IR Heater Types

IR Heater Types

In principle, a material can be warmed or dried using IR radiation with different heating element designs.

The selection of the emitter most suitable for the task of heating has a significant influence on the technical and economic success of the method.

Not only the technical properties of a certain emitter, but also the IR absorbency of the material exposed to heat and the structural design of the overall IR system must be taken into account when selecting a specific heater type.

Ceramic IR heaters, metal rod heaters, quartz heaters, halogen heaters, foil heaters, gas heaters, and heaters with exposed heating coils are all common heaters on the market.

Ceramic Heaters

The ceramic infrared heater consists of a resistance heating conductor that is completely encapsulated in suitable ceramic material. As a result, the energy produced by the heating conductor is uniformly transferred to the ceramic material surrounding it. At the same time, the ceramic material protects the heating conductor from numerous outside influences, thereby extending its operational life. The material used to embed the heating conductor is electrically non-conducting and should possess good emission properties in the desired IR wavelength range. Considering these criteria, ceramic IR heaters with varying geometries can be produced.

Ceramic infrared heaters are thus ceramic bodies with a dedicated panel portion used as a radiating panel with integrated heating coils. Furthermore, ceramic IR heaters make it possible to position a fixed thermocouple right against the heating conductor.

Metal Rod Heaters

The metal rod heater is a heating element that was originally developed for heating liquid media. It is still primarily used for this application. The metal rod heater consists of a metal rod with an internal heating conductor surrounded on all sides by a magnesium oxide powder.

The metal rod heater is also used in individual cases as an IR heater for constructing heating panels. Integrated thermocouples are not state of the art in this case. Due to its construction, the metal rod heater emits in all directions along its circumference. Reflectors are used in order to avoid heat losses; however, they and even the heater itself scale under higher radiant powers and operating temperatures, thereby reducing efficiency.

Depending on the material, metal rod heaters have a lower emissivity than ceramic heaters. The metal rod heater is only suitable to a limited extent for constructing level heating panels or for the manufacture of IR systems, particularly in cases when the radiating panel of a heating panel must be designed to be as thermally homogeneous as possible.

Quartz Heaters

The quartz heater consists of a fused silica or quartz glass tube with an inlaid heating coil. There is no additional filler material between the heating conductor and pipe. There are various forms of construction, one of which is a panel heater in the form of a cartridge with multiple quartz tubes positioned next to each other.

Installation of an integrated thermocouple directly next to the heating conductor is only possible to a limited extent due to the design of the quartz heater. Because of its construction, the heating conductor may undergo a comparatively high thermal load due to individual coils of the heating conductor moving toward one another ("migration of the heating coils") during operation; this is seen as a frequent and premature cause of failure.

Furthermore, the quartz heater may not be operated in the vertical position so as not to damage the exposed heating coils.

The quartz tubes, arranged next to each other in the quartz panel heaters are surrounded on the sides and to the rear by a metal housing that serves as a mounting but is also necessary as a reflector. Quartz heaters with higher power in particular often lose reflector power at an early stage due to scaling.

In comparison to ceramic heaters, the quartz heater is a much more fragile construction not suitable for all mechanical load situations and operating conditions such as vibration, impact and collision with materials.

Halogen Heaters

The halogen heater consists of a bulb with an inlaid wolfram heating conductor protected from oxidisation by a gas mix containing halogen. The halogen heater emits IR radiation with a radiant power maximum in the short-wave IR range. The heating conductor is heated to temperatures of approximately 1,800 °C in a relatively short amount of time. As a result, visible light occurs outside of the IR range. An integrated thermocouple cannot be used with halogen heaters. The heaters are operated with integrated or external reflectors.

Due to its IR emission having a short-wave range, the halogen heater is useful for heating materials that absorb short-wave range IR radiation. This is not the case, however, with most materials. Using a halogen heater to heat a material that absorbs radiation predominantly outside of the range emitted by the halogen heater results in high efficiency losses. IR radiation not absorbed by the material causes the undesired heating of the material's surroundings.

Because halogen heaters also generate visible light, common industrial safety regulations often go unfulfilled (dazzling effect, cataracts). The amount of overall radiant power of halogen heaters emitted as visible light is 5–7 %, which makes no significant contribution to heating.

The additional following criteria are important in assessing a halogen heater:

The electrical operational life of halogen heaters is short compared to ceramic heaters of similar construction size if utilised where cyclic operations play a significant role in the heating unit. The frequent heating and cooling intervals hinder the chemical process of recombination between the heating conductor and the gas mixture inside the bulb.

Mechanical strength, e.g. with regard to vibration, impact, and collision with materials does not fulfil all requirements. Installation in a vertical position must be avoided to prevent compression of the coils and electrical short-circuiting.

Due to the comparatively high operating temperatures, it is necessary to construct high-quality reflectors, insulators and mechanical components and, if applicable, cooling devices such as electric fans.

Foil Heaters

Foil heaters are heating elements that have a metal heating conductor embedded in flat synthetic material. The plastic serves as the substrate and insulating material. Foil heaters are flexible and available in freely selectable lengths and widths. They are used for contact heating and IR heating. The operating temperature depends on the material and is limited to approximately 280 °C.

Space heating is an important area where foil heaters are used as IR heaters because, in this connection, lower power densities and lower surface temperatures are required in comparison to industrial applications. The installed power is distributed across a relatively large area, with the maximum radiant power in the long-wave IR range. If foil heaters are used as space heaters, the objects in the room are predominantly heated through convection due to the relatively low power per unit of panel.

Gas Heaters

Gas heaters are either luminous radiant heaters or dark heaters. With the luminous radiant heater design, gas is conducted through a perforated tube or over a ceramic incandescent grid where it is burned with an open flame. The resulting combustion heat serves to heat the ceramic incandescent grid. The incandescent grid then emits the absorbed energy as IR radiation. With gas-fired dark heaters, a gas-air mixture is burned in a closed pipe system. In this case, the heated pipe is the radiating body. Gas heaters are predominantly used to heat rooms but also have applications in the food business.

In comparison to electrically operated ceramic IR heaters, the operation of gas heaters requires significantly higher installation expenses, for example for the supply and exhaust systems and ignition and burner controls. Gas heaters can only be operated in a location where a suitable gas supply is guaranteed and which fulfils safety requirements (fire protection/explosion protection).

Furnaces are the main field of application for gas heaters in industrial processes. Gas heaters are generally not used in other manufacturing processes because electrically operated heating elements offer economic and technical advantages in this respect.

IR Heaters with exposed heating coils

Heaters with open heating coils have a cross section with flat or round heating conductors that are freely arranged in a housing or embedded in a thermally insulated substrate to keep them in place, the entire heating conductor or a part thereof forming an unencased radiating panel.

The principal reason for heaters having exposed heating coils is their comparatively short reaction time for heating and cooling. This is a unique heater property that is necessary and required in some production processes. With most of these thermally fast-reacting heaters an integrated thermocouple cannot be installed right next to the heating conductor.

Depending on the construction and type of installation in the substrate material or mounting case there are considerable differences with regard to the operational life of this type of heater in comparison to a heater of a different design. One example of this is zigzag-shaped, vertically embedded heating conductors. The primary frontal radiating panel of the heating conductor is only a few tenths of a millimetre wide. The vertical radiating panels related to this are generally arranged in a meandering fashion and have the disadvantage of heating each other. The operational life of the heating conductor is reduced for IR heaters of this type, particularly under high operating temperatures.

Whether the heating conductor is arranged on the substrate vertically or horizontally, there are numerous cases of electrical failure of the heating conductor at the power supply contact point. Exposed heating conductors made of round materials achieve a longer operating life.

Other IR Heater variants

Other IR heater designs are available on the market. They are, however, of much less quantitative importance in the manufacturing industry in comparison to the systems described here.

Summary // Outlook

The comparisons listed here are largely based on observations and findings from practical applications. As a result, electrically operated ceramic IR heaters with completely embedded heating conductors can be used universally and for the most part have a longer operational life in comparison with other IR heaters. This fact is why, in many cases, other heater designs are replaced with ceramic IR heaters. The growing worldwide trend of the economic and ecological optimisation of production facilities begins, in many cases, with the decision to use ceramic IR heaters. Decisions in the other direction are rare.