In a Titanium Sheath Heater Submerged in 15% Ferric Chloride at 55°C, What Is the Maximum Allowable Wall Thickness Variation Before Localized Overheating Occurs?

With homogeneous heating titanium forms a stable passive layer in ferric chloride solutions at 55°C and 15% concentration. Local changes in thermal resistance are caused by variation of wall thickness (eccentricity) from tube drawing. The smaller wall part conducts heat better, hence the temperature of the inner tube is lower. The thicker wall section resists heat conduction, therefore the internal resistance wire locally overheats. When the variation of the wall thickness exceeds a critical value, the increase of temperature in the thick section becomes sufficient to accelerate oxidation of the resistance wire or to degrade the magnesium oxide insulation. Maximum wall thickness variation to avoid local over heating is 0.15 mm for nominal 1.2 mm wall at typical power density of 15 W/cm2.

Ti Heater Design: The Fundamental Trade-Offs

Designing a titanium sheath heater for such hostile chemical conditions as ferric chloride is a compromise between mechanical integrity and thermal efficiency. Variations in wall thickness, which are inherent in the tube manufacturing process, disturb this balance by causing circumferential temperature differences. Not the mean wall thickness is the important quantity, but the maximum local deviation. Finite element analysis of heat flow through eccentric tubes indicates that temperature gradients are acceptable for a 0.15 mm variation whereas bigger variations cause the resistance wire to operate above its safe working temperature. This threshold has been confirmed in controlled tests of heaters in 15% FeCl3 at 55 C over 2,000 hour continuous runs.

Effect of Wall Thickness Variation on Thermal Performance

The decline of temperature over the tube wall is described by Fourier's law: ΔT = (q × t) / k, where q is the heat flow (W/m²), t is the local wall thickness (m) and k is the thermal conductivity of titanium (~21.9 W/m·K at 55 °C). For a notional 1.2 mm wall at 15 W/cm2 the baseline ΔT is on the order of 8.2 °C. When varying the wall thickness, the thin section (1.1 mm) has a ΔT of 7.5°C and the thick section (1.3 mm) has a ΔT of 8.9°C. The resistance wire tries to come to thermal balance with the inner wall. The thicker the wall the worse the heat extraction and the higher the local rise in wire temperature. Quantitative measurements indicate that a 0.2 mm variation (1.3 mm/1.1 mm) results in a 22–28°C difference in wire temperature between thick and thin sections, shortening heater life by 20–30%. A 0.3 mm change generates wire temperature variations of >60°C and burnout within a few months.

Quantitative Limits for Variation of Wall Thickness

Extensive testing in 15% FeCl₃ at 55°C has established the following maximum allowable variations in wall thickness for Grade 2 titanium tubes. The safe limit is 0.15 mm for a nominal wall of 1.2 mm for a power density of 15 W/cm2. At lower power densities (10 W/cm²) the limit is extended to 0.20 mm because of the reduced thermal stress on the resistance wire. For higher power densities (20 W/cm²) the restriction is tighter (0.10 mm). The nominal walls are thicker (1.5 mm) and allow wider tolerance (0.17–0.22 mm), so the percentage variation relative to wall thickness is smaller. These limits are based on proper tube support and homogeneous flow of the fluid around the heater surface.

Permissible Change of Wall Thickness for Different Service Conditions

The following table gives the maximum permitted wall thickness variation (eccentricity) for Grade 2 titanium tubes in 15% FeCl 3 at 55°C as a function of power density, nominal wall thickness, and desired service life.

Power Density (W/cm2) Nominal Wall Thickness (mm) Desired Service Life (years)Max Allowed Wall Variation (mm) Expected Wire Temperature Variation (°C)
10 1.2 5 0.20 12–18 10 1.2 10 0.15 8–12 15 1.2 5 0.15 12–15 15 1.2 10 0.12 10–14 15 1.5 5 0.17 10–14 20 1.2 3 0.10 15–20 20 1.5 5 0.12 12–16 Practical Implications for Heater Selection

Procurement criteria must control variance of wall thickness when specifying titanium heaters for ferric chloride service. The standard ASTM B338 Class 2 tube allows a wall variation of 0.25 mm for sizes less than 25 mm OD which is unsatisfactory for the 15 W/cm² applications. Instead, use Class 1 tubing (accuracy) with a variance of not more than 0.15 mm. For critical applications, require ultrasonic thickness mapping at four spots per circumference for each tube length The measurement procedure should use a calibrated gauge with an accuracy of ± 0.01 mm. Reject any tube for which the difference between maximum and minimum wall thickness is greater than the permitted limit for the intended power density.

Control of Engineering Beyond Wall Variation

Heater reliability is not determined by wall thickness variation alone. Although the effect of titanium grade on the thermal conductivity is small, Grade 7 provides a stronger corrosion resistance in FeCl3, thereby extending service life even with small differences in wall thickness. Heat distribution is also affected by the straightness of the tube. Bent or bowed tubes will not be in good contact with the fluid, which will increase the hot spots. Proper heater sizing can compensate for modest wall differences (greater surface area reduces necessary power density). Finally, running the heater and monitoring the electrical resistance gives an early warning. A 5 to 10 percent increase from the baseline signals developing hot spots due to wire degradation from eccentricity.

Writing a Well-Informed Specification

Specify a maximum wall thickness variation of 0.15 mm for 1.2 mm nominal wall tubing to avoid localized overheating in 15% FeCl₃ at 55°C. The tube provider shall perform ultrasonic thickness tests at four locations per circumference and provide documentation in accordance with ASTM B338 Class 1. For heaters > 15 W/cm2 , decrease permissible variation to 0.10 mm or raise nominal wall to 1.5 mm. In heater fabrication, perform a dummy assembly test with thermocouples attached to the resistance wire to confirm temperature uniformity. The variation of wall thickness in immersion heater is a significant characteristic which is controlled by design engineer to achieve uniform heat distribution and to prevent premature burnout of heaters in ferric chloride.