Article updated on 26 May 2020
Infrared Dring of a car body part. (Image Credits: Aleksandr Kondratov/shutterstock.com)
Lasers and infrared technologies are used in many industries and applications. One of these applications is in heat treatment processes. Whilst both high-powered lasers and infrared heat treatments are used, they are used in completely different industries, with lasers being primarily used to harden metallic materials and infrared radiation being used to destroy microorganisms and keep food fresh in food processing operations. Even though they are both different, they hold distinct advantages over the ‘standard’ heat treatment methods in each sector, and we will look at both examples in this article.
Lasers and Metallic Heat Treatments
Lasers are known to be a high energy and high-temperature heat source. Whilst most of their use is in various optics applications, there are a couple of lasers which are used in heat treatment processes. The two main types of lasers are carbon dioxide lasers—which have been around for a few decades in the heat treatment space—and high-power diode lasers—which are a newer type of laser in heat treatment processes.
Lasers are primarily used to treat metals or metallic alloys and are used so that the surface of the metal becomes hardened and is, therefore, more resistant to wear and degradation. During the laser hardening process, a well-defined laser beam is used to illuminate a localized piece of metal. The light from the laser is readily absorbed by the metal and this causes rapid heating to occur on the surface of the metal (in the localized area), but it does not penetrate below the surface layers of the metal.
The main reasons why a laser is beneficial for this type of heat treatment are the ability to control the illumination on a localized area (and in turn the degree of heat on the metal), it being a rapid processing technique, the precise control over penetration depth, precise control over the degree of hardening of the metal, and minimal distortion of the metal itself. This is in addition to the properties that the hardening process brings, including increased wear and corrosion resistance, and increased fatigue strength.
Carbon Dioxide Lasers
Carbon dioxide lasers are the traditional laser heat treatment method which has now become the standard laser method for treating metals. But it does have some drawbacks, and this is why new types of laser heat treatments are starting to take the fore. Even though they are the traditional laser treatment, the drawbacks include the laser light being in the infrared region of the electromagnetic spectrum and this means that the light is not as easily absorbed by the metals (and this often requires an absorptive coating), the conversion of electrical energy input to light output is low, and the laser beam often needs to be expanded as it is well-collimated and too small for many processing areas.
In recent times, diode lasers, i.e. those composed of semiconducting parts, have been more widely used because they can better convert electrical input into light output. Another reason why is because they emit light at the near-infrared region of the electromagnetic spectrum, so the illuminated light is absorbed by the metal more efficiently. This also negates the need to pre-coat a metal prior to treating it with the laser, which also has knock-on effects with less chemical waste generated during the heat treatment process. Because the power conversion efficiency of diode lasers is much greater, it also means that they use less energy, and this correlates to a cheaper and greener heat treatment process.
However, the localized area in which the laser can illuminate at one time is (usually) still smaller than the total area undergoing the heat treatment. But diode lasers use many different beamlets that form a single laser beam, and this helps to uniformly distribute the laser light over the area being processed. So, in many cases, the laser just needs to move around the metallic component, or the metallic surface needs to be moved across the processing area, to cover large areas uniformly.
Infrared Heat Treatments in the Food Industry
Irradiation with infrared light is a technique used within the food processing industry—especially on fresh food, such as fruit and vegetables—to inactivate microorganisms, improve the shelf-life of the product, whilst enabling the food to keep its nutritional value. Infrared processing has been shown to have many benefits over traditional heat treatment methods—such as blanching, dehydration, freeze-dehydration, thawing, roasting, baking, and cooking—and it can be used in conjunction with these traditional methods if required, as well as in conjunction with microwaving heating, as this has been found to provide a high energy throughput.
In infrared heat treatments, the infrared wavelengths (which are variable) penetrate the foodstuff and gets converted into heat on the surface of the food. This type of heating also suffers less from surface irregularities and this enables more uniform heating. Whilst many wavelengths in the infrared region can be used, those which are present in the far-infrared spectrum are more readily absorbed by foodstuffs. When the food is exposed to the infrared radiation, the structure undergoes electronic, vibrational, and rotational changes, which generates a high amount of heat.
Some of the advantages of using infrared heat treatments over conventional heat treatments include shorter process times, an increased energy efficiency, uniform product temperature, higher-quality finished products, a higher degree of control in the process, higher heat transfer coefficients, lower use of space, and a more environmentally friendly process.
Infrared Drying of Foodstuffs
Infrared heat treatments have also been utilized in the drying of foods. In these scenarios, infrared radiation and hot air are used to dry out multiple high-water content foodstuffs and the heating/drying rate is known to be higher than when other traditional techniques are used on their own. Infrared-assisted drying also provides a more uniform drying process and this correlates to a higher quality dried product compared to more traditional methods.
During a drying process, the infrared radiation penetrates the surface of the foodstuff, and the energy contained within the radiation transfers into heat energy. How far the infrared radiation penetrates the foodstuff is dependent upon a few factors but is particularly dependent upon the composition and structure of the foodstuff and the wavelength used. In any case, when the heat energy is generated, the internal temperature of the foodstuff increases rapidly, and the food reduces in size rapidly. Additionally, intermittent bursts of infrared radiation followed by cooling, in a continuous cycle, helps to bring the moisture from the core of the food to the surface, and this helps to remove water quickly and uniformly.
As infrared is involved with the drying of fresh foodstuffs, there are many fresh foods that can be dried by using infrared-assisted drying, and these include onions, bananas, apples, pineapples, potatoes, herbs, peppers, blueberries, strawberries, cashew nuts, shrimp, mackerel, rice, barley, and other cereal-based foodstuffs.
In addition to using infrared heating in drying processes, infrared-assisted heating can also be used in freeze-drying, thawing, roasting, blanching, baking and cooking processes, but to the same extent as drying.
Sources and Further Reading