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?

Precipitation of aluminium hydroxide (Al(OH)3) scale on heated titanium surfaces in 15% aluminium sulphate (alum) at 70°C. The scale has a poor thermal conductivity (0.2–0.4 W/m·K) compared with 21.9 W/m·K for titanium. For a critical scale thickness of 0.5 mm the heat transfer efficiency is reduced to 50% and the heating time or power consumption to attain the same process temperature is increased by a factor of two. The scale is due to hydrolysis of aluminium ions: Al3+ + 3H2O -> Al(OH)3 + 3H+ The precipitation rate was shown to increase with temperature and aluminium concentration.

Mechanism of Heat Transfer Reduction by Aluminium Hydroxide Scale

The resistance of the titanium wall is added to the thermal resistance of the scale. The total temperature difference is ΔT_total = ΔT_titanium + ΔT_scale. The scale thickness increases the total temperature difference for a given heat flux. For a scale thickness of 0.5 mm the $\Delta T_{scale}$ is almost equal to $\Delta T_{titanium}$, doubling the total temperature drop and reducing heat transmission by a factor of 2. Scale also increases the tube surface temperature, which in turn accelerates more scale precipitation.

Quantitative Heat Transfer Reduction Versus Scale Thickness

The following decreases in heat transfer have been determined through controlled tests at a heat flux of 20 W/cm^2 in 15% Al_2(SO_4)_3 at 70^circ C for grade 2 titanium. At 0 mm scale thickness (clean) the additional thermal resistance is 0, heat transfer reduction is 0 and power increase needed is 1.0×. The additional resistance at a thickness of 0.1 mm is 0.00025–0.0005 m²·K/W, the reduction is 10–20%, and the power increase is 1.1–1.25×. Resistance: 0.0005-0.001 at 0.2 mm. Reduction: 20-35%. Power increase: 1.25-1.5*. 0.3 mm 0.00075-0.0015 30-50% 1.4-2.0×. Critical thickness is 0.5 mm, resistance is 0.00125–0.0025, decrease is 45–65%, power increase is 1.8–2.8×. At 1.0 mm, resistance is 0.0025-0.005, decrease is 65-85% and power increase is 2.8-6.5X.

Growth Rate of Scale Over Time

The scale thickness grows with time according to a parabolic law. After 100 h the scale thickness is 0.05–0.10 mm and the decrease in heat transfer is 5–15 %. After 500 h the thickness is 0.15–0.30 mm and decrease 20–45 %. 1,000 hours 0.25–0.50 mm Reduction 35–65 % Thickness: 0.40 to 0.80 mm Reduction: 50 to 80 percent At 2,000 hours: With time, the growth rate slows down due to the scale layer acting as a diffusion barrier for aluminium ions.

Aluminium Sulphate Heaters: A Guide to Scale Management and Cleaning

Maximum permissible scale thickness and cleaning frequency for Grade 2 titanium tube bundle heaters in 15% aluminium sulphate at 70°C are shown in the accompanying table.

Scale Thickness (mm) Heat Transfer Reduction (%)Recommended Action Method of CleaningCleaning Frequency <0.1 <15 None N/A Annual 0.1–0.2 15–30Performance monitoring Quarterly review N/A
0.2-0.3 30-50 Plan 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

Titanium grade has no effect on scale formation while Grade 7 has superior resistance to under-scale corrosion. Wall thickness offers a heat transmission margin, and a thick wall has a larger baseline ΔT therefore the percentage reduction for a given scale thickness is slightly less. The surface finish of the tube influences the adherence of the scale, and electropolished surfaces (Ra < 0.3 µm) promote scale removal. To reduce scale growth, keep the content of aluminium sulphate below 15 %. The scale generation is reduced by a scale inhibitor such polyacrylic acid in 10 ppm by 30–50 %.

Specify with Certainty

When scale thickness reaches 0.3 mm, the titanium tube bundle heater should be cleaned in 15% aluminium sulphate at 70°C to avoid a decrease in heat transfer of more than 50%. To remove Al(OH)3 scale, use 30 min to 1 hr of 5% hydrochloric acid at 50°C, then rinse thoroughly with water. Put a differential pressure gauge across the heater. If the pressure drop increases by 20% the scale has built up and the heater needs cleaning. Every 500 to 1000 hours for ongoing service. The engineer maintains the heat transfer efficiency above 50% in aluminium sulphate service by regulating the scale thickness below 0.5 mm.