This article presents all the information you need to know about Infrared Heating.
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Infrared heating is a heating method used to warm surrounding bodies by infrared radiation. Thermal energy is transferred directly to a body with a lower temperature through electromagnetic waves in the infrared region. The surrounding air is not heated and is uninvolved in the transfer of heat; this makes infrared heaters energy-efficient, convenient, and healthy. The heat produced is warm and non-drying. Infrared heaters are powered by electricity or fuel.
The electromagnetic waves in the infrared spectrum have a wide range of wavelengths, from 780 nm to 1 micron. The shorter wavelengths in the infrared spectrum have higher frequencies and associated energies. Therefore, the heat produced by infrared waves ranges from hundreds of centigrade up to 3,6000C.
In recent decades, developments were made to harness energy based on these scientific principles for the benefit of mankind. Nowadays, infrared heaters are available with different features and designs to flexibly accommodate our needs. They warm the surfaces in our living spaces, offices, workplaces, garages, and warehouses. Industries benefit from infrared heaters as they can perform several processes such as drying, curing, printing, and thermoforming. In medicine, infrared heaters are used in physiotherapy to improve rehabilitation.
The infrared region was discovered by Sir William Herschel, a British-German astronomer, in the early 19th century. Infrared heating was not commonly used until World War II. The benefit of infrared heating was recognized by the military during World War II, and it was utilized for drying the paints and lacquers of military equipment quickly to replace fuel convection ovens, which were far more expensive. Infrared heaters in those times were frequently seen in workshops and factories. However, the popularity of infrared heaters declined after World War II as more people started to venture into central heating systems.
With the drive for greener technologies, the development of infrared heaters resumed between the late 20th and early 21st century. The range of heating expanded to more regions in the infrared spectrum. Design flexibility was also studied so that infrared heaters could be conveniently installed in strategic locations in our homes and offices. The use of infrared heaters is continuously growing.
Electromagnetic waves are waves composed of two waves oscillating perpendicularly to one another. One of the waves is an oscillating electric field while the other is an oscillating magnetic field.
Electromagnetic waves can be described by their wavelength and frequency. Wavelength is the distance between two adjacent crests in a cycle of a wave. Wavelengths in the electromagnetic spectrum are usually expressed in nanometers or angstroms. Frequency is the number of wave cycles per second and is usually expressed in Hertz (Hz). Electromagnetic waves are classified based on these properties.
Wavelength and frequency are inversely related to each other. Furthermore, the energy of a wave is directly proportional to the frequency but inversely proportional to the wavelength. Waves with higher frequencies and shorter wavelengths carry higher energies and are more transmissive. Waves with lower frequencies and longer wavelengths carry lower energies.
Unlike mechanical waves, electromagnetic waves do not require a medium to propagate. They do not need surrounding molecules to travel unlike sound waves (mechanical waves) that traverse through the air. They can travel through air, objects, and even a vacuum. This is why we feel the warmth of the sun though it is thousands of miles away from the earth as well as the surrounding cold air when we stay under the sun. This principle is also applied in the operation of infrared heaters, in which infrared heaters are analogous to the sun.
The infrared region lies between the visible and microwave region of the electromagnetic spectrum. Infrared waves have a wavelength ranging from 700 nm (430 THz) – 1 mm (300 GHz). As stated earlier, their existence was discovered in 1800 by Sir William Herschel, a British-German astronomer, while measuring the temperature of the invisible region in the spectrum lower than the red light, which exhibited the highest temperature.
The infrared region is broad, as is its associated energy and temperature range. Infrared waves are classified into:
| Region | Abbreviation | Wavelength(µm) | Frequency (THz) | Photo Energy (meV) | Temperature Range (°C) |
|---|---|---|---|---|---|
| Near-infrared | NIR | 0.75 - 1.4 | 214 - 400 | 886 - 1653 | 3,591 - 1,797 |
| Short wavelength infrared | SWIR | 1.4 - 3 | 100 - 214 | 413 - 886 | 1,797 - 693 |
| Mid-wavelength infrared | MWIR | 3 - 8 | 37 - 100 | 155 - 413 | 693 - 89 |
| Long-wavelength infrared | LWIR | 8 - 15 | 20 -37 | 83 - 155 | 89 - -80 (negative temperature) |
| Far infrared | FIR | 15 - 1000 | 0.3 – 20 | 1.2 - 83 | -80.15 – -270.15 |
Radiative heating is one of the many applications of infrared waves. Infrared waves are also useful in spectroscopy, imaging, and communications.
Radiation is the mechanism of heat transfer caused by the emission, absorption, and reflection of electromagnetic waves of bodies. All bodies above the absolute temperature (-2730C) emit thermal radiation. Thermal radiation emitted by a body is caused by the random movement, vibrations, and collisions of atoms and molecules and their constituting protons and electrons. Bodies radiate heat based on their temperature: the hotter objects radiate more thermal energy. Thermal energy transferred by radiation does not affect the surrounding molecules, rather it is dependent on the objects the source "can see." It can easily travel through air, objects, and even a vacuum. It is also independent of the amount of radiation emitted by the receiving body. Other factors affecting radiation are the nature of the surface and the angle of incident radiation.
Other mechanisms of heat transfer are conduction and convection, which can happen simultaneously with radiation. In conduction, heat is transmitted through collisions and vibrations between neighboring atoms or molecules that readily occurs in solids. The direction of heat transfer in conduction is from a region of higher kinetic energy to a region of lower kinetic energy. In convection, thermal energy is transferred through the displacement of molecules in the bulk fluid. When a portion of the fluid is heated, the molecules near the primary heat source expand and travel away from it. Thermal energy is carried along with the molecules‘ movement and is transferred to a cooler portion of the fluid mass.
Infrared heaters can be classified according to their source of energy:
Electric infrared heaters utilize electricity to deliver heat to their surroundings. The heating system produces heat using the principle of Joule heating or resistive heating. Joule heating is the conversion of electrical energy to heat by passing an electric current to an element with high electrical resistance. The resistance of the heating element must not be as high as the resistance of insulators. The common heating element materials are tungsten, nichrome (80% Nickel, 20% Chromium), Kanthal (FeCrAl), cupronickel (CuNi), and carbon fibers.
Radiant gas heaters, also known as gas-fired infrared heaters, depend on chemical energy stored in natural gas, propane, or petroleum for the heat source. They also use a heating element that converts the heat energy from the gas flames into infrared electromagnetic radiation. The heating elements and the combustion chambers are contained in a metal, ceramic, or glass encasing. Some types of radiant gas heaters are:
Infrared heaters can also be classified based on the wavelength of the infrared waves they emit:
NIR heaters produce infrared waves of around 0.78 – 1.5 microns in wavelength and operate at high temperatures above 1,300 – 2,6000C. Since these wavelengths have higher frequencies, they tend to be more transmissive and reflective but less absorptive to the surfaces they strike. Thus, they are less efficient and are not suitable for drying applications. They can produce harsh heat and can be felt 2-3 meters from the source but cannot provide warmth at a deeper level.
NIR heaters instantaneously warm the environment and are typically used in outdoor heating applications.
MWIR heaters produce infrared waves of around 1.5 – 3 microns and operate at 500 – 1,3000C. These wavelengths have lower frequencies, which are better absorbed by objects, but they are still not suitable for comfort heating. They are used in industrial applications such as drying and curing of paints, lacquers, and solvents as well as in the economic processing of plastic foils and sheets.
FIR heaters produce infrared waves of around 3 – 1000 microns in wavelength and operate at lower temperatures. Since these wavelengths have lower frequencies, they are better absorbed by the surface they strike. Water begins to absorb the infrared heat in this spectrum.
FIR heaters produce comfortable heat that is optimally absorbed by our skin, which is further conducted into our tissues, blood, and the rest of our bodies. They take a longer time (around 5 minutes) to warm surrounding bodies. They are used in saunas, incubators, heating cabins, and other indoor heating applications.
Some infrared heaters can be distinguished by their material of construction. A few of such infrared heaters are listed here:
Quartz heat lamps were developed by General Electric in the 1950s. They produce intense heat with a temperature above 1,5000C and emit medium- to short-infrared waves. They heat the surrounding bodies quickly. Quartz is used as the enclosing material protecting the tungsten heating element, which can withstand higher temperatures than glass. It is filled with highly pressurized inert gas containing halogens, gaseous bromine, or iodine that regenerates tungsten atoms in the filament and prolongs the service life of the heating element.
Quartz heat lamps are used as outdoor patio heaters and in several industrial processes such as drying, curing, and thawing of frozen products.
Carbon infrared heaters have heating elements made from woven carbon fibers which are housed in quartz. They are also filled with halogen gas like quartz heaters. They operate at around 1,2000C and emit medium wave infrared. The carbon fibers produce gentler heat, and their light is less intense than tungsten. They also have long service lives.
Carbon infrared heaters are used in heating spaces with large, draughty, and hard-to-heat interiors: public halls, café terraces, and covered outdoor spaces.
Ceramic heaters have a heating element that is directly cast into a ceramic material. They operate at 300 – 7000F and emit medium to long infrared waves with 2 – 10 microns in wavelength. The ceramic casting comes in different shapes: a flat-shaped cast spreads the infrared heat over a wider area, while the concave-shaped cast focuses the infrared heat into one spot. The surface is glazed to prevent corrosion.
Ceramic heaters are used in comfort heating and industrial processes such as paint drying, curing, printing, annealing, thermoforming, and packaging. Food processing industries and medical facilities employ the use of ceramic heaters.
The following are a few types of infrared heaters categorized by their application:
Construction heaters are portable infrared heaters used in outdoor or indoor construction areas, and they can be installed over a tank top. They are used in spot heating. Construction heaters use infrared energy to radiate heat to their surroundings through a ceramic or steel surface.
Over-door heaters are positioned in building entrances and frequently-opened doors where the inside air is noticeably hotter. These heaters use axial fans to generate a high-velocity air stream to rapidly heat the cold entering air; this avoids heat losses and saves energy.
Over-door heaters are also known as air curtains. They can work in the opposite manner during summer to reduce air conditioning costs.
Garage Heaters are used in large spaces like garages and workshops, spaces that are not meant for insulation. They emit high-frequency radiation for the heat to penetrate the large area and warm the working personnel as well.
Warehouse heaters are used to heat large spaces such as warehouses where complete insulation and forced air convection heating are impractical.
Infrared heaters are composed of a heating system and a reflector. The heating system converts electrical energy or chemical energy from fuel sources into thermal energy. The reflector then directs the thermal energy produced by the heating system as radiant heat to the objects in its surroundings.
Reflectors greatly determine the efficiency of an infrared heater. They must have high reflectivity and must absorb minimal radiation from the heating system in order to store less heat. Their shapes and contours are designed to bend the infrared waves to space and prevent them from bouncing back. Other desirable properties of reflectors are high resistance to corrosion and moisture, ability to withstand high temperatures during their service life, and ability to be easily cleaned.
Reflective materials that are commonly used are aluminum, stainless steel, ceramics, and quartz. Some reflectors are plated with gold or ruby to increase their reflectivity and focus more heat on the surrounding objects.
Infrared heaters are versatile, easy to install and maintain, and are available in different designs to suit our needs. The benefits of infrared heating are as follows:
Infrared heaters warm surrounding objects directly. Heat losses are avoided because they don‘t expend energy by heating the surrounding medium. This feature consequently reduces energy costs.
Since the radiant heat is directed to the surrounding bodies, they don‘t spend time heating the air and then transferring it to the objects; that is the process of traditional convection heaters. This feature is helpful in drying applications.
The heat given off by infrared heaters is comparable to the radiant heat from the sun (excluding the ultraviolet waves). They don‘t increase the humidity level and reduce the oxygen content in their environment and do not evaporate moisture in the air. With infrared heaters, we feel warm and refreshed at the same time.
Infrared heaters inhibit the growth of these microbes since the mobility of moisture is limited. This feature reduces stuffy nose, wheezing, and itchy eyes and skin. This is also beneficial for places where food and medicines are handled, stored, and consumed.
Most infrared heaters don‘t rely on fans and blowers to circulate the heated air unlike convection heaters. Those auxiliary devices generate noises that are undesirable for bedrooms and office areas.
Electric infrared heaters don‘t generate gaseous products, toxic fumes, or fine particulates that have adverse effects on the environment. They do not agitate the surrounding air, which carries dust and allergens.
The energy efficiency of infrared heaters also helps to green the environment.
The use of infrared heaters improves living by taking care of our bodies. Infrared heaters promote overall health because:
Despite all this, infrared heaters can be a safety hazard. A hot core material of the infrared heater must be maintained to radiate heat to its surroundings. This may cause serious burns when touched or when one is exposed for a long period at too close a distance. Looking directly at the glow of high-intensity infrared heaters may cause impairment to the vision. Injuries and harm can be prevented by placing engineering controls and practicing vigilance when around an infrared heater. This downside can never outweigh the benefits an infrared heater can bring.
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