HEAT TRANSFER
The thermal conductivity of titanium is roughly 50% higher than 316 stainless steel as shown in Table 8. This contributes to its excellent heat transfer properties.

The overall heat transfer coefficient, U (Btu/hr./ft.squared/degrees F), indicates the ability of a surface to transfer heat from one flowing fluid on one side to another fluid on the opposite side. The inverse of the coefficient, (1/U), can be considered to be the total resistance to heat flow which, as indicated in Figure 3, is made up of five component resistances: tube-side fluid, rt, tube-side fouling, rtf, tube metal, rm, shell-side fouling, rsf, and shell-side fluid, rs. An ideal tube material will resist fouling (minimizing rtf and rsf), permit high tube side velocities (minimizing rt), and be usable in thinnest section (minimizing rm).

A zero corrosion allowance can often be specified for titanium. This, coupled with adequate strength, permits titanium tubing to be used with unusually thin walls.
The high resistance of titanium to corrosion prevents buildup of corrosion products which rob other metals of heat transfer efficiency. Titanium's hard, smooth surface also minimizes buildup of external fouling films and makes cleaning and maintenance easier. The smooth surface on titanium is also thought to promote dropwise condensation. The result, as shown in Figure 4, is high rates of distillation and condensation for titanium, compared to other metals.

The excellent resistance of titanium to turbulence and erosion- corrosion permits use of relatively high flow rates of 18-22 ft./sec. in silt-laden seawater or even up to 100 ft./sec. in clean seawater without damage to the passive oxide film. Tests in 80 degree F. sea water for 60 days at 25 ft./sec. have shown titanium's corrosion-erosion

resistance to be 80 times better than that of the next-best material, a copper-nickel alloy. Other tests in 85 degrees F. sea water for 60 days at 27 ft./sec. proved titanium to be almost 100 times better than stainless steel, the next-best material.

Putting it all together--the resulting overall heat transfer rate of titanium surfaces is often comparable to that of metals with higher thermal conductivity. The data in Figure 5, for instance, illustrate that the overall heat transfer coefficient of titanium in a desalination environment equaled that of 90-10 copper nickel after a short operating period.

The copper-nickel alloy, due to its higher thermal conductivity, had a higher overall heat transfer coefficient when first placed in service with clean surfaces. However, as fouling due to corrosion product proceeded on the 90-10 alloy with time, the heat transfer coefficient dropped to a value equal to that of titanium which did not experience corrosion product fouling. In these tests, sea water moved at 5 ft./sec. inside 3/4" x 19 gage tubes and steam was condensing on the outside. Had thin-walled titanium tubing been used as is present practice, the heat transfer coefficient for titanium would have been higher than that of the copper-nickel alloy almost from the start.