Efficient racks arrangement - Air Flow Patterns

The air-flow through the IT equipment and the facility itself are the crucial factors in terms of managing the correct temperature and hence, avoiding equipment failure. The cold air flow patterns from the bottom to the front part of the server, forced by the small cooling fans integrated and cooling down all the components such as memory device and processor, where the biggest amount of heat is generated.

In a bigger picture, there are three acceptable and efficient flow patterns in a rack system: front to rear, front to top and both of the simultaneously.

Fig 1: AirFlow Protocol

This air flow pattern goes in line with the most used rack arrangement in an air-cooled facility: the hot-cold aisle arrangement. In this configuration, racks are placed so the front part of one rack is never facing the back of another.
By doing this, you are able to create alternating rows of cold air-flow and hot return air, allowing a proper cooling equipment design and avoiding air flows mixing.

RAISED FLOOR CONFIGURATION AIR FLOW PATTERN

The most common implementation is the raised floor configuration with CRAH units.
By using this open aisle arrangement, we can face a potentially huge drawback. As there’s no physical barrier to prevent the IT equipment hot exhaust air, it can be recirculated and mixed with the cool supply air. Increased cooling airflow can reduce recirculation but cannot prevent it, and a low cooling airflow will result in temperatures above the ASHRAE allowable limits.
One accepted solution for this open aisle arrangement issues is the cold aisle containment, achieved by using panels acting as barriers.
It can result in:

• Uniform temperature distribution at the IT equipment inlet.
• Cooled temperatures in the cold aisle as there would be no airflow mixing.
• The energy consumption would be reduced, as there’s no need to increase the cooling airflow to prevent recirculation.

As a conclusion, it is extremely important to find the right balance in the airflows patterns, both hot and cold. IT equipment temperatures should always be inside of the allowable limits recommended by ASHRAE in order to avoid equipment failures.