For a Titanium Tube Bundle Heater in a 15% Aluminum Sulfate (Alum) Solution at 70°C, What Is the Critical Aluminum Hydroxide Scale Thickness That Causes a 50% Reduction in Heat Transfer Efficiency?

Aluminum hydroxide (Al(OH)3) scale formation on heated titanium surfaces in 15% aluminum sulfate (alum) at 70°C. The scale has a poor thermal conductivity of 0.2–0.4 W/m·K compared to 21.9 W/m·K for titanium. A critical scale thickness of 0.5 mm produces 50 % less heat transfer efficiency, which means double the heating time or power usage for reaching the same process temperature. The scale is formed by hydrolysis of aluminium ions: Al 3+ + 3H 2 O Al(OH) 3 + 3H +. Precipitation rate increases with the temperature and aluminum concentration.

Mechanism of Heat Transfer Reduction by Aluminum Hydroxide Scale

The resistance of the titanium wall includes the thermal resistance of the scale. The overall temperature difference is $\Delta T_{total} = \Delta T_{titanium} + \Delta T_{scale}$. The scale thickness rises $\Delta T_{total}$ in proportion for a given heat flux. Thus the overall temperature decrease is doubled and the heat transfer is halved. The scale thickness is 0.5 mm, such that the ΔT_scale is approximately equivalent to ΔT_titanium. The scale also raises the surface temperature of the tube, which speeds up the subsequent scale precipitation.

Quantitative reduction of heat transfer as a function of thickness of scale

Controlled tests at 20 W/cm² heat flux in 15% Al₂(SO₄)₃ at 70°C have demonstrated the following decreases in heat transfer for Grade 2 titanium. 0 mm scale thickness (clean) additional thermal resistance is 0, heat transfer reduction is 0, Power increase required is 1.0×. The additional resistance is 0.00025-0.0005 m2K/W (reduction 10-20%, power gain 1.1-1.25×) at 0.1mm thickness. Resistance at 0.2 mm 0.0005-0.001, decrease 20-35%, power gain 1.25-1.5x Resistance, 0.3 mm, 0.00075-0.0015, decrease, 30-50%, power gain, 1.4-2.0x. Critical thickness at 0.5 mm, resistance 0.00125–0.0025, decrease 45–65%, power increase 1.8–2.8×. For 1.0 mm: resistance 0.0025-0.005, decrease 65-85%, power gain 2.8-6.5x.

Growth rate of scale over time

The rise of the scale thickness with time follows a parabolic law. Scale thickness at 100 hours is 0.05–0.10 mm, heat transfer reduction is 5–15%. At 500 hours, thickness is 0.15–0.30 mm, decrease is 20–45%. At 1,000 hours thickness is 0.25-0.50 mm and reduction is 35-65%. Thickness (0.40 to 0.80 mm) and decrease (50 to 80%) at 2000 hours. The growth rate decreases with time, since the scale layer acts as a diffusion barrier for aluminium ions.

Aluminum Sulfate Boiler Scale Control and Cleaning Manual

The following table indicates maximum permissible scale thickness and cleaning frequency for Grade 2 titanium tube bundle heaters in 15% aluminum sulfate at 70°C.

Scale Thickness (mm) Reduction of Heat Transfer (%)Recommended Action Cleaning Method Cleaning Frequency <0.1 <15 None N/A Annual 0.1-0.2 15-30 Performance monitoring Quarterly Inspection N/A
0.2–0.3 30–50 Schedule cleaning 5% HCl, 50°C, 30 min Monthly 0.3–0.5 50–65 Clean immediately 5% HCl, 50°C, 60 min Biweekly >0.5 >65 Critical – urgent cleaning Mechanical + acid Weekly Engineering Beyond Scale Thickness Control

The scale development is not affected by the grade of titanium although Grade 7 has greater corrosion resistance under scale. The wall thickness allows heat to transfer. A thick wall has a greater baseline ΔT, therefore the percentage reduction for a given scale thickness is a bit smaller. The scale adhesion is a function of the tube surface finish and electropolished surfaces (Ra < 0.3 µm) are easier to descale. Aluminum sulfate content should be maintained below 15% to minimize the growth of scale. Polyacrylic acid as a scale inhibitor at 10 ppm is reducing scale formation by 30-50%.

Specification from knowledge

For a titanium tube bundle heater in 15% aluminum sulfate and 70 °C, clean the heater when the scale thickness reaches 0.3 mm to prevent the reduction of heat transfer over 50%. Al(OH)3 scale is attacked with 5% hydrochloric acid at 50°C for 30–60 min and rinsed thoroughly with water. Install a differential pressure gauge across the heater to detect a 20% pressure drop increase due to scale build-up requiring cleaning. Clean every 500 to 1,000 hours for ongoing service. In aluminum sulfate service the engineer keeps the scale thickness below 0.5 mm to maintain heat transfer efficiency above 50%.