Evolution – A Window to my mind

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:

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).

For simplicity, you could round down 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.

Tags: airflow, CFM, Data Center, Density, Temperature

This entry was posted on Wednesday, September 2nd, 2009 at 21:46 and is filed under Data Center Facility. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.