1.4 Recirculation

What is it?

Recirculation is defined as the percentage of hot exhaust air from active IT equipment that is circulated back into the intake of active IT equipment. Recirculation can be calculated from air temperatures with the following formula:

recirculation = (rack inlet temp - CRAC outlet temp) / (rack outlet temp - CRAC outlet temp)

Why should you care?

Recirculation has a detrimental effect on cooling efficiency through a second order effect. With high levels of recirculation, hotspots are created, this in turn requires much colder air to be supplied by the CRAC in order to compensate for this effect. Secondly, recirculation prevents us from raising the overall temperature of the datacenter. Much of the IT equipment being produced today is capable of handling up to 35 degree Celsius air intake. Running a datacenter at say 30C instead of 20C can save as much as 30% on cooling energy. Hotspots created through recirculation prevent such drastic temperature raises.

How does it relate to the other items in the model?

Recirculation is a very specific KPI that greatly influences cooling efficiency and consequently DCiE values.

Which KPI’s are directly related to this KPI?

• KPI DCiE is impact directly when the airflow is not controlled.
• KPI Bypass is directly related to this KPI as both are part of the set of airflow KPI’s.
• KPI Electrical and Mechanical Management delivers the input to calculate the bypass on a realtime basis.
• KPI Change and Configuration Management influences this KPI because any change made to the data center and IT can impact airflows.
• KPI Product Lifecyle Management ensures that the IT hardware in the data center fits the airflow requirements best.
• KPI Capacity Management is used to plan future airflow requirements.
• KPI Service Level Management supports chargeback and other means to run a data center successfully.