4 things you need to know about the airflow volume

Unless you’ve been living under a rock the last few years, you’ve probably noticed the role of airflow management in the data center become more and more important. However, there are many things to take into consideration before properly implementing an airflow management strategy. Whether you’re considering adjusting raised floor supply tiles, flow rates, cabinet density, modular containment, or room-level airflow management (AFM), here are a few reminders and potentially new fundamentals to bear in mind…

What you Need to Know

•  Volumetric flow rate, also known as flow rate or flow, is the volume of fluid (air) that passes through a given surface per unit time.

The most common imperial unit is cubic feet per minute (CFM) and the most common metric unit is cubic meter per hour (CMH). [1 CFM = 1.7 CMH]

• Perforated tiles and grates for supply air are tested to determine what flow rate they will deliver at various pressures—the static pressure differential from under the floor to above the floor.

The imperial/US unit is inches of water column (WC) and the metric unit is Pascal (Pa). [1″ WC = 249 Pa]

• In marketing brochures, tile manufacturers typically list their tile flow rate at 0.10″ WC. However, 0.10″ WC is most likely found in about 2% of data centers.

In all likelihood, 80% of data centers have raised floor static pressures between 0.03″ and 0.07″ WC (7.5 to 17.4 Pa). This discounts the occasional negative static pressure near cooling units. For example, a Tate GrateAire panel delivers 2,075 CFM at 0.1″ WC, but delivers about 1,475 CFM in a raised floor with 0.05″ WC.

• Flow rate is obviously important since it determines how much IT load (kW) can be cooled by the supply tile, but this is not as clear-cut as it appears. The kW of IT load that can be cooled depends on the ΔT (temperature rise) through the IT equipment.

Server cooling fans consume as much as 30% of total server power consumption. To help reduce power consumption, server manufacturers are implementing slower speed, lower-power fans, and higher ΔTs in their designs. Whereas older IT equipment often has about a 20 ΔT, more modern high-density equipment can have ΔTs around 35 F.

The following equation governs the relationship between power, flow rate, and ΔT:

CFM = 3.16 x Watts
                    ΔT

Where,

CFM = Cubic feet per minute of airflow through the server.

3.16 = Factor for density of air at sea level.

ΔT = Temperature rise of air passing through the server in degrees F.

For example:

A 5-kW blade server chassis with 16 servers and a 35 F ΔT would draw 451.4 CFM to keep itself cool.

451.4 CFM = 3.16 x 5000
                              35F

Ten 500-Watt pizza box servers (also 5 kW total) with a 20 F ΔT would draw 790 CFM to keep cool.

790 CFM = 3.16 x 5000
                           20F

Another way to look at this:

• Servers with a 20 F ΔT require nearly 158 CFM/kW to maintain internal design temperatures (500 CFM / 158 CFM per kW = 3.2 kW).

• Servers with a 35 F ΔT require nearly 90 CFM/kW to maintain internal design temperatures (500 CFM / 90 CFM per kW = 5.5 kW).

A tile delivering 500 CFM can cool 3.2 kW of IT equipment with a 20 F ΔT.

So when thinking through supply tile placement or new AFM measures, always take into account your volumetric flow rate, floor static pressure differentials, ‘best-case’ vs. ‘real-world’ tile flow rates, and IT equipment temperature deltas.