Turning up the heat01 October 2005

At any stage in every manufacturing process, the application of heat to the workpiece or product is required - whether that is in the drying of paper or cellulose films, to the curing of rubber or painted surfaces. Within industry, the use of process heating systems has typically relied on convection heating systems, with the heating elements installed in ovens and air driven through the heating element on its way to making contact with the product to be heated. Of course, such processes are inherently wasteful of energy, plus a proportion of the plant's overall fuel consumption is used to heat anything in the path between heat source and product.

Use of infrared process heating not only has a variety of process applications, but offers benefits in terms of more rapid production, efficient use of energy and reduced factory floor space. Although there are many methods of process heating in use, the potential for using infrared (IR) light in this process technology has, until recently, only been exploited in a small way. Similarly, the use of ultra-violet radiation, also based on quartz lamp technology devised back in 1904, is only partially exploited. The industrial application of ultraviolet as process heating is particularly useful in waste water treatments, curing printing inks, lacquers and adhesives.

Typical uses of IR technology in manufacturing include coating, laminating, stretching and embossing operations. Stretching can include the pre-heating of PET plastic containers, prior to final forming operations, while floor coverings that have embossed features are pre-heated by IR systems, prior to the embossing process. Other examples include the bonding of powder, paint, or varnish coatings to metal surfaces, embracing anything from bicycle components to gas or oil pipelines.

Form and function

Infrared transfers heat by radiation and is the thermal energy emitted when a body is heated, and is generated by the oscillation of the atoms in the heated body or emitter. The energy radiated is a continuous spectrum of frequencies and the spectral distribution, the amount of energy radiated and the rate at which energy is radiated all depend largely on the temperature of the emitting surface.

Infrared radiation occupies that part of the electromagnetic spectrum immediately adjacent to visible light, at a wavelength extending from 0.78µm to 200µm. For industrial heating applications, the useful part of this spectrum extends from 0.7µm to 10µm and is then further subdivided into short-, medium- and long-wave radiation, according to the position of the peak intensity in the emitted spectrum.

Conventional infrared emitters have used 'iconel' or nickel chrome spiral elements in silica glass tubes, with reflective materials at the rear, although today carbon is the most common medium for the basic emitter. The emitters themselves come in a variety of sizes and, in specific examples, have been formed to follow the contours of products. This approach allows heat energy to be produced and targeted exactly where it is needed, in terms of focus, size and spectrum.

In general, infrared emitters are tubular assemblies, with a carbon strip located in the centre of quartz glass tubes, and a gold or other metallic reflector applied to the surface of the tube. The most common arrangements are single or twin tube layouts, and these may be connected in multiple heating arrays, depending on the application. Also, the use of halogen within some of the more recent designs can deliver very fast response times, with maximum outputs of around 1,000 kW/m2.

Transition and efficiency

Before moving to an infrared process heating solution, there are a number of factors that need consideration. Ideally, the absorption characteristics of the material should be matched with the emissivity of the radiator. Broadly speaking, longwave radiation is less sensitive to colour difference while short-wave radiation is more penetrating and can be better for volumetric heating.

However, the wavelength which is most readily absorbed by most materials, and especially by water, is medium wave. With process materials such as plastics, it should be borne in mind that, because of its low absorption, short-wave infrared penetrates more deeply into a material and provides volumetric uniform heating, while medium wave is absorbed rapidly in the material outer layer and so predominantly heats only the material surface.

In terms of return on investment, depending on the complexity of the process, this can easily be achieved within a conventional five-year business planning cycle, and typically within one to two years from installation.

Electricity is the most effective and preferable power source for IR process heating, with efficiency of the overall heating system usually at 50-60%, while gas-powered solutions only reach between 30-40%. This contrasts markedly with a convection oven, where the maximum efficiency may only reach 20%. Conventional systems are, by comparison with an IR solution, not only inefficient, in that they are required to heat large volumes of air that is directed at the product or coating for drying or curing using fans, but are complex to manage.

An important contribution to overall efficiency is the use of electronic controls that enable rapid start-up and shut-down of the drying/curing process. Infrared process heating control systems make extensive use of electronics in power conditioning systems, with high-speed switches such as thyristors to regulate the timing, energy consumption and output of the emitter arrays. However, the control system's contribution to the overall process improvement is not limited to controlling the infrared emitters, but also the flow of products, perhaps on a conveyor belt, through the heating/drying/curing section.

A key reason for adoption of infrared process heating is to reduce, remove or speed up the drying section of the manufacturing process. For plant engineers and managers, competitive pressures within the business will also drive the need to reduce overall operational costs and application of infrared heating to the process can be a major factor in achieving that goal. Infrared systems can be retrofitted in less space than that occupied by existing convection oven heating systems, reducing the need and costs for any factory extension. This, in turn, could free up space without large scale refurbishment.

Within the process itself, benefits include more rapid start-up times for the drying and curing phases, with more precise control of timings, energy consumption and power output, and a much simpler installation, offering significantly lower maintenance costs.

For some applications, the use of the air knife modules can further speed up the drying or curing processes, where cool air is passed between the IR emitters and the product, and extracted as dry, warm air at the end of the process. This has the effect of removing vapours or moisture as the product is heated or dried, and at the same time both shortens the drying process and extends the useful life of the emitters.

Infrared is now used in virtually every industry, from softening rubber components to facilitate fitting in the motor industry, to curing the powder coatings on mountain bikes. It is easy to install, simple to operate, has minimum maintenance requirement and offers a range of operating benefits including, space- and energy-saving and controllability.

Whilst infrared is not the panacea to all production heating, drying and curing problems, it is a technology which seriously merits consideration when viable alternatives are sought to current production practices.

Brake pad bottleneck

At Federal Mogul Friction Products, infrared has eliminated a previous bottleneck and quadrupled the productivity of curing a powder coating of a range of brake pads.

An important stage in the production of brake pads is the application of an anticorrosion coating, which provides protection to the pad against water damage during operation. After this black epoxy coating is applied on the production line, it must then be cured. Previously, this took place in a 48kW medium wave infrared oven. However, with increasing production requirements, on account of growing sales demands, it was found that the curing operation could no longer keep pace with the coating application. The decision was taken to investigate improved powder coating curing technology to eliminate what was becoming a production bottleneck.

As it was a requirement of the curing process that any heat applied should be effective only at the product surface and not within the material of the product, it was decided that infrared, again, offered the best solution to the problem. In addition, as any new curing system had to be fitted within limited existing space on the production line, this also precluded technologies such as convection ovens and reinforced the argument for infrared. Consequently, Federal Mogul contacted Heraeus.

Infrared is ideally suited to the curing of powder coatings and acts in two stages. The pre-heat stage brings the powder to melt and flow temperature, and the coated component is then held at temperature to allow curing to take place. Medium-wave infrared is especially suited to this application and Heraeus carried out tests at its Bromborough Test Centre, using samples from Federal Mogul, to establish how best to meet the curing requirements.

As a result, a 162kW infrared oven, fitted with 30 fast-response medium-wave emitters, a honeycomb wire belt conveyor and a control panel for both emitters and conveyor was installed at the Chapel-en-le-Frith factory. This is divided into two zones to provide the melt and cure, the first zone providing a power density of around 78kW/m over the first 1,200mm of the oven's length, while the second zone provides a power density of 51kW/m over the final 800mm of the oven's length. The control panel features five selector switches, each controlling six emitters so that these can be set to zero, 50% power or 100% power, as required. Similarly, a potentiometer provides speed control of the conveyor between 0.5 and 2.0m/min, so that the power and dwell within the oven can be varied to provide a curing profile to suit a range of pads.

SOE

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