The double pipe heat exchanger is, in its simplest form, one pipe held concentrically inside of a larger diameter pipe. One fluid flows through the inner pipe which also acts as a conductive barrier and the other flows around through the annulus of the inner and outer pipe. The outside flow passes over the inside flow which will cause heat exchange through the inner tube’s walls. It is also referred to as hairpin, jacketed pipe, jacketed u-tube, and pipe-in-pipe exchangers.
Double pipe heat exchangers are not limited to a single tube, they can contain one tube or a tube bundle, but the bundle must be < 30 tubes and the outer pipe must be < 200mm in diameter. The inner tube can also use longitudinal fins, which further increase the heat transfer between the two working fluids.
The double pipe heat exchanger is the exchanger that provides exact co and counter-current flow within the exchanger. If both fluids enter from the same end, they flow in a parallel direction till the exchanger’s second end. None of the fluid changes its direction within the exchanger. Similar is the case for counter-current flow. Both fluids flow exactly in opposite directions with respect to each other.
The double pipe is not only used as a single heir pin. The number of double-pipe heat exchangers can be coupled to make a stack of exchangers to provide adequate heat duty to the process. The DPHE can be generative and non-generative depending upon the application.
Read also: Classification of Heat Exchangers
Parts of the Double Pipe Heat Exchanger
- The first part is the outer shell. This shell may be insulted depending upon the operation. If heat loss is permitted in operation then the exchanger will be without insulation.
- The second major part is the inner pipe. The inner pipe normally contains hot fluid. An exception can be made based on other factors like flow rate and the corrosive nature of the cold fluid.
- The return bend is also an important part that connects the inner pipes of two exchangers. One u-bend exchanger is called a hairpin.
- Other than these three there may be some auxiliary parts as well like shell cover, shell side end piece, twin flange, and fixed, and Sliding Supports.
- They can handle both high pressures and high temperatures well.
- They have standardized parts which make their maintenance easy.
- Their flexible design makes the addition and removal of parts easy.
- They have a small footprint that requires little space while still having a good heat transfer
- They are limited to lower heat duties than other, larger designs
- Leaking can occur, especially when paired with more units
- The tubes are easily fouled and difficult to clean without disassembling the whole heat exchanger.
- If the budget and space exist for a shell and tube exchanger, then a double pipe design is often a less efficient method of heat transfer.
The application of DPHE can be found in sensible heating and cooling systems such as boilers. They are also been used in petroleum refineries, space heating, and refrigeration systems. They are also used widely in the food industry.