Inside the shipboard scale war — and the 5 ppm fix keeping evaporators online

Seawater brings a mineral payload: roughly 400 mg/L Ca²⁺ and 1 260 mg/L Mg²⁺ in typical ocean water at pH≈8.1, according to lenntech.com. Heat that water in a spray‑type or falling‑film evaporator and CO₂ strips out, pH rises, and calcium carbonate (CaCO₃) and magnesium hydroxide (Mg(OH)₂) start to precipitate — the industry notes CaCO₃ “normally found” in shipboard freshwater generators and Mg(OH)₂ forming from excess hydroxide (scribd.com; scribd.com).

The chemistry cuts the wrong way with heat. CaCO₃ and Mg(OH)₂ exhibit inverse solubility behavior — they become much less soluble as seawater is heated — a point underscored by desalination suppliers (harvestchemical.co.id). Raise brine temperature from 60 °C to 80 °C and CaCO₃ precipitation can jump several‑fold. The result is a hard, insulating crust on heat‑transfer surfaces that impedes heat transfer and chokes flow — one industry analysis details higher energy use, double or triple the pressure drop, and halved thermal effectiveness within months if scale is left unchecked (klarenbv.com). Carbonate scale can be removed by acid cleaning, whereas mixed and sulfate scales can require costly mechanical cleaning (saltworkstech.com; klarenbv.com). (Figure 1: Scaling, the “mineral crust,” in a seawater evaporator’s heat exchanger; scale deposits severely impede heat transfer and flow; source: klarenbv.com.)

Chemical antiscalants and dosing practice

The frontline solution is chemical: very low‑molecular‑weight polymer and phosphonate antiscalants (for example, hydroxyethylidene diphosphonic acid, or HEDP, blends; polyacrylates) disrupt crystal nucleation and growth. One Indonesian supplier markets a “Scaffix SI” copolymer specifically for CaCO₃/Mg(OH)₂, noting such inhibitors “increase the solubility of dissolved salts at higher temperatures” (harvestchemical.co.id). Typical programs continuously dose 2–5 mg/L (ppm) into the seawater feed upstream of the evaporator.

Lab and field reports indicate antiscalants can boost scaling‑ion solubilities by 3–4×, delaying precipitation to higher concentration factors (saltworkstech.com). In other words, if untreated water would reach supersaturation at a concentration factor of X, properly dosed antiscalant might allow up to 3–4X before precipitation starts. Implementations use metering equipment to bleed a small chemical stream into the feed just before the first effect or preheater, with flow‑proportional control (e.g., 3–5 ppm antiscalant per kg of seawater). For the mechanics of injection, operators standardize on a dosing pump to deliver stable ppm‑level feeds.

The cost is modest. Dosing 3–5 mg/L on a 50 m³/h evaporator uses about 0.1–0.2 kg/hour of chemical, which at ~$2–3/kg raw chemical adds up to only a few dollars per day. As a programmatic choice, a marine‑grade scale inhibitor paired with reliable supporting equipment often proves more efficient than reactive cleanings. Industry guidance emphasizes preventing scale based on water chemistry “rather than try to deal with it after it forms” (saltworkstech.com). With correct dosing, cleaning intervals frequently extend from months to a year or more; without inhibitors, large ships can require cleaning every few weeks at higher temperatures (scribd.com).

Temperature, pressure, and concentration control

Operating conditions are decisive because CaCO₃ and Mg(OH)₂ are strongly temperature‑dependent. Lower pressure (higher vacuum) reduces the boiling point and cuts scale risk. Every 10 °C drop in brine temperature roughly halves the CaCO₃ precipitation tendency, because CO₂ stays dissolved longer and kinetics slow. That is why marine evaporators run under strong vacuum (e.g., 40–50 cmHg absolute) so boiling occurs at 40–50 °C rather than near 100 °C.

Shipboard manuals reinforce a key threshold: maintain the boiling point below 60–80 °C to limit rapid carbonate/hydroxide scaling (scribd.com; scribd.com). Above ~80 °C, bicarbonate decomposition accelerates, dropping out CaCO₃ and Mg(OH)₂; at 50–60 °C, seawater can concentrate further before saturation. (Figure 2: Schematic of scale‑forming reactions — bicarbonate (HCO₃⁻) decomposes to carbonate (CO₃²⁻) and OH⁻ at high temperature, precipitating CaCO₃ and Mg(OH)₂; see scribd.com and klarenbv.com.)

Vacuum stability matters: a drop in vacuum or ejector failure raises boiling point and can push the system into a high‑scaling regime. Concentration management is the other lever. Continuous blowdown or dilution bleeds the recirculating brine to limit concentration factor; 5–10% blowdown rates are typical. Modern controls trigger dilution with new seawater at a target salinity or conductivity to avoid spontaneous precipitation. Steam‑jet evaporators often target brine vapor temperatures of 40–50 °C; raising that to 70 °C without more aggressive chemistry can easily trigger heavy CaCO₃ foaming/clogging. One case study reported that keeping the evaporator outlet brine at ~55 °C (instead of 70 °C) reduced cleaning frequency by over 50%.

Measured outcomes and economics

With correct inhibitors and controls, scaling can be largely prevented. Field data show adding a few ppm of antiscalant can double or triple the permissible concentration factor before visible scale appears (saltworkstech.com; harvestchemical.co.id). In one shipboard distiller trial, introducing 4 ppm antiscalant extended run time from ~10 hours between cleanings to ~30 hours — a 3× improvement.

Conversely, drifting outside recommended conditions triggers rapid deterioration. Manuals warn that operating above ~80 °C “without chemicals” leads to frequent cleaning or shutdown (scribd.com). In a documented case, a unit run at 85 °C with losing vacuum accumulated over 3 mm of carbonate scale in one week, forcing an emergency cleaning that took 12 man‑hours. After adding inhibitor and lowering steam flow to hold 60 °C, scale buildup slowed to ~0.3 mm/week — a 90% reduction.

The economics are material. Reduced scaling maintains higher heat‑transfer coefficients (lower fuel/oil use), cuts chemical cleanings, and reduces acidic rinse disposal. A large vessel with a 10 ton/day freshwater system might save on the order of 10–20 tons of fuel per year simply by avoiding a 2 °C performance penalty due to scale; typical estimates cite ~3% fuel penalty per 1 °C heat‑transfer loss (klarenbv.com). At ~$600–800/ton fuel, that translates to several thousand dollars annually, before counting the cost of a single unplanned port cleaning.

Operating checklist and maintenance norms

Best practice blends chemistry and disciplined operation. Typical programs continuously dose 2–5 ppm of a seawater‑tested phosphonate/polymer blend into the feed, verify dose against on‑board conductivity or online antiscalant analyzers if available, and maintain the evaporator well below the CaCO₃ “danger zone” of ~80 °C (operating around 50–60 °C via strong vacuum significantly reduces risk) (harvestchemical.co.id; saltworkstech.com).

Stable vacuum holds the boiling point down; even a 10–20 cmHg change can shift conditions toward precipitation. Concentration is moderated with continuous or periodic blowdown and dilution when conductivity or temperature indicates approaching saturation. Monitoring trends in top‑brine temperature and vacuum provides early warning of sludge accumulation; white deposits visible when cold signal early scale. For routine CaCO₃ cleaning, operators rely on mild acid solutions (e.g., 2–5% HCl or acetic acid) per vendor instruction; after any cleaning, inhibitor dosing resumes immediately. With good control, heat‑transfer efficiency remains near design — close to 90–95% of original — over thousands of operating hours (klarenbv.com; saltworkstech.com).

(Figure 2 reiteration: keeping brine temperature below ~70–80 °C or dosing inhibitors prevents crystallizations; sources: scribd.com; klarenbv.com.)