Nonmetallic Heat Exchangers

Another growing field is that of nonmetallic heat exchanger designs which typically are of the shell and tube or coiled-tubing type. The graphite units were previously discussed but numerous other materi- s are available. The materials include Teflon, PVDF, glass, ceramic, and others as the need arises. 

Heaters. The preferred methods for heating are a double-jacket heated tank, nonmetallic heat exchangers, quartz heaters, or Teflon-coated low watt density stainless steel heaters. Localized overheating must be avoided. 

Nonmetallic conductors and corrosion products. Carbon brick in vessels is strongly cathodic to the common structural alloys. Impervious graphite, especially in heat-exchangers, is cathodic to structural steel. Carbon-filled polymers can act as active cathodes. Some oxides or sulfates are conductors, such as mill scale (magnetite Fe304), iron sulfides on steel, lead sulfate on lead can act as effective cathodes with an important area to that of the anodes. Very frequently, the pores of the conductive film are the preferable anodic sites that leads to localized corrosion (pitting).5... 

Metals exhibit high conductivity, although values vary widely. Nonmetallic solids normally have lower conductivities than do metals. For the porous materials of this group, the overall conductivity lies between that of the homogeneous solid and the air that permeates the structure. Low resultant values lead to wide use as heat insulators. Carbon is an exception among nonmetals. Its relatively high conductivity and chemical inertness permit its wide use in heat exchangers. 

Sometimes nonmetallic conductors may act as cathodes in galvanic couples. Both carbon brick in vessels made of common structural metals and impervious graphite in heat-exchanger applications are examples. Conductive films, such as mill scale (Fe203) or iron sulfide on steel, or lead sulfate on lead, are cathodic to the base metal or to some metallic components in their contact. 

Transfer of thermal energy occurs via three basic mechanisms conduction, convection, and radiation. Conduction of heat in metallic solids occurs via electrons. When one end of a metallic rod is heated, the excited electrons essentially run to the other end conducting the heat. In opaque, nonmetallic solids, heat is conducted via the excitation of lattice waves, or vibrations, which exchange quantized packets of energy, called phonons, with each other. 

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