1.3 Bypass

What is it?

Bypass is defined by the ratio between the volume of cooled air that is pushed out of the computer room air conditioner (CRAC) and the volume of air that is circulated directly back into the CRAC’s without passing through any IT equipment. Bypass is easily measured by a series of temperature measurements in the airflow and calculating the mixing percentages of hot and cold air:

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

Why should you care?

Bypass has a detrimental effect on cooling efficiency in two distinct ways, first because fans have to work harder to push a larger then needed volume of air, secondly because of cold air mixing, the return temperature on the CRAC is lowered, decreasing the cooling capacity of the cooling coils in the CRAC. Ideally, bypass is near 0%, obtained through balancing the amount of air needed for cooling in a certain area with the supply of cold air, through i.e. adjustable floor tiles.

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

Bypass 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 Recirculation 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.