Twisted tubes bundles

Figure 27.5 Simplified portion of a twisted tube bundle indicating cleaning lanes c, tubesheet not shown.

At one Midwest refinery, on a 180,000 BSD crude unit, two parallel, 6000 square ft, titanium twisted tube bundles have been in service for a number of years. Crude tower overhead vapors plus steam are condensing on the shell side. Crude is on the tube side. Fouling on both the shell side and the tube side appears to be quite minimal. However, the heat-transfer coefficient is bad only about 20-25 Btu/hr/ ft /°F, even when the exchanger is clean. 

We attribute this mainly to the relatively low (3 ft per second) tube-side velocity and cold (50°F), somewhat heavy (25°API) crude with a high viscosity. The overall performance of these exchangers did not improve all that much, according to our client, after the new twisted tube bundles were installed. However, they also converted the exchangers from four to two tube-side passes, and this decreased the tube-side velocity by half. Presumably, the reduced tube-side velocity offset the benefits of the twisted tube design. 

Special care should be taken on acceptance of a new twisted tube bundle to verify that the tubes are lined up correctly and that all shell-side cleaning lanes are unobstructed. 

We know that although there is concern as to how to clean these twisted tube bundles, in some cases, the run length has been so long that even years after installation there still has been no need to clean the bundle. Apparently when it works well, it works very well. 

The need to take extra care when pulling and reinserting the bundle because, especially with the surrounding shroud, twisted tube bundles are less robust than traditional shell-and-tube exchanger bundles. 

The need to take more care when commissioning twisted tube bundles in very hot service because they are less rugged than traditional shell-and-tube exchanger bundles, and there may be a greater risk to tubes being pulled out from the tube-sheets by uneven thermal expansion. 

The cost of the twisted tube bundle as compared to a new larger traditional shell-and-tube exchanger of comparable surface area with wide rotated square pitch for ease of cleaning. In some cases the traditional shell-and-tube exchanger (shell + bundle + installation) was the least expensive option. 

Putting crude on the shell side of a twisted tube bundle may cause problems with fouling. Do not put vacuum resid on the shell side with twisted tubes. 

Twisted tube bundles are manufactured by a division of Koch-Glitsch and are also marketed by Brown Fintube in Luxembourg and the United Kingdom. 

Figure 10-4G. Twisted tubes with heat exchanger bundle arrangements. (Used by permission Brown Fintube Co., A Koch Engineering Co., Bui. B-100-2.)...

Terminal-tube velocity. Have you ever seen a heat-exchanger tube bundle pulled Perhaps many of the tubes were bent and twisted like partly cooked spaghetti. This distortion could not be very good for the... 

The hotter, fouled tubes must grow. But their horizontal expansion is constrained by the cleaner, colder tubes, since the colder tubes do not allow the hotter tubes to grow, the hot tubes bend. This, then, is the origin of the twisted tubes we see when an improperly designed tube bundle is pulled from its shell during a turnaround. 

Rovings are essentially untwisted bundles of fiberglass strands wound up in parallel on cylindrically shaped packages. They are used in open lay-up moldings, woven fabrics, rods, and tubes. Woven rovings are used for open lay and press moldings. Yams [1] consist of twisted yam bundles. Weaver yarns are used for electrical and aircraft laminates, insulating tape, window shades, and filtration applications. Their major use is in printed circuit boards. Chopped strand serves as a reinforcement in a variety of markets. For example, it is used to... 

Cao Jiang, 2007), wire-in-tube structures (Greiner Wendorff, 2007), What s more, twisted fiber bundles, golfball-like micro particles and multi-chambered hollow spheres (Chen et al., 2008) are also able to be generated via this technique. These fibrous structures and particles obtained by electrospinning have some unmatchable properties, like the... 

The other basic concept that is essential to the functioning of a tube bundle made with these twisted tubes is that in manufacture of the bundle, the tubes have to be grouped or lined up in the bundle so that the ridges do meet and touch to form the curved points of contact described above. Also and most significantly, this alignment... 

The main benefit that is foimd seems to be the increase in surface area, typically 40 percent more surface area in the bundle due to the fact that there are more tubes in the bundle compared with a conventional bundle that would fit the same shell. Actual experience shows that the twisted tubes may foul more readily and will plug if fouling deposits are from particulates in feed. But the twisted tube design does work in this service to combat fouling if deposits are formed as a fouling film inside tubes. 

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