The rapid growth in applications related to artificial intelligence, cloud computing, and high-performance computing is significantly increasing the thermal density of IT infrastructure. In modern data centers, some racks can reach several dozen kilowatts, making air cooling insufficient to effectively dissipate the heat generated.
Liquid cooling has thus emerged as an essential solution for keeping IT equipment within its optimal operating ranges. At the heart of this architecture, the Coolant Distribution Unit (CDU) manages the circulation, regulation, and monitoring of the coolant between the cooling systems and the IT racks.
The performance of a data center CDU depends directly on the stability and accuracy of the flow rate. Reliable measurement makes it possible to balance the various circuits, ensure proper rack cooling, and improve the facility’s overall energy efficiency. Fuji Electric’s S-Flow FSZ ultrasonic flow meter addresses these challenges with precise, non-intrusive measurement that causes no pressure drop.
The widespread adoption of artificial intelligence, cloud computing, and high-performance computing is concentrating ever-greater computing power into smaller spaces. High-density racks now generate levels of heat that air cooling alone is no longer always sufficient to dissipate effectively. Data center designers and operators must therefore adapt their thermal architectures to ensure equipment stability, service continuity, and energy consumption control.
In this context, liquid cooling has emerged as a suitable solution for high-density infrastructure. By capturing heat as close as possible to critical components, it improves thermal efficiency without excessively increasing the footprint. The coolant distribution unit (CDU) plays a central role by serving as the interface between the site’s primary loop and the secondary loop that supplies the racks. In particular, it regulates the fluid’s flow, temperature, and pressure.
The performance of the CDU depends directly on flow rate stability. An insufficient flow rate can lead to localized overheating, while an excessive flow rate unnecessarily increases pump consumption and reduces overall energy efficiency. Reliable flow measurement therefore makes it possible to adjust the operation of pumps and heat exchangers, detect hydraulic imbalances, and limit excessive consumption.
At the same time, operators must increase their computing capacity, reduce energy costs, minimize unplanned downtime, and extend the service life of their facilities. Integrating flow and temperature data into BMS or SCADA systems facilitates real-time monitoring, automated adjustments, and preventive maintenance. In an environment where availability is critical, controlling flow within the CDU thus becomes a key factor in ensuring reliability, energy efficiency, and operational performance.
CDU manufacturers must design increasingly compact systems. This challenge is particularly acute in in-row configurations, where cooling equipment is installed directly between the server racks.
The space available for installing a CDU flowmeter is therefore limited. The straight pipe runs required for certain measurement technologies to function properly cannot always be provided. The instrument must also be capable of being mounted on pipes of various diameters without unduly complicating the CDU’s design.
Any obstruction introduced into a pipe can cause a pressure drop. This pressure drop must then be compensated for by the pumps, which increases energy consumption and can degrade the hydraulic performance of the system.
In a coolant distribution unit, the measurement technology must therefore fully preserve the fluid flow path. An intrusive solution can also create an additional point of weakness, particularly in terms of sealing, fouling, or wear.
Depending on the site conditions and equipment requirements, a CDU may use water, a mixture of water and glycol, or other specific fluids.
In this type of architecture, several measurement points are typically distributed across the CDU’s inlet and outlet circuits, as well as as close as possible to the racks. This configuration allows for continuous monitoring of fluid distribution throughout the secondary loop and ensures optimal cooling balance.
The instrumentation must maintain reliable measurements despite differences in viscosity, speed of sound, and thermal properties among the fluids.
Cooling requirements are not necessarily the same from one rack to another. Situations involving underflow or overflow may occur depending on the IT load, valve settings, or hydraulic configuration.
An underflow can cause the temperature in a rack to rise excessively. Conversely, an overflow can unnecessarily strain the pumping capacity and reduce overall energy efficiency.
Without reliable measurements, it becomes difficult to:
The S-Flow FSZ flow meter uses ultrasonic time-of-flight technology. The flow velocity is determined based on the difference in the propagation time of ultrasonic signals traveling in the direction of flow and in the opposite direction.
This technology provides accurate, continuous measurement with a specified margin of error of ±2%, without introducing any obstruction in the pipeline. The flow meter therefore does not cause any additional pressure drop and does not affect the hydraulic performance of the CDU.
The lack of direct contact with the liquid also reduces the risk of leaks associated with the instrumentation. This feature is particularly important in data centers, where a leak can affect the availability of IT equipment and lead to complex maintenance operations.
In addition to flow measurement, the S-Flow FSZ ultrasonic flowmeter also includes temperature measurement. This allows the CDU manufacturer to enhance monitoring of the cooling loop and gain a more accurate understanding of the thermal performance of its system.
Combining flow and temperature data makes it easier to:
This two-part approach improves our understanding of the CDU's behavior and allows us to fine-tune the operation of the pumps, heat exchangers, and control systems.
The flow meter is designed to measure low flow rates, including under conditions close to zero flow. This capability facilitates the monitoring of startup phases, low-load periods, and transient conditions.
Detecting an abnormally low flow rate may indicate a closed valve, a malfunctioning pump, a blockage, or a hydraulic imbalance. Continuous monitoring thus helps improve the system’s availability and enables action to be taken before overheating occurs.
The S-Flow ultrasonic flow meter can be configured based on the fluid's properties, including its kinematic viscosity and the speed of sound. These settings ensure accurate measurement with various coolants, including antifreeze mixtures.
This flexibility makes it easier to adapt the flow meter to different coolant distribution unit configurations and to the specific pro datasheet of each project.
Thanks to its compact, one-piece design, the flow meter can be installed in tight spaces. It is compatible with high-density CDU architectures and various piping configurations.
Its design facilitates direct installation within coolant distribution units, alongside control equipment such as variable-frequency drives and monitoring interfaces. This integration enables the creation of a complete cooling measurement, regulation, and control system.
Data from the flow meter can be used to adjust pump speed, monitor system performance, and transmit information to a BMS or SCADA system.
ApplicationDownload the application sheet and learn how to measure coolant flow to improve the reliability and monitoring of your CDU units.
