Tubes in shell and tube heat exchangers

Tubes of circular cross section are exclusively used in exchangers. Since  the  desired heat  transfer  in the exchanger takes place across the tube surface, the selection of  tube geometrical  variables  is  important from the performance point  of view.  Important tube geometrical variables include tube outside diameter, tube wall thickness, tube pitch, and tube layout patterns (Fig). Tubes should be able to withstand the following:

1.  Operating temperature and pressure on both sides

2.  Thermal stresses due to the differential thermal expansion between the shell and the tube bundle

3.  Corrosive nature of both the shell-side and the tube-side fluids

There are two types of tubes: straight tubes and U-tubes. The tubes are further classified as

1.      Plain tubes 2.  Finned tubes 3.  Duplex or bimetallic tubes 4.  Enhanced surface tubes

Tube Diameter

Tube size is specified by outside diameter and wall thickness. From the heat-transfer point of view, smaller diameter tubes yield higher heat-transfer coefficients and result in a compact exchanger. However, larger diameter tubes are easier to clean, more rugged, and they are necessary when the allowable tube-side pressure drop is small. Almost all heat exchanger tubes fall within the range of in (6.35 mm)) to 2 in (5.8 mm) outside diameter. TEMA  [31  tube  sizes  in  terms of  outside diameter are  i, i, :,i,$,  i,1,  1.25, 1.5, and  2  in  (6.35, 9.53, 12.70,  15.88, 19.05, 22.23, 25.40, 31.75,  38.10,  and 50.80 mm). Standard tube sizes and gages for various metals are given in TEMA Table RCB-2.21. These sizes give the best performance and are most economical in many applications. Most popular are the;-in and f-in sizes, and these sizes give the best all-around performance and are most economical in most applications. Use in (6.35 mm) diameter tubes for clean fluids. For mechanical cleaning, the smallest practical size is 3 in (19.05 mm). Tubes  of  diameter  1 in  are normally used when  fouling  is expected because smaller ones are not  suitable  for mechanical cleaning,  and  falling film ex- changers and vaporizers generally are supplied with  1.5- and 2-in  tubes.

Tube Wall Thickness:

The tube wall thickness is generally identified by the Birmingham wire gage (BWG). Standard tube sizes and tube wall thickness in inches are presented in TEMA Table RCB-2.21. Tube wall thickness must be checked against the internal and external pressure separately, or maximum pressure differential across the wall.  However, in many cases the pressure is not the governing factor in determining the wall thickness. Except when  pressure governs,  the wall thickness  is  selected on  these bases: (1) providing an  adequate margin  against corrosion, (2)  fretting  and wear  due to flow induced vibration,  (3) axial strength, particularly  in  fixed tube-sheet exchangers, (4) standardized dimensions, and (5) cost.

Low-Finned Tubes:

Shell and  tube  exchangers employ low-finned tubes to increase the surface area on the  shell side when the  shell-side heat-transfer coefficient  is  low  compared to  the  tube-side coefficient-for  example, when shell-side fluid is highly viscous liquids, gases, or condensing vapors. The low-finned tubes are generally helical or annular fins on individual tubes. Fin tubes for a shell and tube exchanger are generally “low-fin” type with fin height slightly less than & in  (1.59 mm). The most common fin density range is 19-40 finding (748- 1575 fins/m). The surface area of such a fin tube is about 2.5-3.5 times that of a bare tube. The finned tube has bare ends having conventional diameters of bare tubing; the diameter of the fin is either slightly lower than or the same as the diameter of the bare ends, depending upon the manufacturer. In addition to the geometrical variables associated with bare tubes, the additional geometrical dimensions associated with a fin tube are root diameter, fin height, and fin pitch.

Tube Length:

For a given surface area, the most economical exchanger is possible with a small shell diameter and long tubes, consistent with the space and the availability of handling facilities at site and in the fabricator’s shop. Therefore, minimum restrictions on length should be observed. However, for offshore applications, long exchangers, especially with removable bundles, are often very difficult to install and maintain economically because of space limitations.  In this case, shorter and larger shells are preferred despite their higher price per unit heat-transfer surface. Standard lengths as per TEMA standard RCB-2.1 are 96, 120, 144, 196, and 240 in. Other lengths may be used.