Double Pipe Heat Exchangers:
Double pipe heat exchangers design is rather straight forward. It uses one heat exchanger pipe inside another. After determining the required heat exchanger surface area, for either counter flow or parallel flow, the pipe sizes and number of bends for the double pipe heat exchanger can be selected. In double pipe heat exchanger design, an important factor is the type of flow pattern in the heat exchanger.
Design: These are can be done using the basic heat exchanger equation: Q = UA ΔTlm, 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 ΔTlm is the log mean temperature difference in of, calculated from the inlet and outlet temperatures of both fluids.
This heat exchangers are simplest form are 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, or 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 or double pipe heat exchanger. These would always be longitudinal fins, rather than the more common radial fins used in a cross flow finned tube heat exchanger.
- Double pipe heat exchangers can handle high pressures and temperatures well.
- When they are operating in true counter flow, they can operate with a temperature cross.
- The cold side outlet temperature is higher than the hot side outlet
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