In brief
Coolant chemistry and cleanliness must suit the cold plates, seals, metals and CDU components in the actual system. Set conductivity, filtration and sampling limits from the approved IT, CDU and fluid supplier specifications. There is no universal threshold for every loop.
A liquid cooling loop looks simple. A closed circuit, a pump, a heat exchanger and water. In practice it is one of the more demanding fluid systems built into a data center, and its service life is rarely decided by the mechanics. It is decided by what circulates inside it and what it is made of.
The reason is scale. Inside a processor cold plate the flow channels are on the order of tenths of a millimetre, and they are deliberately tight so that heat transfers as efficiently as possible. That same tightness makes them vulnerable. A particle that would cause no trouble anywhere else in the plant blocks a channel and does not come back out. This article covers how the loop is kept serviceable over its whole life. The wider picture is in our complete guide to data center cooling.
What a buyer should provide
Share the approved coolant specification, wetted-material list, filter arrangement, OEM cleanliness criteria and flushing acceptance plan. Finkova can discuss the filling, sampling and documentation interfaces.
Three risks, all of them manageable
Problems in a liquid loop almost always trace back to three things.
Particles. Weld scale, swarf, gasket debris from installation, or material released by corrosion travels with the flow into the narrowest point and stays there. Beyond blocking cold plate channels, particles lodge in quick disconnect sealing faces and cause leaks.
Corrosion. A loop typically contains several metals: copper in the cold plates, stainless steel in the pipework and heat exchanger, sometimes aluminium. Dissimilar metals in a fluid form a galvanic pair in which the less noble metal corrodes. Pitting and crevice corrosion also occur, particularly at joints and seals.
Microbial growth. Warm water that moves slowly or stands still is an ideal growth medium. The resulting biofilm blocks channels, degrades heat transfer and causes corrosion beneath the deposit, where it goes unnoticed until the damage is done.
All three are preventable, but only if they are resolved at design stage. Fixing them afterwards means opening the loop and, at worst, replacing cold plates.
Two loops, two levels of requirement
The server loop (TCS) and the facility loop (FWS) carry different requirements, because they differ in both construction and risk.
The TCS is a closed loop filled with an OEM-approved coolant. Its conditions are stable but its specification is tight, because the fluid passes through the cold plates. The FWS is a larger and more variable loop whose requirements depend on how heat is rejected and on local water quality.
The acceptable pH range can differ between FWS and TCS and depends on the approved coolant and wetted materials. What actually governs, though, is the figure given by the hardware manufacturer, the CDU supplier and the fluid supplier, not a general guideline. Those figures belong in the project documentation, not in an installer’s memory.
What to monitor in the fluid
Depending on the fluid, monitor glycol concentration, pH, reserve alkalinity, appearance, inhibitor level, hardness, chloride, sulphate, degradation products, dissolved metals, conductivity and oxygen. Select test frequency and alarm limits from the approved OEM and fluid supplier criteria.
Conductivity is the fastest and cheapest of these to measure continuously, because a rise indicates contamination before the other values have moved. Set conductivity targets and alarms for the actual coolant and materials, based on the approved OEM and fluid supplier specifications. One numerical threshold does not suit every loop. Most CDUs already have conductivity measurement built in, so this is mainly a question of setting alarm limits and watching them.
The first samples are taken at commissioning. That baseline is what every later result is compared against. Without it, a single analysis gives you a value but not a direction.
Material compatibility is settled on paper, not on site
The materials that come into contact with the fluid are defined as a list in the project, and both ASHRAE and the Open Compute Project publish ready-made tables for this. The list covers seals, hoses, membranes and filter media alongside the metals.
For metals, the single most important rule is to avoid aluminium and copper in the same loop. That is the classic galvanic pair: the aluminium corrodes, and the corrosion products travel straight into the cold plate channels. For elastomers the question is whether the seal will withstand the specific fluid and temperature in use.
The practical risk is not at the design desk but in installation and maintenance. One wrong seal or hose can leach material into the whole loop, and the consequences appear months later in an analysis or in a filter differential pressure. This is why the materials list belongs in the hands of the installer and the service engineer, not only in the designer’s folder.
Filtration
Filtration is sized to the narrowest point in the loop, not to the pipe size. The CDU’s standard filter may not meet a particular cold-plate cleanliness requirement. Confirm the filter rating, any side-stream filtration and its flow share against the IT and CDU manufacturers’ approved specifications.
Filter ratings are given as either absolute or nominal. ASHRAE recommends an absolute rating for the server loop, because a nominal rating describes only average retention and does not guarantee that larger particles are stopped. The distinction is not hair-splitting: it decides whether a particle reaches a cold plate.
Filters are fitted with pressure measurement before and after. A rise in differential pressure is the first sign that something is moving in the loop that should not be. The filter medium itself also has to appear on the approved materials list.
Flushing, filling and dead legs
Loop cleanliness is settled before the equipment is connected. Flushing is done before the cold plates are attached, and cleanliness is verified by measurement rather than by eye. This is the single most important quality step in a liquid cooling project, and it is also the step most often left short when the schedule tightens.
The loop has to be designed to drain completely, with no dead legs where fluid does not move. Standing fluid is exactly where microbial growth starts, and where a biocide does not reach because it does not circulate that far.
Oxygen enters even a closed loop by several routes: with make-up water, through gas-permeable hose materials, via the expansion vessel, and every time the loop is opened for maintenance. Make-up water is the most common of these. If the loop is topped up with untreated mains water, the fluid specification loses its meaning.
The maintenance programme
A programme that works in practice contains the following:
- Sample fluid at the OEM-approved interval and after relevant maintenance.
- Monitor conductivity, pH and inhibitor content where specified.
- Trend filter differential pressure and make-up fluid demand.
- Keep the approved materials and fluid specification available to service personnel.
Of these, make-up water quantity is the one most often left untracked, even though it is the cheapest and the most informative. A slowly rising top-up demand is a leak that has not yet reached the floor.
Common mistakes
Topping the loop up with mains water. A quick and easy way to void the entire fluid specification.
Replacing a seal or hose with whatever is on the shelf. The materials list exists precisely for this moment.
Analysing in the first year and then forgetting. Inhibitors deplete gradually, and their exhaustion shows no symptom until damage has already started.
Mixing fluids from different manufacturers. The additive chemistries may not be compatible, and nobody knows the properties of the mixture.
Changing filters on a schedule without watching differential pressure. You then know neither whether the change was overdue nor whether it was unnecessary.
Cutting the flush short for schedule reasons. The cost of this surfaces in production, and by then it is the cost of cold plates.
Loop architecture and the role of the CDU are covered in the article on CDUs in the data center, and the differences between cooling methods in liquid cooling in the data center.
Finkova’s scope and next step
Finkova installs and commissions data center cooling in the Nordics, with instrumentation and lifecycle support. The work scope, OEM requirements and acceptance criteria are agreed for each project.
Contact Finkova with the project inputs above to discuss the relevant installation, testing and handover interfaces.
Frequently asked questions
Why does water chemistry matter in liquid cooling?
Cold plate flow channels are on the order of tenths of a millimetre, which makes them vulnerable to particles, corrosion and microbial growth. The service life of a liquid loop is rarely decided by the mechanics. It is decided by what circulates inside it and what it is made of.
What is monitored in the fluid of a liquid cooling loop?
Typically glycol concentration, pH and reserve alkalinity, appearance, inhibitor concentration, total hardness, chloride and sulphate, degradation acids, dissolved metals, conductivity and dissolved oxygen. Conductivity is the fastest and cheapest of these to monitor continuously.
What pH is appropriate in a liquid cooling loop?
The acceptable pH range can differ between FWS and TCS and depends on the approved coolant and wetted materials. What actually governs, though, is the figure from the hardware manufacturer, CDU supplier and fluid supplier. Those figures belong in the project documentation.
What is the conductivity limit?
There is no universal conductivity limit across coolant types. Set the target and alarm level against the approved OEM and fluid supplier specifications.
Why should copper and aluminium not share a loop?
Copper and aluminium form a galvanic pair in a fluid, and the aluminium corrodes. The corrosion products travel with the flow straight into the cold plate channels and block them. This is the single most important materials rule for a liquid cooling loop.
How fine does liquid cooling filtration need to be?
A CDU’s standard filter may not meet the cold-plate cleanliness specification. Confirm the need for side-stream filtration with the OEM. Select filter rating and side-stream flow against the approved IT and CDU equipment specification.
What is side-stream filtration?
In side-stream filtration part of the flow is continuously routed through a considerably finer filter. The fluid in the whole loop is gradually brought to the finer level without creating a large pressure drop in the main flow. Liquid cooling needs it because cold plate channels are tighter than standard CDU filtration covers.
What is the difference between absolute and nominal filter ratings?
A nominal rating describes a filter’s average retention but does not guarantee that larger particles are stopped. An absolute rating does. ASHRAE recommends an absolute rating for the server loop, because the distinction decides whether a particle reaches a cold plate.
Can a liquid cooling loop be topped up with mains water?
No. Untreated mains water introduces oxygen, hardness and contaminants, and the whole fluid specification loses its meaning. The loop is topped up only with the specified fluid. Mixing fluids from different manufacturers should also be avoided, as their additive chemistries may not be compatible.
How often is fluid analysed?
Typically twice a year, and always after the loop has been opened. Conductivity and pH are also monitored continuously from the CDU instrumentation. Inhibitors deplete gradually and their exhaustion shows no symptom until damage has already started, so regular analysis is essential.
