Account for filtering too! Beware of how CDU rating capacity is specified.

Specifying coolant distribution units or CDUs (sometimes referred to as cooling distribution units) for your data center liquid cooling deployments should not be rocket science or even confusing. Indeed, selecting these should be a clear, transparent process coordinated with your cooling gear vendor to help you get the right cooling capacity. Your liquid-cooled IT will require a specific flow, pressure, and coolant inlet temperature to effectively remove heat. These parameters are specified by your IT provider, but the CDU is what delivers them. Thus, it is critical to work with your CDU vendor and clarify all conditions under which the CDU cooling capacity is specified.

We have heard “horror” stories from customers who buy CDUs and later find out that pressure losses introduced by the CDU’s filters were not accounted for in the “rated CDU external pump pressure”. The net result is a lower CDU pressure capacity, i.e., derated cooling capacity. Unfortunately, this requires you to redesign your technology cooling system (TCS) layout, acquire additional equipment (CDUs and TCS components), leading to installation and commissioning delays.

To avoid surprises, these key points must be considered when selecting your CDUs:

• TCS pressure – beyond the CDU
• Ratings and rating conditions
• Filtering and coolant quality

TCS pressure – beyond the CDU

The TCS loop connects your IT equipment to the CDU. Your CDU pumps move coolant through the TCS loop and eventually through your expensive IT equipment. The TCS is composed of piping and additional components that ensure correct function and optimum performance of the liquid cooling system. See diagram in Figure 1.

Each component in the TCS – cold plate, pipe section, fitting, connector, valve, flow meter, pressure sensor, temperature sensor, etc. – introduces a pressure drop. The sum of all these pressure drops represents the TCS’s total pressure drop. The important thing to remember is that pressure drop increases with increasing fluid flow rate (e.g., liters per minute). See Analogy.

Once you know your TCS’s pressure drop, you need a CDU with enough external pump pressure to overcome it at the required flow rate. For this, you need to understand the CDU’s ratings – and the test conditions for these ratings (i.e., rating conditions).

Rating conditions and ratings

Just like a home’s air conditioner has certain ratings like cooling capacity, a CDU must also have ratings to help specify a CDU solution for your TCS. Fortunately, ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) is actively working on producing a standard to test and certify CDUs and other liquid-cooling equipment. This standard is needed to ensure that different CDUs can be compared fairly – an apples-to-apples comparison that improves transparency and industry practices. The forthcoming Addendum B to Standard ASHRAE 127-2020 specifies the test conditions and procedures used to rate CDUs. These standard rating conditions allow you to compare the performance of different CDU models because their ratings are measured consistently and under the same conditions. Below we first describe the standard rating conditions and then explain the CDU ratings you will likely use to compare different CDUs.

Standard rating conditions

• Ambient dew point temperature: held to no less than 2.0°C below the TCS leaving fluid temperature. Keeping the ambient temperature above the dew point prevents condensation on the piping that will alter thermal energy measurements.
• Load points: measurements are made at different load points: 100% (full capacity), 75%, 50%, and 25%.
• Facility water system (FWS) entering fluid temperature (FWS EFT): 26°C (78.8°F)
• TCS leaving fluid temperature (TCS LFT):  30°C (86°F)
• CDU approach temperature: 4°C (7.2°F). This isthe temperature difference between the cold coolant temperature supplied to the IT (TCS LFT), and the chilled water temperature entering the CDU on the FWS side (FWS EFT). See Figure 1. A smaller CDU approach temperature means that you can get your chilled water temperature (FWS side) closer to the one supplied on the TCS side (required by your IT). This in turn gives you a wider chilled water temperature operating range to optimize your entire cooling architecture.
• TCS entering fluid temperature (TCS EFT):  39.6°C (103.3°F) at 100% load. This temperature decreases as percent load decreases. For example, as the heat load decreases from 100% to 75% load, the TCS entering temperature will eventually reach the rating condition of 37.2°C (99.0°F). The heat load at this point is what determines the CDU’s 75% cooling capacity.
• TCS flow rate: established at 100% load to achieve the TCS entering (39.6°C) and leaving (30°C) temperatures above. This unique flow rate is specific to the CDU under test and, once established, is held constant for the four percent load points above. Note that the vendor must provide a value for their CDU’s cooling capacity at 100% load in order to establish the TCS flow rate.
• Fluids used: FWS side of the CDU uses water and TCS side uses 25% Propylene Glycol and 75% water.

A key caveat to these rating conditions is that the CDU under test is set up according to the vendor’s instructions. This plays a key role in the published ratings. For example, if a vendor instructs the testing lab to remove the CDU filter prior to testing, the ratings will appear more favorable.

Ratings

Published ratings are what vendors may present in their public literature such as specifications. These ratings should be based on rating conditions like those discussed above.

Below are five key ratings that we anticipate from the forthcoming Addendum B to Standard 127-2020.

• Cooling capacity (kW):  capacity is a measurement of the heat removed from the coolant as it passes through the CDU’s heat exchanger. Simply, it’s the heat removed from the TCS returning coolant (i.e., coolant coming from the IT load).
• CDU critical input power (kW): input power is the total electrical power CDU consumes. This includes any pumps, variable speed drives, control systems, etc. the CDU requires.
• Efficiency (COP): coefficient of performance is the ratio between the cooling capacity and the CDU input power. The higher the COP, the more efficient your CDU, since more heat is rejected per unit of energy. Note that the FWS pump energy is not included in the CDU’s critical input power.
• TCS external pump head (pressure) available (kPa):head pressure available is the pressure you can expect from the CDU at the rating conditions discussed above.

Pressure drop CDU introduces to FWS: This is the pressure drop at the facility water system side caused by the CDU. It is also referred to as CDU FWS pressure losses in the proposed Addendum B to Standard ASHRAE 127-2020. The lower this pressure drop, the better; as this translates to lower FWS pump energy consumption, and likely lower energy bills for your entire cooling system.

Having these ratings allows you to compare different CDU models, but there’s a catch. Remember the caveat above, “the CDU under test is set up according to the vendor’s instructions.” Herein lies the issue I alluded to at the start. A CDU requires a coolant filter – if the vendor decides to exclude the CDU’s filter for establishing its ratings, you won’t be comparing apples to apples. The filter acts as an additional pressure drop that the pump must overcome leading to a lower TCS pump head pressure.

Filtering and coolant quality

Proper filtration is absolutely necessary to prevent particles or contaminants from reaching your IT cold plates. Effective filtering helps avoid GPU throttling for a simple reason: cold plates rely on micro-channels to transfer heat from GPUs and other critical IT components to the coolant. If particles or other contaminants reach the cold plates, fouling reduces the cooling performance. Ultimately, this raises GPU temperatures and can lead to GPU throttling, server shutdown, or even damage to IT components. Any reduction in the heat transfer effectiveness in your TCS forces the TCS to increase flow rate (i.e., more pump power), which drives up energy bills. Poor filtration and/or coolant quality also shortens the lifespan of both your IT equipment and cooling gear, leading to added maintenance costs from service visits, equipment replacement, and downtime. ASHRAE recommends selecting a filter size no larger than half the smallest micro-channel dimension (e.g., cold plates with 50um channels require 25um filter size).

What to look for in a CDU

Working closely with your cooling vendor is key to choosing the right CDU. In addition to the ratings above, here are key attributes to look for:

• CDU monitoring and control capabilities: This includes the CDU communication and control scheme, along with its manual, automated, and remote operation modes. Also consider how the CDU integrates and communicates with other equipment and software in your data center.
• Filtering and coolant pressure monitoring: These are essential features. Proper filtration helps maintain optimum coolant quality and prevents clogging and wear issues across the IT equipment, CDUs, and connected TCS components. Pressure monitoring helps identify potential issues, including a clogged filter.
• Material compatibility list: Clear guidance on materials compatible with your CDU (and TCS) is a must to ensure proper operation of the liquid cooling system and to prevent premature failure of your IT and TCS connected components. Your CDU vendor should be able to answer any questions about materials used in wetted-surface components – those in direct contact with the TCS coolant.
• Hot swappable filter and pump: Assuming redundant pumps, this allows concurrent maintenance and simplifies servicing.
• Redundant power supplies: Redundant components such as power supplies help significantly reduce the risk of downtime.
• Short circuit current rating (SCCR): This protects CDU components during a fault. The SCCR must never be lower than the available fault current of the circuit feeding the CDU. The higher the SCCR, the easier it is to power the CDU in an existing data center.
• Harmonic filter: Electric motors can generate harmonics. Integrated filters help limit the harmonics injected into your data center’s electrical system.
• Transparent published ratings: Look for ratings that clearly indicate the configuration of the tested CDU, or at a minimum whether components like the filter were included in the test.
• Factory acceptance test (FAT): Published ratings rarely match application ratings, which depend on the design conditions of your specific data center. For an additional cost, some vendors offer the option of FAT, allowing you to test the CDU under the same conditions as your data center. This helps validate whether your CDU will perform as intended and reduces the risk of surprises during installation.

Wrapping up

All this to say, don’t let inconsistent or misleading capacity specifications confuse or distract you. Ask vendors to disclose the CDU configuration during rating tests, including whether a filter was installed, so you can fairly compare CDU external pump head (pressure) and flow rate across CDUs. Consider requesting a FAT to verify that the CDU will perform as expected in your data center. And don’t overlook proper filtration and coolant quality. Ask us for guidance, we want your liquid cooling deployment to benefit from our experience.

Next Steps?

Find here more information on our End-to-End Liquid Cooling Solutions for AI and HPC Data Centers, and our Coolant Distribution Units.

See here more resources to facilitate your data center cooling journey:

White Paper 211, 10 Ways to Harness the Energy and Water Efficiencies of Direct Liquid Cooling

White Paper 210, Direct Liquid Cooling System Challenges in Data Centers

White Paper 133, Navigating Liquid Cooling Architectures for Data Centers with AI Workloads

Schneider Electric University Course, Liquid Cooling: Essential Architectures for AI-Driven Data Centers