For a Titanium Immersion Heater in a 3% Sodium Bicarbonate Solution at 80°C, Why Does the Scale Composition Shift from Calcite to Aragonite When the Wall Temperature Exceeds 95°C, and How Does This Affect Cleaning?

Calcium carbonate (CaCO3) scale deposits on titanium heaters in 3% sodium bicarbonate (NaHCO3) at 80°C. The scale's crystal shape is controlled by the tube wall temperature. Calcite-a rhombohedral, moderately adhering form-is the predominant mineral below 95 C. Aragonite occurs over 95°C in a needle-like, extremely adhering form. Aragonite is tougher and more difficult to remove by acid cleaning. The transition temperature is an important parameter for the choice of cleaning frequency and procedure.

Wall Temperature vs. Quantitative Scale Composition

At different heat flux and bulk temperature of 80 °C:

Wall temperature <90°C: Calcite >80%, aragonite <20%. Scale thickness 0.1-0.3 mm/yr. Removable by HCl 5%.

Wall temperature 90-95°C. Calcite 50-80%. Aragonite 20-50%. Mixed scale. Moderately tough to clean.

At wall temperatures of 95–100 °C, aragonite is >80%. Scale thickness: 0.3-0.6 mm/yr Mechanical cleaning or HCl + inhibitor.

Wall temperature >100°C Aragonite >95%. Very hard, persistent scaling. Hard to get off.

Scale Management Manual

Wall Temperature (°C) Scale Type Cleaning Method RecommendedCleaning Frequency <90 Calcite 5% HCl, 30 min, 50°C Quarterly 90–95Mixed 5% HCl + inhibitor 60 min Monthly 95–100Aragonite Mechanical (brushing) + HCl Bi-weekly >100 Aragonite (hard) Replace heater N/A
Engineering Advice

For sodium bicarbonate service, design heater to maintain wall temperature below 95°C by increasing surface area (lower watt density) or increasing flow velocity. The engineer controls wall temperature to retain calcite scaling for easy clean up.