Coolant distribution units (CDUs) and liquid cooling loops

When a data center adopts liquid cooling, a new piece of equipment becomes central to the design: the coolant distribution unit, or CDU. The CDU sits between the IT equipment's liquid loop and the building's facility water system. It controls the temperature, pressure, flow and cleanliness of the coolant that reaches servers, while keeping the large, less pristine facility loop separate from delicate cold plates.

Understanding what a CDU does, and how to select and commission it, is essential for anyone planning direct-to-chip liquid cooling or rear-door heat exchangers on a secondary loop.

The two loops

ASHRAE describes liquid cooling in terms of two loops:

• Facility water system (FWS): the building-side loop from the CDU to the heat rejection plant (chillers, dry coolers, cooling towers or a waterside economizer).
• Technology cooling system (TCS): the IT-side loop from the CDU through rack manifolds to the cold plates or other liquid-cooled components.

The CDU's heat exchanger transfers heat from the TCS to the FWS without mixing the two fluids.

What a CDU contains

• Heat exchanger: typically a brazed or gasketed plate heat exchanger
• Pumps: usually redundant, variable speed
• Control valve: on the facility side, modulating to hold TCS supply temperature
• Filtration: fine filters on the TCS side to protect cold plate microchannels
• Expansion tank and fill system: to manage pressure and make-up fluid
• Sensors: temperatures, pressures, flow, and often fluid quality (conductivity, pH)
• Leak detection: in the CDU drip tray and sometimes along connected piping
• Controller: with local display and communications (Modbus, BACnet, SNMP or Redfish, depending on the product)

Types of CDU

Liquid-to-air CDUs let operators deploy cold plate servers without bringing facility water to the row, but the heat still goes into the room, so the room's air cooling must handle it. They are a transition tool rather than a long-term high-density solution.

Sizing and selection

Capacity

Capacity depends on TCS flow rate, facility water temperature and flow, and the approach temperature. A CDU rated at a headline capacity at one set of conditions may deliver much less at warmer facility water. Always ask for selections at your actual conditions.

Approach temperature

Approach is the difference between facility water supply and TCS supply. Typical approaches are in the range of about 2 to 5 K. A smaller approach lets you run facility water warmer, which increases free cooling hours, but requires a larger heat exchanger.

Flow and pressure

IT vendors specify flow per kilowatt or per server and pressure drop through the servers. The CDU pumps must overcome server, manifold, hose and piping pressure drops at the required flow, with margin for filter loading. Over-pressurizing the TCS risks leaks at connectors, so pressure limits matter.

Fluid

The TCS fluid is chosen to suit the IT vendor's requirements, often treated water or a propylene glycol solution around 25 percent. Glycol reduces heat transfer slightly and increases pumping power. Materials throughout the loop must be compatible with the fluid.

Redundancy

The CDU is a single point of failure for every rack it serves. Common strategies:

• Redundant pumps within each CDU (N+1), with automatic changeover
• Multiple CDUs on a common secondary header (N+1 CDUs) so any unit can be taken offline
• Dual facility water connections or A and B facility loops
• UPS power for CDU pumps and controls, since cold plates have very little thermal mass

See data center cooling redundancy.

Controls

A CDU typically controls:

1. TCS supply temperature by modulating the facility water valve, keeping supply within the IT vendor's range and above room dew point
2. TCS flow or differential pressure by varying pump speed
3. Pump rotation and failover
4. Alarms for high temperature, low flow, high pressure, leaks, low fluid level and filter differential pressure

Some CDUs can also control based on dew point, raising supply temperature automatically if room humidity rises, to prevent condensation. Integrate CDU alarms and trends with the BAS and DCIM. See data center cooling controls and DCIM integration.

Installation essentials

• Install isolation valves on facility and TCS connections for each CDU.
• Provide drains, vents and flushing connections.
• Use pressure-rated hoses and dripless quick-disconnects at racks.
• Locate leak detection beneath CDUs, manifolds and piping.
• Protect facility water with strainers before the CDU heat exchanger.
• Label everything clearly, including flow direction and loop identification.

Flushing, filling and water quality

Construction debris is the most common cause of early liquid cooling problems. Before connecting any IT equipment:

1. Flush the facility and TCS piping thoroughly, with temporary bypasses around CDUs and racks.
2. Clean with an appropriate cleaning chemical if required, then flush again.
3. Fill with the specified fluid and add inhibitors.
4. Test water quality: pH, conductivity, inhibitor levels, particulates and microbiological counts.
5. Record baseline values for ongoing monitoring.

An ongoing water quality program, similar to that for any closed hydronic loop but with tighter limits, keeps the system healthy.

Commissioning

CDU commissioning should include factory testing where possible, site startup by the manufacturer, flow balancing of rack manifolds, control loop tuning, alarm verification and failover testing under load. Heat load simulators connected to rack manifolds allow testing before IT equipment arrives. See commissioning data center cooling systems.

Frequently asked questions

Why not connect servers directly to the facility chilled water?

Facility water is a large, open-to-the-world loop with more particulates, varying chemistry and higher pressures. Cold plates need clean fluid at controlled temperature and pressure. The CDU provides that separation and limits the volume of fluid that could leak into the white space.

How many racks can one CDU serve?

It depends on the CDU's capacity and the rack loads. In-rack units serve one rack, in-row units commonly serve a handful of racks, and centralized units can serve many rows.

Key takeaways

• The CDU separates the IT liquid loop (TCS) from the facility water system (FWS).
• It controls supply temperature, flow, pressure and filtration for cold plates.
• Select CDUs at real operating conditions, paying attention to approach temperature and pressure limits.
• Redundant pumps, multiple CDUs and UPS power protect against loss of flow.
• Thorough flushing, water quality monitoring and load-based commissioning are essential.