Closed loop & chilled water treatment

What is a closed loop water system?

A closed loop is a sealed circuit. The same water goes round between a chiller, boiler or heat exchanger and the coils it serves, and only a small amount of make-up water is added to replace what is lost. The three kinds you will find in a Hong Kong building are:

• Chilled water — from the chillers to the air-handling units and fan coils. In a hotel or office tower this is the loop that carries the cooling to every floor; in a hospital it is treated as critical plant.
• Low-pressure heating and hot water loops — from boilers to heating coils and calorifiers.
• Closed condenser loops — where heat is rejected through a sealed circuit rather than an open cooling tower.

That last distinction matters. An open condenser loop through a cooling tower evaporates water, concentrates minerals and breathes air, which is why it needs the scale, corrosion and Legionella programme described under cooling tower treatment, and why the EMSD Code of Practice thresholds are written for fresh water cooling towers. A closed loop has none of the evaporation. What it has is metal, water and — as the next section explains — more oxygen than it should.

We look after closed loops in hotels, office buildings, hospitals and factories, and we flush new tenant pipework in shopping malls before it ties into the landlord's chilled water.

Why does a closed loop still corrode?

Corrosion is an electrochemical reaction. Where water and oxygen meet steel, iron dissolves at tiny anodes on the surface and dissolved oxygen consumes the electrons at the cathode. Take the oxygen away and the reaction starves — which is exactly why a sealed loop should be nearly corrosion-free, and why oxygen control matters so much in closed systems and boilers.

In practice closed loops corrode anyway, and the reason is usually make-up water. Every leak that is quietly topped up brings in fresh oxygen and fresh minerals, and dilutes the inhibitor a little more each time. Four other things make it worse:

• Mixed metals. Copper coils and steel pipework joined in one circuit form a galvanic couple, and the steel pays for it.
• Debris from construction. Mill scale, swarf and flux left in new pipework settle out and start under-deposit attack that a surface inspection cannot see.
• Bacteria. Biofilm creates its own corrosive micro-environment beneath it — microbiologically influenced corrosion — which is why a closed HVAC loop is dosed with a non-oxidising biocide as well as an inhibitor.
• Conductivity. Corrosion is electrical, so the more dissolved solids the make-up water carries in, the faster the cell runs.

The result is rarely dramatic. It is pitting — deep, localised penetration while most of the pipe looks fine — which is why corrosion in a closed loop tends to announce itself as a leak in a ceiling. Deposits do the quieter damage: just 1/32 of an inch on a heat-transfer surface can cut heat exchange effectiveness by over 10%. The chemistry is set out in full in metal corrosion in water systems.

Nitrite or molybdate: which corrosion inhibitor for a closed system?

Both work the same way: they lay down and maintain a thin protective film on the metal so the corrosion cell cannot operate. The difference is what forms the film. Gotech's industrial range carries both types for closed chilled and heating loops.

Which one your loop gets depends on the metals in it, the make-up water, the operating temperature and whether the system carries glycol. That is settled at the free site survey, and the written quotation confirms the product and the residual we will hold it at. Product datasheets and safety data sheets are available on request through the SDS library.

Closed loops with glycol

Some loops carry glycol to lower the freezing point of the water. Glycol changes the job in two ways: the inhibitor has to be compatible with it, and the residual still has to be measured rather than assumed.

All three closed-system inhibitors in the Gotech range are compatible with glycol antifreezes, and Haber A522 is formulated for glycol-dosed systems used to lower the freezing point. The residual is still confirmed by nitrite test on each visit. If you already run glycol, tell us the type and concentration at the survey so the dose and the test range are set for the mixture in your loop, not for plain water.

Start clean: pre-commission flushing for new pipework

New pipework is dirty. It carries mill scale from manufacture, welding debris, swarf, flux residue and installation oils, and if that is still in the loop when the inhibitor goes in, the film forms over debris instead of clean metal. Whatever is left at handover circulates, lodges in strainers and coils, and seeds under-deposit corrosion for the life of the system.

So a new chilled or heating loop is flushed first. Chemical flushing and pre-commission cleaning lifts the debris out at velocity, chemically removes the mill scale and oil that water alone will not shift, passivates the clean metal and hands the system over already inhibited, with a flushing report for the commissioning file. The same applies to a shop fit-out in a mall: new tenant pipework should be flushed before it ties into the building's chilled water.

An existing loop heavily fouled with iron oxide gets the same treatment in miniature: a preoperational cleaner such as Freshol 1960 is circulated for eight hours to several days to remove the oxide and pre-condition the metal before the inhibitor is re-established.

What a closed loop treatment programme includes

1. Free site survey and water analysis. We look at the loop, its metals and its make-up, and test the water before recommending anything.
2. Cleaning or flushing where needed. New pipework is flushed and passivated; a fouled existing loop is cleaned first, because inhibitor dosed over deposits protects the deposits.
3. Inhibitor selection and initial dose. Nitrite or molybdate chemistry chosen for your metals, make-up water and glycol, slug-dosed to the site-specified concentration.
4. Service visits with residual testing. Our technician tests the inhibitor residual, pH and conductivity on site, tops up to the target, doses biocide and records the readings — a film that is allowed to lapse stops protecting.
5. Corrosion coupons. Pre-weighed metal strips placed in the flow and removed after a set period give a measured corrosion rate rather than an opinion.
6. Laboratory analysis where the picture needs it, by a HOKLAS-accredited independent laboratory, reported as a formal dated certificate — see laboratory testing.
7. A dated written report after every visit — what was tested, what it read, what was done — for your maintenance file or group audit.

Where you want the residual watched between visits, real-time monitoring gives continuous data on pH, conductivity and inhibitor residual, with alarms when a reading drifts. Boilers feeding a heating loop are covered under boiler water treatment, and the full programme model is described under treatment programmes.

Signs of trouble in a closed loop

• The inhibitor residual keeps falling. The most important sign. A closed loop whose nitrite keeps disappearing is telling you it is not closed: make-up water is coming in, diluting the inhibitor and bringing oxygen with it. Persistent inhibitor loss means a leak, not a dosing problem — find the leak before buying more chemical.
• Constant topping up. The same message from the other direction.
• Rust-coloured water at drain points. Iron oxide from active corrosion, or from debris that was never flushed out.
• Blocked strainers and coils that will not cool or heat. Deposits travelling to the worst possible places, and heat transfer paying for it.
• A leak in a ceiling. Pitting has finished its work on one pipe. Assume it is under way on others.

If you already have a contractor and still see these, an independent audit gives you a documented view of the loop's condition before you decide anything. If you are losing inhibitor now, message us on WhatsApp and we will come and look — the site visit is free.

Frequently asked questions