(85b) Impact of High-Emissivity Coatings on Process Furnace Heat Transfer

High emissivity coatings have been shown to enhance heat transfer in high temperature process furnaces. This paper examines the theory behind the enhanced heat transfer, applications where such heat transfer is desirable, approaches for simulating the effects of high emissivity coatings, and field results from furnaces which have installed one such coating.

For gas-fired furnaces, normal furnace refractories tend to reflect a majority of arriving energy back into the furnace flue gas at the same spectral wavelength at which it is emitted from the flue gas. This energy is then again absorbed by the flue gas, limiting the amount of energy transferred to the process (tubes or materials). High emissivity coatings on furnace walls absorb more of the incident radiant energy and re-emit this energy across the wavelength spectrum, including emission through transparent windows in the spectrum.  This spectral redistribution of emitted energy allows more radiant energy to pass through the flue gas and be transferred to the process surface.

Simulation of this process requires spectral based heat transfer models which can represent the spectral absorption bands and transparent windows in the flue gas. This is accomplished using a Spectral-Line-Weighted (SLW) model with multiple grey gases. CFD simulations of a millisecond furnace process furnace shows that changing the refractory emissivity from 0.4 to 0.9 resulted in negligible differences when using only a grey gas model. When the same change was made using a spectral gas property model, the radiant furnace efficiency increased by two percent and the arch temperature decreased by 30 °C. Simulating this behavior is helpful when evaluating adjustments to furnace operation, for example when changing from a liquid to gas process feedstock.

Data from successful applications of the Emisshield high emissivity coating technology in a glass furnace and VCM furnace will be presented showing enhanced heat absorption, increased production and lower fuel usage.