Using Infrared to Cure and Dry
One advantage is that the length of the oven is much shorter, providing savings in both capital cost and plant space. The short length normally will allow it to be installed vertically rather than horizontally, further reducing plant space requirements. When installed vertically, the oven eliminates catenary and/or camber concerns and allows the generators to be placed with a minimum gap to the strip or web.
When a coating is completely dried or cured in the infrared oven, the substrate temperature exiting the oven is lower than the same substrate and coating exiting from a convection oven. The lower substrate temperature is due mainly to the fact that water or solvents have a high absorptivity for infrared waves, which often results in the moisture being evaporated completely in a short time and eliminating the need to heat the substrate to the evaporation temperature. Additionally, the lower substrate temperature reduces the amount of heat to be removed -- in some cases, eliminating the need for a cooling zone.
Infrared ovens may use less energy than convection ovens, for a number of reasons. The shorter infrared oven operates at a lower temperature, reducing heat loss up the exhaust stack. Less heat transfer to and from the work is required due to the lower substrate temperature. In some cases, large recirculation fans with high horsepower motors are eliminated, resulting in substantial electrical power reduction.
Gas Infrared vs. ElectricIn the past, most of the infrared ovens in the field were electric infrared. However, a properly designed gas infrared oven will provide the same operating features as electric infrared, including modulating control and flexibility, with many additional operating advantages.
Depending on the local fuel price and operating procedures, fuel costs using natural gas might be less than when using electric to do the same job.
Maintenance costs for a gas infrared oven may be less than an electrically powered oven. The radiant face of a gas burner has an average operating life of four to five years minimum. The gas infrared oven will require some replacement of gas valves, flame safeties and safety switches, but the amount required on a yearly basis is less than some electric infrared ovens such as those using T-3 lamps, which require regular maintenance to replace broken or burned out bulbs.
The gas infrared burners can operate in the oven environment without requiring any cooling air to the burners. Cooling is sometimes a requirement for electric infrared heaters. With gas infrared burners, the only exhaust air needed is the amount required to remove the products of combustion from the gas infrared burners and any evaporated moisture and solvents.
Because the gas burner radiates all of its heat from one flat surface that looks directly at the strip, reflectors are not required. Most electric infrared ovens require reflectors, and with some emitters such as T-3 tubes, up to 50% of the infrared is redirected by reflectors. In time, these reflectors may become less reflective (figure 1) and reduce the amount of energy that is re-radiated.
Gas Infrared HeatingInfrared is part of the electromagnetic spectrum. Infrared transfers heat in wavelengths varying from 0.75 to 400 micron. The short infrared wavelengths close to the wavelengths of visible light are classified as "near" infrared wavelengths and ranges from about 0.75 to 2 micron. Electric infrared emitters operating at 3,000 to 4,000oF (1,649 to 2,204oC) produce much of their output in these shorter wavelengths; hence, they are called short-wave infrared heaters. Gas infrared heaters normally operate at a maximum radiant face temperature of 1,600 to 1,650oF (871 to 899oC). Gas emitters produce most of their infrared wavelengths in the "far" infrared range with wavelengths above 2 micron (figure 3); hence, they are called long-wave infrared heaters. Between far and near are medium-wave emitters, which have emitter face temperatures of approximately 2,300oF (1,260oC). At 1,650oF, minimal light wavelengths and no ultraviolet wavelengths are produced; at 2300oF, some visible light and ultraviolet wavelengths are produced; and at 3,000 to 4,000oF, more visible light and ultraviolet wavelengths are produced. Ultraviolet wavelengths can affect pigmentation and cause damage to the eyes if exposed over a period of time.
Far infrared wavelengths are absorbed readily by most common material coatings and water, and the absorptivity of far infrared by coatings of different colors from white to black is fairly small. Table 1 shows the absorptivity of various color enamel coatings for infrared energy produced by a short-wave electric infrared lamp and a long-wave gas radiant burner.
Because the gas burner produces all of its radiation from a flat surface, it usually is possible to install more heat per square foot than with a lamp that requires reflectors to redirect some of the radiation. At 1,600oF, the gas infrared burner converts about 60 to 65% of its available heat into long-wave infrared heat while the remaining 35 to 40% is convective heat that enters the oven work chamber to contribute to the overall heating and drying. Higher gas infrared radiant temperatures are available, but the efficiency begins to decrease quickly as the percentage of infrared heat produced from the available heat in the fuel decreases.
Gas infrared heaters are environmentally friendly as they burn with a relatively low flame temperature that results in low production of NOX levels in the waste flue gases. Most designs easily meet the NOX emission standards for gas-fired equipment.
Gas Infrared Burner ConstructionA number of different types of gas infrared burners are available. Some use direct impingement to create a radiant face, usually on a ceramic surface. Others fire into a radiant tube. Yet another type uses a porous radiant surface in which the gas and air mixture comes from a plenum behind the burner through a porous face consisting of many small holes. During normal operation, the radiant face glows red hot. Visible flames are not noticeable as the fuel burns evenly over the entire radiant surface.
The porous radiant face can be made from a number of materials. Most infrared heaters of this type use a ceramic face with many small holes or fine Inconel wire mesh screens. Although the ceramic material will withstand higher temperatures, due to production restraints, the minimum hole size in the ceramics is larger than in the Inconel wire mesh screens, which have up to 1,600 openings/in2.
An Inconel wire mesh burner provides the following benefits:
- Ability of the all-metal generator to withstand thermal shock.
- Ability to operate at high-heat flux without flashback.
- Fast response in radiant face temperature from 1,600oF to less than 1,000oF (538oC) black in less than 4 sec. This response time prevents metallic substrate destruction or color change during strip stoppages.
Where infrared equipment is used to process paper or cloth webs, an added measure to ensure the nondestruction of the web is the addition of a steam or inert-gas quench triggered by line stoppages.
Burner ControlGas infrared burners can be operated using either inspirated (venturi-type) or premixed air-gas mixing. For simple applications, the inspirated burner will meet the requirements; however, for high volume continuous coil strip drying or coating curing, it is recommended that premix burners be used. Premix burners can provide full gas-air modulation, allowing tight temperature control as required to suit changes in line speed and strip gauges.
By modulating the burner input, the burner can operate from a radiant face temperature of 1,600oF (cherry red) to less than 1,000oF (black). However, as the rate of infrared heat transfer varies by the differential temperature between the infrared radiator and the object being heated to the power of four (ΔT4), the turndown in the infrared heat transfer is very high. With the Inconel wire mesh screen burner, the speed and flexibility of turndown is equal to or better than that of electric infrared heaters.
The effect of the differential temperature to the power of four on the radiant heat transfer is shown in the following formula for calculating radiant heat transfer.
Q = σ As (T84 - T14)
Q represents heat transfer rate
σ represents the Stephan-Boltzman constant, 0.173 x 10-8
As is the area of radiating body
T8 is the temperature of radiating body
T1 is the temperature of load (object being heated)
Note that this equation assumes that both the source and the load are black bodies and that both are parallel plates.
Oven DesignGas infrared ovens can be designed to heat one or both sides of a continuous strip running either horizontally or vertically through the oven. The minimum oven length is a function of the amount of heat that can be transferred to the work in a given period of time with the infrared burners operating at full fire. If practical and advantageous for the application, a vertical oven design reduces floor space demands and eliminates strip catenary and/or camber in the oven (figure 3).
Because multiple burners are installed on both sides of the strip, it is possible and preferable to zone control the burners in banks over the length of the oven. This allows the burner banks at the oven entrance to operate at maximum infrared efficiency, followed by modulating control on the oven-exit banks, which can be controlled automatically by a radiation pyrometer reading the strip exit-temperature. This minimizes both fuel consumption and substrate heating during drying or curing. A lower substrate temperature reduces the cooling load, saving plant space.
A combustion air blower supplies clean (and filtered, if necessary) air to the burners. If premixed burners are used, each bank of burners has its own gas-air premixer and control system to allow individual bank control. In horizontal ovens, the bottom and top burners in each bank are controlled separately. Each burner or pair of burners is equipped with automatic spark ignition and flame safety supervision.
The exhaust of products of combustion, water vapor or solvents from coatings are controlled by a mechanical exhaust-fan-and-damper arrangement. Often a combination exhaust/circulation air system is provided to assist drying and, through the damper arrangement effect, offer faster cool down during a stoppage.
For solvent-based coatings, a combination exhaust, recirculation and dump system is required to maintain a dilution of the air in the oven at four times the lower explosive limit as per NFPA safety ventilation requirements for continuous curing of solvent coatings.
Gas infrared ovens can be used to dry and cure both water- and solvent-based coatings (figure 4). They can be used to cure the prime and finish coats and to dry chemical coatings. They can also be used with a convection oven to provide a combination convection/radiation oven to utilize the advantages of both types of heating.