Measuring FEGT In Industrial Furnaces

Furnace Exit-Gas Temperature (FEGT): A Crucial Parameter

Industrial furnaces are used in a wide range of applications, including metal production, chemical processing, and heat treatment. They are designed to reach high temperatures to achieve the desired results (Figure 2). The combustion process in these furnaces involves the mixing of fuel and air, which generates heat that is transferred to the material being processed. Measuring the FEGT is essential in ensuring the efficient and safe operation of the furnace. The FEGT is a critical parameter that indicates the performance of the combustion process and can help identify potential issues such as overheating or underheating.

Figure 2: Heat Absorption Chart

Figure 3: FEGT Measurement Chart

The FEGT measurement (Figure 3) offers a precise measure of the heat transfer to the water walls of the furnace under a given load condition and establishes the anticipated performance of the superheat and reheat procedures. When FEGT is high, it can cause fly ash to stick to the tubes and pendants, creating slag and decreasing heat exchange efficiency with the tube walls. This can result in frequent soot-blowing, tube corrosion, lower load operation, and potential safety hazards.

Conversely, a low FEGT value may indicate excessive radiative losses to the water walls or an incomplete combustion process resulting in lost efficiency. Fuel quality, excess air, burner selection and tilt, low NOx operation, and heat transfer issues all affect the exit-gas temperature.

Conversely, a low FEGT value may indicate excessive radiative losses to the water walls or an incomplete combustion process resulting in lost efficiency. Fuel quality, excess air, burner selection and tilt, low NOx operation, and heat transfer issues all affect the exit-gas temperature.

Methods for Measuring FEGT

There are several methods for measuring FEGT in industrial furnaces, including contact and non-contact methods. Contact methods involve the use of a thermocouple or other temperature sensor that is placed directly into the exhaust gas stream. Non-contact methods involve the use of infrared (IR) radiation to measure the temperature of the exhaust gas.

Figure 4: RTD – Thermocouple – Thermistor

Contact Methods for Measuring FEGT

Thermocouples are commonly used to measure FEGT in industrial furnaces. They are small, durable, and can be inserted directly into the exhaust gas stream. They can measure temperatures up to 4,532°F (2,500°C) and are highly accurate. It is recommended that these thermocouples are calibrated annually to maintain functionality and accuracy.

Other contact methods include resistance temperature detectors (RTDs) and thermistors. RTDs are highly accurate and have a stable output over a wide temperature range. They are suitable for use in harsh environments and can measure temperatures up to 1,832°F (1,000°C). Thermistors are less expensive than RTDs and thermocouples but have a limited temperature range.

Figure 5: Infra-View Pyrometer

Contact Methods for Measuring FEGT

Thermocouples are commonly used to measure FEGT in industrial furnaces. They are small, durable, and can be inserted directly into the exhaust gas stream. They can measure temperatures up to 4,532°F (2,500°C) and are highly accurate. It is recommended that these thermocouples are calibrated annually to maintain functionality and accuracy.

Other contact methods include resistance temperature detectors (RTDs) and thermistors. RTDs are highly accurate and have a stable output over a wide temperature range. They are suitable for use in harsh environments and can measure temperatures up to 1,832°F (1,000°C). Thermistors are less expensive than RTDs and thermocouples but have a limited temperature range.

The Placement Of The Temperature Sensor:

The position of the temperature sensor in the exhaust gas stream can affect the accuracy of the measurements. The sensor should be placed in a location that is representative of the temperature of the exhaust gas stream.

The Type And Size Of The Temperature Sensor:

The type and size of the temperature sensor can affect the accuracy of the measurements. The sensor should be selected based on the temperature range of the exhaust gas stream and the accuracy required.

The Emissivity Of The Material Being Measured:

The emissivity of the material being measured can affect the accuracy of the measurements. IR radiation is absorbed differently by different materials, which can affect the readings of infrared thermometers.

The Presence Of Contaminants In The Exhaust Gas Stream:

The presence of contaminants such as soot or ash in the exhaust gas stream can affect the accuracy of the measurements. Contaminants can block the view of the temperature sensor or affect the emissivity of the material being measured.

Figure 6: Infrared Thermometers

A Safe and Reliable Solution: Infra-View® Infrared Thermometers for FEGT Measurement

Infra-View® Infrared Thermometer (Figure 6) is a state-of-the-art infrared system that offers a reliable and secure solution for measuring flue gas temperature. The device is equipped with an Infra-View® sensor that can detect heated CO2 gas generated during combustion processes in boilers or furnaces using fossil fuels. The sensor is designed with a thin film thermopile and a spectra filter that blocks out other infrared emissions, allowing it to detect CO2 infrared energy with high sensitivity. This makes it possible to directly measure the temperature of the CO2 gas within its field of view.

Infra-View® Infrared Thermometers are remotely-sensed infrared detectors that can become a permanent fixture that is installed on any door, port, or into the boiler or furnace (Figure 7). The device is lightweight and can also be used as a portable system for testing purposes. It comes with a patented design that includes a protective-cooling jacket, air cooler, purging and filtering system, making it suitable for use in harsh environments.

Figure 7: Installation Location Diagram

The Infra-View® infrared “non-contact” sensors can monitor flue gas temperatures in the boiler or furnace within the range of 250°F (120°C) to 3,000°F (1,650°C), with even higher temperature measurements also available.

Other Notable Features of Infra-View® Infrared Thermometers include:

• Operators can set adjustable alarms that sound off in the presence of FEGT changes (e.g., deviations from the set-point.
• Detect the occurrence of fouling and slagging.
• Digitally adjust the alarm set-point to suit different run and fuel conditions.
• Pyrometers can continuously measure temperatures between 250°F (120°C) to 3,000°F (1,650°C), with an accuracy of better than 1% (Max. Error + 30°F).
• The analog output is linear 0 to 20 mA OR 4 to 20 mA, and can be fully sub-scaled.
• Offers a Field of View that is 30:1 (i.e., 3 ft spot @ 90 ft).
• Response Time: 100 msec(600 readings/min.)
• Data Output: Digital 4-20 mA Loop Powered.
• A NIST Calibration Certificate can be requested.