17 January 2024
Matthew Blackmore, Head of Business Development at Armstrong Fluid Technology, explains the key principles for design and operation of ultra-efficient chilled water systems for data centre applications.
Data centre applications present specific challenges in relation to the design and operation of chilled water systems. These challenges are driven by higher than average cooling loads, the need to design for over-capacity/modular expansion, and shorter upgrade cycles. This article discusses the key principles for ultra-efficient design and operation.
Control and sequencing principles
Since a data centre cooling system is required to be reliable and efficient over a greater operating design envelope, meeting variable demand, performance of the system at part-load is critical. The most effective method to satisfy the continuously variable, critical cooling demands of data centres is to utilise all variable-speed components – chillers, pumps and fans – and a control strategy specific to the unique operating characteristic of variable-speed devices. There are no exceptions to this, because constant-speed devices cannot solve the challenges of a varying application such as data centre cooling.
When a variable frequency drive (VFD) is added to a compressor, pump or fan to improve part-load efficiency, the energy savings potential is huge due to the pump fan laws which state: power is proportional to rotary speed cubed (PαN3). If a rotating device is allowed the flexibility to operate along its Natural Curve, a 50% reduction in flow would be equivalent to (.53) or 12.5% nameplate power draw.
This would equate to 50% / 12.5% = 400% increase in operating efficiencies. This efficiency is only possible if the pump fan law relationship between pressure and rotary speed, along the Natural Curve, is maintained at the decreased speed. A 50% reduction in flow would be equal (.52) or a 25% reduction in pressure.
Figure 1: comparison of pump curves. Pump operating at fixed differential pressure compared with pump operating along its natural curve
Traditional control practices, however, often fail to optimise this potential. Pumps, for example, are often set to maintain a fixed or minimum differential pressure across the pump supply and return headers (see Figure 1 above). This means the pump will not have the freedom to operate along its Natural Curve and will consume more energy. Best practice is to utilise advanced integrated control across the system. In this scenario, with the pump placed at the CRAC unit, the flow/pressure relationship is maintained automatically for optimum efficiency. In the case of variable speed chillers, integrated control ensures operation along the chiller’s Natural Curve for all operating scenarios, ensuring optimum efficiency at all loads.
Another important design principle is the employment of capacity-based (rather than demand-based) sequencing. With capacity-based sequencing, each chiller would be taken up to 90% loading, for example, before the next chiller was introduced. Demand-based sequencing, however, balances the load across the system as a whole, unlocking additional energy efficiencies which might otherwise remain under-exploited.
Modular/incremental approach to increasing building loads
Chilled water system design also needs to reflect the specific incremental increases in demand relating to data centre operation. Heat load densities in IT-intensive facilities are typically increasing by 25% to 30% annually, and upgrade cycles for data centre processing and storage equipment are currently three to five years. This provides a challenge for the related critical cooling systems, which have 20-year potential lifecycles. Over-sizing at the outset in order to accommodate the full cooling load at a later date is hugely inefficient and does not align with the typical financing models employed in the data centre industry. The control and sequencing principles discussed above are therefore central to optimisation of energy efficiency at each stage of the data centre’s evolution.
Importantly, however, the design strategy also needs to harness the opportunities inherent in modular packaged plant solutions, to provide repeatability and ease of expansion. Modular all variable speed HVAC solutions, such as Armstrong’s iFMS modular packaged pump solutions for example, are capable of ‘bolting-on’ additional cooling in line with expansion of IT processing capacity. In addition to avoiding the energy wastage of over-sized plant, this assists profitability by preventing the need to front-load capital investment, and guarantees reproducibility.
Digital twinning and Active Performance Management
Finally, best practice chilled water design for data centres involves harnessing advanced technology, during the developmental phase and throughout component lifecycles, to optimise and maintain energy efficiency performance. Digital twinning, for example, is a powerful tool for effective chilled water system design. It involves the creation of a virtual representation which can function as the real-time digital counterpart of a process. Practical application of this technology was pioneered by NASA to improve spacecraft design, and the first physical-model simulation was announced in 2010.
Since then digital twinning has been introduced in a number of other technology sectors to continually improve product design and development. In data centre applications, for example, the Hysopt Hydraulic System Support technology employed by Armstrong delivers numerous benefits via digital twinning. In addition to the usual data such as room dimensions and building load, the Hysopt technology creates simulations employing far more operational characteristics than before, from anticipated temperature, pressure and flow throughout the hours of the day, to the specific weather-related data for the specific location across the entire year.
Extremely detailed calculations of system operation, energy consumption and carbon savings can therefore be created. The technology is also powerful enough to allow alternative scenarios to be compared, by replacing key system components. For example, a system designer can rapidly receive a quantified report on the impact of replacing a specific pump model, with actual calculations of the resulting savings in energy consumption, energy costs and carbon reductions provided by the technology in minutes. Using these insights the simulation also calculates the payback period and return on investment.
After installation of new or upgraded systems, the performance and resulting savings can be verified/proven using the same digital twinning technology.
Advanced connectivity and visibility of system performance are also important throughout the lifetime of ultra-efficient critical cooling systems for data centres. Without information on fluid flow, across the system, it’s difficult to diagnose and optimise efficiency. With accurate flow information, the picture changes entirely. The Active Performance Management developed by Armstrong Fluid Technology, for example, helps to optimise HVAC systems at any stage of a data centre’s life-cycle, responding to changing cooling requirements. The combination of smart commissioning with real-time alerts and system transparency addresses performance drift and maintains occupant comfort. With Active Performance Management you can make annual energy savings of up to 40%.
One of Armstrong’s Active Performance Management solutions is Pump Manager, which ensures that pumps continue to operate efficiently and reliably throughout their complete lifecycles. Pump Manager is a cloud-based application that uses the embedded intelligence and connectivity of Armstrong Design Envelope pumps to provide performance reports to system operators. With this information, operators can make changes and address issues to optimise HVAC performance.
Online trending and analysis across multiple parameters on single pumps, or on an aggregated basis for multiple pumps, assists in identifying performance degradation and facilitates a predictive and proactive approach. Pump Manager will, for example, report issues such as excessive vibration, pump in hand, risk of cavitation or a dead head should they start to occur.
Compatible with industry-standard BMS, EMS or CMMS solutions, Pump Manager helps reduce operating costs by providing data to support continuous optimisation of pump performance. Pump Manager also increases pump availability and reliability, reducing unexpected failures and providing early problem detection. Lastly it helps organisations report their energy use and environmental performance.
To conclude, ultra-efficient critical cooling involves specific challenges, but the application of the key design principles discussed here, and the utilisation of advanced technologies such as digital twinning and Active Performance Management, can meet and surpass the exacting standards demanded by the data centre industry.
3D rendering of a chilled water plantroom