Cooling |
Indirect free cooling |
Liquid asset |
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direct free cooling should only be used where filtering can control the quality of outdoor air . Also , if the outdoor air is too cold , a certain proportion of warm air from the data centre must be mixed in with it to supply controlled , tempered air to the IT equipment .
Humidification also has to be controlled and methods to humidify or dehumidify air can also be very expensive and complex , with large ductwork systems often required . In addition , large openings in the building fabric are needed to bring in the outdoor air , plus an equally large ductwork system and fabric opening for the exhaust air . These openings and ductwork bring a security risk to the data centre that has to be considered and in the event of an external situation such as a fire , the system must have the ability to operate independently of the air inlet .
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Figure 3 : Indirect free cooling with heat exchanger thermal wheel .
Figure 4 : Indirect free cooling with CRAC and dry cooler .
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With indirect free cooling , no outside air enters the data centre and CRAC systems can be configured to suit requirements . However , indirect free cooling is theoretically less efficient than direct free cooling , as at least one heat transfer must always take place between the air in the data centre and the outdoor air .
Single stage indirect free cooling is based upon an air / water heat exchanger placed on the hot water return of the chiller . Outside air is blown across the heat exchange and helps remove the heat before it gets to the chiller . Therefore , with a return water temperature of 20 ° C , if the outside temperature is 19 ° C or less , it starts reducing the activity of the chiller and reduces running costs .
Two stage indirect free cooling systems have a considerably smaller footprint than their single stage counterparts , but are less efficient , as they function with two heat transfers . In the first heat exchanger , the heat from the air in the data centre is transferred to a liquid , normally glycol . This is then pumped to the outside in relatively small pipes , where heat from the data centre is rejected to the outdoor air via a second heat exchanger .
Indirect dynamic free cooling
To further improve the efficiency of indirect free cooling , dynamic technology controls the mode in accordance with the current heat load in the data centre , and so increases the time in free cooling mode . Moreover , it has another operating mode – extended free cooling – that further lengthens the operational time and drastically reduces operating costs by cutting energy intensive compressor cooling to a minimum .
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Air contains a certain amount of water and the total amount depends on the air temperature and barometric pressure . As water absorbs into the air , an adiabatic process takes place – this means the temperature of the air decreases , while the energy content of the air remains unchanged .
By using adiabatic assistance in direct free cooling systems the air that enters the data centre is cooled before entering . Indirect free cooling and adiabatic assistance is more conducive however , as the adiabatically assisted outdoor air never enters the data centre .
In single stage indirect free cooling systems , adiabatic assistance reduces the temperature of the outdoor air entering the air / air heat exchanger and reduces the amount of mechanical cooling required to satisfy the supply air temperature to the IT equipment . In two-stage indirect free cooling systems , the warm outdoor air can be cooled by adiabatic assistance before entering the dry cooler to either increase the hours of usable free cooling , or extend the mixed mode at the upper limit to reduce the use of mechanical cooling .
Last word
As the density of installed equipment in the data centre has risen , so too has the amount of heat generated . While being able to fit more kit into a smaller space is generally considered a good thing , the need to control temperature has led to the growing use of free cooling . It should be remembered that no free cooling system is perfect though , and all have limitations . It is therefore important to evaluate what kind of free cooling is best suited to the application on a caseby-case basis .
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28 | August 2017 |