Infrared heating for human comfort

Introduction

Infrared heating provides warmth in a fundamentally different way from conventional convection heating. Rather than relying primarily on heating the surrounding air, infrared radiation transfers energy directly to surfaces and objects within its field.

For people, this can create a distinctive sensation of radiant warmth. The experience depends on a number of factors, including the infrared wavelength, emitter temperature, radiant intensity, distance from the source, exposure time, clothing and the surrounding environment.

Infrared heating is therefore particularly interesting for applications where targeted and controllable heat is required.

Ceramicx has extensive experience in the design and manufacture of infrared heating elements and systems for controlled, application-specific heating.

Infrared on the electromagnetic spectrum

Infrared radiation forms part of the electromagnetic spectrum, between visible light and microwave radiation. The infrared region itself covers a broad range of wavelengths and is commonly divided into near-, mid- and far-infrared regions.

Thermal infrared heating is concerned with the transfer of radiant energy from a warm emitter to a receiving surface. The wavelength distribution of that radiation is strongly influenced by the temperature of the emitter.

The radiation produced by a thermal emitter is determined largely by its temperature. As emitter temperature increases, total radiant output increases and the peak emission shifts towards shorter wavelengths. These relationships are described by Planck's law, Wien's displacement law and the Stefan–Boltzmann law.

For infrared heating applications, this means that emitter temperature and wavelength are important considerations when selecting a heating technology.

Different materials also absorb infrared radiation differently. The performance of an infrared heating system therefore depends not only on the emitter, but also on the material being heated, the distance between emitter and target, the geometry of the system and the surrounding environment.

Human radiation

Like all objects with a temperature above absolute zero, the human body both emits and absorbs electromagnetic radiation.

The exact methodology can be worked out using a number laws of physics, including Plancks constant, Boltzmann constant, temperature and light calculations, The central equation can be presented thus:

The basic principle is that objects, including the human body, both emit and absorb infrared radiation. The balance between emitted and absorbed radiation contributes to the body's thermal balance.

Thermal imaging provides a practical example of this principle. Infrared cameras detect thermal radiation emitted by objects and can be used to visualise differences in surface temperature. Systems commonly used for human thermal imaging operate within infrared wavelength ranges where the body's thermal radiation can be effectively detected.

The human body is therefore far from a blank slate when considering infrared heating. It is both a source and receiver of thermal radiation, with its emission characteristics determined primarily by temperature.

Infrared and water

Water has strong but highly wavelength-dependent absorption characteristics in the infrared spectrum. As human beings are comprised of at least 70% water – this relationship is relevant when considering how infrared radiation is absorbed by biological tissue.

The nature of infrared wavelengths can in fact be defined by the water absorption behaviour. Different wavelengths agitate the molecules in different ways. The size of the wavelength can be measured by how the molecules vibrate.

All molecules vibrate – as long as they are at a temperature above (theoretical) absolute zero (-273.15 °C). When they vibrate they give off a vibrational (infrared) frequency and scientists have worked out that when molecules are at a certain temperature each different type of molecule emits a different infrared frequency which can be measured by an infrared spectrometer – a machine which measures the frequency a molecule is vibrating at.

Most of us are aware that our bodies should be made up of 70-90% water, so therefore it may be of some interest for us to know what the spectral absorption rate of water is. Complications can arise from temperature, atmospheric pressure solid liquid or gas. Different wavelengths agitate water (H2O) molecules in different ways. Below are some examples of that agitation (known as stretching or bending). The pale blue represent the ‘H2’ and the dark blue represents the ‘O’.

Water absorption therefore provides an important example of why wavelength selection matters when designing an infrared heating system. The amount and distribution of energy absorbed will depend on the wavelength of the radiation as well as the properties of the receiving material.

Infrared heating and human comfort

Radiant heating can provide targeted warmth without requiring the entire surrounding air volume to reach the same temperature.

This can make infrared particularly useful for:

• personal or localised heating
• intermittently occupied spaces
• outdoor and semi-outdoor environments
• workplaces
• hospitality and leisure areas
• zoned heating applications

The comfort experienced depends on the balance between radiant heat received by the body and heat lost to the surrounding environment. Emitter wavelength, power, distance, positioning and exposure time can all influence the resulting thermal experience.

Disclaimer

Please note: There are variable interpretations on where short/medium/long wave IR fall on the electromagnetic scale.

These test results should be carefully considered prior to a determination on which type of infrared emitter to use in a process. Repeated tests conducted by other companies may not achieve the same findings. There is a possibility of error in achieving the set-up conditions and variables that may alter the results include the brand of emitter employed, the efficiency of the emitter, the power supplied, the distance from the tested material to the emitter utilised and the environment. The locations at where the temperatures are measured may also differ and therefore affect the results.