Raised Floor vs. Overhead Cabling for Data Centers

Tag : Thermal Fabric
As part of data center design activities, design professionals areoften posed with the question of “raised floor vs. overheadcable tray” when designing air handling systems and cablingdistribution systems within data centers.
This question creates considerable debate, but when accompaniedwith the challenge on corporations to be environmentally conscious,an affordable “green” solution is obviously thepreferable solution. With data center white space and serversdriving such a significant part of the global IT economy, there isan increasing need to design in energy conservation andefficiencies within the data centers space.
Energy Costs are on the rise
Data center power consumption and cooling are two of the biggestenergy issues that confront IT organizations today. In many cases,over 50 percent of the power in data centers is used fortemperature reduction. By 2015, it's expected that the costs tooperate servers will exceed the costs to purchase server hardware. And as energy use grows, IT bills that once accounted for 10percent of overall enterprise budgets may soon account for morethan half. As a result, organizations need more efficienttechnology to better manage increased computing, network, andstorage demands.
Raised Access floors can play a significant role in precise airflowmanagement while maintaining the physical integrity of the DataCenter environment. Access Floors provide the ability to directthermal energy without the use of space-consuming overheadductwork. Additionally, under floor cooling can ease the complexityand facilitate the option of using overhead cable handling space.
When shallower access floor heights are used, overhead cablingpathways tend to be the preferred cabling method as most mechanicalengineers would prefer to reduce the dampening of airflow in thespace underneath the floor. There are additional design metrics toconsider when placing low-voltage cabling and the necessary pathwaysupports under raised floors. Providing adequate space existsbetween the top of the rack and between ceiling-mounted sprinklerheads, designing the pathway overhead can provide easier access formoves, adds, and changes to take place without a disruption of theunderfloor cooling environment. One design metric that is oftenoverlooked is the significant delta in the height ofracks/cabinets. Significant variances in heights of cabinets, theexistence of ceiling-mounted fire suppression, when combined with alow ceiling can prove to be a very challenging and risky design. TIA-942 mandates a minimum ceiling height of 8.5ft above finishedfloor to allow for 7 foot racks and cabinets. As cabinet heightsinch upwards, the TIA-942 mandated clearance of 18 inches canincrease the data center ceiling height, or force an under floorinstallation.
The decision to employ an overhead cable pathway is a balancebetween functionality and aesthetics. Many corporations look attheir data centers as technology showcases and prefer a“clean” overhead appearance. In these instances, alllow and high voltage cabling would be installed in pathways createdwithin the raised floor space.
In these cases, the best practice would be to allow additionalheight to accommodate the cable pathway while insuringuncompromised airflow. Some data center designers have chosen totake the functional route to new levels and have begun to mountcabinet-serving patch panels on the underside of overhead cabletray, directly above the cabinet or rack. This overhead patchpanel design frees up valuable space and permits permanenthorizontal links to remain in place during major server, storageand cabinet upgrades. In these creative designs, the patch cordsare unplugged from the data switching, storage manager, server orstorage device and pulled through the access holes in the top ofthe rack. After removal of the old rack or cabinet, the new orrepopulated enclosure can be put in place and patch cordsreconnected with a minimum of downtime and no reinstallation ofhorizontal cables. The cost-savings and reduction in carbonfootprint using this method of overhead cabling are obvious.
Other data center designs are taking a higher operating efficiencyapproach. When the requirement for moves, adds, or changes arelow, the equipment cabinets can be viewed as an architecturalfixture. This allows airflow management to be installed in thecabinets to segregate the hot and cold air. In this configuration,all cabling is installed in the raised floor space along theexhaust side of the cabinets and requires adequate vertical spacein the raised floor.
Cold air is supplied through perforated floor tiles on the supplyside of the cabinets. With the addition of return air ducting onthe cabinets and cooling units all exhaust air is confined to thereturn air path, delivering a higher return air temperature to thecooling units.
This allows the cooling units to perform at higher efficiencieswhile preventing the mixing of hot and cold air in the data centerspace. Installations of this type have seen greater than 40%reductions in operating costs over standard hot isle / cold isleinstallations.
Hot Air Isolation (Cabinet Ducting)
The fundamental principle in using chilled air to cool servercabinets is to separate hot and cold air as much as possible. Thisis the primary reason for laying out a data center in hot aislesand cold aisles. In a effort to achieve the highest levels inthermal efficiency, many different practices have been employed,including cabinets with ducted exhausts directing heated air into asuspended ceiling return plenum, cabinets ducting directly back tothe cooling unit, even ducting heat directly out of the building.
Traditional hot aisle/cold aisle convection theory requires thathigh pressure cold air be delivered in close proximity to theserver cabinet (perforated floor tiles). With a ducted hot airevacuation system, the delivery points for the chilled air nolonger matters – chilled air can enter the room anywhere andas long as the room remains pressurized.
These newer advances in thermal technology provide efficiencies ata reasonable cost but, require a dedicated room design as ductedcabinets may prohibit the use of some overhead cable tray.
Green Impact On Where to Route Cables.
One metric gaining importance in the designer’s considerationon where to place cables is its impact the carbon footprint of thedata center. First it should be noted that USGBC does notspecifically address “green” best practices in the datacenter. In fact, the high availability and performance levelsrequired in the modern data center are at the opposite end of thespectrum from green best practices, and are more likely to see LEEDpoints taken away rather than awarded. In fact some USGBCcommittees have discussed assigning negative points for theinclusion of PVC-jacketed cabling any place in the rated commercialoffice space, including data centers. Although this ruling isunlikely to become reality, it does point out the opposingobjectives for data centers and green buildings.


In spite of this, there are design and operational practices thatcan make the modern, highly available data center greener. Forexample, the single biggest offender in the waste of coolingefficiency centers around the treatment of raised floorpenetrations for low and high voltage cabling. The use of properairflow management, including seals, brushes, air dams and othercommonly available enclosure accessories can increase the coolingefficiency of a data center by 15% or more. This makes the designand construction of the floor systems and overhead cabling systemscritical to the overall thermal efficiency. Comprehensive toolsare now available to assist in data center design process.
Virtual Thermal Engineering with CFD
Computational Fluid Dynamics (CFD) is a powerful software designtool offering 3 D modeling capabilities for the operationalanalysis and maintenance modeling of cooling systems for datacenters and computer rooms. These software programs provide avirtual tool for the analysis of cooling system for provide forimproved design and construction of data center cooling systems andenhances the administration and management of mission criticalfacilities. Additionally, CFD modeling gives designers theopportunity to consider several data center design cabling andpathway options as well as raised floor vs. overhead cablingdesigns prior to construction within a reasonable amount of time.As a result, the final design is not based on a tentative approach,but is a result of a professional design process consideringseveral options and selecting the optimum solution. This can saveon capital and operational costs as well as save time by avoidingmistakes and during commissioning.
Cabling Infrastructure Design and the Green Data Center
In order to achieve an efficient cooling design, cabling must beproperly designed and installed to assure the air to flow in anunobstructed manner. TIA-942 and other standards suggest thathorizontal and vertical cabling be installed with the anticipationof growth and/or change so that these areas are not repeatedlydisturbed in the future. When considering underfloor cablingsolutions there are distinct reasons for this including eliminatingthe adverse affects of opening floor tiles and decreasing staticpressure under raised floors during MAC work as well as insistingthat pathways are run in a manner that will allow the flow of coldair in cold aisles to be unobstructed by cabling and pathwaymaterials. Cabling channels can sometimes create air dams which mayobstruct air flow, which could result in higher energy consumption.
When considering overhead cabling pathways, there is generalagreement that overhead tray systems offer little obstruction withthe flow of cool air into the data center space. With this minimalimpact on the delivery of cold air directed at servers, the onlyrequirement is to assure that sufficient space is allowed aboveserver cabinets as to not trap or obstruct the exit of heated airfrom the cabinets. Another significant advantage of locatinginfrastructure overhead is that it enables you to forego a raisedfloor system for your server environment. Overhead installation istherefore less expensive, occupies less floor space, and fits moreconveniently in shorter building spaces. Cable trays, ladder racks,and raceways are less expensive than a complete raised floorsystem, contributing to the cost savings.
Final Selection
As the economy continues to shift from paper-based to digitalinformation management, data centers have become common andessential to business, communications, academic, and governmentalsystems. Data centers are now found in nearly every sector of theeconomy and will continue to proliferate to meet the expandingdemands of a digital world. Virtualization promises some relief tosizing and cooling concerns but the overall appetite for fasterprocessing and greater storage will continue to challenge datacenter design. The construction and functionality requirements ofdata centers that demand these complex and often conflictingobjectives will drive product innovation and encourage developmentof systems and solutions that provide greater efficiencies andlower operational costs. The final selection of either anunderfloor or an overhead pathway cabling system is a balancebetween construction costs, available technology, operationalmetrics, aesthetics, physical constraints, and the evolutionaldemands of the enterprise. That final selection will have tosurvive the ever changing shape of the fabric it supports.
About the Author: Dan Reynolds is the Director NAR/CALA Regions for CommScope DesignServices Organization