Air Management is a discipline to address and minimize re-circulation of warm air and by-pass of cold air in the data center. The main objective of air management is to achieve energy savings and a better thermal condition for the data center. The picture below illustrates what by-pass and re-circulation means:
By-pass air refers to cold air which does not participate in cooling the equipment, i.e. wasted cooling capacity. Typically, by-pass air may be caused by an excess of supply air or leakage through cut-outs on floor tiles, or inefficient positioned perforated tiles. The implication of by-pass is that it requires higher system airflow (higher fan energy) and leads to lower return air temperatures (lower chiller efficiency).
Re-circulation air refers to hot exhaust air being channelled back to the intake of the equipment rack and going through the IT equipment multiple times. This should be minimized. It may be caused by a deficit of supply air. Re-circulation air leads to less control of the equipment intake conditions and may result in reduced equipment reliability and longevity. It also may lead to a “perception” of hot spots and a need to increase the overall supply airflow (higher fan energy) or reduce the supply temperature (lower chiller efficiency).
Air Management is important because:
- Efficient air management helps to reduce overall operating costs by enhancing economizer utilization, improving chiller efficiency, and reducing fan energy. (Energy Management)
- More than 50% of cooling capacity in data centers today are wasted due to random mixture of air. Therefore, improved air management will bring about energy efficiency and this means that we can reduce capital investments for cooling equipment and air-moving equipment as it is no longer necessary to over-provision. (Capital Management)
- Adequate thermal conditions (intake temperatures) are important for the reliability and longevity of electronic equipment. (Thermal Management)
There are a few metrics generally used to measure the performance or efficiency of air management strategies in a data center, however, these metrics requires accurately measured data and are usually used in conjunction with a detailed CFD analysis.
The Rack Cooling Index (RCI) index is a measure of how effectively the equipment are cooled and maintained within an intake temperature specification. The index helps evaluate the equipment cooling effectiveness.
The Return Temperature Index (RTI), or sometimes referred to as Return Heat Index (RHI), is a measure of the level of by-pass air or re-circulation air in data centers.
The Supply Heat Index (SHI) is a dimensionless measure of re-circulation of hot air into the cold aisles. SHI not only provide a tool to understand convective heat transfer in the equipment room but also suggest means to improve the energy efficiency. SHI is a number between 0 and 1, the lower the better (ideal to be <0.40).
Best Practices / Strategies to achieve efficient Air Management:
- Arranging racks layout in alternating hot and cold aisles configuration.
- Consider separation of cold and hot aisle by capping or erecting a containment around the cold / hot aisle.
- Bring the supply of cold air as close to the equipment (heat source) as possible without prior mixing with ambient air and return hot exhaust air without prior mixing with ambient air.
- Minimizing unncessary and inefficient placement of perforated floor tiles.
- Minimizing leakages from floor cut-outs, exposures or empty equipment racks through use of brushing strips and blanking panels.
- Draw return from as high as possible.
- Use CFD to inform design and operation.
- Design for provision of variable airflow fans for AC unit supply.
Tags: Air Management, airflow, by-pass, Efficiency, re-circulation