A double-pipe exchanger is made up of one pipe containing the tube fluid concentric with another pipe, which serves as the shell. The tube is often finned to give additional surface area. The double-pipe exchanger was developed to fit applications that are too small to economically apply the requirements of TEMA for shell and tube exchangers.
Double-pipe exchangers can be arranged as in Figure 3-12 such that two shells are joined at one end through a “return bonnet,” which causes the shell-side fluid to flow in series through each of the two shells. In this configuration, the central tube is bent or welded into a “U” shape, with the U-bend inside the return bonnet, The principal advantage to this configuration is that a more compact exchanger can be designed, thus simplifying installation.
A variation of the U-tube exchanger is the hairpin style of exchanger. In the hairpin exchanger, multiple small tubes are bent into a “U” shape in place of the single central tube. This variation allows for more surface area to be provided in the exchanger than would be obtained with a single tube. U-tube exchangers may be designed with or without fins.
The advantages of double-pipe exchangers are that they are cheap and readily available, and because of the U-tube type of construction, thermal expansion is not a problem. Double-pipe exchangers are normally designed and built in accordance with the applicable requirements of TEMA. Thus, they can be applied to most services encountered in oil and gas production facilities as long as the required surface area can be fit into the physical configuration of the exchanger. Although they can be built in almost any size, double-pipe exchangers are most frequently used when the required surface area is 1,000 ft2 or less.