Duplex tubing

Bimetallic Tubes When corrosive requirements or temperature conditions do not permit the use of a single alloy for the tubes, bimetalhc (or duplex) tubes may be used. These can be made from almost anv possible combination of metals. Tube sizes and gauges can be varied. For thin gauges the wall thickness is generally divided equally between the two components. In heavier gauges the more expensive component may comprise from a fifth to a third of the total thickness. 

The relatively long service life of the duplex tubes suggested that similar tubes be used for retubing. Because of supplier delays in securing duplex tubing, however, alternative materials, including admiralty brass, were considered. 

The duplex tube (Figure 10-11) is a tube within a tube, snugly fitted by drawing the outer tube onto the inner or by other mechanical procedures. 

Figure 10-11. Duplex tube. Note inside liner is resistant to tube-side fluid and outer finned tube is resistant to shell-side fluid. (Used by permission Wolverine Tube, Inc.)...

If conditions are such as to require a duplex tube, it is quite likely that a plain end detail for the tube will not be satisfactory. Grooved or serrated joints are recommended for this type of tube, and the ends should be flared or beaded. Table 10-8 gives recommended flare or bell radii for copper-based alloys. Also see Table 10-8A. In service where galvanic corrosion or other corrosive action may take place on the outside material used in the tube, a ferrule of inside tube... 

Duplex Tube Beaded or Belled This Tube May also be Installed Plain End (No Ferrule)or Flared With or Without Ferrule. 

The I.D. of this plain surface duplex tube is determined hy deducting the gage thickness from the O.D. = 0.804 in. I.D. of cupro-nickel. 

Duplex tubes 1-in. O.D. X 16 ft, 0 in. long, 16 BWG steel outside, 16 BWG cupro-nickel inside No. = 578,1 V4 -in. triangular pitch Shell 36-in. O.D., 4 tube pass Film coefficients ... 

Fig. 7.6 An intensifier vessel (a) a double-walled pressure vessel (b) the top plug (c) the bottom plug, which admits the high-pressure fluid (d) duplex tubing (e) the high-pressure fluid (f) the low-pressure fluid (ca. 500 bar) (g) the hydraulic fluid (h) the sample (i) the pressurizing fluid.

SVBR-75/100 (Russian Federation) duplex tube, no superheater and reheater ... 

Heat transfer. A number of options exist for the heat exchanger/chemical reactor. Examples of alternative interfaces include traditional heat exchangers, radiation heat transfer (thermal infrared between tube banks), duplex tubes (tubes constructed of two metals), and intermediate heat exchanger loops. It is not clear what the preferred option is. The very high temperatures does create new options such as the use of a heat exchanger that operates on radiation heat transfer (Fig. 5). Such options provide very high degrees of separation between the nuclear and chemical facilities. 

The compatibility of INOR-8 and sodium is adequate in the temperature range presently contemplated for molten-salt reactor heat-exchanger operation. At higher temperatures, mass transfer could become a problem, and therefore the fabrication of duplex tubing has been investigated. Satisfactory duplex tubing has been made that consists of Inconel clad with type-316 stainless steel, and components for a duplex heat exchanger have been fabricated, as shown in Fig. 13-20. 

The fabrication of duplex tubing is accomplished by coextrusion of billets of the two alloys. The high temperature and pressure used result in the formation of a metallurgical bond between the two alloys. In subsequent reduction steps the bonded composite behaves as one material. The ratios of the alloys that comprise the composite are controllable to within 3%. The uniformity and bond integrity obtained in this process are illustrated in Fig. 13-21. 

The problem of welding INOR-8-stainless steel duplex tubing is being studied. Experiments have indicated that proper selection of alloy ratios and weld design will assure welds that will be satisfactory in liigh-tempera-ture service. 

Fig. 13-21. Duplex tubing consisting of Inconel over type-316 stainless steel.

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