Posted by: Electric Thoughts™ | April 2, 2012

Thermal Instability

Reducing Thermal Instability

Today, the traditional means of data center cooling is through decentralized air conditioner control. And yet, decentralized control is inherently unstable. This instability occurs because most data centers are redundantly cooled. In these scenarios, there is a lot of cross talk between air conditioners. Cross talk causes positive feedback.

Decentralized control leads to at least three different types of thermal instability. The first is when air conditioners fight against one another creating the afore-mentioned crosstalk, based on redundant cooling. The effect of this real-time crosstalk is that temperatures cycle up and down, which can cause the temperature to exceed the rate of change limits specified by ASHRAE.

The second type is short-cycling of compressors for air conditioners which have direct expansion cooling. When these air conditioners operate at low load, the compressors will often short cycle. This causes the temperature at the rack inlet to oscillate in an unacceptably high frequency, which again can cause the temperature to exceed ASHRAE guidelines. The third type derives from the combination of points one and two, in which some air conditioners overpower others and cause them to deliver warm air – versus cool air – to raised floor plenums or other areas that should receive cool air.

An additional problem stemming from temperature instability is the increased wear and tear on air conditioning and IT equipment. For example, compressors that are continually cycling wear out faster than those that run at a steady speed. Perhaps more significantly, cycling temperatures impose a higher level of mechanical stress on electronic components – which can result in either intermittent or catastrophic failure of IT equipment.

Smart coordinated control of cooling equipment eliminates or attenuates a number of these common instabilities. This improved control can have a dramatic, positive affect on both mechanical and IT equipment life and ultimately, on system reliability.

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