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Hot Aisle/Cold Aisle Containment Drives Changes to Fire Protection in the Data Center - Part 2 of 2

Lee Kaiser posted this on February 2, 2012

Advances in computing technology, worldwide growth in data consumption, and the need for more efficient cooling of IT equipment is driving data centers to change rapidly. A significant part of these changes include the use of Hot Aisle or Cold Aisle (HACA) containment systems. The partitions that form these containment systems are affecting fire protection systems in these spaces. In the first part of this series of blog posts, I introduced how HACA systems are changing fire protection approaches in data centers. In this second part I will continue the discussion of how HACA systems are specifically challenging both fire detection and fire suppression systems in the Data Center. Challenges to Fire Detection

Hot Aisle or Cold Aisle (HACA) containment systems

The National Fire Alarm and Signaling Code (NFPA 72) gives guidance to engineers on the spacing of smoke detectors in rooms with different air change rates. The fire alarm code only provides data for smoke detector spacing in rooms up to 60 air changes per hour (ACH); this equates to data centers loaded to roughly 5 kW per rack. According to the Intel Corporation, HACA containment systems are implemented in data centers with densities of 12 kW per rack and higher. With cooling airflows sufficient for typical HACA cooling loads, air change rates within the contained aisles range from 500 to 1000 ACH and higher. These high airflows will challenge ceiling mounted spot smoke detectors due to the velocity of the air and dilution of smoke.

It has been common for fire protection engineers to specify air sampling smoke detection (ASSD) in rooms exceeding 60 ACH because of their increased sensitivity to smoke. Because of higher velocities experienced within contained aisles it makes even more sense to utilize ASSD. Engineers should consider installing ASSD sample pipes/ports arranged to sample the hot return/exhaust openings in the contained aisle. This would be done in the same manner as is commonly applied to traditional CRAC unit return air grilles.

Data center designers would do well to take notes on best practices of semiconductor manufacturing clean rooms where ASSD is often employed. Clean rooms have similar challenges of high velocities, turbulent air flows, and directional routing of air. There is an easy translation to data centers where air sampling detectors should be installed at the return air inlet to air handling units and somewhere immediately downstream of the hot side of the server at the ceiling level.

Despite the fact that more research is needed on how to detect fires in high airflow environments, many professionals believe that the detection techniques needed are already available to the industry.

Challenges to Fire Suppression

Whether the barriers included in HACA containment systems are applied horizontally, vertically or both, they can affect sprinkler pattern development and clean agent dispersion. NFPA 13: Standard for the Installation of Sprinkler Systems is very explicit in how to apply fire sprinklers to overcome obstructions in the protected space. These rules should be applied to sprinklers where HACA barriers exist in Data Centers. Many of these containment systems have provisions for “automatic” removal when a fire occurs; usually by means of a fusible link. For removal to work, the fire must grow to a point where it can melt the link(s). If the link that removes the barrier is not positioned perfectly over the location where the fire starts, the fire must grow larger to build heat in the location of the link. This also applies to systems that require fusing of multiple links for barrier removal. Be wary of containment systems that require a large fire to remove the barrier before the fire sprinkler system is given the opportunity to activate.

Plastic drop out ceiling panels, used most often in cold aisle containment systems, are another type of barrier that “automatically” removes. These panels are UL listed and melt around 135°F so that the sprinklers above the panels can operate. Data center managers should know that these panels are designed for use with standard response sprinklers that operate at a higher temperature near 155°F. Unfortunately, this sprinkler type is not installed in data centers; usually quick response sprinklers which fuse at 135°F are installed. The temperature difference is important and could lead to issues with sprinklers operating before these ceiling panels have dropped out. Installing quick response sprinklers inside the contained aisle may be the best way to avoid this issue.

In Data Centers protected by clean agent fire extinguishing systems, containment barriers must be removed prior to agent release. Containment systems which rely upon fusible action for removal are a problem because of the large fire size needed to obtain the action. Clean agent systems in data centers most often activate upon detection of smoke, not heat; and are designed to extinguished small developing fires. The problem of barrier removal can be overcome by adding extra clean agent nozzles within the contained aisle.

Clean agent nozzles have several of the same obstruction distance requirements as sprinklers. When retrofitting an existing Data Center with HACA containment, a qualified fire protection firm should be consulted to ensure the required extinguishing concentration can be obtained given the new barriers installed in the space.

It is a valid assumption that clean agents will disperse to spaces which are not in line-of-sight of the agent nozzle, such as the ability to reach the inside of server cabinets. The high airflows associated with containment systems challenges our current assumption and more research must be done on this topic.

While more research is needed to address these new challenges, one thing is for certain; hot aisle/cold aisle containment systems have the attention of data center managers, designers, and fire protection professionals. All are working to ensure a reliable means exists to detect and suppress fires in these environments. If you have added HACA to your data center and not had a qualified professional evaluate your fire protection system, you could be risking higher losses than what your business can tolerate. To learn more on this important topic, download my article that appeared in the 7×24 Exchange Fall 2011 Magazine.

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