I
recently received a query regarding the relationship between CFM
(cubic feet per minute) and density of heat load that can be supported
in a data center using a room-oriented cooling architecture.

CFM
is a non-SI unit of measurement of the flow of a gas or liquid that
indicates how much volume in cubic feet pass by a stationary point in
one minute. rate of air-flow. In a typical data center facility that
has conditioned air being delivered through a subfloor and via
perforated tiles to cool down IT equipment in racks, it sure helps to
understand the relationship between CFM (air-flow), temperature rise,
heat dissipation and the amount of cooling capacity required. The
diagram below charts the relationship between the required cooling
capacity (in kW) per tile versus the required air-flow (in CFM),
assuming a delta temperature of 20′F.

So,
for example, in this case, a 4kW of IT load would require almost close
to 600 CFM of air-flow. This relationship can be characterized by the
equation of:

So, for
example, in this case, a 4kW of IT load would require almost close to
600 CFM of air-flow. This relationship can be characterized by the
equation of:

CFM = (3412 x kW) / (1.085 x delta-T)
where
CFM
: an indicative amount of cumulative air consumption required by the IT
equipment in the rack for heat dissipation, measured by how much volume
in cubic feet per minute;
kW : the heat load of the rack in kilo-watts;
delta-T : the difference between the IT equipment intake temperature and its exhaust temperature (in Fahrenheit).

Sometimes, you could simplify the equation to just CFM = 3.1W / delta-T (i.e. each 3.1W per delta-T will require one CFM).

But
if we think about it, delivering conditioned air in a data center via
the subfloor has a limit. There is only so much air you can push
through a perforated tile or grate. Whatever design parameter has to
take into considerations the distance to deliver the conditioned air to
the IT rack, static pressure, amount of leakages in the facility,
amount of perforation (on tile), blockages under the subfloor, etc. So
therefore, the CFM part of this equation pretty much defines the upper
limit on air cooling. For example, if we were only able get 700 CFM
through a perforated tile, then we should ask ourselves how much heat
load (maximum) can this cool? This can be determined by

W = 700 CFM x 20′F delta-T / 3.1 = 4516.1

So, 4.5kW is practically as much as you can afford to cool in the IT rack given a 700 CFM.
That
is per design specs. To really understand if you’re getting 700 CFM
(actual), you could measure it. There are equipments out in the market
that specifically does this (examples are the CFM hood on the right).

So,
in summary, we need to understand this relationship to be able to
workaround the physical limitations and put in place design features
that can help us achieve a higher density load in the data center. Some
of these features includes minimizing leakages, maintaining an
appropriate subfloor pressure of between 5 to 12.5 Pa, eliminating
mixing of recirculated return air, and if possible, bringing cooling
closer to the heat source.

Achieving a higher density load in
the data center would mean reduction in footprint and an increase of
energy efficiency. Both are important to businesses in today’s
challenging climate.

Source/Kaynak : http://blogs.sun.com/waikit/entry/cfm_and_density_in_data