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?