How is the thermal efficiency of a steam boiler calculated? Understanding the inlet/outlet heat method and the heat loss method.

Factory boilers silently burn gas, diesel, and heavy oil every day, generating steam that is then fed into equipment. But did you know that in Taiwan, over half of industrial energy is discharged as waste heat ? In other words, we're not just "heating products," we're also incidentally "heating the atmosphere." Therefore, "thermal efficiency," while sounding like an academic term, actually reflects the most pressing issue for businesses: how much of your fuel money actually turns into steam, and how much is lost in the chimney?

1. What is "boiler thermal efficiency"?

In short:

"The ability to turn fuel into steam."

To be more precise, it is:

Effective heat generated by the boiler ÷ Total heat input from fuel

Higher efficiency means less waste. Low efficiency? That means you're burning money every day 💸.

According to the national standard CNS2141 / B1025 and the Japanese standard JIS B8222-1986 , there are two main methods for calculating boiler efficiency: 1️⃣ Input-Output Method.

2️⃣ Heat Loss Method

2. The secrets of the two algorithms: calculating "profits" vs. calculating "losses".

Inbound and outbound heat method: as simple as bookkeeping

Count how much comes in (fuel calorific value), then count how much goes out (steam calorific value). This method is the most intuitive and accurate when the instrument is good enough. But be aware that fuel humidity, steam moisture content, and temperature measurement error—each of these factors can make the calculation inaccurate.

Heat loss method: Calculate how much heat you "leak".

It doesn't calculate how much you earn, but rather how many calories you lose. This includes:

• Exhaust heat loss

• Incomplete combustion loss

• Heat dissipation from furnace walls

• Unburned carbon in ash

• Radiation loss

Then use this formula:

Boiler efficiency = 100% − (Total losses ÷ Total input heat) × 100

For example, a 3 million kcal heat transfer boiler was measured to have the following losses: exhaust loss 13.3%, combustion loss 0.2%, ash loss 1.1%, radiation loss 1.3%, and finally an efficiency of 84.1%.

What does that mean? For every 100 dollars of fuel burned, 16 dollars evaporate from the chimney.

3. What kind of boiler is considered "highly efficient"?

According to the standards of the Japan Boiler Association, the exhaust temperature of a 30-ton coal-fired boiler should be below 180°C , and the efficiency will drop by about 1% for every 20°C increase in exhaust temperature.

So why does Chuang Ding Boiler often emphasize "energy savers" and "smoke exhaust heat recovery design"? Because the difference of 1%, 2%, or 3% can amount to hundreds of thousands of dollars in fuel costs over the long term.

4. 8 Secrets to Improving Thermal Efficiency

This part is the most practical. The following techniques combine the CNS2141 standard with the Chuang Ting team's more than 30 years of field experience:

1️⃣ Lower exhaust temperature 🌬️

The exhaust temperature is usually 20-30°C higher than the steam temperature. If it is too high, it indicates that the heat transfer area is insufficient, scale buildup inside the pipes, or incomplete combustion.

👉Solution:

• Regular maintenance

• Install energy-saving device

• Check if the combustion atomization is uniform.

Lowering the flue gas temperature by 20°C immediately increases efficiency by about 1%.

2️⃣ Reduce the oxygen content in the exhaust (control the air-fuel ratio)

Ideally, the oxygen content in exhaust air should be between 3% and 5%. Too much air → blows away the hot air; too little air → fills the sky with black smoke.

👉Solution : Adjust the air intake under high load to ensure sufficient combustion without wasting air – this is the key to adjusting boiler combustion.

3️⃣ Ensure proper furnace insulation.

If the furnace shell surface is more than 30°C hotter than the ambient temperature, it indicates heat leakage. This is not only wasteful, but it also makes the work area hotter and reduces work comfort.

👉Inspection method: Scan the surface of the furnace with an infrared thermometer. Areas with excessively high temperature differences should be insulated.

4️⃣ Fuel preheating

If the fuel is too cold and viscous, it will atomize unevenly, leading to afterburning and black smoke. The ideal preheating temperature is 80-120°C . Too high a temperature will cause the fuel to carbonize and clog the injectors.

👉Tip : Maintaining a stable preheating temperature is one of the most overlooked details in improving the efficiency of fuel-fired stoves.

5️⃣ The water must be clean.

Scale buildup in boiler water = decreased heat transfer efficiency = increased exhaust temperature = decreased efficiency. Calcium and magnesium ions in the water are the culprits.

👉Practical advice:

• Use an automatic water softener

• Regularly test water hardness (<0 ppm).

• Recycle more condensate for reuse

6️⃣ Replace with a closed-loop condensate recovery system

Open condensate tanks often emit "secondary steam," and that white smoke is all heat escaping! By adopting a closed recovery system, the heat energy can be regenerated and returned to the boiler for water, saving energy, reducing pressure, and being environmentally friendly.

7️⃣ Avoid low-load operation ⚙️

If a boiler is kept at a low flame for a long time, the surface heat loss will increase significantly.

👉Recommendation : If the equipment is frequently under low load, consider replacing the burner with a smaller one or using an inverter control system.

8️⃣ Reduce black smoke production 🏭

Regulations stipulate that the opacity of black smoke must not exceed 40%, also known as "Ringelmann II". But the real key point is not about meeting the standard, but rather—black smoke means you're burning money!

👉Tips : Adjust the fuel injectors, check the burner pressure, monitor the O₂ and CO concentrations, and maintain a blue-yellow flame for the best results.

5. Efficiency is not just a matter of equipment, but also a matter of "management culture".

A high-efficiency boiler isn't something you buy; it's something you manage. When Chuang Ting often assists customers with "efficiency checks" on-site, he discovers that the efficiency difference between two factories producing the same model and brand of boiler can be as high as 10-15% .

The problem isn't with the machine, but with—

✅ Precise control of the combustion air ratio

✅ Operator's daily habits and sensitivity

✅ Preventative maintenance to maintain optimal performance

✅ Precise control of water quality management and chemical dosing

✅ Efficiency of condensate recovery and reuse

These are the key factors that truly determine efficiency.

6. Make every kilowatt-hour of gas count for maximum value.

In an era of ever-increasing energy prices and increasingly stringent carbon emission regulations, high-efficiency boilers are not only about saving fuel, but also the starting point for corporate ESG practices .

Thermal efficiency = Economic benefits + Environmental responsibility + Safe operation

🔹Contact Chuang Ting Boiler now to explore new possibilities for steam boilers! Why Choose Chuang Ting Boilers? ✔ High-quality, stable, and reliable steam ✔ Energy-saving design, reducing costs ✔ Safe and reliable, worry-free operation ✔ Compact size, flexible installation ✔ Multiple units can be connected in parallel for flexible expansion ✔ Quick start-up, improving efficiency ✔ No license or inspection required, saving time and providing convenience ✔ Color HMI, intuitive operation ✔ Nationwide after-sales service, peace of mind and assurance ✔ Top reputation, top quality 📞 Call now: Kaohsiung 07-787-2258 | Taipei 02-8287-6999