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Data Centre Humidification

Condair humidifiers are used in data centres around the world to provide humidity control, prevent electrostatic discharge (ESD) or offer high capacity, low cost evaporative cooling. Some of the world’s largest brands put their trust in Condair humidification systems to help them fulfil their data centres’ environmental control strategies.

Humidity control in data centre environments has come a long way over the years. From the early computers which used punched cards that needed humidification to maintain the physical properties of the card to magnetic tape data storage that needed a level of humidity to prevent dust and debris adhering to the reels.

As technology has moved on so has the need for data centre humidification. The main reason to humidify data halls has been to combat electrostatic discharge (ESD) that can damage sensitive electronics. However, servers have now evolved to minimise the risk of damage from ESD, reducing the need for humidification of data halls.

This led ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) to change its recommended guidelines on the acceptable lower level of humidity in data centres in 2008. Previously the recommended humidity level included a minimum of 40%RH (relative humidity) but was reduced and is now measured in dew point rather than relative humidity as studies suggested absolute humidity has a greater impact on ESD. The new recommended lower limit is a line from 18°C dry bulb temperature and 5.5°C dew-point temperature to 27°C dry-bulb temperature and a 5.5°C dew-point temperature. Over this range of dry-bulb temperatures and a 5.5°C dew point, the recommended RH varies from approximately 25% to 60%.

The reduction in the minimum recommended humidity level has resulted in a fewer number of hours per year when humidification (and its associated energy use) to combat ESD is necessary. However, humidification is still needed in winter months, when cold outside temperatures result a very low internal humidity and increase the potential risks from ESD.

Traditionally electrode boiler steam humidifiers incorporated in the CRAC (Computer Room Air Conditioning) units have been used to combat ESD in data centres. However, this type of humidifier has a high energy consumption and maintenance requirement, giving it a high overall operating cost. More recently, low energy cold water humidifiers have been used in innovative energy saving strategies.

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.

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/hr. By supplementing free air cooling systems with evaporative cooling from cold water humidifiers, the use of free air cooling systems can be extended reducing the reliance on compressor driven cooling to the very hottest days. 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 Useage 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, cooling equipment, etc. If a data centre has a PUE of 2.0, then for every kW used by the servers another kW is being used by the other services within the data centre. Ideally the PUE should be as close to 1.0 as possible. The revised ASHRAE guidelines and the resulting adoption of free air cooling with evaporative cooling have resulted in many modern data centres being built with a greatly reduced PUE.

Although the revised guidelines have reduced the need to humidify to combat ESD, the use of humidifiers in data centres has actually increased as a result. Low energy cold water humidifiers are not only delivering energy efficient humidification in cold winter months but are helping reduce PUE in warmer months when free air cooling alone doesn’t deliver enough cooling to satisfy the demand.

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