…And how this influences data center airflow management.
Sorry sports fans… this is not about your favorite team. Instead we are going to explore the fascinating world of mechanical fans.
How many times have you seen vender illustrations of fans pushing air in long blue lines from perforated raised floor tiles into the intake of a rack? The truth is that air does not move in such a way. Calculating the airflow induced by one particular fan at any given distance away from the fan, about any point of the fans face is a very involved set of calculations.
Traditional thermal designs for fans were originally measured as jet velocity of water jets. This presented a close estimate, but inaccurate data. A recent study in 2012 helped in creating very accurate formulas as to fan outlet velocity and distributions. (— Fan Outlet Velocity Distributions and Calculations Eli Gurevich, Michael Likov (Intel Corporation, Israel Design Center, Haifa, Israel) David Greenblatt, Yevgeni Furman, Iliya Romm (Technion Institute of Technology, Haifa, Israel)
Generally, volumetric flow rate and distance traveled decreases when contained air enters ambient room air, and this is why mechanical air contractors use ductwork or a contained plenum to direct supply air to the thermal load. Increasing the velocity of air in order to reach the thermal load, instead of using a duct system, is considered inefficient.
It’s important to understand the relationship of mechanical air movement from fans and what actually happens to the airflow. The issue with fans is the manufacturer’s stated CFM capacity, and the distance of air movement that the fan is capable of will carry it. This value reflects what the fan is able to produce in a given test environment. Manufacturer stated air displacement (CFM) is based on what is called normal temperature and pressure conditions (NTP).
The actual volume of air that a fan can displace varies due to two factors:
1) Air density (hot, low density or cold, high density)
2) Air pressure (positive or negative)
Thus it is important to determine the manufacturer’s test conditions for the fan, and then compare the data to the actual planned environment in which the fan will operate.
For example, when considering the installation of a fan in the subfloor to move subfloor air into the cold aisle, the first question that should be addressed is: “what is the temperature of the air and head pressure that the fan will operate in?”
Why? The temperature of the air will determine its density when confined to a constant volume. In most cases, the subfloor air is denser, which is good. Thus the more important question will be about the subfloor pressure. It is not unusual to have negative pressure areas in the subfloor due to high velocity air steams. The Bernoulli principle explains our concern, in that an increase of air speed will result in a decrease of air pressure. Additionally, when two air streams of high velocity air intersect from opposing directions, the result is often a subfloor vortex, resulting in the reversal of current.
So what’s the point? Imagine putting a raised floor fan system over an area with negative pressure. This would negatively affect the fan’s ideal operating conditions.
Consider this, what is the typical reason for using additional fans to move air into the cold aisle? Most likely the unassisted perforated tile or grate is not able to deliver sufficient airflow to the thermal load of the racks. What if this is based on inadequate subfloor pressure? If that is the case, adding a fan assisted raised floor panel will require taking into consideration the fan NTP. Also it will can drastically and unpredictably impact other areas of the data center as you “rob Peter to pay Paul” so to speak.
Consider the following subfloor airflow management strategies:
1) Eliminate high velocity air: This will ensure a more balanced delivery of air due to a nominalized subfloor pressure.
2) Cold Aisle Containment: Instead of designing rack cooling by placing an airflow-producing raised floor tile at the feet of each rack, why not create a cold aisle that is not dependent on perforated tile placement?
Cold aisle containment creates a small room of supply air that can be accessed by all IT equipment fans. Instead of managing each supply raised floor tile, the only requirement is ensuring positive air pressure in the aisle. Cold aisle containment systems provide several benefits: most contained cold aisles will only have a one-degree differential from the bottom to the top of the rack, and the cold aisle containment does not require high air velocity, which can create other airflow management problems, such as bypassing IT equipment intake.
Understanding the NTP conditions of IT equipment cooling fans is an important aspect of data center airflow management. For example, in order to properly adjust CRAC unit set points, it is important to know the temperature at which the supply air’s density will drop below each fan’s NTP conditions. It is possible to lower the supply temperature to a level at which an increase in fan speed would be required to make up for the less dense airflow, potentially offsetting any energy savings from a higher cooling set point.
Simply adding fans to cool IT equipment is not a quick fix; it is imperative to first understand why sufficient airflow is not available. It is important to understand the fan’s NTP in the proposed environment, and to see if you can supply IT equipment with consistent airflow by simply separating supply and return air through data center containment. Containment can prevent the unnecessary use of additional electricity that is required to operate fans, saving money and electricity in the long term.