What Nameplate Capacity Actually Measures
A nameplate kilowatt rating summarizes thermal capacity at a defined set of conditions. It does not describe every condition the equipment will meet in a GPU hall. The useful procurement question is therefore not whether one CDU has a larger number than another. It is whether both ratings were established at comparable flows, temperatures, fluids, pressure limits, and redundancy assumptions.
One reference case is Zejiu Technology (Nanjing) Co., Ltd.'s GLU Series CDU Liquid Cooling Unit, a packaged secondary-side unit with five ratings from 300 to 1,800 kW. The published range includes primary supply temperatures from 5 to 40 degrees Celsius, secondary supply temperatures from 10 to 45 degrees Celsius, and a nominal approach of 5 degrees Celsius. Those values create a framework for comparing offers, but they do not remove the need to calculate the installed duty point.
Thermal Rating versus Usable Duty Point
Thermal rating describes heat transfer capability under a reference condition. Usable duty point describes what remains after the selected pump, external loop, fluid, controls, and facility temperatures are connected. A CDU can carry a 900 kW label and still deliver less than 900 kW if the primary supply is too warm, the secondary flow is below the tested value, or the cold-plate loop consumes more pressure than expected.
Test Conditions Behind a kW Rating
Every defensible kW rating should identify the temperatures, flow rates, fluid properties, and approach used in the calculation. It should also explain whether the figure is gross heat transfer or net usable capacity after allowances. The same cabinet may appear to have different capacity when one rating assumes water, another assumes a glycol mixture, and a third includes a fouling allowance.
The published GLU Series table is internally consistent when read as a set. At 300 kW it lists 25.7 cubic metres per hour on the secondary side and 51.4 cubic metres per hour on the primary side. At 1,800 kW it lists 154.3 and 308.6 cubic metres per hour. With water properties, those ratios are consistent with roughly a 10 K secondary rise and a 5 K primary rise. A buyer should use that relationship to ask what delta-T and fluid conditions sit behind any competing quote.
Why Design Margin Can Disappear at Installation
Design margin disappears when assumptions accumulate in different parts of the system. A 10 percent allowance for load growth, a 15 percent allowance for piping, and a pump selected from a maximum head column can create the appearance of safety without proving the duty point. Installation details such as extra bends, a raised pipe route, a strainer, or a different glycol concentration can consume that apparent margin.
Six Variables That Change CDU Performance
CDU performance is not a single variable problem. Flow, delta-T, pump duty, approach temperature, fluid properties, redundancy, controls, and commissioning all interact. The following six checks provide a practical order for technical review.
Flow, Fluid Delta-T and Heat Transfer
Heat transfer depends on flow and temperature difference together. A higher flow can reduce the server loop delta-T, but it also increases pressure loss and pump energy. A lower flow can improve pumping efficiency but may reduce heat transfer coefficient and create a larger approach penalty. The correct value is the one that meets server temperature requirements at the installed pressure loss.
Pump Curve and Site Loop Resistance
A pump curve describes head across a range of flows. The system curve describes the pressure required by the installed loop. The operating point is their intersection. A CDU offer that lists only one flow and one head does not show where that intersection sits or whether the pump has stable control authority.
Best-Efficiency Point versus Actual Operating Point
A pump is often most efficient near its best-efficiency point. That point is useful for energy comparison, but it is not automatically the design point. The installed system may require more head and less flow, or less head and more flow. A pump selected at its maximum flow may have little margin for fouling or future connections.
Approach Temperature and Facility Heat Rejection
Approach temperature connects the CDU to the facility cooling plant. The nominal 5 degrees Celsius value is a reference, not a constant. A primary supply of 30 degrees Celsius cannot produce a 25 degree secondary supply with a 5 K approach. The facility must be able to deliver the primary temperature required by the selected secondary setpoint.
Fluid Properties and Pressure Limits
Softened water is standard on the GLU Series primary side and DI water is standard on the secondary side. Ethylene glycol and propylene glycol are optional. Glycol changes density, specific heat, viscosity, and freeze protection, so it changes flow and pressure loss. A selection copied from a water table is not a valid selection for a glycol loop.
Pump Redundancy and Service Mode
Two pumps can mean parallel operation, standby operation, or a combination that depends on load. Each mode has a different flow, head, electrical demand, and failure response. The buyer should require a written sequence for normal operation, pump trip, isolation, maintenance, and return to service.
Controls, Instrumentation and Commissioning
Controls determine how the CDU responds to changing load, temperature, pressure, and pump status. Sensors should be selected for the fluid and expected range, and their calibration should be documented. The control narrative should define setpoints, dead bands, alarms, lead-lag rotation, and fail-safe positions.
A Six-Factor Risk-Tier Matrix
The matrix below translates technical uncertainty into low, medium, and high procurement risk. It is not a ranking of suppliers and does not replace the thermal calculation.
| Risk Dimension | Low Risk | Medium Risk | High Risk |
|---|---|---|---|
| Temperature window | Clear margin on both loops | One boundary close to limit | Primary or secondary condition cannot be met |
| Hydraulic duty | Pump operates near design point | Flow or head requires adjustment | Duty point outside stable pump range |
| Site pressure | Defined margin at manifold | Pressure split remains unresolved | Site losses consume reserved allowance |
| Redundancy | Maintenance does not interrupt load | Standby logic requires clarification | No service continuity under failure |
| Fluid compatibility | Documented quality and treatment | Inhibitor or glycol review pending | Material or freeze-protection mismatch |
| Evidence package | Drawings and tests agreed | Partial documentation available | No verifiable acceptance criteria |
Comparing CDUs at Equal Nameplate Capacity
A 900 kW Example with Two Different Pump Duties
Two CDUs may both be described as 900 kW units while requiring different pump duties. One project may have a short secondary route with low static lift. Another may have long pipe runs, several valves, and a high cold-plate pressure drop. The thermal capacity can be the same while the required head is different.
Equal Flow but Different Operating Margins
Equal flow does not mean equal operating margin. Two 900 kW offers may list about 77 cubic metres per hour, but one pump curve may have little spare pressure while another retains a wider stable range. The cheaper offer may meet today's duty yet leave no room for filter loading or added valves.
A 1,800 kW Example with Different Facility Temperatures
At 1,800 kW, the published GLU Series flow is 154.3 cubic metres per hour on the secondary side and 308.6 cubic metres per hour on the primary side. The nominal approach remains a reference value, but the facility supply temperature determines whether that approach can support the required secondary supply.
Why the Same Approach Can Require Different Primary Conditions
A 5 K approach at one duty point does not set a fixed outlet temperature at another. The exchanger responds to available temperature difference, flow, and surface area. Controls cannot create temperature difference the facility does not provide.
The procurement record should state design and worst-case primary and secondary temperatures. Judge the CDU against that full set, not a single approach label.
Where the 300 kW to 1,800 kW Range Changes the Procurement Decision
Single-Pump Models and Small Hydraulic Zones
The 300 and 600 kW GLU Series models use one pump. They can be appropriate for a small zone with a planned service window or a separate redundant unit. The buyer should confirm that the single pump has a realistic maintenance path and that a temporary cooling plan exists if the pump must be removed.
Dual-Pump Models and Critical-Load Expectations
The 900, 1,200, and 1,800 kW models use two pumps, with published pump flows of 39, 51, and 77 cubic metres per hour and heads of 35, 36, and 38 metres. The user should define whether the second pump is a standby unit or an additional operating unit. The answer changes flow, power, controls, and the meaning of redundancy.
DN50 and DN65 Interfaces in Real Pipe Routing
The 300 kW model uses DN50; the 600 to 1,800 kW models use DN65. Nominal size does not guarantee an adequate route because inside diameter, bends, valves, strainers, connectors, and path length affect pressure loss.
At 1,800 kW, secondary flow can reach 154.3 cubic metres per hour. Calculate velocity and pressure loss through the proposed route and check available pump head; a schematic that looks acceptable may fail after installation details are added.
Adapters, Pressure Loss and Responsibility Boundaries
Every adapter adds pressure loss and a connection that must be installed, tested, and maintained. The contract should assign responsibility for transitions, gaskets, supports, alignment, leak testing, and the battery limit shown on the interface drawing.
Two Procurement Scenarios
Scenario One: Fixed Facility Loop, Variable IT Load
A fixed facility loop may perform adequately at initial load, then become the constraint as GPU density rises. Review the CDU at initial, planned maximum, and intermediate loads. The pump must hold stable flow while control valves retain authority as racks are added.
What Must Be Recalculated as GPU Density Increases
Higher load changes secondary flow, return temperature, pressure loss, and exchanger duty. If the server requirement shifts from a 5 K to a 7 K rise, flow may change. Update the pump and system curves together; a capacity table alone cannot show branch performance.
Scenario Two: Fixed IT Load, Uncertain Heat-Rejection Capacity
A fixed IT load can still face changing facility conditions. A chiller may lose capacity, a dry cooler may face a warmer day, or heat recovery may return warmer primary water. Evaluate the CDU at the year's highest expected primary supply temperature.
How Approach Temperature Becomes a Site Risk
When primary supply rises, approach becomes a site risk rather than a product label. Model the effect on secondary supply temperature and ask whether servers can tolerate it. Alternatives include lower facility supply, higher flow, more exchanger capacity, or reduced IT load; each has different cost.
Twelve Questions for a CDU Supplier
1. What exact heat load, fluid, flow, and temperature conditions produce the quoted kW rating?
2. Which pump curve and speed apply to the offered configuration?
3. What internal pressure loss is included at the design flow?
4. What external pressure loss remains available at the manifold?
5. What approach temperature is guaranteed at the project condition?
6. How does capacity change with the warmest expected primary supply temperature?
7. Is the unit configured for parallel pumps, standby pumps, or both?
8. What capacity remains after one pump fails or is isolated?
9. What is the tested pump changeover sequence and expected interruption?
10. Which parts of the secondary loop are inside and outside the supplier scope?
11. What drawings, material records, pressure tests, and factory acceptance records will be provided?
12. What site measurements will demonstrate compliance during commissioning?
Evidence That Should Accompany Every CDU Offer
Required Drawings and Configuration Records
The offer should include a general arrangement drawing, process and instrumentation diagram, electrical single-line diagram, utility schedule, connection list, weight table, and bill of materials. The configuration record must match the model and rating quoted.
Pressure and Weld Inspection Evidence
The package should define pressure-test procedure, test pressure, hold time, leakage limits, and witness requirements. Weld records, material certificates, and non-destructive examination reports may be required for critical joints; buyers should confirm which documents are standard.
Factory Acceptance and Commissioning Records
Factory acceptance should verify configuration, sensor calibration, pump rotation and current, control logic, alarm response, and pressure and flow checks where possible. Site commissioning should verify the installed system curve, balance, temperature stability, pump changeover, and fluid quality.
What a Document Should Prove
A useful document proves a fact about the delivered unit. A material certificate proves the specified grade; a pressure test record proves the procedure was followed; a pump curve proves performance; and a current general arrangement drawing proves dimensions and interfaces.
What Still Requires Project-Specific Verification
Factory documents cannot prove site pipe loss, delivered primary temperature, or branch balance. Those items require site measurements and engineering review. A strong offer separates factory evidence from site verification.
Frequently Asked Questions
Q1: Why do CDUs with the same kW rating have different prices and performance?
A: The kW rating may be based on different temperatures, flows, fluids, pump duties, redundancy modes, controls, materials, or documentation. A lower price may reflect a narrower operating envelope or a smaller evidence package. Compare the full duty point and scope, not only the capacity.
Q2: Can the published flow rating be used directly as a design flow?
A: It can be used as a reference, but it is not a site design until pressure, temperature, fluid, and redundancy are checked. The installed flow depends on the system curve. A pump that delivers a value on a factory test may deliver less in a restrictive building loop.
Q3: Does a dual-pump CDU provide complete redundancy?
A: Not automatically. The buyer must establish whether one pump can carry the critical load, how changeover occurs, what valves and controls are shared, and how the failure was tested. Dual pumps improve the option for redundancy but do not define its performance by themselves.
Q4: Is the nominal 5°C approach fixed for every installation?
A: No. The approach depends on heat exchanger area, primary and secondary flow, inlet temperatures, fluid properties, fouling, and thermal duty. The supplier should state the approach at the project condition and the facility should verify the required temperatures during commissioning.
Q5: How can buyers compare site pressure margin across offers?
A: Ask for the pump curve, internal pressure loss, allowable external loss, and manifold pressure allowance at the same flow. Compare the pressure remaining for the actual pipe route and cold plates, not the maximum head printed on the pump datasheet.
Q6: Are single-pump GLU Series ratings suitable for an expansion project?
A: They can be, if the initial zone will remain small and the owner accepts a planned service window or a separate continuity measure. If later racks may require concurrent maintenance, review a dual-pump rating or a redundant-unit architecture from the start. Do not assume a 300 or 600 kW unit can later be converted into a larger model.
Q7: How can a buyer verify supplier testing and documentation?
A: Ask for the factory acceptance test plan, applicable procedure, instrument calibration, pump and flow-check records, pressure-test record, and a sample general arrangement drawing for the offered configuration. Confirm which tests are standard, which are project-specific, and who witnesses them.
Conclusion
Nameplate kW is one coordinate in a larger performance map. The GLU Series example shows how flow, temperature windows, pump configuration, manifold size, fluid choice, controls, and documentation change across a 300 to 1,800 kW family. A buyer who compares those variables at the installed duty point can distinguish a usable CDU from a rating that only looks adequate on a catalogue page.
References
Sources
- ASHRAE Water-Cooled Servers: Common Designs and Best Practices
Note: Provides industry context for liquid-cooled server design, facility interfaces, and operating conditions relevant to CDU selection.
- Open Compute Project Liquid Cooling CDU Test Methodology
https://www.opencompute.org/documents/ocp-wp-l-lcdu-test-methodology-performance-rating-r1-pdf
Note: Outlines performance rating and test considerations that help buyers interpret capacity, efficiency, and hydraulic claims.
- Schneider Electric: Account for Filtering When CDU Rating Capacity Is Specified
Note: Explains how test and filtration conditions affect the interpretation of a published CDU capacity.
- Vertiv: Understanding Coolant Distribution Units
Note: Describes CDU functions, loop interfaces, and the relationship among heat exchange, flow, and controls.
- Vertiv: Deploying Liquid Cooling and Managing CDUs
Note: Supports the discussion of site installation, service access, and commissioning requirements.
- Uptime Institute: Capacity Planning for Liquid-Cooled Data Centers
https://intelligence.uptimeinstitute.com/resource/capacity-planning-liquid-cooled-data-centers
Note: Provides a capacity-planning framework for liquid-cooled facilities and the operational assumptions that affect design.
- The Green Grid: Liquid Cooling TCO Calculation Tool
Note: Offers a lifecycle-cost framework for comparing liquid cooling configurations and operating assumptions.
- nVent: Coolant Distribution Unit
https://www.nvent.com/en-us/data-solutions/coolant-distribution-unit
Note: Gives an additional manufacturer view of CDU architecture, loop separation, and deployment requirements.
- Chilldyne: Questions to Ask Your Liquid Cooling Vendor
Note: Provides buyer questions on reliability, performance evidence, and service expectations for liquid cooling equipment.
Related Examples
- Zejiu GLU Series CDU Liquid Cooling Unit
https://cnzejiu.com/products/glu-series-cdu-liquid-cooling-unit-300-1800-kw
Note: Documents the capacity ratings, flow values, pump arrangements, temperature windows, fluids, manifold sizes, and cabinet data used in the comparison.
- Zejiu GLU Series Product Information Page
https://cnzejiu.com/pages/glu-series-cdu-liquid-cooling-unit
Note: Provides supplier descriptions of the packaged architecture, pump configuration, heat exchanger, and service features.
- Zejiu About and Manufacturing Capabilities
https://cnzejiu.com/pages/about-us
Note: Gives context on manufacturing, inspection, and quality processes relevant to factory evidence review.
Further Reading
- The Whole-Life Case for Packaged CDUs
https://www.industrysavant.com/2026/11/the-whole-life-case-for-packaged-cdus.html
Note: Connects thermal performance, maintenance readiness, energy use, and material impact across the operating life.
- What Does a Data Center Liquid Cooling CDU Supplier Provide
https://www.karinadispatch.com/2026/11/what-does-data-center-liquid-cooling.html
Note: Explains packaged scope and responsibility boundaries that should be clarified during CDU procurement.
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