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Introduction to Radiant/Infrared Heating

Interesting links:

  1. How Infrared Works
  2. Taking the Mystery Out of Infrared Heating
  3. Selecting Infrared Wavelength
  4. Drying & Curing with Infrared
  5. How Can Infrared Save my Company Money?
  6. Charts for Infrared Heating

Infrared through the ages
Infrared radiation was discovered by Isaac Newton when he separated the electromagnetic energy from sunlight by passing white light through a glass prism that broke up the beam into colors of the rainbow. In the 1800's, William Herschel discovered energy beyond the visible. In the 1900's, Planck, Stefan, Boltzmann and Wien further defined the activity of the electromagnetic spectrum and developed equations to identify IR energy. This research makes it possible to define IR energy using the basic blackbody (a perfect emitter) which shows that an object with a temperature greater than -273 deg C emits radiant energy in an amount proportional to the fourth power of their temperature.

Today Infrared Heating is used in the industry for a variety of applications as well as for comfort heating.
Spectrum showing the infrared wavelengths
How does it work?
Infrared energy is heat that can be applied to many different things for manufacturing, finishing, drying and heat processing. To find out what infrared energy can do, and where it can and cannot be used to its full advantage, here is a short guide.

Essentially, infrared is an electromagnetic phenomenon, which is measured in wavelengths (�m = microns). Electromagnetic energy particles attack the surface of materials to be processed after which conduction takes over. To use infrared energy successfully, we have to understand this reaction. Materials can act as a good heat conductor (example: gold, copper). In many cases, however, the conductivity is less than desired resulting in absorbing and retarding the penetration of heat. Some of the materials may even work as an insulator. (example: foam, ceramic)

The infrared heater selection is essential to success. The infrared spectrum can be divided into roughly three types of density: short wave, medium wave and long wave. It is really important to understand that infrared energy is a surface phenomenon. For example, processing foam is poor radiant energy conductor so it should be radiated from both sides. Also, if the surface is shiny (example: aluminum), the surface may reflect the infrared waves and bounce back and forth which will result in a delayed heating action. Objects with surfaces that readily absorb energy usually give satisfactory results.

In industrial heating, the main thing to know is what is being processed and at what speed it is being processed. Basically, any product has its own inherent reaction to infrared. This is called "heat absorption factor". Each type of material can be categorized in a certain wavelength. Naturally, that can be translated into degrees of temperature. As a result, we are able to obtain the fastest possible reaction from materials if exposed to an infrared radiation peak corresponding to its absorption factor.

In all cases, infrared is a hotter energy source than convection heat. Infrared heat is always applied directly to materials. The exposure to this direct heat source has to be timed in order to not overheat and destroy the material.

To remember: for each material the right wavelength (= IR heater) must be chosen.

Selecting a IR heater
Consider these points when selecting an Infrared Heater:

  1. Temperature
    The highest temperature (and shortest wave length) can be achieved with a quartz lamp. (up to 4000�F) On the other side, if your material can be heated with longer waves, why not consider a gas catalytic heater (up to 800�F)? Ceramic, panels, quartz tubes, metal sheath heaters can achieve temperatures in the 1300 - 1600�F range.

  2. Response time
    If your application requires achieving the process temperature within fractions of a second, then quartz lamps would be appropriate. If you have seconds, then quartz tubes could be an option. If you have more time (for example in an oven), flat panels, and ceramic could be a choice.

  3. Environment
    Is the application paint drying or computer chips manufacturing? Can anything be falling on the heater face? Is it dusty? Are you designing a new system or work with an existing one? These factors could determine the emitter's surface.

  4. Control
    A well designed IR application uses a PID. Are you going to use contact temperature measurement (TC, RTD) or would a non-contact measuring method (IR / Pyrometer) be beneficial? If you're using quartz lamps or heating air, you should also consider a SCR to switch power.

     

What is IR comfort heating?

The usual temperature to feel comfortable is around 70�F. The human body in a 70�F room has a 92�F skin temperature (84� clothing temperature); so you're not really "heating" the body with that 70�F air.

The average person seated or resting will dissipate about 400 BTU's per hour (his metabolism)..or the equivalent of a light bulb. By exercising or doing other fun stuff the human body can dissipate up to ten times that. The body must lose heat to be comfortable but it must lose it at a proper, controlled level.

Radiant heaters quickly provide heat to those closest to the heater, rather than providing heat for an entire room. They're heating objects and not the air, so even in a large space partial heating is possible. (for example only the shipping and receiving area instead of the entire warehouse) Oil-filled radiators look like old-fashioned radiators that are found in many older homes. However, these are radiant electric heaters that are permanently filled with oil. Many of the models available come with wheels so they are easy to move around your home. This type of heater provides a steady heat source for an entire room.

Convection: In a 70�F room, a person at rest will lose about 100 BTU per hour through convection.the hotter the room, the less he loses.

Before you can select the appropriate heater, you must know:

  1. Indoor or Outdoor application (to determine watt density per square foot)
    Examples:
    • indoor comfort heating requires 30-40 Watts per square foot
    • indoor moisture removal requires 15 - 30 Watts per square foot
    • outdoor in a protected area requires 80 - 120 Watts per square foot
    • Area that needs to be heated (to determine Watts required)

    If you want to heat an indoor area (comfort heating) of 20 feet x 20 feet, you need 11'000 Watts if your ceiling height is 10ft. (And this 11'000 Watts you can't get from 110V!)

  2. Mounting height and method (ceiling or wall)
    Standard heaters are made with 45, 60 and 90 degree reflectors.
  3. Available power (electric or gas) If the available power (for example in private homes) isn't enough, gas (Natural gas or Propane) infrared heaters could be considered.

     

Comparison of different Infrared Heaters

Comparison of Infrared Heaters

Infrared wavelength
The infrared wavelength spectrum is divided into three groups:

  1. short wavelength
  2. medium wavelength
  3. long wavelength
The Division of the Infrared Wavelengths

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Infrared Image of Human Body Heat
This is an infrared image of a Jet Propulsion Lab engineer holding a burning match. The image is color-coded to show differences in temperature: note the white and deep red in the flame and the engineer's palm (where his warm blood vessels are close to the surface of the skin) and the blue of his cool glasses. This picture demonstrates that infrared images predominantly show heat energy and its distribution.

Infrared Heater from Delta T