Air Preheater Efficiency Calculator
Calculate APH gas-side or air-side efficiency based on flue gas and air temperatures.
APH efficiency looks like a single percentage on the screen; in the field, it is not read that way. If the gas inlet temperature, gas outlet temperature, air inlet temperature, and air outlet temperature do not come from the same operating moment, the resulting value is only a nicely formatted estimate. Especially if there is leakage in a rotary air preheater, the gas outlet temperature can mislead you at first glance. Seeing a low flue gas temperature on the DCS feels good, but if the seal side is leaking, that benefit stays on paper.
Which temperature difference does the efficiency percentage come from?
An air preheater is a heat exchanger that brings hot flue gas and cold combustion air closer to each other. The gas side cools down, and the air side heats up. What the calculator does is read this temperature change from two different directions: gas-side efficiency or air-side efficiency.
The common point of the two methods is the temperature difference between the inlets:
$$
\Delta T_{head} = T_{g,in} - T_{a,in}
$$
If the gas enters the APH at 340 °C and the air enters at 30 °C, the available temperature difference is 310 K. Here, the difference in Kelvin and Celsius gives the same magnitude; whatever 340 - 30 is, 613,15 - 303,15 is the same. The calculator also shows this difference in the result, because that is the basis on which the efficiency percentage rests.
For gas-side efficiency, the calculation is set up as follows:
$$
\eta_{gas} = \frac{T_{g,in} - T_{g,out}}{T_{g,in} - T_{a,in}} \times 100
$$
Air-side efficiency looks at the temperature gained by the air:
$$
\eta_{air} = \frac{T_{a,out} - T_{a,in}}{T_{g,in} - T_{a,in}} \times 100
$$
Despite having the same denominator, the two results do not have to be the same. In fact, in most real data they are not the same. Gas and air flow rates are not equal, specific heats are different, leakage measurement is not included in this calculation, and the in-duct temperature distribution is not always uniform. Therefore, it is more accurate to think of gas-side and air-side efficiency as two separate readings.
If an engineering note says “efficiency 80%”, the method should also be added next to that line. Gas side or air side? The same percentage can mean something completely different with a different method.
Gas-side calculation: how much did the flue gas cool down?
The gas-side method measures how much temperature the flue gas loses while passing through the APH. If the gas inlet is 340 °C, the gas outlet is 150 °C, and the air inlet is 30 °C, the formula works as follows:
$$
\eta_{gas} = \frac{340 - 150}{340 - 30} \times 100
$$
The result is 61.29%.
This calculation gives a very practical first reading for the gas side. If the APH is dirty, if the basket surfaces are filled, if the sootblower effect has weakened, or if blockage has started on the cold-end side, the gas outlet temperature remains high. The gas does not cool enough. The numerator in the formula becomes smaller, and the efficiency drops.
However, the gas outlet temperature should not be viewed too romantically. In a rotary APH, if air leaks from the air side to the gas side, the measured gas outlet appears cold. This can be perceived as if the real heat transfer has increased. The low value on the DCS screen looks good in the report; then when the leakage measurement comes to the table, the same value is no longer so trustworthy.
This calculator does not apply a leakage correction. It also does not generate a no-leakage gas outlet temperature. If you have a raw gas outlet temperature, the gas-side efficiency is calculated with the raw temperature data. That is how it should be written. Otherwise, the percentage appears too assertive.
Gas-side efficiency is especially useful when tracking the same unit over time. Yesterday, last week, before maintenance, after maintenance. But for the comparison to be meaningful, the load point should be similar. If the fuel has changed, excess air has shifted, or the fan damper has stayed in another position, it is not easy to put two efficiencies side by side and make a judgment. In Excel, two rows align; in operation, they do not.
Let one more example look more realistic: gas inlet 346.7 °C, gas outlet 158.4 °C, air inlet 32.1 °C. The gas-side efficiency is about 59.85%. The neatness of rounded examples is not present here. It is usually not present in the field either.
Air-side calculation: how far did the combustion air heat up?
The air-side method looks at how much the APH raises the combustion air temperature. If the calculation is made with gas inlet 340 °C, air inlet 30 °C, and air outlet 280 °C:
$$
\eta_{air} = \frac{280 - 30}{340 - 30} \times 100
$$
The result is 80.65%.
In the same temperature set, the gas side was 61.29% and the air side was 80.65%. This alone is not a fault indication. The temperature drop on the gas side is 190 °C, and the temperature rise on the air side is 250 °C. Since flow rate and \(C_p\) are not included in the calculation, it is wrong to read these two percentages as an energy balance.
Air-side efficiency speaks more directly for someone interested in the temperature of the air going to the burner. If the air outlet temperature is low, it may be a condition that should be tracked in terms of fuel, flame stability, or boiler efficiency on the combustion side. But here too, the sensor location comes into play. In large air ducts, the mixture is not always homogeneous. A temperature measured from a single point may not represent the entire duct.
Air inlet temperature is affected by the season. Air entering at 5 °C in winter and air entering at 35 °C in summer can give different percentages in the same APH. Therefore, when making comparisons between months, it is necessary to ask, “Did the air inlet change, but did the efficiency remain the same?” Some reports skip this distinction. Then it is not clear why the values moved.
In the air-side calculation, the gas outlet temperature is not in the formula. Still, it is monitored out of the corner of the eye. If the air outlet looks good while the gas outlet is also unusually low, the leakage suspicion does not leave the table. Just because the air heated well does not mean the gas-side story is closed.
Where do errors most often occur in the inputs?
There are four temperature fields in the calculator: gas inlet, gas outlet, air inlet, and air outlet. There is also a method selection. When gas side is selected, gas inlet, gas outlet, and air inlet values are used. When air side is selected, gas inlet, air inlet, and air outlet values come to the front. In every case, the temperature difference comes from between the gas inlet and the air inlet.
The unit can be °C, °F, or K. If the conversion is correct, the percentage does not change. The error is more often in the value itself: writing 340 K instead of 340 °C, for example. 340 K is around 66.85 °C. The calculator cannot understand this from the user’s intention; it takes the number, performs the calculation, and gives the result. Then a strange percentage appears in the report.
Measurement timing is as troublesome as the unit. If the gas inlet is the 10:00 average, the air outlet is the 10:30 instantaneous value, and the gas outlet was taken while the load was dropping, the calculation looks proper, but the data does not tell the same story. At stable load, a 15-30 minute average usually looks better for most practical checks. Shorter intervals may sometimes be enough, especially in a quick shift check. For a performance report, a little more patience is needed.
Sensor placement should also be noted. If the temperature profile at the APH inlet is not uniform, a single probe may not be enough. If there is stratification at the gas outlet, a value taken from one corner does not represent the whole cross-section. The air outlet has a similar problem; one side of the duct may remain hotter and the other side cooler. The formula does not know this.
Sometimes what we call a calculation error is actually a measurement arrangement error.
This percentage is not an energy balance
This should be stated clearly without dragging it out: this calculator does not use flow rate or specific heat. It only derives gas-side or air-side efficiency from temperature differences. No X-ratio. No leakage correction. No no-leakage temperature.
This does not mean the calculation is useless; this is its scope.
If heat duty is to be calculated, flow rate and \(C_p\) are required. On the air side, a rough heat gain is found as follows:
$$
Q_{air} = \dot{m}_{air} \times C_{p,air} \times (T_{a,out} - T_{a,in})
$$
A similar reading is made on the gas side:
$$
Q_{gas} = \dot{m}_{gas} \times C_{p,gas} \times (T_{g,in} - T_{g,out})
$$
These two calculations are not the same thing as temperature efficiency. Air-side efficiency may come out as 80%, and gas-side efficiency may come out as 61%; without knowing the flow rates, it is not correct to jump from this to “energy was lost” or “the calculation is wrong.” An energy balance cannot be established before air flow rate, gas flow rate, specific heats, and leakage information enter the table.
The gas \(C_p\) assumption behaves differently in a coal-fired boiler, differently in natural gas, and differently again in biomass. Excess air ratio, moisture, fuel composition, load; all of them affect the gas side. What this calculator does is narrower: it gives a quick APH efficiency percentage using the temperature difference.
Reading the result in the field
Low efficiency first brings fouling to mind. Basket surfaces may be filled, the sootblower routine may be weak, it may be time for APH washing, or blockage may have started on the cold-end side. These are real possibilities. Still, opening a maintenance work order directly when efficiency drops would be premature.
If the gas inlet temperature came in low, the denominator changes. If the air flow rate has increased, the air outlet drops. If leakage has increased, the gas outlet may look strangely good. If pressure drop has increased, the fouling suspicion becomes stronger. The same percentage can be reached through different paths; this is the uneasy side of the APH calculation.
High efficiency alone is not a certificate of success either. The air outlet sensor may be reading high. The gas inlet probe may have landed in a hot zone. The averaging time may have been kept short. A value that looks bright on the DCS is sometimes the product of a weak measurement arrangement in the field.
If a comparison will be made against a design or acceptance value, a deviation calculation can also be used separately:
$$
Sapma\ (\%) = 100 \times \frac{\eta_{mevcut} - \eta_{tasarım}}{\eta_{tasarım}}
$$
If the current gas-side efficiency is 61.29% and the design reference is 65%, the deviation is approximately -5.71%. This line can go into the report. If load, fuel, excess air, and measurement interval are not written next to it, it will not be very convincing.
If it were me, I would put a few hard data points in the report instead of a long explanation: method, four temperatures, load, pressure drop, and leakage note if available. Six lines are often more useful than two paragraphs. Especially in a maintenance meeting, no one reads long ornate sentences; they look at where the value came from.
Short note when using the tool
Start with the method selection. If gas-side efficiency is required, focus on gas inlet, gas outlet, and air inlet temperature. For air-side efficiency, gas inlet, air inlet, and air outlet temperature carry the calculation. The fourth temperature still remains on the screen; it is useful for interpretation.
340 °C gas inlet, 150 °C gas outlet, and 30 °C air inlet give 61.29% on the gas side. With the same gas inlet and air inlet values, if the air outlet is entered as 280 °C, the air side gives 80.65%. These two test calculations are enough to check the temperature difference logic of the tool.
If data will be entered with a different unit, first check the unit selection. If °F and °C are mixed, the result deviates badly. When using Kelvin, it is also necessary to make sure the absolute values are entered correctly; writing data without converting it just because “K looks more technical” is not a good idea.
It is more honest to leave the last line like this: do not read the resulting percentage alone; read it together with trends at the same load point. If gas outlet temperature, air outlet temperature, and pressure drop are all saying the same thing, the calculation is useful. If one of them is going somewhere else, first dig into the measurement side.