Infrared or IR waves form part of the electromagnetic spectrum. Electromagnetic waves with wavelengths from 0.78 µm to 1000 µm are called infrared waves. You are already familiar with electromagnetic waves of different wavelengths. Microwaves, X-rays, radio waves and visible light are all electromagnetic waves. Infrared waves produced inside the furnace lie predominately in the near (NIR or near-IR) and medium infrared range with wavelengths ranging between 0.5 and 3.0-µm.
When using infrared lamps, higher heat-lamp temperatures emit higher radiant energy. This elevated energy translates to a shorter electromagnetic wavelength of emitted IR radiation. While the IR waves of a heat lamp come from a continuous range of wavelengths, in theory the dominant wavelength (ldom) as given by Plank’s distribution principle is the wavelength transmitted with the highest occurrence. So for a given temperature, only one ldom exits. See Figure 1 below.
The relationship between heat-lamp filament temperature T and ldom is given by the fixed relationship:
ldom mm = 2897 µm·K / T K
To convert from degrees Celsius (°C) to Kelvin (K) add 273 to the Celsius temperature value.
At 1000°C the respective material dominant wavelength is:
T = 1000°C + 273
T = 1273 K
Substituting back into the given equation:
ldom = 2897 µm·K / 1273 K
ldom = 2.28 µm
Table 1 shows dominant wavelengths for some common temperatures.
If you know the resonant frequency of a particular substance, matching the furnace dominant wavelength the product resonant frequency ensures maximum energy transfer via IR radiation. In most cases, rapid product heating can be achieved more efficiently through frequency matching rather than with temperature increases.
Infrared (IR) heating is electromagnetic radiation emitted from the surface of IR lamps or emitters. Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation. In the furnace, radiant heating from IR lamps provides heat directly to objects without first heating the surrounding air. IR waves excite molecules within a substance (product) thus generating heat, but pass generally undisturbed through the surrounding atmosphere. Other substances such as glass, ceramics and some organic materials are also transparent to IR waves. Objects suspended in these media can, therefore, be heated directly by IR waves without directly heating the supporting media.
Not all heating in producition furnaces occurs via direct IR radiation. The belt and air inside the furnace are heated via the IR lamps. Also edge heaters (resistance heaters installed along the furnace length) can introduce heat into the furnace. Your product also acquires heat from the edge heaters, conveyor belt and surrounding heated gas in the chamber via conduction.
The amount of direct heating via IR radiation is determined by three factors:
1) The level of IR radiation emitted from the heat lamps.
2) The amount of IR absorbed by a product.
3) The level of edge heat introduced into the furnace
Heating via conduction and convection operates by transferring heat to object surfaces. Heat is then transferred from the surface to the layers beneath. Heat transfer, however, is not uniform, causing temperature differences and unequal expansion across an object. The unequal expansion due to the uneven heating is called thermal stress and can cause objects to fracture called thermal shock.
IR radiation heats molecules below an object’s surface and allows for more uniform heat distribution than can be provided by conduction and convection heating alone.
Rapid heat up time is also achieved with IR technology due to the high energy-transfer rate of IR waves. The speed of conduction and convection heating is proportional to the temperature difference between the object and heating environment, whereas the speed of IR heating is proportional to the difference between the fourth powers of the object and environment temperatures.
Suppose the temperatures of an object were 100ºC.
If a convection heating furnace were heated to 500ºC, the proportional difference would be:
500 - 100 = 400
If an IR furnace were heated to 500ºC, the proportional difference would be:
5004-1004 = 6.25 E10 – 1.00 E8 = 6.24E10
Other factors such as the emissivity of objects are taken into account when calculating energy transfer rates.