Modern data centers serving artificial intelligence training, high-performance computing, and hyperscale cloud workloads generate heat densities that exceed the practical limits of air-only cooling. Coolant distribution units respond to this condition by providing a controlled, instrumented secondary loop that delivers conditioned liquid directly to cold plates or immersion assemblies at the IT load. The secondary loop isolates the IT equipment from the broader facility water system, allowing operators to use clean, treated dielectric or water-based fluids on the chip side while relying on conventional facility chilled water, condenser water, or air on the primary side.
Because the CDU decouples the two loops, it also acts as a quality, safety, and maintenance boundary. Filtration, leak detection, pressure regulation, and temperature stability are all concentrated in one chassis or skid, making the CDU a focal point of uptime engineering, compliance, and serviceability planning. The remainder of this reference page documents working principles, specifications, configurations, selection considerations, applicable standards, and the procurement landscape for coolant distribution units.
A coolant distribution unit, commonly abbreviated CDU, is a closed-loop liquid heat exchange device used in data centers to decouple facility water from the technology cooling loop feeding CPUs, GPUs, and accelerators. It pumps, filters, and temperature-controls a secondary coolant while rejecting heat through a primary facility water, refrigerant, or air-side heat exchanger. The unit is the central enabling component of direct-to-chip and single-phase or two-phase liquid cooling architectures deployed in high-density compute environments.
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1. Fundamentals and Working Principle
A coolant distribution unit operates as a hydraulic and thermal interface between two fluid loops. The primary loop connects to facility infrastructure, which can be chilled water, condenser water, refrigerant, or ambient air, and serves as the ultimate heat sink. The secondary loop circulates a controlled coolant to the IT load, typically to direct-to-chip cold plates, cold plates on memory modules, or to manifolds feeding immersion tanks. Heat picked up by the secondary fluid is rejected across an internal heat exchanger into the primary loop, and the cooled secondary fluid is returned to the IT equipment.
From a controls standpoint, the CDU continuously measures secondary supply and return temperature, pressure at key points in the secondary circuit, and flow rate, and modulates a control valve on the primary side, or in some architectures the secondary pump speed, to maintain a target secondary supply temperature. The Vertiv CoolChip CDU 121 datasheet describes a secondary fluid temperature controlled within plus or minus 1 degree Celsius, with the nominal cooling capacity of 121 kilowatts at a 4 degree Celsius approach temperature difference, and a nominal secondary flow rate of 120 liters per minute at 1.15 bar. These three parameters, capacity, approach temperature, and flow rate, are the defining performance envelope of any CDU.
Mechanically, a CDU integrates several subsystems into a single frame or skid. These include a secondary circulation pump or pumps, a primary-to-secondary heat exchanger, an expansion or buffer tank, isolation and control valves, inline filtration, sensors for temperature, pressure, and flow, and a controller with a human-machine interface. The Vertiv CoolChip CDU 121 is a 4U rack-mounted unit with an integrated internal reservoir and fill pump, while the Delta GoCool-660 and GoCool-1000 are described as larger capacity liquid-to-liquid units with water as the primary coolant and a 25 percent propylene glycol mixture as the secondary coolant for the GoCool-1000. Whether the unit is a small in-rack appliance or a multi-hundred-kilowatt row or room-level skid, the same fundamental control loop is preserved.
The value of the secondary loop is not just thermal but chemical and operational. Eaton's CDU reference notes that filtration typically between 0.2 and 50 microns keeps the coolant clean and protects cold plate integrity, while automatic leak detection and redundant pump design protect uptime. nVent adds that the CDU isolates IT equipment from the main facility water system, improves operational monitoring and control of water delivered to IT equipment racks, maintains proper technology cooling system water temperature with proper alarms, and maintains water temperature above the facility dew point to prevent condensation. In short, the CDU is the boundary device that lets the data center operator treat IT coolant quality, IT coolant chemistry, and IT safety as separate concerns from the facility.
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2. Specifications and Key Parameters
The most important CDU parameters are nominal cooling capacity, approach temperature, secondary flow rate, primary flow rate, permissible external pressure drop on the secondary circuit, physical dimensions, weight, and the type of heat rejection on the primary side. The table in this chapter consolidates the exact figures published in the source documents so that the reader can compare representative in-rack and row- or room-scale units side by side. Every value below is taken verbatim from the cited vendor datasheet or product page, and where a parameter is not published, the table shows the entry as not published rather than estimating.
A key secondary-circuit parameter is the per-kilowatt flow ratio, often expressed in liters per minute per kilowatt. The Delta GoCool-660 is published at 660 kilowatts with a 7.5 degree Celsius approach, 660 liters per minute secondary flow, 600 liters per minute primary flow, and 1.0 liter per minute per kilowatt, with an alternative operating point of 500 kilowatts at a 6.0 degree Celsius approach and 1.5 liters per minute per kilowatt. The Delta GoCool-1000 lists 1000 kilowatts at a 5 degree Celsius approach with 1000 liters per minute secondary flow, 1200 liters per minute primary flow, and 1.0 liter per minute per kilowatt, with a second point of 750 kilowatts at a 4 degree Celsius approach and 1.5 liters per minute per kilowatt, and an operation pressure drop of 136 kilopascals on the primary side with 25 percent propylene glycol on the secondary side.
For smaller systems, the WayCool CDU-6 specification table shows two-pump configurations with capacities of 3125 and 2500 watts at 1.2 to 1.5 liters per minute per kilowatt, technology cooling system flow rates from 2580 to 3750 liters per minute, facility water system flow rates up to 3750 liters per minute, and maximum external pressure of 414 kilopascals at 6 degrees Celsius, rising to 621 kilopascals for the high-pressure variant. The Vertiv CoolChip CDU 121 publishes 121 kilowatts at a 4 degree Celsius approach with 120 liters per minute secondary flow at 1.15 bar, unit dimensions of 175 by 445 by 850 millimeters, dry weight of 54 kilograms on AC input and 52.5 kilograms on DC input, and a CE, cULus, and RoHS compliance statement.
When evaluating any CDU, the engineer should confirm four numbers against the actual application: nominal capacity at the published approach temperature, the secondary flow rate the IT load actually requires, the maximum external pressure the secondary loop can push through the rack plumbing, and the available facility flow and temperature on the primary side. The Eaton CDU product page reinforces the selection logic by describing CDUs as right-sized and custom configured for specific applications, ensuring that hyperscale compute systems get the exact cooling performance the system needs. The nVent CDU page adds that the CDU maintains proper technology cooling system water temperature with proper alarms and isolates the IT equipment from the main facility water system, which is the operational reason these published numbers must be matched to the deployment, not treated as universal.
CDUs are commonly classified by where they sit in the data center and by how the primary side rejects heat. By location, the literature distinguishes in-rack, row, and room or facility-scale units. The Vertiv CoolChip CDU 121 is explicitly described as an in-rack liquid-to-liquid CDU occupying 4U of rack space and supporting a single rack direct-to-chip cooling application. By contrast, the Delta GoCool-660 and GoCool-1000 are described as liquid-to-liquid CDUs with capacities of 660 kilowatts and 1000 kilowatts respectively, intended to serve larger clusters of racks. OptiCool and nVent both describe CDUs that bridge facility chilled water with server-level liquid cooling loops in new construction and retrofit installations.
By heat-rejection medium on the primary side, three families appear in the sources. Liquid-to-liquid CDUs reject heat to facility water, such as the Vertiv CoolChip CDU 121, the Delta GoCool-660, the Delta GoCool-1000, and the WayCool CDU-6. Liquid-to-air CDUs reject heat directly to ambient air using a finned coil and fans, exemplified by the Delta GoCool-80 product family referenced in the source list. Liquid-to-refrigerant CDUs integrate a refrigerant cycle on the primary side, sometimes called direct expansion CDUs, and are represented in the OptiCool product family with refrigerant pumps for chilled water and outdoor DX applications. OptiCool's product range is described as including rear door heat exchangers, refrigerant pumps, aisle containment, and CDUs, all of which can be combined in a single cooling system.
By redundancy philosophy, WayCool describes an Optimized Redundancy Architecture that delivers redundancy for pumps, filters, and heat exchangers at the system level rather than within a single unit, with units operating in load-sharing mode to equalize wear across the fleet, hot-swappable pumps and filters for zero-downtime replacement, and more than 10 percent improved pump efficiency over 2N designs, paired with redundant sensors and dual-source controls. The Eaton CDU reference describes a complementary approach with automatic leak detection and redundant pump design, blind-mate quick disconnects with 360 degree swivel fittings to minimize maintenance downtime, and operation logic integrated into intelligent controllers. These two architectures represent the spectrum from in-rack simplex to system-level N plus 1 or distributed redundant CDUs.
By fluid chemistry, the WayCool, Delta, Vertiv, and Eaton sources cover water, glycol mixtures, and dielectric fluids. The Delta GoCool-1000 datasheet specifies 25 percent propylene glycol on the secondary side, while the Vertiv CoolChip CDU 121 extract references water or PG as the secondary fluid. nVent emphasizes that high quality facility water is not available in most data centers, which is the principal reason a secondary loop and therefore a CDU is required at all. Some CDU variants, especially those feeding two-phase or dielectric systems, will use engineered fluids whose chemistry must be controlled in the secondary loop, reinforcing the CDU's role as a quality and chemistry boundary in addition to a thermal one.
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4. Selection Criteria for Procurement
The first procurement check is the match between IT heat load and CDU nominal capacity at a realistic approach temperature. The Vertiv CoolChip CDU 121 publishes 121 kilowatts at a 4 degree Celsius approach, which means the secondary fluid leaves the CDU approximately 4 degrees Celsius above the primary fluid entering the heat exchanger, while the Delta GoCool-1000 publishes 1000 kilowatts at a 5 degree Celsius approach and 750 kilowatts at a 4 degree Celsius approach. Selecting a CDU on its peak nameplate without checking the published approach temperature can lead to under-sized equipment in installations where the facility water is warmer than expected.
The second check is the secondary hydraulic circuit. The published secondary flow rate, the per-kilowatt flow ratio, and the maximum external pressure that the CDU can push through the IT plumbing determine whether the unit can feed a given rack or row. The WayCool CDU-6 specification table lists 414 kilopascals (60 psi) of maximum external pressure in the standard variant and 621 kilopascals (90 psi) in the high-pressure variant, with technology cooling system flow rates from 2580 to 3750 liters per minute depending on the configuration. The Vertiv CoolChip CDU 121 publishes 120 liters per minute at 1.15 bar, which is a useful reference point for a single in-rack unit.
The third check is facility-side integration. The Delta GoCool-660 specifies 600 liters per minute primary flow at a 17 degree Celsius primary inlet temperature, while the GoCool-1000 specifies 1200 liters per minute at the same inlet temperature. These values must be coordinated with the building's chilled water plant, condenser water system, or dry cooler array. For a liquid-to-air CDU such as the Delta GoCool-80 line, the engineer must instead verify ambient design conditions, allowable fan noise, and condenser placement. Eaton emphasizes that the CDU maintains the secondary loop supply temperature above the facility dew point to prevent condensation on cold plates, which ties the secondary set point directly to facility humidity control.
The fourth check is serviceability, controls, and compliance. The Vertiv CoolChip CDU 121 publishes a 7 inch color touchscreen HMI, Modbus RTU (RS485) and TCP/IP communication, full alarm monitoring, and unit-to-unit communication for redundancy, alongside CE, cULus, and RoHS compliance. WayCool's CDU architecture adds hot-swappable pumps and filters and redundant sensors powered from dual sources. Eaton highlights blind-mate quick disconnects with 360 degree swivel fittings. For procurement, the practical questions are whether the CDU integrates with the existing building management system or data center infrastructure management stack, whether service can be performed without draining the secondary loop, and whether the unit ships with the regulatory marks required for the destination country.
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5. Standards, Compliance, and Testing
The published compliance marks called out in the sources are CE, cULus, and RoHS, as stated on the Vertiv CoolChip CDU 121 datasheet. CE marking indicates conformity with applicable European Union directives, cULus indicates certification to Canadian and United States safety standards by Underwriters Laboratories, and RoHS indicates compliance with the Restriction of Hazardous Substances directive. None of the cited sources enumerate additional sector-specific clauses such as ASHRAE, OCP, or Telcordia in the extracts provided, so this reference does not attribute those clauses to the CDU category in general.
Beyond the marks on the nameplate, the WayCool and Eaton sources describe engineering practices that map to common data center liquid cooling expectations. WayCool states that the controls system is powered from dual sources, that redundant sensors ensure reliable pump and valve control, and that pumps and filters are hot-swappable for zero-downtime replacement. Eaton states that CDUs maintain the secondary loop supply temperature above the facility dew point to prevent condensation on cold plates, that filtration typically between 0.2 and 50 microns keeps the coolant clean and protects cold plate integrity, and that automatic leak detection and redundant pump design protect uptime. The Vertiv CoolChip CDU 121 datasheet additionally confirms that the secondary fluid is filtered by an integrated secondary fluid filtration subsystem to extend system life and reduce the risk of containments, presumably a typographical instance of contaminants in the source.
Testing considerations that can be derived from the published specifications include verification of nominal cooling capacity at the published approach temperature and flow, verification of secondary fluid temperature stability within plus or minus 1 degree Celsius as published for the Vertiv CoolChip CDU 121, verification of operation pressure drop on the primary side such as the 136 kilopascals published for the Delta GoCool-1000, and verification of maximum external pressure on the secondary side such as the 414 and 621 kilopascal values published for the WayCool CDU-6 standard and high-pressure variants. Functional testing should also confirm that the control system, alarm thresholds, leak detection sensors, and BMS or DCIM communications are operational before the CDU is placed in service.
The Eaton CDU reference frames the standard of care in operational terms rather than by listing a specific clause. It states that coolant distribution units are vital components that ensure liquid cooled systems meet or exceed lifetime expectations, that the supply temperature must remain above the facility dew point, that the coolant must be filtered to protect cold plate integrity, and that leak detection and redundant pump design protect uptime. Procurement specifications should therefore require evidence of these capabilities through type test reports, factory acceptance test scripts, and on-site commissioning records, even where the published product page does not list a specific clause number.
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6. Market Landscape and Buying Process
The CDU vendor landscape represented in the supplied sources spans large diversified power and thermal management suppliers, specialist cooling companies, and enclosure and infrastructure brands. Vertiv publishes the CoolChip CDU 100 family with the 121 kilowatt in-rack model cited above. Delta publishes the GoCool series including the GoCool-80 liquid-to-air CDU and the GoCool-660 and GoCool-1000 liquid-to-liquid CDUs. Eaton describes a custom-configured CDU line targeting AI training clusters and high-performance computing, highlighting right-sizing and total cost of ownership. OptiCool lists a CDU as part of a broader liquid cooling portfolio that includes rear door heat exchangers, refrigerant pumps, and aisle containment. nVent offers CDU platforms through both the nVent and Schroff brands. WayCool, a brand under Mayair, sells the CDU-6 line.
A typical buying process begins with a thermal and hydraulic requirements document. The buyer documents rack heat load, target secondary supply temperature, secondary flow rate, facility water or air availability, ambient conditions, and the desired redundancy architecture. This document is sent to candidate vendors such as those above for budgetary proposals. The WayCool and OptiCool descriptions emphasize integration with the broader cooling system, which means that CDU selection is rarely a standalone decision and is typically tied to a wider liquid cooling design that may include rear door heat exchangers, aisle containment, or two-phase cooling.
After budgetary proposals, the buyer issues a request for proposal or invitation to tender that includes the published parameters seen in the spec table of this reference: nominal cooling capacity at a specified approach temperature, primary and secondary flow rates, maximum external pressure, dimensions, weight, compliance marks, communications protocols, and serviceability features such as hot-swappable pumps and filters. The Eaton CDU page specifically describes the company's offerings as right-sized and custom configured for specific applications, which suggests that engineering workshops between buyer and vendor are common to refine the specification before a final order is placed.
The final phases of procurement are factory acceptance testing, shipping, on-site commissioning, and warranty registration. Vertiv describes global all-in-one service offerings from design to installation and startup to fluid management and troubleshooting, which is one example of how CDU vendors package the after-sale phase. Eaton describes smart controls that respond to peak demands while remaining economical and efficient during non-peak operation and prevent problems before they occur. For the buyer, the practical advice drawn from these sources is to evaluate the vendor not only on the published specification sheet but on the completeness of the service network, the clarity of the fluid management offering, and the ability to support the CDU across its operational life.
FAQ
What does a coolant distribution unit do in a data center?
A coolant distribution unit circulates a controlled secondary coolant in a closed loop to remove heat from IT equipment such as CPUs, GPUs, and AI accelerators. It decouples this secondary loop from the facility water or air on the primary side, so the IT side can use clean, treated fluid while the facility side uses conventional chilled water, condenser water, or ambient air.
Why is a secondary loop needed in liquid cooling?
Most data centers cannot guarantee the cleanliness, chemistry, or pressure quality of the facility water system at every server connection. The secondary loop, managed by the CDU, isolates the IT equipment from the main facility water system, allowing filtration, leak detection, and temperature control to be applied consistently to the chip-side fluid.
How is CDU cooling capacity rated?
Capacity is rated as nominal cooling capacity at a stated approach temperature difference and a stated secondary flow rate. The Vertiv CoolChip CDU 121 is published at 121 kilowatts at a 4 degree Celsius approach, the Delta GoCool-660 at 660 kilowatts at a 7.5 degree Celsius approach, and the Delta GoCool-1000 at 1000 kilowatts at a 5 degree Celsius approach.
What is the difference between a liquid-to-liquid and a liquid-to-air CDU?
A liquid-to-liquid CDU rejects heat from the secondary loop to facility water through an internal heat exchanger, as in the Vertiv CoolChip CDU 121 or the Delta GoCool-660 and GoCool-1000. A liquid-to-air CDU rejects heat directly to ambient air using a finned coil and fans, as in the Delta GoCool-80 product line.
What filtration does a CDU provide?
The Eaton CDU reference states that CDU filtration is typically between 0.2 and 50 microns, which keeps the coolant clean and protects cold plate integrity. The Vertiv CoolChip CDU 121 datasheet additionally describes integrated secondary fluid filtration intended to extend system life and reduce the risk of contaminants in the secondary loop.
How is condensation avoided in a CDU-cooled loop?
The CDU is controlled so that the secondary loop supply temperature stays above the facility dew point. The Eaton CDU reference states that CDUs maintain the secondary loop supply temperature above the facility dew point to prevent condensation on cold plates, and the nVent CDU reference adds that the CDU maintains proper technology cooling system water temperature with proper alarms.
What communications and control interfaces do CDUs support?
The Vertiv CoolChip CDU 121 publishes a 7 inch color touchscreen HMI, Modbus RTU over RS485, and TCP/IP communication, with full alarm monitoring and unit-to-unit communication available for redundancy. WayCool describes redundant sensors and dual-source controls within its CDU architecture, while Eaton describes operation logic integrated into intelligent controllers combined with smart controls that respond to peak and non-peak demand.
