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Serpentine Tubes The low dispersion serpentine tube (6), an alternative to the coiled tube, was designed to increase secondary flow above that which occurs in a simple coiled tube by reversing the direction of flow at each serpentine bend. A diagram of a serpentine tube is shown in figure 12. Ref (P) J. Chromatogr. 268(1978)681 Internal radius, 0.010 in. (0.0127 cm), external radius 0.020 in. (0.025 cm), linear length 17 in., serpentine length 15 in., serpentine amplitude, 0.050 in. Figure 12. The Low Dispersion Serpentine Tube  

The serpentine tube is protected by an outer sheath which also provides some mechanical strength. In a coiled tube, the flow of fluid is continually deflected in the same direction, but in the serpentine tube the reversal of the flow at each bend also induces some turbulence, which strongly augments the increase in diffusivity. As a result low dispersion is produced at relatively low solvent velocities.   Curves for a serpentine tube having the dimensions given in figure 12 relating the variance per unit length of the tube (H) against flow rate are shown in figure 13. The flow rate is employed as the independent variable as an alternative to the more usual linear velocity

Despite the apparent advantages, low dispersion serpentine tubing appears to have been employed in only one commercial LC detector. It should be pointed out that any conduit system that has low dispersion will also provide very fast heat transfer rates. Serpentine tubing has been also used in commercial column ovens to heat the mobile phase rapidly to the column oven temperature before it enters the column. The serpentine tubing allows effective heat exchange with a minimum of heat exchanger volume to distort the concentration profile of the

nbsp; Low dispersion tubing has a another feature that follows directly from its operating principle. The secondary flow that results from its serpentine form greatly increases its thermal conducting properties. It follows that serpentine tubes are also highly efficient heat exchangers. Serpentine tubing has been used to preheat the mobile phase before entering a thermostatted column. A few centimeters of serpentine tubing were found to be sufficient to achieve complete thermal equilibrium between the column and the mobile phase. The different forms of dispersion profiles that are obtained from various types of connecting tubes used in LC are shown in figure 14. The peaks shown were obtained using a low dispersion

coiled tubes by the continual change in direction of the fluid as it flowed round the spirals (his theory will be considered in detail in Book 9). Tijssen found that by coiling the tubes significantly reduced dispersion, particularly at high flow rates However, the coils were a little clumsy to form as the radius of the coil was required to be less than 3 times the internal radius of the tube for optimum performance. A more practical system was introduced by Katz and Scott (10), who developed a serpentine form of connecting tube that met the requirement that the radius of the serpentine bends (a/2 in the diagram) was less that 3 times that of the internal radius of the tube. A diagram of a serpentine tube is shown in figure 3. Figure 3 Low Dispersion Tubing During passage through the tube, the direction of mobile phase flow  changed by 180o as it passed from one serpentine bend to another. This violent change in direction resulted in extensive radial flow which

nbsp;    Figure 4 Graphs of Peak Variance against Flow Rate for Straight and Serpentine Tubes This effect is clearly shown by the curves relating the variance against flow rate for straight and serpentine tubes shown in figure 4. It is seen that at high flow rates, the dispersion is reduced by over an order of magnitude by the serpentine tubing relative to the dispersion that occurred in the straight tube