Summary
The central problem is that of measuring tube-wall surface temperature in direct fired process heaters under production conditions. Two methods are examined: thermocouples directly contacting the tube and radiation thermometers. The goal of the study is to assess current practices and to formulate recommendations for further development of the measurement technology.
Thermocouple installations are thought to be unreliable for extended operation because of the rapid drift in the calibration and deterioration of protective materials in the furnace gas atmosphere. Radiation thermometers are widely used in plants with beneficial results when attention is given to proper/frequent calibration procedures and to the effects of in-furnace conditions tube emissivity, reflected irradiance, and furnace gas emission/absorption on the indicated radiation thermometer readings. Through a radiation analysis and a series of field experiments, the effects of in-furnace conditions are demonstrated. To provide further background on the problem, an extensive literature search was performed, and the information summarized.
The study leads to the conclusion that absolute measurement accuracies of ±10°c (±18°F) at 950°c (1750°F) are possible only if corrections for in-furnace conditions can be estimated. However, at present, only a few plants are capable of routinely obtaining re-producibilities (precision) of ±10 C (±18 F), which can be achieved by well-prescribed calibration procedures, good operator technique, and systematic data analysis including, in some instances, correlative models employing other furnace parameters.
Recommendations to develop the technology are offered. Educational activities at the engineering level need improvement in order to gain a better understanding of the principles of measurement methods, to train operating personnel and to define operating procedures. Investigative studies are called for to generate new information on radiation thermometry practices.
Also, similar studies on improved thermocouple technology are warranted. The industry will greatly benefit by dissemination of data and measurement technology through participation in national technical society meetings.
