A heat exchanger is a mechanical device that exchanges or transfers heat between two unmixed fluids. The heat exchangers have different types according to the space requirements, applications, designs, and fluid flow inside the system. All heat exchangers contain different barriers that separate the liquid and transfer heat at the same time. A double pipe heat exchanger is the most famous type of heat exchanger, which has a very flexible configuration. The simplest form of double-pipe heat exchanger currently available has a small pipe surrounded by another large pipe. This article mainly describes different aspects of the double pipe heat exchanger.
What is a Double Pipe Heat Exchanger?
A double pipe heat exchanger is a type of heat exchanger which consists of concentric pipes parted by a mechanical closure.
As the name of this heat exchanger represents that it uses two pipes to exchange heat between two fluids. One pipe has hot fluid while the other pipe has cold fluid. This heat exchanger is also called a jacketed u-tube, jacketed tube, hairpin, and pipe-in-pipe heat exchanger.
The double pipe heat exchanger has a small pipe that is surrounded by another large pipe. One fluid flow inside the small pipe, and the other fluid moves by the annulus between the two pipes.
The inner wall of the small pipe is the heat transfer surface. The entire heat transfer process takes place in the large tube. In this way, the inner pipe works like a conductive barrier. The external side contains fluid flow through the tube side or inside.
This heat exchanger may have one pipe or bundle of pipes (but lower than 30 pipes). The large pipe has lower than 200 mm in diameter. In some conditions, the inner pipe has vertical fins to improve the heat transfer coefficient between working fluids.
Double pipe heat exchangers are used for small heat transfer areas (e.g., 14 m2). The series connection to raise the heat transfer area needs more space and more accessories, which leads to a higher pressure loss. In addition, it is not possible to increase the number of fluid passages on both sides.
This heat exchanger is not best suitable for dirty liquids because the dirty liquid will clog it, and the cleaning of this exchanger is very hard. The main advantage of the double pipe heat exchanger is that it has an easy operation and simple design.
The construction cost of the double tube heat exchanger is relatively low and requires minimum maintenance. These exchangers are best for sensible cooling or heating in high-pressure and high-temperature applications.
Specification of Double Pipe Heat Exchangers
The double pipe heat exchanger is one of the simplest types of heat exchanger. The below-given diagram represents this heat exchanger. As shown in the below diagram, one shell or tube is surrounded by another shell or tube.
These types of shell and tube heat exchangers have simple and clear designs, giving customers a wide variety of options to choose a heat exchanger according to their requirements. There are multiple custom heat exchangers in the market that are designed according to the requirements of different industries and projects.
The double pipe heat exchanger recommends for use in small applications. They are utilized to transfer heat in small areas (i.e., less than 45 square meters).
For using this exchanger, you must know the types of fluids in your system. Due to the large flow space on the shell side, you must use only viscous fluids on the shell side. Therefore, when utilizing steam as a working fluid in the heat exchanger, it is best to use pipes for the flow.
Before selecting the right heat exchanger, you need to determine the characteristics or conditions of your project. You must also determine the outlet and inlet temperatures as well as the desired heat transfer rate. With this information, it will be easier for you and your seller to present you to the commercially available exchangers and to design the right tube pair for your exchanger.
As mentioned above, the double tube heat exchanger has a modular and simple construction, but you have to bear in mind that an increase in the surface area increases heat exchanger cost.
In simple words, if you want to buy an exchanger to transfer heat in a large surface area, you will need to pay a high price for such a heat exchanger.
Read Also: Working of Plate Heat Exchanger
Types of Double Pipe Heat Exchangers
The double pipe heat exchanger is divided into multiple types on the basis of fluid flow. The double pipe heat exchanger has the following two major types:
- Counter flow heat exchanger
- Parallel flow heat exchanger
1) Counter flow Double Pipe Heat Exchanger
The counter flow heat exchanger is one of the double tube heat exchanger types. In a counter-flow heat exchanger, the one working fluid has an opposite flow direction to the flow direction of the other working fluid.
In this design, the heat exchanger has the highest rate of heat transfer, and we can easily heat or cold the outlets according to our requirements.
The above-given diagram represents the locations of the outlet and inlet. As shown in the diagram, the fluids flow in opposite directions, and the fluids have the maximum temperature difference at the ends of both heads.
Look at the above-given diagram and imagine that Liquid 1 is hot and Liquid 2 is cold. The cold end temperature at the outlet (T2out) can be achieved close to inlet temperature T1in.
As you know, the inlet side (T1in) has more temperature than the outlet side temperature (T1out). However, in the case of the counter-flow heat exchanger, the temperature of the cold side fluid may go higher than the temperature at the outlet of the hot side, which is not possible with the parallel flow heat exchanger.
The maximum shell and tube heat exchangers have the counterflow design because this is one of the most effective designs. The counter-flow design permits the maximum temperature changes between the working liquids.
2) Parallel Flow Double Pipe Heat Exchangers
As the name of the parallel heat exchanger represents, both the fluids flow in the same direction in this heat exchanger. Parallel flow double pipe heat exchanger has the same direction for the inlet and outlet fluids.
This design of the heat exchanger has lower efficiency and heat transfer rate than the counterflow design. Though, we need this parallel design for some specific applications.
Read Also: Different types of Shell and Tube Heat Exchanger
Advantages and Disadvantages of Double Pipe Heat Exchangers
Advantages of Double Pipe Heat Exchanger
- The double pipe heat exchanger has a low cost.
- You can achieve excellent efficiency with very low capital costs
- These heat exchangers have a small size than the shell & tube heat exchangers.
- They don’t require much installation space.
- They have low maintenance costs than the shell and tube exchangers.
- They have an easy assembly and parts replacement.
- These exchangers have simple and compact designs.
- This exchanger is also best for high-pressure and temperature applications like compressors and boilers.
- Due to the very popularity of these exchangers, these have all standardized parts that make their maintenance and repairing easy.
- The structure of these exchangers permits more thermal expansion without the expansion of the joints.
Disadvantages of Double Pipe Heat Exchanger
- The double tube heat exchanger has low tubes than the shell & tube. It means it has less heat transfer rate than the shell & tube heat exchanger.
- It is not available in crossflow design. Therefore, you can’t use these exchangers for some specific applications.
- When you add any extra unit, the chances of leakage increase due to an increase in the number of connections.
- There are high chances of leakage from the removed connections.
- These heat exchangers have heat transfer limitations and can be used for small areas applications.
Applications of Double Pipe Heat Exchanger
The application range of double tube heat exchangers is very broad and difficult to cover due to its practical, beautiful, and concise design. Some of them are:
- The double pipe exchanger is used in the compressors and boiler because they have very high pressure and temperature
- Cooling and heating systems of the process plant.
- Wastewater treatment
- Oil refining
Double Pipe Heat Exchanger Design
If you know the characteristics of your double pipe heat exchanger and your requirements, you can design a heat exchanger according to your heat transfer requirements.
You can determine the heat transfer rate by the below-given equation:
In the above equation:
Q = Heat transfer rate between fluids
U = Heat transfer coefficient
△Tlm = Temperature difference
A = Heat transfer surface area
The below-given formula can calculate the temperature difference (△Tlm):
Why counter flow heat exchanger is more effective?
A parallel heat exchanger has lower efficiency than the counter-flow heat exchanger because the counter-flow exchanger creates a more constant fluid temperature differential over the whole fluid path length.
What is a double-wall heat exchanger?
The working principle of the double-wall plate heat exchanger is very similar like the conventional plate heat exchanger, but the main difference is that the individual plate between the two fluids is exchanged by the plate pair constructed with two identical plates placed on each other and welded around the porthole.
What is the main disadvantage of a double pipe heat exchanger over the shell & tube heat exchanger?
The main disadvantage of the double pipe heat exchangers is their large size. They have high metal consumption per unit of heat transfer area (i.e., five times more than a shell & tube heat exchanger).
What is the difference between double pipe and shell and tube heat exchanger?
A double pipe heat exchanger has two pipes. One pipe has a large diameter which surrounds to small diameter pipe. One pipe uses for cold fluid, while the other pipe uses for hot fluid.
In contrast, a shell and tube heat exchanger has a tube bundle that is surrounded by a large shell. In this exchanger, one fluid flows inside the tubes while other fluid flows between tubes and shell.