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?

Scale of aluminium hydroxide (Al(OH)3) is precipitated on heated titanium surfaces in 15% aluminium sulphate (alum) at 70°C. The scale has low thermal conductivity (0.2-0.4W/m.K vs 21.9W/m.K for titanium). A crucial scale thickness of 0.5 mm leads to 50% reduction in heat transfer efficiency, requiring 2× increase in heating time or power consumption.

Quantitative Decrease of Heat Transfer with Scale Thickness

Grade 2 Ti, 15% Al2(SO4)3, 70°C Heat flux 20 W/cm2:

Scale Thickness (mm) Added Thermal Resistance (m2.K/W) Heat Transfer Reduction (%) Power Increase Required
0 (clean) 0 0% 1.0× 0.1 0.00025-0.0005 10-20% 1.1-1.25× 0.2 0.0005-0.001 20-35% 1.25-1.5× 0.3 0.00075-0.0015 30-50% 1.4-2.0× 0.5 0.00125-0.0025 45-65% 1.8-2.8× 1.0 0.0025-0.005 65-85% 2.8-6.5× Scale Growth Rate vs. Time

Time (hours) Scale Thickness (mm) Heat Transfer Reduction (%) 100 0.05-0.10 5-15% 500 0.15-0.30 20-45% 1,000 0.25-0.50 35-65% 2,000 0.40-0.80 50-80%
Scale Management Guide

Scale Thickness (mm) Reduction in Heat TransferSuggested Action Frequency of Cleaning <0.1 <15%Annual 0.1-0.2 15-30% NoneMonitor Quarterly 0.2-0.5 30-60%Clean Now Monthly >0.5 >60% Critical Clean Now Weekly Engineering Recommendation

Clean heater for aluminium sulphate service when scale thickness is 0.3 mm to avoid >50% heat transmission loss. Al(OH)₃ Scale Dissolve in 5% hydrochloric acid at 50ºC for 30 minutes. The engineer measures the thickness of the scale to keep the heat transfer efficient.