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answered Oct 26, 2014 by sabdullayev (5,290 points) | selected Dec 6, 2014 by odadmin
 
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Graphite is one of three crystalline forms of carbon, the other two are diamond and charcoal. Graphite has a hexagonal crystal structure, diamond is cubic, and charcoal is amorphous. Graphite is inert to most chemicals and resists corrosion attack. It is however porous and to be used, it must be impregnated with a resin sealer. Two main resins used are phenolic and PTFE with furan (one currently being phased out of production). Selection of resins includes chemical compatibility, operating temperatures, and type of unit to be used.

Graphite heat exchanger equipment is made in a variety of forms, including outside packed head shell and tube exchangers. They are fabricated with impervious graphite tubes and tube side headers and metallic shells. Single units containing up to 1300 m2 (14,000 ft2) of heat transfer surface are available.

Shell-and-tube units in graphite were started by Karbate in 1939. The European market started using block design in the 1940s. Both technologies utilize the high thermal conductivity of the graphite material to compensate for the poor mechanical strength. The thicker materials needed to sustain pressure do not adversely impede the heat transfer. Maximum design pressures range from 0.35 to 1.0 kPa depending on type and size of exchanger. Design temperature is dependent on the fluids and resin selection, the maximum is 230 °C.

The graphite heat transfer surface is contained within a metal structure or a shell (graphite lined on process side) to maintain the design pressure. For shell and tube units, the design is a packed floating tubesheet at both ends within a shell and channel. For stacked block design, the standardize blocks are glued together with special adhesives and compressed within a framework that includes manifold connections for each fluid. The cylindrical block unit is a combination of the above two with blocks glued together and surrounded by a pressure retaining shell. Pressure vessel code conformance of the units is possible due to the metallic components of these designs. Since welding of graphite is not possible, the selection and application of the adhesives used are critical to the proper operating of these units. Tube to tubesheet joints are glued since rolling of tubes into tubesheet is not possible. The packed channels and gasketed manifold connections are two areas of additional concern when selecting sealants for these units.
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answered Oct 26, 2014 by pasmith (3,320 points)

The major applications for graphite exchangers are in the acid-related industries. Sulfuric, phosphoric, and hydrochloric acids require either very costly metals or impervious graphite. Usually graphite is the more cost-effective material to be used.

Services are coolers, condensers, heaters, re-boilers, evaporators, interchangers, graphite columns for distillation, absorption and scrubbing, ejector systems and centrifugal pumps, basically all services requiring this material.

Types of units are shell-and-tube, poly block-type (circular and rectangular), and plate-and-frame-type exchangers. The design of the shell-and-tube units are the same as ordinary but the design characteristics of tubes, spacing, and thickness are unique to the graphite design. The block and plate and frame also can be evaluated but the unique characteristics of the graphite materials require input from a reputable supplier. Most designs will need the supplier to provide the most cost-effective design for the immediate and future operation of the exchangers. Also, consider the entire system design as some condensers and/or evaporators can be integral with their associated column.

Applications are increasing in the following industries as new products with chlorine and fluorine compounds expand:

  • photovoltaic
  • fine chemicals and pharmaceuticals
  • organic chemicals
  • inorganic chemicals
  • polysilicon
  • metallurgy
  • mining
  • oil and gas
  • water and wastewater treatment
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answered Oct 26, 2014 by wangroc (3,280 points)

Advantages of using Graphite tubes:

  • High resistance to corrosion and good temperature resistance
  • High efficiency and high heat transfer capacity
  • High performance combined with small heat exchange area
  • Low pressure drops
  • Excellent refractory qualities and mechanical properties
  • Non-contaminating properties
  • Optimum design for Cleaning-in-Place procedures
  • Allows single or multiple counter-current flow, or several different applications in a single unit
  • The resulting savings in heat exchange area can be as much as 40%
  • Wide range of material combinations for shell and header (FRP, rubber-lined steel, enamel, steel/PTFE)

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