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Feature
The change in the ASHRAE guidelines for environmental control included an expansion of the recommended temperature window , from 20-25 ° C to 18-27 ° C and an increase in the maximum humidity level from 55 % RH to 69 % RH and 15 ° C dew point . This loosening of the guidelines made the use of low energy ‘ free air cooling ’ systems more practical in data centre environments . Fresh air brought in from outside is used to provide cooling rather than compressor driven CRAC units . In a UK climate this type of cooling can maintain a data hall environment to the recommended levels for around 70 % of the year .
Alongside the use of free air-cooling systems , humidification took on a different role within the data centre environment . A cold-water humidifier will provide around 680W of cooling for every 1kg of moisture it evaporates and can operate on as little as 550W while providing up to 840kg per hour . By supplementing free air-cooling systems with evaporative cooling from cold water humidifiers , the use of free air-cooling systems can be extended from 70 % of the year to 95 %, reducing the reliance on compressor driven cooling to just 5 % of the year – the hottest days .
There are two types of evaporative cooling using cold water humidifiers – direct and indirect . Direct evaporative cooling sees the humidifier providing moisture to the incoming air , increasing its humidity while decreasing its temperature . Indirect evaporative cooling is when the humidifier operates on the exhaust air . Before being expelled , the air is humidified and cooled , then passed through a heat recovery unit to transfer the cool thermal energy to the incoming air . This cools the incoming air without adding any additional moisture .
Packaged heat recovery AHUs are starting to be manufactured with this type of innovative indirect evaporative cooling technology incorporated . The AHU passes the return air through a HumEvap MC3 evaporative humidifier , humidifying the air and cooling it by several degrees . The AHU ’ s heat recovery unit then transfers a percentage of this temperature drop to the incoming fresh air reducing the load on energy-hungry chillers . Other packaged AHUs using evaporative humidifiers in a direct evaporative cooling capacity have been able to achieve up to 12 ° C cooling from the humidifier when used in data centre environments in hot climates .
The efficiency of a data centre is measured in what is called the Power Usage Effectiveness ( PUE ), which is a ratio of the energy consumed by the servers compared to the energy consumption of other services such as the lights or cooling equipment , for instance . If a data centre has a PUE of 2.0 , then for every kW used by the servers another kW is being used
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RACA Journal I September 2023 www . refrigerationandaircon . co . za