Scaling in Evaporators and Crystallizers: Supersaturation Is the Only Lever That Matters
Scale forms when the liquor is supersaturated at the heat transfer surface. You cannot eliminate supersaturation — that is how crystallization works — but you can control where it is released. Plants that foul weekly are almost always releasing it on the tubes instead of in the bulk.
Ranked causes
| Cause | Evidence | Fix |
|---|---|---|
| Excessive ΔT | Hard scale on the hottest tubes; capacity falls as scale grows | Lower steam temperature; increase surface area rather than ΔT |
| No seed / low seed | Fines carry-over, sudden nucleation, erratic crystal size | Maintain seed inventory; classify and return fines |
| Insufficient circulation | Scale concentrated in low-velocity zones | Raise circulation velocity above 1.5–2 m/s in tubes |
| pH drift | Scale composition changes; carbonate or sulphate bands | Control pH within a narrow band |
| Hardness in feed | Calcium/magnesium scale independent of product | Pre-soften or purge; consider antiscalant |
Why ΔT is the usual culprit
Operators raise steam temperature to recover capacity. That increases the supersaturation at the wall faster than the bulk can absorb it, so more scale forms, capacity falls further, and the loop tightens until the unit is cleaned weekly. The fix is more surface area or lower throughput — not more heat.
Seed and classification
Crystals in suspension give the supersaturation somewhere to go. A crystallizer with adequate seed and a fines-destruction or classification loop produces larger, more uniform crystals and far less wall scale. If your crystal size distribution is bimodal with a heavy fines peak, seed control is the problem.
Cleaning is a design parameter
Specify cleaning access from the start: flanged bends, spray balls, and a CIP route that reaches the tube sheet. A unit that takes three days to clean will be run past its fouling limit because production pressure wins.
Send your liquor analysis, duty, ΔT and current cleaning interval, and we will propose a circulation and seeding arrangement that stretches the interval.