Tubes:
A shell and tube exchanger consists of a number of tubes mounted inside a cylindrical shell. Two fluids can exchange heat, one fluid flows over the outside of the tubes while the second
fluid flows through the tubes. The fluids can be single or two phase and can flow in a parallel or a cross/counter flow arrangement.
Tube material:
            Initially select the tube material that is available with the local manufacturer.
Tube outer diameter and thickness:
            Now select the tube diameter from the following TEMA standards (table 2). The most
commonly used tube diameters are ¾ inch and 1 inch.
 
For the corresponding Tube OD and material, select the B.W.G (Birmingham Wire Gauge) number. For Example, if you select the tube material as carbon steel and tube OD as 1 inch then the B.W.G number will be either 14 or 12.
Tube diameters in the range 1/4in. (6.4 mm) to 2 in. (50 mm) are used. The smaller diameters 5/8 to 1 in. (16 to 25 mm) are preferred for most duties, as they will be more compact resulting in cost economy (cheaper exchangers). Larger tubes are easier to clean by mechanical methods and would be selected for heavily fouling fluids.
The tube thickness (gauge) is selected to withstand the internal pressure and give an adequate corrosion allowance.
When U-bends are formed, it is normal for the tube wall at the outer radius to thin. The minimum tube wall thickness in the bent portion before bending shall be:
Where,
To -Required tube wall thickness prior to bending, in. (mm)
t1 -Minimum tube wall thickness calculated by Code rules for a straight tube subjected to the same pressure and metal temperature, in. (mm)
do -Outside tube diameter, in. (mm)
R -Mean radius of bend, in. (mm)
For example, if your B.W.G number is 14 then the corresponding value 2.108mm will be your tube thickness.
Tube Length:
            After choosing the tube diameter and the tube thickness, we must choose the tube length according to TEMA standards. While choosing we must consider with the availability of the space with the client. If nothing is specified about the length of the tube by the client then choose the tube length according to TEMA standards as shown below.
For a given surface area, the use of longer tubes will reduce the shell diameter, which will generally result in a lower cost exchanger, particularly for high shell pressures.
Now for the corresponding B.W.G number select the tube thickness value in ‘mm’ from the table below
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