Electric Infrared Ovens.

 
 

Infrared ovens may be either gas infrared or electric infrared. We have the unique ability to discuss both: Because we use both. We have a wealth of infrared experience to assist you.

Electric infrared ovens use electric infrared elements, or sources. Electric infrared sources are heated by current flowing through a resistance heating element. The element and the material surrounding the element are heated to an incandescent temperature. The following descriptions highlight the most common sources for electric infrared ovens.

Quartz lamps. Quartz lamps generally include a fused quartz tube containing an inert gas and a coiled tungsten filament held straight and away from the tube by tantalum spacers. Standard lamps must be mounted horizontally, or nearly so, to minimize filament sag and overheating of the sealed ends. A modified design is available for vertical mounting. Often called "T3s", the source offers a temperature of 2,000 to 4,000°F and a heat intensity of 5,000 to 30,000 BTUs per hour per square foot. They typically appear yellow to white during operation, heat up in 1 to 3 seconds, are not very resistant to thermal or physical shock, and have a typical radiant efficiency of 70 to 80%.

At full voltage, they have an average life of 5,000 hours and can withstand higher oven ambient temperatures than bulbs or tubes. Thus, quartz lamps can be mounted close to each other to provide high intensities. Reducing voltage even slightly increases lamp service life greatly. Quartz lamps are usually mounted on banks of reflectors, which form the sidewalls of the oven. Heat-up and cool-down times are short because of the low filament mass. Quartz lamp ovens are insulated to help keep lamp terminals and wiring cool. In some very high intensity applications, air cooled reflectors, water cooled reflectors, or both are used.

Quartz tube infrared sources. Quartz tube infrared sources contain a coiled nickel-chrome wire lying unsupported within a fused quartz tube. The use of quartz, being more transparent to infrared rays than other materials, allows higher heat intensities. Typically, the source offers a temperature of 1,300 to 1,800°F and an intensity of 2,000 to 7,500 BTUs per hour per square foot. They typically appear red to orange during operation, heat up in 20 to 60 seconds, are not very resistant to thermal or physical shock, and have a typical radiant efficiency of 70 to 80%.

Because quartz tubes are not sealed or filled with inert gas, the oxidation temperature of the air limits the operating temperature of the resistance wire. This also limits how closely the tubes can be mounted in a curing oven to achieve intense heating. Life expectancy depends primarily on how close the element operating temperature approaches its oxidation temperature. Although impact or vibration can easily damage quartz-tube units, they stand up relatively well to thermal shock. They must usually be mounted horizontally or the internal coil will sag and short circuit. Because the element radiates in all directions, quartz tubes are usually mounted in a fixture that contains a reflector.

Metal sheathed elements. Metal sheath elements include resistance heating wire embedded in an electrically insulating ceramic material enclosed by a tube of steel or alloy. Similar elements are used in the broilers of electric ranges. The oxidation temperature of the resistance wire embedded in the tube limits operating temperatures. Metal sheath elements are rugged, have excellent resistance to thermal shock, vibration and impact, can be mounted in any position, and have longer lives than vamps, tubes, or bulbs. Typically, the source offers a temperature of 1,200 to 1,600°F and an intensity of 2,000 to 6,000 BTUs per hour per square foot. They typically appear red or have no color during operation, heat up in 60 to 180 seconds, are very resistant to thermal or physical shock, and have a typical radiant efficiency of 60 to 75%. Higher radiant efficiency is achieved when these elements are shielded from direct airflow. Thermal storage of the insulation and sheath yield long heat-up and cool-down times.

Radiant panels. Radiant panels contain resistance heating wire grids, or ribbons, sandwiched between a thin plate of electrical insulation on the radiating side and thermal insulation on the back side. Low temperature panels often use thin ceramic papers, boards, or steel as the radiant surface. High temperature panels often use alloy, quartz, or ceramic plates. Panels are generally available in widths of 10 to 30 inches and lengths of 12 to 96 inches. Typically, the source offers a temperature of 1,200 to 1,600°F and a heat intensity of 6,000 to 10,000 BTUs per hour per square foot. They typically appear dull red or have no color during operation, heat up in 60 to 300 seconds, are very resistant to thermal or physical shock, and have a typical radiant efficiency of 55 to 70%. The maximum temperature the radiant surface can withstand or the oxidation temperature of the resistance wire limit operating temperatures. Because the entire surface of the element serves as a radiator, no reflectors are generally needed.

Since the entire surface emits infrared radiation, relatively high infrared intensities can be achieved at lower source temperatures compared with lamp or tube sources. Panel elements generally cost more per KW than other elements. Life expectancies are long, typically 5,000 to 10,000 hours, unless elements are overheated or damaged.

Electric infrared ovens are common for an endless array of coating and polymer applications as a result of the degree of temperature control provided (no pun intended). Powder coating, liquid coating, adhesive bonding, and preheating are ideal applications. And all well designed electric infrared ovens exhibit the following characteristics:

• Vertical and horizontal zoning. To provide an effective, flexible, and efficient application of electric infrared heating to a specific process.

• Precise layout and distribution of elements. To incorporate shape factors, overcome an edge effect, and provide greater flexibility.

• Insulated reflective panels to reradiate heat. To provide reradiation even when panels may be dirty.

• Insulated element wiring to provide additional life. To extend significantly the life of the infrared elements.

• Non-contact temperature sensors for control. To provide the optimum in temperature control.

• Rigid, non-vibrating structure. To lengthen the life of the elements.

• Custom control. To meet the specific needs of the process and the operators.

• Safety. Adherence to all NFPA, FM, IRI, and OSHA standards and regulations.

See also gas infrared ovens.