Heat transfer surfaces developed by Air Preheater Company engineers, combined with state-of-the-art manufacturing techniques, assure the highest thermal performance and durability available. The selection of the optimum heat transfer surface configuration for each application results in the highest level of thermal efficiency for the available pressure drop which is cleanable and otherwise compatible with the fuel that is being fired. The now superceded DU surface profile for hot end applications and the NF profile for cold end applications had been the industry standards for over 40 years and are still among the most widely used profiles in the world. The DU profiles are now known to exhibit performance-related deficiencies that lead to flow related fouling problems, whereas the NF profiles have been simply outdated. If the performance, cleanability and life of your current heating elements are not acceptable, or if there are operational changes planned for the future, then our newer profiles may offer a significant benefit.  The Air Preheater Company has a variety of surfaces including; DUN, DN7™, DL7™, DNF® and FNC® each of which has a unique set of attributes for thermal performance, pressure drop, cleanability and depending upon material type and gauge, life. Depending upon the age and design of your Ljungström® air preheaters, further modifications may be available to; reconfigure the rotor and housing structure to accommodate additional heating element depth, further improve cleanability and reduced air to gas leakage. Our experienced performance and design engineers can help select the optimum profile and arrangement to meet your requirements. Corrosion-Resistant Materials Experience has shown that corrosion potential can be directly related to the rate of conductivity increase as may be measured with an acid dew point meter. Tests on coal-fired units typically reveal a measurable potential for corrosion starting at the acid dew point and increasing steadily as the temperature drops. Oil-fired units tend to see their maximum potential for corrosion at a temperature level some 40ºF below the occurrence of the acid dew point. It has been proven to be both feasible and cost effective to operate regenerative air preheaters with metal below the acid dew point. Close attention, however, must be given to selection of appropriate fabrication materials. In applications where heat transfer surfaces are exposed to highly corrosive atmospheres and low exit gas temperatures are desired for maximum efficiency, alloy steels or porcelain enamel coatings can be utilized.  A high level of success has been realized with porcelain enamel coatings as the most cost-effective means of providing an extremely high level of acid resistance for this application.  Additionally, this enamel coating has been found to provide a highly smooth glass-like surface, more readily facilitating the cleaning of ash deposits.