This is the Infra-View® application note, rewritten for the page from the PDF. Download the PDF (6.5 MB).
Why furnace exit gas temperature matters
Furnace exit gas temperature, FEGT, is the temperature of the combustion gas as it leaves the furnace and enters the convection passes. It is the single number that says how much of the fuel’s heat was absorbed by the water walls under a given load, and it sets what the superheater and reheater will see.
When FEGT runs high, fly ash softens and sticks to the tubes and pendants. Slag builds, heat transfer to the tube walls falls, and the result is more frequent sootblowing, tube corrosion, load restrictions and, at the extreme, a safety problem. When FEGT runs low, the furnace may be losing too much heat to the walls or burning incompletely, and efficiency is lost the other way.
Fuel quality, excess air, burner selection and tilt, low-NOx operation and heat-transfer problems all move the exit gas temperature. Watching it continuously is how operators balance and optimize combustion and protect the boiler.
Ways of measuring FEGT
Contact methods
A thermocouple inserted into the gas stream is the traditional approach. Thermocouples are small, durable and accurate, read to about 2,500°C (4,532°F), and should be calibrated annually. Resistance temperature detectors are accurate and stable to about 1,000°C (1,832°F); thermistors are cheaper but limited in range. All of them share one problem in a furnace: the sensing element sits in the hot, abrasive gas and erodes, so it is a point reading with a service life.
Non-contact methods
Infrared instruments read the gas from outside the wall. An infrared thermometer measures a small area of the gas stream through a port and is simple to apply; it needs a clear line of sight and is sensitive to what it is actually looking at. Thermal imaging cameras give a picture of the temperature distribution across the gas stream at a much higher cost.
What skews a reading
- Sensor placement. The position in the gas path has to be representative of the exit gas, not a corner or a wall effect.
- Sensor type and size. The instrument’s range and accuracy have to suit the gas temperature expected.
- Emissivity. Different materials radiate differently. An infrared instrument aimed at a surface reads that surface’s emissivity, which is why a sensor that responds to the gas itself is a different proposition from one that reads a wall.
- Soot and ash. Contaminants in the gas stream can block the view or change what the sensor sees.
The Infra-View® approach
The Infra-View® infrared thermometer sidesteps the emissivity problem by reading the gas, not a surface. Its detector is a thin-film thermopile behind a spectral filter that passes only the infrared band emitted by hot carbon dioxide, the product of burning any fossil, biomass or waste fuel. Because it is blind to every other wavelength, it measures the temperature of the CO₂ in its field of view directly.
The sensor is a permanent fixture on any door, port or penetration, supplied in a patented protective jacket with air cooling, purging and filtering so it lives on the boiler wall through the run. The same sensor is also supplied as a portable instrument for surveys.
Key figures:
- Range 250°F to 3,000°F (120°C to 1,650°C), with higher end temperatures available.
- Accuracy better than 1% (maximum error 30°F).
- Analog output linear 0–20 mA or 4–20 mA, fully sub-scalable; digital 4–20 mA, loop powered.
- 30:1 field of view (a 3 ft spot at 90 ft).
- Response time 100 ms (600 readings per minute).
- Adjustable alarms on FEGT deviation from set-point, with the set-point digitally adjustable for different run and fuel conditions, so fouling and slagging show up as they start.
- A NIST-traceable calibration certificate on request.
Plants use the continuous FEGT signal for sootblower control, startup, slag control, low-NOx tuning, fireball centring and general FEGT monitoring.
Conclusion
FEGT is the parameter that ties combustion to heat transfer. Contact sensors give an accurate point that wears out; infrared instruments read from outside the wall but are only as good as what they are looking at. An instrument that responds to the hot gas itself, mounted permanently and cooled on plant air, gives the continuous, representative reading that optimization and protection both need.
Tell us about your boiler in the Pyrometer Selector, or contact us.
Measuring boiler or furnace gas temperature?
We manufacture the Infra-View® infrared boiler and furnace thermometer — a non-contact reading of flue exit gas temperature through any boiler penetration, 4–20 mA to the DCS. The selector takes your application and fuel and an engineer comes back with the right configuration.