How to Specify a Filter Cartridge for a Data Center Liquid Cooling CDU
As AI servers and high-density computing increase the use of liquid cooling, coolant filtration has become an important part of system design. For a Coolant Distribution Unit (CDU) manufacturer or liquid-cooling system integrator, selecting a filter cartridge is not simply a matter of choosing a micron number. The filter must match the cooling loop, coolant chemistry, flow requirement, pressure-drop allowance and installation interface.
A suitable CDU coolant filter cartridge helps control suspended particles in the circulation loop and protects downstream components such as cold plates, heat exchangers, pumps, valves and quick-disconnect connections. However, the correct filter structure and rating depend on the design of the complete system. There is no single micron rating that applies to every data center liquid-cooling installation.
1. Identify the Filter Location in the CDU Cooling System
Before selecting a replacement filter element, identify where it will be installed. In many liquid-to-liquid CDU arrangements, the facility water system (FWS) and the technology cooling system (TCS) are separated by a heat exchanger. The filter location and its purpose can therefore differ between the facility-water side, the technology-cooling side and any side-stream or bypass arrangement.
- Facility-water side: A filter or strainer may be used to control particles entering the CDU heat exchanger from the facility loop.
- Technology-cooling side: A filter element may be installed in the coolant circuit serving the rack, manifold and cold plates.
- Side-stream or bypass line: A separate filtration branch may be used where the system design calls for continuous partial-flow treatment.
These are possible arrangements, not a universal CDU piping diagram. Confirm the actual filter position, flow direction and service method from the equipment drawing. The Open Compute Project's liquid-cooling documents describe the TCS loop and emphasize that filtration requirements depend on system architecture and components.
2. Select the Filtration Rating from the System Requirement
The required filtration rating should be related to the particles that need to be controlled and the narrowest relevant passages in the cooling circuit. Cold-plate microchannels, heat-exchanger gaps, valves and quick-disconnect fittings may have different geometries and contamination sensitivities.
ASHRAE's Water-Cooled Servers: Common Designs, Components, and Processes explains that cold-plate passage dimensions and connector design are important inputs. It gives an example in which a 50 μm passage could be paired with a filter in the range of 25 μm to 5 μm, depending on the intended protection ratio and the pressure drop the system can tolerate. This is an engineering example, not a specification for every CDU.
The Open Compute Project's 2025 Modular Technology Cooling System paper discusses filtration at or below 25 μm for microchannel heat exchangers and describes 25 μm or 50 μm inline filtration combined with finer side-stream filtration as one possible approach. The actual requirement still needs to be confirmed by the CDU and rack-equipment manufacturers.
3. Clarify Nominal Rating, Absolute Rating and Filter Efficiency
Micron ratings are not always directly comparable. ASHRAE explains the difference between nominal and absolute ratings and discusses the beta ratio as a way to describe particle removal at a stated particle size.
- Nominal rating: A rating associated with a stated level of particle capture under a defined basis. The term should be accompanied by the relevant efficiency or test information.
- Absolute rating: A more restrictive description, but the term should still be supported by a defined test method and stated conditions. It should not be treated as a guarantee of identical field performance.
- Beta ratio: A particle-count ratio comparing upstream and downstream counts at a specified particle size. A beta value is meaningful only when the particle size and test basis are stated.
When comparing filter quotations, ask whether the micron value is nominal or absolute, what particle size it refers to, and what test method or efficiency data supports the claim. Do not compare two filter elements solely because both are labelled with the same micron number.
4. Balance Filtration, Flow Rate and Pressure Drop
A finer filter can capture smaller particles, but it can also create a higher pressure drop and may require more filter area or more elements to handle the required flow. For a CDU, filter selection should therefore consider both filtration performance and hydraulic requirements.
Provide the filter manufacturer with the design flow rate, coolant viscosity over the operating temperature range, allowable clean-filter pressure drop and the maximum permitted pressure drop during service. If the system designer has a pump curve or a defined pressure-drop budget for the filter assembly, include it with the inquiry.
Filter placement and configuration also matter. The OCP modular TCS paper discusses the trade-offs between series and parallel arrangements: a series arrangement provides full-flow filtration but can create a higher pressure drop, while a parallel arrangement can reduce pressure drop but does not process the entire flow through the filter at once. These decisions belong to the system design and should not be made from the filter element dimensions alone.
5. Confirm Coolant Chemistry and Wetted-Material Compatibility
Data center liquid-cooling systems may use water with additives, water-glycol mixtures or other engineered cooling liquids. Coolant composition affects material compatibility and may also affect fluid properties such as viscosity. The filter media, support layers, end caps, welds and any wetted connection components must be considered as part of the complete assembly.
The Open Compute Project's cold-plate requirements document states that compatibility between the cooling liquid and all wetted materials needs to be established for the particular installation. A material grade should not be selected only because it is commonly used in industrial filtration.
Wan-Filter can discuss metal material options such as Stainless Steel 304, 316 and 316L, as well as Hastelloy and Monel, subject to the customer's requirements. The correct choice must be checked against the actual coolant formulation, additives, temperature and other wetted materials in the CDU loop. No single alloy should be assumed compatible with every coolant.
6. Match the Filter Structure to the Application
Once the system requirements are clear, the filter structure can be evaluated. Different metal constructions offer different combinations of filtration media, support, geometry and cleanability. The final selection should be based on the required rating, flow, contaminant loading, available space and maintenance plan.
Stainless Steel Pleated Filter Elements
A pleated construction provides a larger geometric media area within a given cartridge envelope than a comparable flat sheet. It may be considered where the equipment design requires a compact element with a defined filtration area. Pleat geometry, support and end connections must match the housing and operating conditions.
Sintered Wire Mesh Filter Elements
Multilayer sintered wire mesh provides a metal filtration structure with integrated support layers. It can be considered when the design calls for a formed metal media structure. The layer arrangement and specified filtration rating should be confirmed for the actual element.
Powder Sintered Metal Filter Elements
Powder-sintered media uses a porous metal structure. It may be evaluated for applications that require a porous metal filtration medium, but pore rating, flow resistance and compatibility must be confirmed for the specific design.
Multilayer Composite Metal Filter Elements
Composite structures combine different metal layers to provide filtration and support functions. The layer sequence, material and geometry should be defined according to the target particles and mechanical requirements.
For pipeline protection upstream of a finer cartridge, a stainless steel filter basket may also be considered if the equipment design includes a suitable basket strainer. Its opening size and construction should be matched to the expected debris and the housing.
Explore Wan-Filter's custom metal filter elements, sintered filter tubes and industrial filter baskets for examples of metal filtration product directions.
7. Confirm Dimensions, End Connections and Flow Direction
A replacement filter element must fit the filter housing and seal correctly. Before requesting a quotation, confirm the following dimensional and interface details:
- Element outside diameter and inside diameter
- Overall length and effective filtration length
- End-cap style, connection type and sealing arrangement
- Flow direction: inside-to-outside or outside-to-inside
- Support core, perforated tube or outer protective structure, if required
- Installation clearance and the space available for element removal
If an existing element is being replaced, a sample or clear photographs with measured dimensions can help clarify the construction. The approved drawing should define the critical dimensions and interfaces before repeat production.
8. Consider Maintenance and Filter-Service Requirements
Filter selection should account for how the element will be inspected, cleaned or replaced. The maintenance method may affect the required media structure, end-cap design and housing arrangement.
For the equipment designer, useful questions include:
- Can the filter be accessed without disturbing unrelated equipment?
- Does the system require a bypass or parallel arrangement during filter service?
- Is differential-pressure monitoring provided by the filter housing or CDU?
- Will the element be replaced, cleaned or regenerated, and what procedure is approved for the coolant and media?
Do not assume that a filter element includes a differential-pressure sensor or a hot-swap function. These are system or housing design features and must be specified separately where required.
9. What Information Should a CDU Manufacturer Send to a Filter Supplier?
A complete technical inquiry helps the manufacturer review the design and reduces avoidable clarification during quotation and sampling. Where possible, provide:
| Information | What to specify |
|---|---|
| Application and location | CDU model or loop position; facility-water side, technology-cooling side or side-stream branch |
| Coolant | Fluid type, product name or composition, glycol concentration and additives where applicable |
| Filtration requirement | Target particle size, micron rating, nominal/absolute basis and required capture efficiency |
| Operating conditions | Flow rate, operating temperature, working pressure and allowable differential pressure |
| Element construction | Pleated mesh, sintered mesh, powder-sintered media, composite layers or another defined structure |
| Material | Required metal grade and any coolant-compatibility restrictions |
| Dimensions and interface | Drawing, outside/inside diameter, length, end caps, connection, sealing and flow direction |
| Order requirements | Sample quantity, production quantity, packaging and any agreed inspection or documentation requirements |
If some values are not yet available, send the current drawing, sample or preliminary equipment information first. The CDU manufacturer or system integrator should confirm the final filtration and operating requirements before the filter design is approved for production.
Custom Metal Filter Elements for Liquid-Cooling Equipment
Wan-Filter manufactures customized metal filter elements for industrial filtration applications. Product structures include pleated metal filter elements, sintered wire mesh filters, sintered felt filters, powder-sintered metal filters, reinforced and multilayer constructions, and other special designs. Material options include Stainless Steel 304, 316 and 316L, as well as selected special alloys according to the project requirements.
For a CDU or data center liquid-cooling project, the filter element should be developed from the customer's drawing, housing interface and confirmed operating conditions. Filtration rating, material compatibility, flow and pressure-drop requirements must be agreed for the specific application rather than assumed from a general product description.
Looking for a Custom CDU Coolant Filter Cartridge?
Send Wan-Filter your filter drawing, existing sample details or preliminary technical requirements. Our team can review the required metal filter structure, dimensions and manufacturing details for your application.
For a faster technical review, include the coolant type, target filtration rating, flow rate, operating temperature, pressure conditions and filter-housing interface.
Frequently Asked Questions
Does every data center liquid-cooling CDU need a 1 μm filter?
No. Filtration requirements depend on the CDU design, coolant, cold-plate and heat-exchanger passages, system cleanliness target, filter location and allowable pressure drop. A 1 μm rating should only be specified when the equipment designer's requirements justify it.
Which metal is best for a CDU coolant filter element?
There is no universal best material. The selected metal must be compatible with the actual coolant formulation, additives, operating temperature and other wetted materials. Confirm material requirements with the coolant and equipment suppliers.
Should a CDU use a pleated or sintered metal filter element?
The choice depends on the required filtration rating, flow, pressure-drop allowance, particle loading, available housing space and maintenance method. Compare the complete element design rather than selecting by media name alone.
What should I provide when requesting a custom filter quotation?
Provide a drawing or sample, filter dimensions, end connections, flow direction, coolant information, filtration rating and rating basis, flow rate, operating temperature, pressure conditions and order quantity. If some information is unknown, send the available drawing or sample first.
Technical references
- ASHRAE TC 9.9 — Water-Cooled Servers: Common Designs, Components, and Processes.
- Open Compute Project — Modular Technology Cooling System for Cloud Scale, Revision 1 (2025).
- Open Compute Project — ACS Liquid Cooling Cold Plate Requirements, Revision 1.0.
Technical note: Published guidance and examples do not replace the CDU manufacturer's approved specifications. Final filter requirements must be confirmed for the specific cooling loop and application.