Shell and tube heat exchangers

Shell and Tube Heat Exchangers

The tube and shell heat exchanger is a surface heat exchanger in which the heat flows through tubes enclosed within a cylindrical container or surrounding space (called the shell). Compared to heat exchangers, it is by far the most widely used model and allows for the exchange of large quantities of heat, with exchange surfaces that can reach tens of thousands of square meters.

In a shell and tube heat exchanger the following parts can be distinguished:

• the end heads (rear head and front head), which delimit the volume constituted by the internal part of the tubes and which is called the tube side (represented in blue in the following figures)
• The tubes themselves are attached to a thick perforated sheet metal called a tube sheet. The tubes and tube sheet can be joined by "rolling" and then welding.
• the external casing (or jacket), which delimits the volume external to the tubes, called the jacket side (represented in red in the following figures)

Four flanged "nozzles" connect to the shell, two for the service fluid (i.e., the cooling/heating fluid used as a heat exchange medium, generally water) and two for the process fluid (i.e., the fluid to be cooled/heated, which is directly involved in the industrial process). A distributor is located outside the shell, used only for gaseous fluids on the shell side. The shell may contain transverse metal plates called baffles, which serve to control the hydraulic flow within the shell itself. The heat exchanger shown in Figure 1 is called a single-pass or, more commonly, single-pass, since the fluids enter the exchanger once on both the tube and shell sides and exit once. Figure 2 shows a similar heat exchanger, but with two tube-side passes, or, as they are known, a 2-1 type (where 1 stands for one pass on the shell side). Note the vertical baffle in the upper head, which forces the fluid to flow down the tubes and then back up. In theory, there is no limit to the number of passes; in practice, it is rare to find exchangers with more than 16 tube-side passes and more than 4 shell-side passes. Figure 3 shows a 2-2 exchanger.

The tube and shell heat exchanger is a surface heat exchanger in which the heat flows through tubes enclosed within a cylindrical container or surrounding space (called the shell). Compared to heat exchangers, it is by far the most widely used model and allows for the exchange of large quantities of heat, with exchange surfaces that can reach tens of thousands of square meters.

In a shell and tube heat exchanger the following parts can be distinguished:

• the end heads (rear head and front head), which delimit the volume constituted by the internal part of the tubes and which is called the tube side (represented in blue in the following figures)
• The tubes themselves are attached to a thick perforated sheet metal called a tube sheet. The tubes and tube sheet can be joined by "rolling" and then welding.
• the external casing (or jacket), which delimits the volume external to the tubes, called the jacket side (represented in red in the following figures)

The shell is connected to four flanged "nozzles," two for the service fluid (i.e., the cooling/heating fluid used as a heat exchange medium, generally water) and two for the process fluid (i.e., the fluid to be cooled/heated, which is directly involved in the industrial process). A distributor is located outside the shell, used only for gaseous fluids on the shell side. The shell may contain transverse metal plates called baffles, which serve to control the hydraulic flow within the shell itself. The heat exchanger shown in Figure 1 is called a single-pass or, more commonly, single-pass, since the fluids enter the exchanger once on both the tube and shell sides and exit once. Figure 2 shows a similar heat exchanger, but with two tube-side passes, or, as they are known, a 2-1 type (where 1 stands for one pass on the shell side). Note, in the upper head, the vertical baffle that forces the fluid on the tube side to descend into the tubes and then rise again. In theory, there is no limit to the number of passes; in practice, it is rare to find exchangers with more than 16 tube-side passes and more than 4 shell-side passes. Figure 3 shows a 2-2 exchanger. TEMA Standards

Shell and tube exchangers, being the most widely used, are also the best defined in terms of standardization and regulations. In this context, the guidelines of the American association TEMA (Tubular Exchanger Manufacturers' Association) are authoritative, which publishes a comprehensive collection of standards relating to the classification, sizing, and construction of exchangers. The exchanger tubes do not necessarily have to be straight (coils have already been discussed, but these are improper tube bundles). In fact, it is common practice to use U-bent tubes (TEMA U type), with variable radii, joined by the tube plate to form tube crowns. These particular models of shell and tube bundle exchangers are called "crown" and feature a single head. The advantage lies in the removable tube bundle, which can be easily separated from the shell for inspection and cleaning, at the expense of reduced mechanical stability of the bundle, which, being cantilevered, is subjected to greater stress, including dynamic stress due to vibrations. They are not recommended for corrosive or dirty fluids, as localized erosion of the tubes and fouling may occur at the U-bend. Shell-and-tube exchangers are also used as heating elements within larger equipment, such as kettle-type reboilers. The simplest type of shell-and-tube exchanger is the double tube, essentially a single tube surrounded by a jacket. As will be seen later.