A double pipe heat exchanger, in its simplest form is just one pipe inside another larger pipe. One fluid flows through the inside pipe and the other flows through the annulus between the two pipes. The wall of the inner pipe is the heat transfer surface. The pipes are usually doubled back multiple times as shown in the diagram at the left, in order to make the overall unit more compact. The term 'hairpin heat exchanger' is also used for a heat exchanger of the configuration in the diagram. A hairpin heat exchanger may have only one inside pipe, it may have multiple inside tubes, but it will always have the doubling back feature shown. Some heat exchanger manufacturers advertise the availability of finned tubes in a hairpin / double pipe heat exchanger. These would always be longitudinal fins, rather than the more common radial fins used in a crossflow finned tube heat exchanger.
Types of Double Pipe Heat Exchangers ;-
1. Counter flow
2. Parallel Flow Heat Exchanger
1. Counter flow;-
The main advantage of a hairpin / double pipe heat exchanger is that it can be operated in a true counter flow pattern, To get More Efficiency, In the mean Time, it will give the highest overall heat transfer coefficient for the double pipe heat exchanger design.
2. Parallel Flow;-
Parallel Flow double pipe heat exchangers are focused to handle high pressures & temperatures applications. Also we can Achieve High Log mean Temperature using this.
3. Double Pipe Heat Exchanger Design;-
Determination of the heat transfer surface area needed for a double pipe heat exchanger design can be done using the basic heat exchanger equation: Q = UA ?Tlm, where:
Q is the rate of heat transfer between the two fluids in the heat exchanger in Btu/hr,
U is the overall heat transfer coefficient in BTU/hr-ft2-oF,
A is the heat transfer surface area in ft2, and
?Tlm is the log mean temperature difference in oF, calculated from the inlet and outlet temperatures of both fluids. These parameters in the basic heat exchanger equation are discussed in 'and they are used in an example in ' After determination of the required heat transfer surface area, the diameter and length of the inner pipe can be selected and then the diameter of the outer pipe. Finally, the length of the straight sections and the number of bends can be selected.
A primary advantage of a hairpin or double pipe heat exchanger is that it can be operated in a true counterflow pattern, which is the most efficient flow pattern. That is, it will give the highest overall heat transfer coefficient for the double pipe heat exchanger design. Also, hairpin and double pipe heat exchangers can handle high pressures and temperatures well. They are operating in true counterflow, they can operate with a temperature cross, that is, where the cold side outlet temperature is higher than the hot side outlet temperature.