Page 101 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

98 Chapter 4 Shell and tube heat exchanger

diameter of tubes perpendicular to the flow direction. De, the shell equivalent diameter, is defined
in Eq. (4.12) for triangular and square pitch.
Some useful relationships for quick evaluation of the different design alternatives around a base
design are given below

1. in turbulent flow Dp s ðor Dp t Þ a u 2 (4.26)
2. in laminar flow Dp s ðor Dp t Þ a u (4.27)
3. Dp s ðDBLÞ¼ Dp s ðSEGÞ=8 (4.28)
4. Dp s ðJÞ¼ Dp s ðEÞ=8 (4.29)

3
(4.30)
5. Dp t a ðN p Þ
where
u: linear velocity
Dp s ; Dp t : shell- and tube-side pressure drop excluding nozzle
DBL, SEG: double segmental baffle, segmental baffle
J & E: TEMA shell type
N p : No. of tube passes

4.6 Mechanical detailing
The mechanical details to be passed on to the fabricator are the general arrangement and the fabri-
cation detail drawing(s). This would completely specify the material(s) of construction; dimension of
component parts that make the tube bundle assembly, shell and heads and the nozzles. While such
details are finalised by the mechanical engineering group, the process design team needs to be familiar
with the codes and generate the basic specifications dealt as follows. Intricacies of fabrication steps/
procedure and inspection are intentionally omitted.

4.6.1 Exchanger material
A major consideration for selecting exchanger material is corrosiveness of the working fluids. Carbon
steel is a commonly adopted exchanger material. Mechanical strength becomes important in high
temperature applications. Process industry often uses admiralty brass tubes when the environment is
neither too acidic nor too alkaline, e.g., overhead condenser of crude distillation columns. Stainless
steel is used for food processing or fluids that require corrosion resistant and smooth surface. The best
guidance for selection of material is provided by past experience with similar services and materials.
Table 4.9 shows typical HE material used for different services.

4.6.2 Tube length
Tube bundles are assembled out of tubes, tube sheets, baffles, tie rods and spacers. Standard tube
length according to IS 4503:1967 are 0.5, 1, 2.5, 3, 4, 5 and 6 m. Choosing standard tube length