High emissivity coatings applied to the internal walls of furnaces may help to increase thermal radiation transfer to the load and improve thermal efficiency.

The radiation interchange between a non-grey gas and refractory and load surfaces in a furnace.

Read on....

How does a high emissivity coating work?

In a gas or oil fired furnace, the high temperature non-luminous and non-grey combustion gases absorb and emit radiation at discrete wavelengths corresponding to the spectra for the CO₂ and H₂O molecules.  Of particular interest are the ‘windows’ in the spectrum through which radiation can pass unattenuated. Radiant energy from the hot gases that is reflected off the wall surface, does not change its wavelength and hence can be significantly absorbed by the gas before reaching the load surface. Radiation that is absorbed by the wall on account of its emissivity is re-emitted. A proportion of this energy passes unattenuated through the ‘windows’ in the gas spectra. If the wall is ‘black’ or of a high emissivity, then more radiation is able to reach the load surface without re-absorption by the gas. Thus a net redistribution of energy occurs across the spectrum resulting in an increase in heat transfer to the load.

Independent work by Leeds University [1] and by British Gas [2] came to the same conclusions - that models can predict improvements in heat transfer, when wall emissivity is increased, but only if non-grey gas behaviour is taken into account. Zone models using the 'weighted sum of grey gases' model for combustion products, allow for non-grey behaviour and in particular the effect of ‘windows’ in the spectrum.

The level of improvement predicted depends on many other factors such as the original furnace efficiency, the emissivity of the load, the temperature, the conductivity of the wall, and whether a steady-state or a transient model is being applied. Docherty and Tucker [2] showed that the effect is more significant for steady-state processes when wall heat loss is insignificant.

1. Elliston D.G. et al., ‘The effect of surface emissivity on furnace performance’, J.Inst.Energy, Dec 1987, pp155-167.
2. Docherty P., Tucker R.J., The influence of wall emissivity on furnace performance, J.Inst.Energy, March 1986.

For further information, contact Brakeglen Ltd (specialist consultancy in refractories and coatings)

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