The material herein is reproduced with permission of the Infrared Equipment Division of the Industrial Heating Equipment Association from the “Infrared Process Heating Handbook for Industrial Applications,” Copyright c 2005. All rights reserved. To purchase a copy of the of the IR Handbook, visit the IHEA website: www.ihea.organd click on BOOKSTORE.
Comparison of Infrared Heating & Other Forms of HeatingWhat is Infrared?Comparing infrared with other methods of heat transfer can help you understand the infrared heating method. All heat is transferred by one of three methods:– Conduction heating is the transfer of heat by physical contact between a heat source and the object to be heated.– Convection heating is the transfer of heat using heated air as the transfer medium between the heat source and the object to be heated.– Radiation heating is the transfer of heat using invisible electromagnetic waves of energy from a heat source to the object to be heated.Infrared is one of several ways to accomplish radiation heating along with ultraviolet, microwave, radio frequency, and induction. This handbook is only concerned with infrared heating and therefore we will use the terms radiant and infrared interchangeably.
Radiant HeatOne of the first forms of heat transfer each of us encounter is radiant. The sun beam that warms us is radiant heat.. Radiant energy is not absorbed by air and does not actually become heat until an object absorbs it. While radiant energy does generally show up as heat, this is because it vibrates and rotates the atoms in the absorbing object, which results in a rise in the temperature of that object. However, radiant energy may also show up as a chemical change in the absorbing object (polymerization) or evaporation of water or solvents (drying).
How is Infrared Radiation Produced?Every object with a temperature above absolute zero emits infrared energy. This is because there exists in every object a measured amount of heat, so each object has the ability to radiate heat from itself. The object that radiates heat is called the emitting source, and the object to which it radiates heat, having a lesser amount of heat content, is called the target.There are several physical laws that explain the properties of infrared radiation. The Stefan-Boltzman law of radiation states that as the temperature of a heat source is increased, the radiant output increases to the fourth power of its temperature. The conduction and convection components increase only in direct proportion with the temperature change. In other words, as the temperature of a heat source is increased, a much greater percentage of the total energy output is converted into radiant energy.For the purpose of this handbook, we will address only those sources of infrared heat used in industrial heating applications. This generally means looking at emitting source temperatures in a range from 500 degrees Fahrenheit to 4,200 degrees Fahrenheit. (These temperatures are not to be confused with oven set point temperatures or any other temperature requirements related to your product or process). As the emitting source temperature moves from 500 degrees to 4,200 degrees, the radiant output increases with a corresponding increase in peak wavelength. At each temperature point, there is a unique set of wavelength characteristics and peak wavelengths. An additional set of physical laws helps us to understand this relationship. By applying Planck’s Law and Wien’s Law, it is possible to calculate both the distribution of wavelength (spectral distribution) and the peak wavelengths of a given emitter, operating at a given temperature.
Characteristics of Infrared RadiationInfrared heating is the transfer of thermal energy in the form of electromagnetic waves. It is related to visible light and other forms of electromagnetic energy shown in the electromagnetic spectrum below. The infrared portion of this spectrum has been expanded to show that we can further divide infrared into long wave, medium wave and short wave.
The electromagnetic spectrum describes the various types of electromagnetic energy based on wavelength.By describing an infrared emitter as long wave, medium wave, or short wave, one can quickly determine the approximate temperature range an emitter is operating, as well as an approximate wavelength range measured in microns. Since the temperature of a source determines the wavelength characteristics of that source, the peak wavelength of a given emitter can be controlled only by changing the temperature of the emitter. All emitters can be adjusted for wavelength simply by adjusting their temperatures. However, not all emitters are designed to attain the complete spectrum of long, medium, and short wavelength.There are some heat processing applications that are rather forgiving and will work with long, medium, or short wavelength infrared. On the other hand there are applications where it’s important to choose an emitter so that its wavelength distribution and peak wavelength match the absorptive, reflective and transmission characteristics of the coating or substrate. In these processes, choosing the right wavelength can make a tremendous difference in the overall efficiency and speed of the process, and may even determine whether or not the process works.