Volume 28, number 1, June 2014

Infrared for industrial heating processes

Many industrial processes benefit from the use of infrared heating for multiple applications, including drying finishing products, protective metal coatings, paper, coatings, inks and textiles, as well as cooking foodstuffs or forming plastics and acrylics.

Infrared energy: temperature to the power 4

Infrared is a radiant energy which is not absorbed by air and which converts into heat when it hits an opaque object. Water and solvents evaporate rapidly when they absorb infrared heat, and the radiant energy can also chemically modify materials through the effect of polymerization, which is needed for powder paints.

The most important property of infrared is revealed when the temperature is increased at the source: radiant energy is proportional to the fourth power of the absolute temperature, versus convection and conduction, where the energy transmitted is directly proportional to the temperature.

Overview of different categories of natural gas infrared emitters

Several manufacturers offer infrared emitters for heating applications in industrial processes. They can be grouped into four categories:

  • Metallic fiber matrix

  • Ceramic fiber matrix

  • Direct flame burner

  • Catalytic

In the case of metallic- and ceramic-fibre emitters, a pre-mixture of natural gas and air is released across the fibre and burns on the surface. The heated fibre emits radiant energy on the product to be treated. The direct flame burner emits a flame focused on ceramic plates that are oriented toward the products to be treated. The temperature of these emitters is typically around 650°C to 1,100°C.

In catalytic emitters, the natural gas is introduced in the rear of the unit and passes across a buffer coating on the catalytic converter. In the front, where oxygen is in contact with the buffer, a oxydation reduction follows, generating radiation. There is no flame since the surface temperature of the emitter is lower than the auto-ignition temperature of natural gas. The surface temperature of the emitter operates in a range varying from 230°C to 480°C.

Heat transfer: Emissivity of materials and radiation absorption

Once the radiation is emitted, the object to be heated could partly absorb it, reflect it, or transmit it, since radiation can also pass through certain materials, such as water and solvents (see Figure 1). The energy absorbed is then transferred through conduction to the entire object being heated.

Figure 1

In several applications, it is important to match the radiation emission spectrum to the absorption spectrum of the material to be heated in order to optimize energy efficiency. The emissivity data chart should be consulted for the material to be heated to obtain a relative measure of its potential to absorb radiant energy. The higher the emissivity of a material, the more it can absorb radiation. The following table indicates the emissivity of some materials.

Material

Emissivity

Water

0.96

Glass

0.94

PVC

0.92

Rubber

0.86 – 0.95

Wood

0.9

Polished stainless steel

0.85

Galvanized steel

0.80

Iron and steel

0.70 – 0.75

Brass and copper

0.60

The colour and finish (matte, satin, gloss) also influence radiation absorption. Given the great variety of materials, coatings, colours and finishes to be treated, it is recommended testing different appliances in order to select the type of emitter that will give the best radiation absorption characteristics.

Advantages of radiation

While heating by natural gas convection is still just as popular for industrial oven processes, infrared heating offers several undeniable advantages:

  • Higher heat transfer rate

  • Greater thermal efficiency

  • Faster heating response

  • Better temperature control

  • Simpler control of zones

  • Fewer temperature spillovers into adjoining work zones

  • Lower capital expenditure and installation cost

  • Less floor surface needed

  • Lower maintenance cost

The infrared heating of processes is often more efficient than convection since a higher percentage of the energy input is transferred to the products to be heated rather than to oven walls or lost due, for example, to opening the intake door or removing the products.

Also, infrared ovens do not require a long pre-production time for temperature build-up.

Radiation has a higher heat transfer rate than convection, so the exposure time can be reduced. If the objective is to dry or bake a surface, then the reduced exposure time avoids unnecessarily heating the entire part being treated. The high heat transfer rate of radiation means the required temperature is reached faster.

In the case of drying paints, with radiation the heat can be transferred to the part from the outside, which avoids the formation of a surface skin and bubbles created by the solvents as they evaporate, which phenomenon can occur in convection ovens.

Infrared controls

In pre-mixing infrared emitters, the air intake is controlled by a butterfly valve that modulates according to the temperature required. The natural gas is then introduced proportionally by aspiration into a zero-governor combustion system in order to maintain a good mixture of fuel and air over the emitter’s modulation range (see Figure 2).

Figure 2

For catalytic emitters, the introduction of natural gas can also be modulated to obtain the emission intensity the process requires.

Since an infrared dryer or oven has several emitters, it is easy to zone the oven into several sections, which gives an advantage over convection. Typically, the parts put into the first section of an oven can be subject to strong radiation intensity. Then, as the parts move through the oven, the radiation intensity and temperature can be reduced, thereby improving the quality of the products being treated. Zoning can also be done from high to low, depending on the geometry of the parts. Generally, having several zones means that the process temperature can be profiled, thus reducing energy consumption (see Figure 3 for possible arrangements).

Figure 3

Successful cases: Infrared in industrial heating processes

A multitude of cases benefit from the advantages of infrared for industrial processes, for example, baking powder paints on metallic caissons, or on MDF cabinet doors, or drying liquid paint on steel posts. Catalytic infrared emitters also offer advantages for drying both the water- or solvent-based paints required by office furniture companies. With metallic-fibre emitters, the burners can be configured in the various sizes and forms used in several areas of activity, such as cooking or grilling foodstuffs, or forming plastic products. Thus, several current and future processes can benefit from the advantages of radiation for thermal treatments.

Example of different infrared units in metallic fiber

Guy Desrosiers, Eng., CEM, CMVP
DATECH Group