the hardest.

A/B O₂ comparison

A/B comparison is especially useful in two-sided APH arrangements. Sometimes the total leakage rate alone does not give an alarm, but one side is clearly performing worse. This difference gives a good clue to the maintenance team.

For example, on side A, inlet O₂ is 2.8%, outlet O₂ is 5.7%. On side B, inlet O₂ is also 2.8%, outlet O₂ is 4.5%. With a correction factor of 0.9, side A is approximately %17.06, side B is approximately %9.27. The difference is %7.79.

With such a difference, the radial seal, axial seal, sector plate adjustment, rotor gap, or local duct leaks on side A enter the checklist. Saying "side A is definitely faulty" would be too harsh; but side A deserves a closer look.

Interpreting Results

If the leakage rate is low, the air preheater is considered healthier. As the value increases, fan load, flue gas flow, heat loss, and measurement interpretation may be affected. Still, it is not correct to make a firm decision with a single percentage value. The same APH may behave differently at different loads. The seal setting may change in hot/cold conditions. Rotor deformation or ash accumulation can affect the measurement.

Therefore, the result should be read as an indication, not a diagnosis. A good field approach generally proceeds as follows: first verify the measurement, then take it again under the same conditions, then compare O₂ and if possible CO₂, and finally proceed with mechanical inspection. If the measurement is not proper, mechanical crew is unnecessarily sent into the APH; no one wants that.

What value is considered high?

There is no single answer to this question. APH type, boiler design, operating load, and manufacturer acceptance values are important. Still, in practice, as the leakage rate increases, the attention level also increases. Therefore, the evaluation note in the calculator should be read not as a definitive acceptance criterion but as a quick field warning.

For example, a value around 5% and a value around 17% are not evaluated in the same sentence. When 17% is seen, it is logical to repeat the measurement, check the A/B difference, and proceed with seal inspection. But this must be interpreted together with the operating procedure.

If O₂ and CO₂ results diverge

If the O₂ and CO₂ methods give very different results, the first suspicion should be on the measurement side. Dry/wet basis might be mixed. Inlet and outlet measurements might not have been taken simultaneously. One of the analyzers might have drifted. The sampling probe might be located at a point with poor representativeness in the duct.

In such a case, before discussing the calculation result, discuss the measurement chain. In APH leakage calculation, data quality carries more weight than the formula.

Quick Usage Notes

In the O₂ calculation, the value 21 should be considered a practical assumption for the oxygen ratio in air. Some sources show 20.9; this difference can be discussed in sensitive tests. The O₂ formula in this calculator works with the assumption of 21. It is important to use the same assumption consistently and state it in the report.

Dry basis and wet basis should not be mixed. I know this point sounds repetitive, but it is truly the most common source of error in APH leakage calculation. If the measurement instrument gives dry basis, enter both inlet and outlet values on dry basis. If wet basis, both should be on wet basis.

If the leakage comes out high, first check the data. Is the measurement point correct? Is the analyzer calibrated? Is the load stable? Do the O₂ and CO₂ values represent the same time period? If these are clean, proceed to mechanical inspection. Seal adjustment, rotor gaps, sector plates, duct connections, and local leak possibilities are examined in order.

In short, APH leakage rate calculation is not just about generating a percentage from a few numbers. The formula is fast; the real effort is to ensure the measurement is reliable. When used with correct data, it gives the maintenance team a fairly clear direction: is there a general leakage, or does one of the A/B sides appear more problematic?