|
Assembly and technical evaluation of a steel chimney both require determining shape of its axis. Because permissible deflection of the top of a steel chimney shaft relative to the base according to the PN-B-06200 norm is: 30 mm when height < 50m and 0.0006 * height when height > 50 m, geodetic measurements should be performed with high accuracy. Additionally, the measurements should be performed when deflection of the chimney is not influenced by sun and wind. Deflection measured in a sunny weather is influenced by elastic deflection caused by uneven temperature distribution in the chimney shaft. In practice, the measurements should be done before the sunrise or on a cloudy day. However, during chimney assembly it is often not possible to choose conditions of measurement. For steel chimneys of cylindrical shape without thermal insulation inside, the direction of elastic deflection is not very different from the direction of sun's rays, and its value can be calculated from dimensions of the chimney and temperature differences on its circumference. The problem discussed in this article is measuring the real temperature of the outer surface of a chimney in a possibly fast and accurate way. Thermography allows acquiring the surface temperature distribution quickly (in a few minutes). The accuracy of the measured values is influenced by: emissivity coefficient of the chimney surface, air temperature and humidity and background temperature. The biggest problem is determining background temperature if the surroundings consist of a few objects with different temperatures. The equivalent background temperature was determined from thermo graphic and contact temperature measurements of a specially prepared sample. The sample was produced from the same material as the steel chimney, covered with the same paint and the measurements were performed near the chimney, so the surroundings were similar to the surroundings of the chimney. Conducted experiment shows that on a sunny day the background temperature cannot be assumed equal to the air temperature. In cloudless conditions in the early morning or the late afternoon the background temperature is lower than the air temperature, and around the noon it is much higher. Only in the low overcast conditions the background temperature becomes close to the air temperature
|