HEAT TRANSFER DATA

GENERAL
In the design of heat exchangers, the overall heat transfer coefficient (U) in BTU/hr ft^2 degrees F indicates the ability of the tubes to transfer heat from one flowing fluid to another.

The concept of heat transfer in shell and tube heat exchangers can be easily grasped by considering the inverse of the overall heat transfer coefficient (1/U) as a total resistance of the sum of five resistances; tube-side fluid, tube-side scale, tube metal, shell-side scale, and shell side fluid.

Once the minimum wall thickness required by the working pressures is ascertained, the designer will base his selection of tube material and size upon reduction of the five resistances to heat flow (see Figure 8). The ideal tube will resist scale deposit (minimizing rsf and rtf), permit high tube side velocities (minimizing rt) and be usable in thin walls (minimizing rm).

Industry has not yet taken advantage of titanium's excellent fouling resistance. It is so effective that manufacturers of heat exchangers can reduce fouling factors by as much as 75% in some cases (scale deposits from brine commonly account for 30 to 40 percent of total resistance to heat flow). Further, the passive oxide film on titanium permits much higher flow rates without erosion-corrosion.

When titanium tubing is being used to retube an existing exchanger, the interest is to have an overall heat transfer coefficient similar to or better than the original design value. The conductivity of titanium is lower than that of the copper alloys, but is higher than that of the austenitic and ferritic stainless steels. In practice, an improved resistance to fouling, and particularly sulfidation, has been observed. In combination with thinner tube walls, the overall heat transfer coefficient has been more than adequate in all installations. Using the metal resistance values shown in Table 10 (see page 24) it is possible to calculate the expected heat transfer rate a heat exchanger will have if titanium tubes are substituted for tubes of another material. Use of this method is illustrated in Examples 1 and 2.