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.
