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The cross-section of the twisted tube is elliptical, which is heat exchange element and the flow path in the tube is spiral, so it is called twist flat tube. In order to facilitate the processing of the connection between the heat exchange element and the tube sheet, the two ends of the elliptical tube are still kept as round. The arrangement of the twist flat tubes in the shell is very compact. The outer edges of adjacent heat transfer tubes keep the spiral point contact to reduce the volume of the heat exchanger and increase the flow space between the heat exchange elements.
Twisted tube is one kind of enhanced heat transfer tubes that were firstly presented by Swiss Allares Company, and then improved by the United States Brown company.
The characteristic of twisted tube is that every cross section of the tube is oval, when assemble heat exchangers they can be mixed bundle (that is, the mixed use of twisted tube and bare tube), and can also be a pure twisted tube bundle.
The manufacturing process includes two forming steps: “partial pressure” and “distortion”. The tube cross section is ellipse type the ratio of long and short axis is designed according to the flow velocity in heat transfer tube. When the flow rate in tube is low, we can increase the ratio of the long and short axis, or reduce the flow surface.
Seamless steel twisted tubes used for producing heat exchangers are complete ones without joints. Common materials used in steel tubes shows in bellow chart.
| Material Type | Material Grade | Produce Standard | Supply condition |
| Carbon steel | 10、20/A1 | ASTM A179/A192/A210 | Annealed condition |
| Low Alloy | T5/T11/T22 | ASTM A213 | Annealed condition |
| Nickel Alloy | UNS6625/UNS6852 | ASTM B444 | Annealed condition |
| Titanium | Grade 2 | ASTM B338 | Annealed condition |
| Alloy of copper | C68700/C70600/C71500 | ASTM B111 | Annealed condition |
| Austenitic stainless steel | 304/304L | ASTM A213 | Pickling & passivation |
| 316/316L | |||
| 310 | |||
| 321 | |||
| 347 |
The unique structure of twisted tube can make the flow in tube pass and shell pass in the spiral motion at the same time, in this way it enhanced the turbulent intensity. The heat transfer coefficient of the twisted tube is 40% higher than the normal one, but the pressure drop is almost the same.
| 1 | Lower pressure drop | The longitudinal flow allows for a lower relative pressure drop when compared to segmental baffle designs. |
| 2 | Higher heat transfer efficiency | Allowing for more surface area in a given shell size, the throughput can potentially be increased based on the application. Increasing the tube-side heat transfer coefficient can provide increased heat transfer, based on the application. |
| 3 | Less deposit | No blind zone in the shell |
| 4 | Elimination of Damage-Induced Vibration | Due to the generally longitudinal flow pattern of the shell-side fluid and the multi-point support structure of the bundle, the potential for flow-induced damaging vibration is mitigated. |
| 5 | Potential Fouling Mitigation | The elimination of shell-side baffles eliminates dead spots where sediment can accumulate and cover heat transfer surface area. |
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