Challenges in Using Outside Air Economizers and How to Overcome Them

Robert F. Sty, PE, SCPM, LEED AP is Principal of SmithGroupJJR’s Technologies Studio in Phoenix, AZ and focuses on the Architecture and Engineering design of mission-critical facilities. Robert is on LinkedIn.

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With the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) expanding the window of operating temperature and humidity ranges inside the data center, many designers and operators clearly see the benefits in the use of outside air economizer strategies. Also known as “free cooling,” the economizer operation allows the facility to shut down the chillers or DX compressors, and cool the data center by introducing outside air at the proper conditions. Although many times the need for mechanical cooling is not completely eliminated from the data center, there are significant energy and cost savings associated with economizer strategies.

The concept of outside air economizer seems simple. Turn the chillers off, open the outside air dampers to let the fresh air in to cool the cabinets, and let the air back out through some means of relief. Unfortunately, if not designed properly bringing in outside air can actually put the data center operation at risk. Listed below are four common issues with the design of economizers and the strategies used to minimize the risk to operation.

Outside Elements Inside the Data Center

It is obvious that with bringing in outside air into the data center to cool the racks a number of other particulates such as dust and particulate matter will also enter the facility. Dust can enter the data center and clog server fans, and under the right conditions can also become wet and therefore conductive (although this is a rare occurrence). ASHRAE published an update in 2011 to its original 2009 white paper “Gaseous and Particulate Contamination Guidelines for Data Centers.” This white paper goes on to recommend that room air, which is continuously recirculated be filtered with MERV 8 and that any outside air that is introduced into the data center be filtered with MERV 11 or 13 filters. Dust will always be present in the data center, but proper filtration of outside air can minimize a number of operational issues.

Proper Mixing of Outside and Return Air

Depending on the local climate, there could potentially be many times throughout the year when the outside temperature is below the ASHRAE recommended low temperature limit of 65°F. These times of the year the HVAC system would operate under what’s known as a “partial economizer.” During this mode a portion of the warm return air typically in the 90°-95°F range is mixed with the outside air to reach the desired supply temperature (typically in the 75°F range).  The balance of hot air is exhausted or relieved to the outside.  If this is done inside a central station air handling unit mixing plenum, it’s almost a guarantee that the return and outside air will mix properly.

In cases where it is done outside the air handling unit, such as a large ducted mixing plenum or room, care must be taken that warm return air properly mix with cold outside air to ensure that cold/hot spots do not enter the air handling units.  Cold spots could potentially cause coil freeze and burst in cold weather climates, and warm spots will impact IT cabinet performance.

Temperature and Humidity Changes

Throughout the day the outside air temperature and humidity can fluctuate based on incoming weather patterns or during the “shoulder seasons” of spring and fall.  Control systems should include weather stations that will enable the BMS to react to the changing weather patterns.

In certain climates such as Salt Lake City, UT, morning temperatures could fall within acceptable ranges, but by early afternoon can be much hotter than the allowable recommendations of ASHRAE TC 9.9.  As the data center changes modes from free cooling to chilled water cooling, there must be enough time allotted for the central plant to drop the water temperature to the appropriate set point before the outside air dampers are completely closed.

Given the large volume of chilled water in some piping systems, it is beneficial to understand the time it takes to reach this desired set point so that a proper reaction time is established in the system control strategy.  In addition, the ASHRAE recommendations give a maximum rate of change for temperature and humidity inside the data center.  HVAC sequence of operations must be written to take these limits into consideration as well.

Building Pressurization

Introducing outside air to the data center requires exhaust from the return air plenum or the building can become over pressurized.  Some level of positive differential pressure in the data center is not necessarily a bad thing as it helps with moving the air into IT cabinets while using strategies such as hot aisle containment. The HVAC designer should take caution in just how much positive pressure is set as to avoid issues with opening/closing doors.  The means of economizer relief is an important consideration in the system design.  It is tempting to allow air from the hot aisle or return plenum to gravity relief without the use of relief fans.

The elimination of fans reduces electrical consumption and lowers the PUE.  With gravity relief designs instances have occurred where return air plenums have been over pressurized to open a gravity relief damper to relieve air from the building.  This can create issues with “ballooning” of membrane roofing systems or “oil canning” (popping) of sheet metal roof decking.  Both of which are undesirable results.  Relief fans with variable speed drives set to maintain proper pressure differentials between hot and cold aisles are effective, provide finer pressure control, and do not add significant energy demands to the facility.

Address Issues in the Design Phase

Correctly designed outside air economizer systems can result in significant energy and economic savings for data centers. Without proper attention to the above items, the introduction of outside air can pose additional risks to data center operation and uptime. However, if one takes into consideration these issues, a properly designed HVAC system will yield positive results.

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