34 Chillers
Raising the chiller bar A
T CONCEPTUAL STAGE, advanced design techniques such as computational fluid dynamics (CFD) are used to analyse a range of coil types and orientations to maximise the heat exchange area and airflow. The modular ‘V block’ arrangement and use of microchannel condensers instead of round tube plate fin alternatives, provides a highly efficient surface area for refrigerant heat exchange, enabling increased cooling capacities and lower condensing temperatures within a smaller footprint.
A further performance benefit is that these coils have a lower airside pressure drop which increases airflow; this helps to increase the total heat rejection and makes the fans more energy efficient at full and, particularly, part-load.
In addition to significantly improving performance, the ‘V block’ arrangement also makes it easier to access the unit for maintenance.
The variable speed control – between 25% and 100% – of the centrifugal compressors allows cooling duty to be precisely matched to demand for tighter water set point management, allowing significant part-load efficiencies to be achieved under low load conditions or in low ambient temperatures. Being oil-free, the heat transfer coefficient is improved because there is no oil coating the heat exchangers. In addition, the absence of vibration and fewer moving parts within the compressor reduces operational wear and tear and risk of downtime.
The inherent low starting current of a centrifugal compressor removes the transient starting spikes associated with a similar sized screw compressor which means that electrical supply components need not be oversized on site. The use of flooded evaporators with built-in suction gas heat exchangers improves compressor energy efficiency by up to 15%, particularly at part-load, via the highly efficient method of boiling heat transfer.
EC – electronically commutated – fans reduce power in comparison with equivalent AC fans at both full and modulated fan speeds by as much as 70%. The in-built EC fan control module allows for fan speed modulation from 15%-100%;
ACR News August 2015
in contrast, the modulating range of a standard AC fan is typically between 40% and 100% of full fan speed. In addition, current generation chillers also benefit from lower noise levels on compressors and fans, an important factor where chillers are situated close to residential areas.
Free-cooling
Free-cooling saves vast amounts of energy, particularly when water temperatures are high. To take effect, free-cooling needs a temperature difference between the outside ambient air and return water temperature of just 1°C.
A concept pioneered by Airedale, concurrent free-cooling provides substantial energy-saving opportunities by delivering free-cooling at the same time as DX cooling when free-cooling alone cannot meet the demand. There are two types of free-cooling methodology: thermosiphon and the more efficient, higher performing ‘dedicated water coil’ approach adopted by Airedale. Both methodologies have their pros and cons but, weighing them up, the dedicated water coil approach always comes out on top especially at higher return water temperatures which many applications are now adopting.
With the thermosiphon free-cooling system, mechanical and free- cooling cannot occur simultaneously within the same circuit and free-cooling is typically limited to 30%-35% of the mechanical capacity compared with around 70%-80% with the methodology
applied by Airedale. The energy
savings are achieved through reduced demand for DX cooling and by maximising the part-load
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efficiencies of components. The variable speed control of the fans allows load to be very precisely matched to cooling duty, reducing energy consumption and unnecessary wear, extending unit life.
Elevated supply and return temperatures In the northern hemisphere, free-cooling opportunities are relatively abundant. Raising supply and return temperatures can however generate additional free- cooling opportunities well within the ASHRAE (Class 1 & 2) recommended data centre upper operating temperature of 27°C.
A free-cooling chiller linked with an air handling unit or rack-mounted unit in a 24/7 data centre with a typical room temperature of 24°C and a high supply water temperature of up to 17°C could
Since the launch of the UK’s first water cooling industrial chiller, systems have become increasingly advanced, delivering higher energy efficiency per footprint and capable of ever greater cooling loads, currently up to 1.83MW. Mike Peachey, new product devel- opment manager for Airedale International, discusses some of the principal technologies applied to the latest generation of chillers.
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