U-BENDS

Titanium tubing is routinely bent on conventional tube bending equipment. Mandrel benders are recommended, particularly for tight bends. Wiper dies and mandrels should be smooth and well lubricated to minimize titanium's tendency to gall. Bending rate should be slow.

The minimum bend radii with mandrel for cold bent Grade 2 commercially pure titanium (TIMETAL 50A) tubing are given in Figure 10. Bends made without a mandrel require larger radii. If smaller radius bends are required than given in the table, it may be necessary to heat the tube and perform the bending with the tube metal at a temperature of 400 degrees F - 600 degrees F. Consideration should be given to using heavier wall tubing for tight bends to compensate for wall thinning which takes place on bending.

ENHANCED SURFACE
Under some conditions, the use of enhanced surface titanium tubing can result in much improved heat transfer efficiency, where efficiency can be defined as higher heat transfer per lineal foot of tube. This can mean a smaller heat exchanger is required with resulting lower cost structural supports and lower operating costs. Nominally, enhanced surface tubing should be used when the shell side film resistance to heat transfer is two times or more the tube side film resistance.

The most common types of enhanced surface tubes are:

1. Integral fin tube
2. Applied fin tube
3. Corrugated and rope tube

INTEGRAL FINNED TUBE
This is the most widely used type of enhanced surface. It is made by cold working the tube wall under pressure to form the extended fin as an integral part of the tube. This process results in a tube with 2.5 to 3.0 times the outside surface area compared to the plain or smooth starting tube. The O.D. dimensions of the finned section are identical to that of the plain ends. Because of this, lands or unfinned areas are not required at the baffle or support plates; although lands can be provided if specified. The tube ends are always plain for roller expanding into the tubesheets.

Integrally finned tubes generally have less shellside fouling than bare tubes in the same service. If a scale forms on a finned tube, it breaks up easily and washes off the fins in small flakes. It has been reported^(3) that even in asphalt service the heat transfer capability of the finned tube exchanger exceeds that of the bare tube unit.

Generally, integrally finned tube designs prove to be advantageous when the controlling resistance to the heat transfer is shellside. Good applications for finned tubes are in shell and tube heat exchangers which condense vapors, cool or heat gas, or have a shellside fouling condition which is considered severe. Refinery applications for finned tube include Bottom Coolers, MEA Overhead Condensers, Interstage Coolers and Splitter Reflux Condensers^(4) and LNG Plants.

A very important use of titanium integrally finned tube is in refrigeration and air conditioning, water-cooled freon condensers, particularly in areas where the cooling water is corrosive to other materials. It should be noted that tests^(5) have shown that inside fouling factors with titanium tubes in seawater are often below .0001 as compared to factors of .001 or even higher, used by some engineers when other materials are considered. This difference alone very often justifies the use of titanium as the cost-effective material to be specified when corrosion is a concern.

APPLIED FIN TUBE
This type of fin is used where the shellside heat transfer coefficient is very low, as for example, when a gas or air moves by the finned tube at relatively low velocities. In order to make up for the low coefficient, the heat transfer surface is greatly increased by making the fins quite high (1" to 2"). The fin material is usually copper or aluminum.

The most popular type of applied fin tube has the fin material in the shape of an "L" with the foot of the L acting as a spacer for the helically-wound fin which is under tension and is usually brazed to the tube on both ends.

Another type of applied fin is the sleeve or muff type wherein an extruded section containing a hole for the tube has fins machined on the outer surface. The sleeve is placed over the tube and the tube expanded into tight contact with the sleeve. Due to its much higher cost, it is not used as much as the "L" type.

When using applied fin tube, the engineer should be aware that if the fin material is anodic to the titanium tube, as are both copper and aluminum, accelerated corrosion of the fin material will take place under moist conditions where the wet gas forms an electrolyte.

The most common use of applied fin tubing is in air-cooled heat exchangers wherein the process fluid to be cooled is inside the tube. The cooling is accomplished by fans blowing air up through the finned tube nest. Another application is in hydrogen coolers for power plant generators. In this case, the hydrogen is circulated by the rotation of the generator rotor and either condensate or raw cooling water is inside the tubes.

CORRUGATED AND ROPE TUBE
This type of tube has been more widely used abroad than in the U.S. The distinction between corrugated tube and rope tube is that corrugated tube has a single helix configuration, while rope tube has multiple helix configurations similar to that of a manila hemp rope.

In 1980, the Tennessee Valley Authority completely retubed a 300 MW power plant condenser.^(6) It has been reported^(7) that in Europe there is more than 10 years successful experience with different types of configurations in 15 plants ranging in size from 25 to 200 MW.

The advantage of corrugated and rope tube is the improved heat transfer rate over smooth tubes. However, a disadvantage of corrugated and rope tubing is their higher cost and the water- side pressure drop which is almost double that of smooth tube.