Data Centre Cooling - Humidity

Relative humidity (RH) is becoming less of an issue as most equipment is specified to operate in a range of 10-90% RH with non condensing conditions.  ASHRAE allow a range of 20-80% with a recommended operation of the air dew point within 5.5°-15°C and maximum 60% RH.  Many data centres have now ceased RH control completely.  Close control to the 45-55% RH legacy standard can be an extremely energy intensive and costly process which can result in a PUE exceeding 2.5.

The performance of relative humidity is dependent upon the cold aisle/supply temperature.

We can control the maximum level of relative humidity by dynamically changing the supply set point.  The set point would normally be at 18.5°C but, when the RH goes over a set point, the temperature is increased and the RH falls by ~5% per deg C.  In this way we can control the humidity level in the room.

As an example using Leicester weather data 2011 and a data centre with a 10°C rise this would be the RH profiles for the hot and cold aisles with fixed set point.  From this you can see that the RH can exceed 80%.

Data Centre humidity - evaporative cooling

With a 22°C cold aisle then the RH profiles would be as shown below.  Using this method we can ensure full compliance with your requirements.  We can therefore run at 18°C for most of the time and only move the set point for about 7% of the time to maintain a RH<80%

Data Centre humidity - evaporative cooling

The electrostatic issues found with low RH are no longer an issue provided equipment is correctly grounded and operational procedures to minimise Electro Static Discharge (ESD) are followed.  The advanced control systems used in CRECs dynamically change the air supply set point to control the maximum RH level.  With this technique full compliance of temperature and upper RH can be achieved in the UK.
Tape products require a more stable environment.  In many cases it is economic to provide a storage area separate to the main data centre with its own appropriate temperature and RH controls.

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