CDU Replacement Filter Element Dimensions: What to Include in a Custom Drawing

When a data center liquid cooling system needs a replacement filter element, matching the filtration rating is only part of the job. The replacement element must also fit the filter housing, locate correctly, seal at the intended interface and allow the coolant to follow the designed flow path. For CDU manufacturers and liquid-cooling equipment buyers, a clear dimensional drawing is one of the most useful ways to reduce avoidable specification errors.

This guide explains the physical details to include when requesting a custom metal filter element for a Coolant Distribution Unit (CDU) or another liquid-cooling filtration assembly. It is written for buyers sourcing the replaceable element installed inside a filter housing—not for the purchase of a complete CDU.

Why dimensional accuracy matters in a CDU replacement filter

A filter element is part of an assembled filtration arrangement. Its outside envelope, internal opening, end features and sealing surfaces all interact with the housing. An element that appears close in overall size may still be unsuitable if its end cap is different, its seal sits in the wrong position or its internal support geometry does not match the housing.

The Open Compute Project's liquid-to-liquid CDU test methodology describes the filter as a final protection point against solids or foreign-object debris that could restrict flow through cold-plate microchannels or affect soft components such as O-ring seals. It also emphasizes selecting the appropriate filter material and size. That system-level guidance makes it important to specify the actual replacement element and its interface carefully; it does not establish one universal cartridge dimension for every CDU.

1. Overall length: define exactly where the measurement starts and ends

State the overall length from the same reference surfaces used by the original element or housing drawing. Clarify whether the measurement includes end caps, threaded extensions, locating spigots or other projecting features. If the two ends are different, show each end separately rather than giving only one overall figure.

For a replacement element, the installed length matters as much as the nominal body length. The drawing should identify the surfaces that contact the housing or retaining parts.

2. Outside diameter: include the maximum assembled envelope

Specify the outside diameter (OD) of the finished element. For a pleated construction, clarify whether the dimension refers to the maximum diameter across the pleat peaks or to another defined surface. Also identify any local projections, weld beads, protective layers or support features that could affect insertion into the housing.

Where the housing clearance is limited, a drawing should state the required tolerance rather than relying on a rounded nominal value alone.

3. Inside diameter and central opening

If the element has a central core or open bore, specify its inside diameter (ID), usable depth and end condition. Confirm whether the opening is continuous, closed at one end, stepped or interrupted by an internal support component.

The inside geometry can affect how the element locates in the housing and how fluid enters or leaves the filter. Do not infer the required ID from the outside diameter or from a photograph.

4. End-cap configuration and connection features

Show the top and bottom ends individually. The drawing should identify whether each end is open or closed and describe the actual geometry of any collar, shoulder, thread, slot, locating pin or other engagement feature. If the element uses different end configurations, label them clearly as End A and End B.

Photographs are helpful, but a dimensioned section view is more useful when the connection includes steps, recesses or internal shoulders. If an original part number or housing drawing is available, include it as a reference.

5. Seal location and sealing geometry

Sealing details deserve separate attention. Identify the seal type and its position, including the relevant groove or seating dimensions. Depending on the housing design, the sealing interface may be on an end face or around a cylindrical surface. State the required seal cross-section, groove dimensions and compression arrangement where these are defined by the equipment design.

Also identify the seal material specified by the CDU or coolant-system designer. The compatibility of an elastomer depends on the actual coolant formulation, concentration, temperature and operating conditions. A metal filter manufacturer should not select a seal material based only on the words “water cooling” or “glycol cooling.”

6. Support core, perforated tube and filter-layer build

For a metal filter element, the visible outside dimensions do not describe the complete construction. A drawing or technical note should identify the support arrangement and the intended filter structure. Depending on the design, this may include woven wire mesh, pleated mesh, sintered wire mesh, a perforated support tube or a multilayer composite structure.

Where a multilayer construction is used, identify the layer sequence and which layer is intended to provide the filtration function. The finished dimensions must refer to the assembled element, not only to an individual mesh layer before forming or welding.

7. Flow direction and open/closed ends

Mark the intended flow direction on the drawing or provide a clear note. State whether the design is inside-to-outside or outside-to-inside, if that is specified by the housing arrangement. Identify any end that must remain open, any closed end and any internal passage that must not be obstructed.

Flow direction is not a cosmetic detail. It affects how the element is installed and how its support structure is arranged. If the direction is unknown, confirm it from the housing or equipment documentation before finalizing the drawing.

8. Tolerances, datums and critical dimensions

A production drawing should distinguish nominal dimensions from acceptable tolerances. Mark the dimensions that control fit and sealing as critical. Where practical, use clear datum references so that the manufacturer and buyer measure from the same surfaces.

If the buyer has an existing inspection drawing, provide it with the inquiry. If not, identify which dimensions are essential for assembly and which are reference dimensions only. This avoids treating every dimension as equally critical while overlooking the features that determine fit.

9. Housing clearance and replacement access

Consider the space available when the element is installed and removed. The housing may have enough internal diameter for the filter body but insufficient clearance at a retaining ring, cover, locating feature or extraction path. If the element is replaced during scheduled maintenance, the required removal direction and available service space should be considered in the equipment design.

For a replacement project, a housing cross-section or a photograph of the element installed in the housing can help clarify these interfaces.

Dimensional information checklist for a supplier inquiry

What if you do not have a dimensional drawing?

If a formal drawing is not available, provide the original filter element, its part number, clear photographs from several angles and the housing model or relevant housing drawing. Include a photograph of both end connections and the sealing area. If possible, provide a section view or a simple sketch with the main measurements marked.

A used element may have been compressed, distorted or damaged during removal. For that reason, do not rely only on a damaged sample to establish the original dimensions. Compare it with the housing interface and any available equipment documentation before approving a replacement drawing.

Dimensions are only one part of the technical specification

After the physical interface is clear, provide the operating information needed to select the filter construction: the coolant name and formulation, temperature range, operating pressure, flow rate, filtration target and the location of the filter in the loop. State whether the filtration rating is nominal or absolute and provide the required test basis if a specific efficiency claim is needed.

OCP's liquid-to-liquid CDU methodology gives typical examples of 25–50 μm filtration on the technology cooling side and 150–250 μm on the facility side, depending on design or customer requirements. These are contextual examples from that document, not universal specifications for every CDU or a substitute for the equipment designer's requirements.

Do not choose a finer rating simply because a smaller number sounds more precise. The filter, available flow, allowable pressure drop, contamination load and system design must be considered together.

How Wan-Filter supports custom metal replacement elements

Wan-Filter manufactures customized metal filter elements and filter baskets. The company supplies replacement filtration components rather than complete CDU systems. Production can be discussed against customer drawings, samples and confirmed working conditions.

Available metal-filter construction directions include pleated metal mesh elements, sintered wire mesh elements, metal powder-sintered elements and multilayer composite structures. Material options include stainless steel 304, 316 and 316L, as well as selected alloys such as Hastelloy and Monel where the application requires them. The final material must be checked against the actual coolant and all relevant wetted materials; no alloy should be assumed compatible with every coolant.

The manufacturing workflow may include incoming mesh inspection, material cutting, preparation of pleated or sintered structures, element assembly and end-cap welding, followed by checks of welds and the finished filter condition before packaging. Product dimensions, construction and filtration specifications should be confirmed for each drawing and application.

Frequently asked questions

Are CDU replacement filter elements made to one standard size?
No single cartridge size should be assumed for every CDU. Confirm the housing design and original element interface before ordering.

Can I order a replacement using only the housing diameter?
Usually that is not enough. Overall length, internal opening, end configuration, seal location, flow direction and tolerances may all affect compatibility.

Can a manufacturer work from a physical sample?
A sample can help establish the construction and interface, especially when accompanied by clear photographs, the housing information and the required operating conditions. A damaged or deformed sample should be checked against the housing before a drawing is approved.

Does a smaller micron number mean the replacement is automatically better?
No. The filtration rating must match the system's particle-control objective, flow and allowable pressure drop. The rating basis and supporting test method should be stated clearly.