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Optimizing Data Center Cooling Using Differential Pressure Sensing

Posted on Monday, June 2, 2014 by Alan Katz

While cool air is one of the most valuable commodities in a data center, optimizing air flow and cooling can be difficult without a good picture of air movement through your data center. By using differential pressure sensors, it becomes easy to get a clear understanding of the air flow patterns throughout your data center. The cost and ease of implementation can translate to an ROI in as little as one month.

Understanding differential pressure zones and airflow allows you to:

  • Identify wasted or misdirected air pressurization / flow
  • Make the changes to optimize flow utilization
  • Focus cooling resources and air flow on areas where it is most needed
  • Easily verify the effects of cooling and air handling efficiency efforts
  • Be alerted to any changes that may occur in pressurization indicating potential leaks or changes in air handling performance
  • Correlate air pressure / flow changes with temperature changes (using available temperature probes)

Using Packet Power's environmental monitor with differential pressure sensor, you get comprehensive, real-time data on temperature, humidity and differential pressure. The monitor can be placed in strategic locations. Installation is further simplified with all monitors automatically connecting to the Gateway. No expertise is required as the network is self-configuring and data can be viewed immediately on Packet Power’s EMX Portal or integrated into any BMS or DCIM.

E306 with diff pressure tubes

What Differential Pressure Tells You

Differential pressure is valuable in monitoring the efficiency of airflow or cooling through a facility. It is important to note that differential pressure is not a measure of airflow, but rather the air pressure that drives the airflow, specifically the difference in air pressure between one area and another.

If the differential pressure between two areas is high, that means that the flow rate of air between the high pressure and low pressure area will be faster. It does not tell us the volume of air flow, only the force.

Differential pressure can be a useful indicator when trying to optimize cooling systems and balancing airflow. Changes in differential pressure validate improvements in airflow such as better sealing of underfloor tiles or racks. It can point out unseen changes in air handling performances that may otherwise go unnoticed (i.e. a decreased differential pressure can be caused from leaks, fan performance, or obstructions).

Being able to observe differential pressure in key parts of the data center, enables airflow to be better understood, contained and directed, creating a more efficient and reliable environment.

Installing Differential Pressure Sensors

To install a differential pressure sensor, find two areas with different air pressures; an area with higher air flow will always have a different pressure to an area with a lower flow rate (i.e. inside an air plenum versus outside an air plenum). Typical areas for monitoring differential pressure include:

  • air plenums and vents
  • server racks
  • between aisles and under floor perforated tiles

Getting differential pressure data from Packet Power's E306 monitor is simple. Place one of the sensing tubes in a higher pressure zone like the base of a server cabinet by the air intake. Place the other tube in an ambient area with little airflow like the top of exterior of the server cabinet.

Note the area monitoring the higher pressure (or faster airflow) should use the tube marked with the (+) positive symbol. The area with the lower pressure or slower flow should use the tube marked with the (-) negative symbol.

E306 differential pressure label

The resulting pressure differential will be indicative of the degree of airflow coming through the cabinet. Differential pressure is expressed in Pascals (±500 Pa in 0.2Pa or ±3% accuracy). Remember that this is not a measurement of airflow but rather a relative indication.  A lower differential pressure will indicate a lower airflow rate. A higher differential pressure value indicates a higher flow rate between the high and low pressure areas. Airflow and thermal management goes beyond the scope of this article which is intended as a quick overview.

Packet Power's environmental monitor also allows up to six temperature points to be monitored via remote probes. Adding temperature measurements is an easy way to determine the cooling effectiveness of airflow changes. All measurements can be viewed on the local LCD display of the  monitor, or obtained via Modbus TCP/IP or SNMP through the Gateway (integrates with any BMS or DCIM), or via the Packet Power EMX Portal.

Installation Examples

data center temp flow

Penum air flow with baffle: The effect of baffle flow control can be monitored using a high pressure tube placed on the source side of the baffle and the low pressure tube on the opposite side of the baffle. A change in differential pressure indicates the effect of the baffle position or changes in the volume of air flow if the baffle position does not change.

Plenum air flow: Plenum air flow can be measured relative to ambient air. The high pressure tube is placed inside the plenum with the low pressure tube in the ambient air. An increase in differential pressure indicates higher a higher flow rate through any vents (assuming all vent openings remain the same).

Underfloor perforated tile: Placing the high pressure tube underfloor in the proximity of the perforated tile, and low pressure tube above the floor, the differential pressure will indicate the force of air through the floor tiles. Having a higher differential pressure will increase the flow through the perforated tiles. The pressure reading can assist in identifying leaks and confirming the effectiveness of improved underfloor air handling.

Server rack air flow: Measuring the differential pressure between the inside of the server rack and outside the rack (away from direct air flow such as the top exterior of the rack) can indicate changes in the airflow to the rack. A change in differential pressure can indicate a decrease in the pressure of the airflow into rack assuming the perforations or air flow channels have not changed. Likewise an increase in differential pressure can indicate a higher air flow rate (again assuming no change to the air path).   Since the EM3 support up to six remote temperature probes, it is possible to correlate the change in air pressure with changes in cooling performance, monitoring both the hot and cold zones of the rack.

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