Why Heat Exchangers Need Descaling
Marine heat exchangers operate in some of the most demanding scaling environments on earth. Seawater carries dissolved calcium, magnesium and other minerals that precipitate as hard calcium carbonate scale on heat transfer surfaces as water temperature rises across the exchanger.
Scale buildup acts as an insulator — even a thin layer of 1–2mm can reduce heat transfer efficiency by 20–40%. Left untreated, scale accumulation leads to:
- Reduced cooling capacity — engine, generator or process temperatures rise beyond normal operating range
- Increased fuel consumption — cooling systems work harder to compensate for lost efficiency
- Accelerated corrosion — scale traps chlorides against metal surfaces, promoting under-deposit corrosion
- Equipment failure — severe cases can cause tube blockage, plate damage or catastrophic overheating
Regular chemical descaling is the most effective and least disruptive maintenance method — restoring heat transfer efficiency without dismantling or replacing components.
Types of Marine Heat Exchangers
The descaling method applies to all common marine heat exchanger types, with minor variations in setup:
Plate Heat Exchangers (PHE)
Gasketed plate packs (Alfa Laval, APV, Sondex). Most can be descaled in-situ by circulating acid solution through both sides. Gaskets are compatible with phosphoric acid at working concentrations.
Shell & Tube Exchangers
Common in engine cooling circuits. Circulate descaler through the tube side (seawater) while the shell side remains isolated. For severe scaling, the tube bundle can be removed and soaked.
Box Coolers
Mounted in the vessel sea chest. Descale by sealing the sea chest and flooding with descaling solution, or by fitting temporary circulation connections to the box cooler headers.
Keel Coolers
External hull-mounted coolers. Descale by circulating acid solution through the internal channels via the engine room connections while the vessel is afloat.
Choosing the Right Descaler
Not all descaling acids are equal. The choice of acid affects cleaning speed, material compatibility, corrosion risk, environmental impact and logistics:
| Factor | Phosphoric Acid | Hydrochloric Acid |
|---|---|---|
| Scale Dissolution | Effective on calcium carbonate, lime, light rust | Fast on calcium carbonate |
| Metal Passivation | Yes — forms protective iron phosphate layer | No — can cause flash rusting |
| Titanium Compatibility | Safe at working concentrations | Risk of pitting corrosion |
| Copper/CuNi Compatibility | Safe — protective layer reforms in service | Aggressive — requires inhibitor |
| Fume Generation | Minimal | Produces HCl vapour — ventilation required |
| Transport Classification | Formulation-dependent — CFS Descaler RTU is Non-DG, concentrate is DG | Typically Class 8 Corrosive |
| Environmental Impact | Spent solution is calcium phosphate (fertiliser component) | Spent solution contains chlorides |
CFS Descaler uses food-grade phosphoric acid with a proprietary corrosion inhibitor package. The 4:1 concentrate ratio means one 20L drum produces 100L of working solution — significantly reducing freight costs versus ready-to-use alternatives.
Step-by-Step: CIP Descaling Procedure
Clean-in-place (CIP) descaling is the standard method for most marine heat exchangers. Here is the full procedure:
Shut off seawater supply valves. Close isolation valves on both sides of the heat exchanger. Drain the seawater side if possible. Ensure the process side (freshwater/glycol) remains closed to prevent contamination.
Connect a portable descaling pump to the heat exchanger inlet and outlet via flexible hoses. Use a suitable mixing tank or drum. Ensure all connections are secure and check for leaks before adding chemicals.
Fill the mixing tank with fresh water. Add CFS Descaler at 4:1 ratio (1 part product to 4 parts water). For heavy scale, use 2:1 ratio. Always add product to water, not water to product. Calculate total volume needed to fill the exchanger and circulation loop.
Start the pump and circulate at a moderate flow rate. Avoid high pressure — the goal is consistent contact time, not mechanical force. Ensure the solution flows through all passages. For plate heat exchangers, circulate through both channels if scale is present on both sides.
Active descaling produces CO₂ gas — visible as bubbling in the return flow or mixing tank. Monitor solution pH periodically: active cleaning shows pH below 2. When pH stabilises above 5–6, the acid has reacted with available scale and treatment is nearing completion. Typical treatment time is 1–4 hours.
Drain the spent solution. Flush the system with clean fresh water — minimum 2x the system volume. This removes dissolved scale residue and any remaining acid. Dispose of spent solution according to local environmental regulations.
If accessible, visually inspect heat transfer surfaces. They should be clean and free of scale. Phosphoric acid-treated surfaces will show a light grey iron phosphate conversion layer — this is beneficial passive corrosion protection. Reconnect the system, open isolation valves and return to service.
Pro Tips for Better Results
- Warm water improves speed — dissolving the descaler in water at 30–40°C accelerates the reaction. Do not exceed 60°C.
- Reverse flow direction — mid-way through treatment, reverse the circulation direction to ensure even coverage of all surfaces.
- Top up if needed — if the solution pH rises above 5 but scale remains, drain and prepare a fresh batch. Severely scaled systems may require two cycles.
- Document your results — record before/after differential pressures and coolant temperatures. This data helps establish optimal descaling intervals for your equipment.
- Consider a descaling schedule — preventative descaling at 6–12 month intervals is more effective and less costly than reactive descaling after performance has degraded.
Frequently Asked Questions
How often should marine heat exchangers be descaled?
Descaling frequency depends on water conditions and operating hours. In tropical or high-sediment waters, descaling every 6–12 months is typical. In temperate waters with moderate hardness, annual or biannual descaling is usually sufficient. Monitor differential pressure and coolant temperatures — rising ΔP or declining cooling performance are the clearest indicators that scale has built up.
Can I descale a heat exchanger without removing it?
Yes. Clean-in-place (CIP) descaling is the most common method for marine heat exchangers. Isolate the exchanger, connect a portable recirculation pump to the inlet and outlet, and circulate the descaling solution through the system. This reaches internal passages that mechanical methods cannot access.
What is the best descaler for titanium heat exchangers?
Phosphoric acid-based descalers like CFS Descaler are recommended for titanium heat exchangers. Phosphoric acid is compatible with titanium, stainless steel and copper alloys at working concentrations. Hydrochloric acid descalers can cause pitting corrosion on some alloys and are not recommended for titanium without specialist advice.
How do I know when descaling is complete?
During chemical descaling, the acid reacts with calcium carbonate scale producing CO₂ gas — visible as bubbling or fizzing in the solution. When the reaction ceases (no more bubbling), the available scale has been dissolved. You can also monitor solution pH: active cleaning shows pH below 2, and when pH stabilises above 5–6, the acid has been consumed.
Is phosphoric acid descaler safe for the environment?
Spent phosphoric acid descaling solution (after reacting with calcium carbonate) is largely calcium phosphate in water — a common agricultural fertiliser component. Dispose of spent solution according to local environmental regulations. Phosphoric acid is generally considered less hazardous than hydrochloric or sulphuric acid alternatives.
Ready to Descale?
CFS Descaler is available in 5L, 20L, 200L and 1000L pack sizes. Contact us for pricing, technical advice or to request a sample.