Thermography in the Paper Mill -
Detecting Moisture Irregularities.

ABSTRACT
In growing numbers, paper companies are realizing the benefits of Infrared Thermography as a troubleshooting tool, especially as it relates to determining the source of cross direction moisture variations in the sheet. The detailed Infrared Analysis, in most cases, can quickly determine the origin of the process-related problem causing the moisture variation. The variations in sheet moisture at the reel are usually found to be occurring at the wet end, directly correlate with temperature differences. The application of this phenomenon to the paper making process is based on the fundamental temperature/moisture relationship in the sheet. Paper with a lower moisture content is relatively warmer than paper with a higher moisture content therefore, the darker thermographic images are "cooler" and represent areas of higher moisture content. Conversely, the brighter the area, the drier the sheet. The sheet temperature profile correlates very well with the moisture profile scans at the reel. No matter where they originate, the wet areas in the sheet remain cooler than the drier areas. This paper discusses the use of the Infrared Thermographic analysis to enable the mill to identify the source of irregularities in moisture without shutting down the machine or otherwise affecting production; thus making problem-solving both efficient and economical.

INTRODUCTION
The application of Infrared Thermography as a diagnostic tool, to look at the entire paper making process allows a unique opportunity to literally "see" the affects of the various processes on a paper machine have on product quality and machine efficiency.

HERE'S HOW IT WORKS
The principles of infrared imaging are based on the fact that all objects emit infrared radiation. This level of radiation increases directly as temperature increases. Infrared imaging equipment provides real time presentation of temperature patterns being emitted by an object. These patterns are displayed as standard video images in either black & white, or color. The darker colors represent "cooler" temperatures, while the lighter colors would be "hotter".

SHEET MOISTURE/TEMPERATURE RELATIONSHIP
The advent of portable infrared imaging equipment has proven useful in determining the cause of cross-direction and machine direction moisture variations on paper machines. Variations in temperature of the sheet at the reel directly correlate with variations in sheet moisture. Areas with higher temperatures indicate dry conditions. Conversely, areas that are cooler contain higher moisture content. We found through the use of high-resolution thermography, that the sheet temperature profiles correlate very well with the moisture profile scans at the reel.

THE SURVEY
When conducting an infrared survey of a paper machine looking for moisture irregularities, I begin by looking at the sheet being wound at the reel and follow the sheet to the wet end. The following sequence of photographs, show how the cool (wet) area can be followed down the machine to determine its source. Figure 1 show a real time photograph of the sheet being wound at the reel, infrared photographs (in color and black & white) of the reel, and a copy of the mill's moisture scan of the reel. The infrared photographs correlate quite well with the poor moisture profile.

Figure 1

The survey continues toward the wet end looking at the sheet anywhere there is a clear unobstructed view. Figure 2 shows the top side of the sheet exiting the 3rd press, it still shows to be cooler (wetter) on the tending side, same as what was noted at the reel.

Figure 2

The sheet passing over the center press roll is looked at next, (figure 3) this shows the same cool (wet) front 1/4 th of the sheet, the same as at the reel.

Figure 3

The stock passing down the wire and over the couch roll (figure 4) shows the temperature to be quite even ahead of the couch roll, but as the stock passes over the couch roll it becomes cool on the front 1/4th

Figure 4

CONCLUSION
The Cool (wet) front 1/4th of the sheet we followed from the reel to the former, was being caused by a worn wipe on the couch roll.

SUMMARY
While infrared thermography does not replace good paper making know-how, it does provide information that can help pinpoint complicated or persistent moisture imbalance problems.

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