Standard text books learn that radiation is the main heat transport system at higher temperatures. For that reason, most people don’t realize that radiation can be important at lower temperatures. The most popular example at room temperature is the low-E double glazing. Indeed, installing a low-E coating on the warm pane of the double glazing halves the thermal conductivity of the double glazing.
The emissivity of glass and stainless steel
A material may be able to absorb and radiate very good radiation while another material is a good reflector of the heat. Glass is a good radiator or absorber of radiation while stainless steel is generally a poor absorber/radiator and good reflector. This ability to be a good radiator / absorber is expressed by the emissivity of the material. This (surface) property v ranges between 0 and 1, with v=1 the emissivity of a perfect radiator/absorber and v=0 a perfect reflector.
Emissivity is a surface property and for that reason, we can change the emissivity of a material with no more than a coating. In the following, we explain the possibilities and opportunities of a high emissivity coating.
For reasons of temperature and corrosion, a lot of heat exchangers are built from stainless steel. But we know that stainless steel is a good reflector, which is negative for the efficiency of the heat exchanger. By using a high emissivity coating on the stainless steel, we improve the efficiency to a maximum value, while the other advantages, high temperature and corrosion resistance remain intact.
As an example, CNUD EFCO uses stainless steel heat exchangers in the hot sections above 450°C for reasons of temperature stability and corrosion resistance against SO2. However, a larger radiation cooling capacity would be welcome for a lower residual stress and improved glass surface at the roller side. With a high emissivity coating, we obtain a large improvement. Also in the case of on line coated low-E glass and ultra-thin glass, an improved heat exchanger has a large added value. The coating for a heat exchanger must be able to absorb perfectly temperature cycling. EMISSHIELD® is the first (ceramic) coating on stainless steel, which is able to absorb perfectly cycling between 600°C and room temperature.
High emissivity coating
Another example in the high temperature range is a glass melting furnace with combustion heating (and not electric melting). The burner flames, walls and crown are heating the colder batch flowing through the glass melting furnace. High emissivity coating of the walls and crown lowers the temperature of the walls, crown and flue glasses by an improved radiation to the colder batch. Improvements up to 7% are reported if the glass meting furnace operator adjusts the set point of the temperature controllers, like proven by Owens Corning.
At ambient temperature, there is the genial application of a high emissivity coating on high voltage cables. Due to the coating, the cable becomes a few degrees colder due an improved radiation. The copper cable has a higher electrical conductivity at a lower temperature. As a consequence, the electrical energy is transported at a higher efficiency and the EMISSHIELD® coating has a payback, expressed in weeks.
EMISSHIELD® is a ceramic coating with a Nano structure after sintering at 800°C. This Nano structure is responsible for the extreme high temperature cycling resistance and also is responsible for a large corrosion resistance of the coated material. On top of that, a catalytic function avoids sticking of all kind of particles on the EMISSHIELD® coating. This NASA coating was developed for an improved cooling of the space ships returning to earth. By an improved radiation to the cold environment, the ceramic tiles remain on a safer temperature when reaching the earth atmosphere.
CNUD EFCO has a license to coat stainless steel structures with EMISSHIELD® and is also able to construct these stainless steel structures at sharp prices. Especially heat exchangers in rather corrosive atmosphere can be largely improved in efficiency and life time. Get in touch with us to have a better look at what CNUD EFCO could do for you.