JNTU B.Tech III INSTRUMENTATION PRACTICES IN INDUSTRIES Previous Question Paper (August - 2022 - R18)

Temperature measurement in a blast furnace is critical for monitoring the reduction process and ensuring structural integrity. Radiation pyrometers are preferred because they allow for non-contact measurement, which is essential given the extremely high temperatures (up to 2000°C) and the harsh, corrosive, and high-pressure environment inside the furnace.

The measurement principle relies on the Stefan-Boltzmann Law, which states that the total energy radiated by a black body is proportional to the fourth power of its absolute temperature:

In a blast furnace, radiation pyrometers are typically installed at the tuyeres (where hot blast air enters) or at the furnace stockline (the top surface of the burden). The implementation involves the following steps:

• Sighting and Optical Path: A sighting tube or a protected port is used to provide a clear line of sight to the target area. Because the furnace atmosphere is filled with dust, smoke, and gases, these ports are often purged with high-pressure nitrogen or clean air to prevent lens fouling and ensure an unobstructed optical path.
• Radiation Collection: The pyrometer lens focuses the infrared radiation emitted from the hot surface or the molten iron onto a sensitive detector, such as a thermopile or a photodiode.
• Emissivity Compensation: Since the interior of a blast furnace is not a perfect black body, the pyrometer must be calibrated for the specific emissivity of the coke, ore, or molten metal being measured. Multi-wavelength or ratio (two-color) pyrometers are often used to minimize errors caused by varying emissivity and partial obscuration by dust.
• Signal Processing: The detector converts the radiant energy into an electrical signal, which is then linearized and processed by a transmitter to provide a standard output (e.g., 4-20 mA) for the control system.

The primary advantage of this method is the ability to monitor the thermal profile of the furnace without being damaged by the molten slag or metal, thereby providing real-time data for optimizing fuel consumption and iron quality.