Fundamentals of Infrared
Means of Transferring Heat

Conduction

• The transfer of heat by either contact between the heat source and the object to be heated or within the object from one point to another.
• An example is a coffee pot on a warming plate.

Convection

• The transfer of hea, from the heat source to the object being heated via a fluid medium. That medium is commonly air.
• An example is a preheated oven used in front of an infrared tunnel on a thermoforming machine or a convection oven used in a paint curing application.

Radiation

• The transfer of heat via electromagnetic radiation between the heat source and the object to be heated.
• Radiation is broken down into many subsets divided by different wavelengths. These wavelengths include:
• Ultra Violet
• Infrared
• Microwave
• Radio Frequency

The Electromagnetic Spectrum

The Infrared Spectrum

Infrared Heating Applications

Infrared Specifications

• The wavelength spectrum ranging from .72 to 1000 microns.
• The infrared region is divided into 3 subsets
• Short-wave (near): .72 - 2 microns (7000-2150°F)
• Medium-wave (middle): 2 - 4 microns (2150-845°F)
• Long-wave (far): 4 - 1000 microns (845-<32°F)
• The useful infrared region for industrial process heating ranges from 1.17 to 5.4 microns (4000°F - 500°F).
• The wavelength is inversely proportional to the temperature. As the temperature goes up, the wavelength goes down.

History of Infrared Heating

• First used back in the 1930s for automotive paint curing applications.
• It wasn't until World War II that infrared heating came into heavy use. It sped up the production of military equipment.
• After WW II, the use of infrared heating once again slowed down.
• Today, the use of infrared heating is growing rapidly around the world. Utility technology centers have helped to spawn that growth.

Why use infrared heating systems?

• Reduces floor space
• Lowers energy consumption
• Increases line speed
• Reduces maintenance
• Clean operating environment

Daily examples of infrared heating

• Toaster
• Bathroom heat lamp
• Barbecue grill
• Light bulb (90% heat - 10% light)
• The granddaddy of all infrared heaters - the Sun. Half of the sun's energy is infrared radiation

What is infrared heating?

• The electromagnetic energy that is emitted by all bodies above -273°C (0°K or absolute zero).
• When infrared energy strikes an object it causes the surface electrons to excite and oscillate.
• This oscillation creates heat.
• It travels in straight lines from the source
• It can be directed into specific patterns with the use of properly designed reflectors
• It decreases in intensity as it travels outward from its source

How does heater output wavelength effect the process?

Infrared radiation is either:

• Reflected
• Absorbed
• Transmitted
• Materials have different absorption curves
• Ideally, you would like the heater to output the majority of its energy in the area where it is best absorbed.

Plank's Law

• Plank's Law defines the relationship of wavelength output to temperature based on a point source in a vacuum. Raising the output power increases the temperature of the point source. This results in the peak wavelength shifting to a shorter wavelength, as displayed in the above curve.

Stefan Boltzmann Law

• F = s T4 s = 5.73 x 10-8 W/m 2 x K 4
• The total energy radiated is equal to the black body temperature to the fourth power
• That is to say, if the temperature of an infrared heater is doubled, then the power output will increase by sixteen-fold
• The peak wavelength will shift to a shorter wavelength

Wien's Law

• This curve and formula express the relationship between wavelength and absolute temperature.

The Inverse Square Law

• This law is applicable to a point source, not necessarily a real life infrared emitter
• The radiant intensity at the product to be heated varies inversely as the square of it's distance from the emitter surface
• In real life applications the law does not hold true. View factor is a better determinate of the radiant loss due to distance from the product to the heater

View Factor

Compliments of CMF - Center for Materials Fabrication

Heating Technologies for Thermoforming

Definitions

Emissivity is the relationship between reflectivity and absorption. A perfect absorber (black body) has an emissivity of 1.0. The perfect reflector has an emissivity of 0. All products fall somewhere in between this range.

Color Sensitivity refers to different curing or heating rates based on the emitter wavelength. White coatings are more reflective and therefore do not absorb as much infrared energy. Therefore, white coatings take much longer to heat up. This factor is more acute with shorter wavelengths.

An example of emissivity

An example of color sensitivity

A quick comparison between emitters at different wavelengths

• Short-wave
• Medium-wave
• Long-wave

Comparison between 3 heaters, each covering a 10" x 10" area at 1000 watt

• Heater A (short-wave) is one 1000 watt short wave lamp (T3) operating at 4000°F.
• Heater B (medium-wave) is two 500 watt medium wave quartz tubes operating at 1800°F.
• Heater C (long-wave) is a ceramic face heater with ten 100 watt coils operating at 800°F

Heater Output Differences

Wavelength discussion

• The peaks for most plastics are at 3.5 and 6-10 microns
• 3.5 microns equates to approximately 1030° F
• 6 microns equates to a temperature below 500° F
• In order to reduce the heating cycle time, the heater output is set at the highest possible temperature, without burning the sheet.
• The goal is to put in as much heat as possible, without damaging the product surface - at any wavelength
• Wavelength, radiant efficiency, and power output all determine how quickly the sheet can be heated. It also determines how much energy is required.

Absorption curves

Absorption curves Courtesy of EPRI/CMF Technology Guidebook for Electric Infrared Process Heating

Typical drying and hardening curves Courtesy of EPRI/CMF Technology Guidebook for Electric Infrared Process Heating

A standard convection oven transfers 500-2,000 BTU/hour - square foot while IR ovens transfer from 3000 - 25,000 BTU/hour - square foot
	IR vs Convection Heat Transfer Comparison This curve was found in Electric Process Heating By Maurice Orfeuil, Battelle Press

Chart comparing IR to Convection Heating Courtesy of EPRI/CMF Technology Guidebook for Electric Infrared Process Heating 0.05" Steel & Aluminum
	Chart comparing IR to Convection Heating Courtesy of EPRI/CMF Technology Guidebook for Electric Infrared Process Heating 0.25" Steel & Aluminum

Chart comparing IR to Convection Heating Courtesy of EPRI/CMF Technology Guidebook for Electric Infrared Process Heating 1.0" Steel & Aluminum
	Chart comparing IR to Convection Heating Courtesy of EPRI/CMF Technology Guidebook for Electric Infrared Process Heating 0.25" Plastic & Wood

Typical Infrared Misconceptions...

Misconception No. 1
Courtesy of EPRI/CMF
Technology Guidebook for Electric Infrared Process Heating

• IR Radiation may be harmful to oven operators.
• There is no immediate danger associated with the use of IR radiation compared with ultraviolet radiation or microwave. However, as a precaution one should avoid prolonged viewing of high intensity IR emitters at close distances (less than 15 feet). Repeated, long term, near exposure to high intensity IR radiation may cause cataracts in some individuals.

Misconception No. 2
Courtesy of EPRI/CMF
Technology Guidebook for Electric Infrared Process Heating

• IR ovens are effective in heating only flat surfaces.
• Flat surfaces are ideally suited to heating by IR radiation. They can be heated rapidly and effectively in an IR oven. However, more complex, three-dimensional shapes can also be heated in an IR oven. Three-dimensional parts can be rotated so that all sides are evenly exposed to radiation as they pass through the oven. The heating rate can also be varied from zone to zone to allow sufficient soak times to heat internal regions of a part.

Misconception No. 3
Courtesy of EPRI/CMF
Technology Guidebook for Electric Infrared Process Heating

• IR radiation works better in a vacuum with little or no air moving.
• Air is virtually transparent to IR radiation. IR radiation is neither absorbed nor scattered by air. However, water vapor, carbon dioxide, and other greenhouse gases do absorb IR radiation. If the air between the emitter and the product contains water vapor or other absorbing gases, it could absorb a portion of the IR radiation. For distances between the emitter and the absorber of a few feet or less, the energy absorbed by the gas will be negligible.

Misconception No. 4
Courtesy of EPRI/CMF
Technology Guidebook for Electric Infrared Process Heating

• Short-wavelength infrared penetrates more than medium and long-wavelength infrared.
• Although this statement is true in many cases, it is not universally true. For example, metals do not transmit infrared radiation of any wavelength. All the IR radiation incident on a metal is absorbed or reflected at the surface. On the other hand, some non-metals transmit radiation. These include water, glass, quartz, and some ceramic and polymer materials. These same materials also may transmit longer wavelengths to some degree.

Misconception No. 5

• Only one wavelength is best for a given application.
• This statement is blatantly false. There are many factors that need to be considered. All wavelengths will most likely work for a given application. But you need to consider not only the heating rate, but also the available floor space, maintenance requirements, heater durability, response time, heater and system efficiency, initial oven cost, energy consumption cost, conveyor speed, part size variation, controllability, and aggravation cost. All of these items need to be considered in order to pick the right solution.

Bibliography

• Maurice Orfeuil, Electric Process Heating, Battelle Press 1987
• JR O'Connell, EFB Croft, WC Hankins, Electric Infra-red Heating for Industrial Processes, EA Technology 1990?
• Technology Guidebook for Electric Infrared Process Heating, CMF Report No. 93-2 1993
• Jay Siedenburg, Heating Technologies for Thermoforming, CMF Report No. 95-1 1995
• Shelby F Thames, Ph.D., Presentation on the Use of IR with Polymer Applications, IREA meeting 1997
• Thomas A. Stryker, The Heat Processing Handbook for Paint & Powder Applications, 1997
• Philips Lighting Application Information, 1994