Thermography is a non-contact technique that uses infrared cameras to capture images of surface heat variations, called thermograms.

Thermography is a non-contact technique that uses infrared cameras to capture images of surface heat variations, called thermograms.

1. Aircraft Condition Monitoring UNIT-III Condition Monitoring Analysis-Thermography Prepared by Jhumki Nandy Department of Aeronautical Engineering, Sanjay Ghodawat University Kolhapur

2. Industrial Applications of Thermography Thermography has found many industrial applications for two obvious reasons: First, thermography is noncontact in nature, and temperature can be measured from a distant location. Second, light, portable, and precise IR image cameras are commercially available. The recent advances in instrumentation along with the availability of user-friendly software has made thermography useful in many engineering applications. Figure 10.2 shows a hand-held thermal imaging camera being used for temperature monitoring of an engine.

3. Electrical and Electronic Component Heat Generation In many electrical and electronic systems and circuits, due to flow of current in a conductor, heat is generated that is proportional to the square of the current. In many instances, due to an increase in the load resistance, the current in the circuit increases as a result of which there is a temperature rise, which can be easily detected with infrared imaging. The increase in resistance could be due to oxide formation at the electrical contact points, electrical sparking due to loose contacts, and so on, which in turn increases the current flowing through the conductors. In a plant control room or a system of machineries where several electrical conductors connect the various power sources with electrical loads through switchgears, thermography aids in a quick survey to detect a conductor at a higher temperature. Electrical motor defects like broken rotor bars, stator short circuits, and so on, can also be detected by thermal imaging. Figure 10.3 shows the heated electronic components in an amplifier detected by thermal imaging.

4. Building Condition In many instances, several defects can occur in buildings, like roof leakage, cracks at building joints, mold growth due to dampness, and so on. These result in different temperatures in different parts of the building. Through infrared imaging of such building structures, variations in temperature can be measured and clues as to cause of the building defect can be obtained. Machineries IR imaging is used for thermal imaging of processing equipment. Figure 10.4 shows the thermal image of a reciprocating compressor. It can be seen that

5. Applications of Thermography in Condition Monitoring Shafts are important machine components that are supported on bearings. The bearings are designed to provide low frictional resistance to the rotating shafts and to support the loads on the shafts. Due to defects in the rotating machines, many times the loads on the shaft increase, which in turn increases the normal reaction at the bearings. Due to an increase in the normal load, the components in the bearing undergo deflection along with an increase in frictional torques. The heat generated at the bearings thus increases, which is responsible for the increase in temperature at the bearings. The rate of heat generation is proportional to the rotational speed of the shafts. With an increase in the generated heat at the bearings, the grease that is present to lubricate the bearings liquefies and flows out. Due to excessive heat generated at the bearings, the remainder of the grease can become baked and form hard particles. These particles score the bearing races, which leads to further deterioration of the bearing. To extend the life of the bearings, the temperatures of rolling element bearings are not allowed to rise above 70°C.

6. Infrared thermal imaging can be used to monitor the temperature of bearings, particularly in distant locations. Similar to the phenomenon of heat rise in rolling element bearings, flexible couplings, in which flexure elements are present in the coupling, experience frictional heat generation because of excessive forces at the couplings due to shaft misalignment. This heat generation is responsible for the increase in the coupling temperature.

7. Infrared Thermometer On its most basic design an  infrared thermometer  consists of a lens to focus the infrared (IR) energy on to a detector, which converts the energy to an electrical signal that can be displayed in units of temperature after being compensated for ambient temperature variation. This configuration facilitates temperature measurement from a distance without contact with the object to be measured. As such, the infrared thermometer is useful for measuring temperature under circumstances where thermocouples or other probe type sensors cannot be used or do not produce accurate data for a variety of reasons. Some typical circumstances are where the object to be measured is moving; where the object is surrounded by an EM field, as in induction heating; where the object is contained in a vacuum or other controlled atmosphere; or in applications where a fast response is required.

8. T YPES OF I NFRARED I NSTRUMENTS Most infrared-monitoring systems or instruments provide special filters that can be used to avoid the negative effects of atmospheric attenuation of infrared data; however, the plant user must recognize the specific factors that will affect the accuracy of the infrared data and apply the correct filters or other signal conditioning required to negate that specific attenuating factor or factors. Collecting optics, radiation detectors, and some form of indicator are the basic elements of an industrial infrared instrument. The optical system collects radiant energy and focuses it on a detector, which converts it into an electrical signal. The instrument’s electronics amplifies the output signal and processes it into a form that can be displayed. Three general types of instruments can be used for predictive maintenance: infrared thermometers or spot radiometers, line scanners, and imaging systems.