New fabric duct innovations solve air distribution challenges

by Katie Daniel | September 11, 2017 2:48 pm

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by Kevin J. Gebke
Fabric HVAC ductwork, also known as fabric air dispersion, has always offered specifiers choices when it comes to mechanical system efficiency and efficacy, but various new product innovations over the last eight years have resulted in superior alternatives to traditional metal ducts. The most notable of these material advancements have been developed to solve challenges in data center cooling, underfloor air distribution (UFAD), and the aesthetic shortcomings of deflated wrinkling and sagging during any idle air-handler periods.

Contrary to the common phrase ‘fabric duct,’ the term refers more to a diffuser system than a ductwork assembly. Due to the intimately included diffusion component of fabric duct, they are always designed into spaces being conditioned with the air that is being dispersed from it. In other words, they are never used as a component to simply convey air from one room to another.

Air diffusion occurs in these systems via discrete orifices, linear vents, and through the fabric porosity. A common similarity among projects in which fabric ducts are employed is the open-ceiling design style. (For more on fabric ductwork, see the article, “Trends in Fabric Air Dispersion for HVAC,” by Jeff Klopfenstein from the February 2008 issue of The Construction Specifier. Visit kenilworth.com/publications/cs/de/200802/files/54.html[2]. Also, see the article, “Designing Fabric Duct Ventilation,” by Nick Kaufmann, which appeared in the March 2015 issue of Construction Canada. Visit www.constructioncanada.net/designing-fabric-duct-ventilation[3].)

Overcoming the ‘deflated’ look
Fabric duct has gained significant market share in the open architectural ceiling market during the last decade due to certain advantages over metal duct. Fabric duct installs faster, can be more energy-efficient, and disperses air more uniformly with multiple vent and outlet choices, while also offering improvements in aesthetics, colors, and streamlined design over spiral round metal duct and registers (the latter of which tend to create drafts).

However, fabric duct has traditionally been associated with a ‘visual’ challenge—its sagging or deflated appearance during air-handler idle periods. This negative aspect has now been remedied with cylindrical in-duct tensioning systems that can be used for both new construction or fabric duct retrofits. This gives fabric duct an inflated appearance, free of wrinkling, sagging, and roll-out popping sounds regardless of air-handler operation. The elimination of roll-out during air-handler startup significantly increases the life of the system because there is less wear-and-tear from no fabric movement. As a result, some fabric duct manufacturers lengthen their product warranty when these tensioning systems are incorporated.

Premium tensioning models use 360-degree cylindrical supports attached to a central skeletal spine with an adjustment nut that is ratcheted during onsite installation to spread the beginning and end support rings apart and hold the material taut. Diameters range from 305 to 2540 mm (12 to 100 in.)—the larger sizes can accommodate assemblies for huge spaces, such as the recent arena retrofit of Texas Christian University’s (TCU’s) 55-year-old, 6503-m² (70,000-sf) Ed and Rae Schollmaier Arena. This arena retrofit presented many HVAC engineering challenges for Baird Hampton & Brown (BHB) Inc., a Fort Worth, Texas-based consulting engineer/architecture firm that spearheaded the HVAC portion of the $72-million renovation.

Unlike the outdated HVAC approach of back-wall air distribution grills it replaced, BHB’s fabric duct ring hangs approximately 12 m (40 ft) out from the back wall and disperses air more evenly throughout the entire seating and court areas. The ducts’ linear array of laser vents dispersing air at four different air throws at3 o’clock, 4 o’clock, 9 o’clock, and 8 o’clock to match the different target areas for the conditioned air.

Most importantly, the 14-m (47-ft) high circular fabric duct was a significantly safer product to install for the mechanical contractor, because metal is 90 percent heavier and posed tremendous logistical risks at that height.

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Improving data center efficiency and efficacy
Myriad air-distribution strategies have evolved simultaneously with the data center industry’s development in order to get air-conditioning to electronics racks. Early trends flooded electronics rooms with as much cooling as possible to ensure
it would eventually infiltrate the racks to prevent equipment overheating. More recently, greater attention is paid to air distribution efficiency and efficacy than refrigeration tonnage, because of ongoing concerns for greater energy conservation and operational costs reduction.

Just a one-degree set-point temperature reduction could result in thousands of dollars of annual energy savings even for a small-sized data center room. The challenge is to lower discharge temperatures via reductions in HVAC refrigeration circuits and fan energy, but still maintain the same equipment cooling through better air distribution.

The following paragraphs outline common methods of air distribution in data centers.

Freestanding units
Computer room air-conditioner (CRAC) units are typically upflow units that are evenly spread around the perimeter of the space to distribute air throughout the room. This type of application was more common in legacy locations that were more concerned with maintaining tight environmental conditions than energy efficiency.

Rectangular or spiral round metal duct
A central rooftop air-handling unit (RTU) or air-handling unit (AHU) positioned outside the data room itself supplies rectangular or spiral metal duct in the data hall. Traditional metal ductwork disperses air from registers positioned 1.5 to 3 m (5 to 10 ft) apart. Like conventional HVAC, these registers have manual adjustment capability for airflow volume, but have limited direction. As air dispersion ‘results’ became more critical, back-to-back registers were incorporated to eliminate hot and cold spots within the data hall, but at a significant cost.

Underfloor ventilation
The same underfloor space that distributes cabling can also be sized properly to distribute air supplied from downflow CRAC units. While underfloor offers a clean, aesthetic appearance to a data center, it can be challenging to provide proper airflow—especially after newer equipment with larger cooling loads replaces older equipment. If floor tiles are moved to aid in cooling the new equipment, it can create issues elsewhere in the cold aisle where other equipment is being starved of required airflow. Thus, some data centers with underfloor ventilation supplement it with overhead duct systems and/or additional HVAC systems, which carry additional capital as well as operational costs.

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Conventional fabric air dispersion
In an effort to reduce high velocities inherent in metal duct/register systems, data center designers began looking into fabric air dispersion for air distribution. Developed more than 40 years ago, such systems were not designed specifically for data centers, but rather for applications such as food processing, indoor pools, retail stores, and office buildings. They resemble round spiral metal duct
in appearance, except their construction is fabric offered in either nonporous or porous options.

Porous models are ideal for data centers because of the low air velocities that result when air passes through the fabric. The porosity can be combined with linear vents, which are laser-cut holes, typically less than 25 mm (1 in.) in diameter and factory engineered in any configuration. In most cases, the linear vent pattern spans the length of the duct. These systems provide the required airflow into the cold aisle with velocities ranging between 152 and 244 m/minute (500 and 800 fpm)—an air dispersion improvement over metal duct systems.

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Combination fabric diffuser/spot cooling
The fabric diffuser/spot-cooling hybrid method was recently developed specifically for data centers. The fabric-based diffusers employ porosity combined with field-adjustable nozzles for spot cooling.

While conventional fabric duct porosity is too miniscule to be seen with the naked eye, data center fabric diffuser micro pores are definitively visible. The fabric diffuser porosity slows the potential higher air velocity down to allow a gentle descent toward the equipment rack façades. Air velocities below 1.2 m/minute (400 fpm) allow electronics equipment fans to draw it into the cabinet. These systems are positioned directly above the cold aisle. Cold air not drawn into racks will settle near the floor, pushing (or displacing) warmer air out of the cold aisle and up to the ceiling.

The diffusers also incorporate adjustable nozzles that can direct air toward high-wattage servers and higher-density racks—both growing industry trends. The 360-degree rotatable hemispherical diffusers secured inside 50 or 76-mm (2 or 3-in.) diameter housing (i.e. 15.5-L/s, 124.4-Pa [33-cfm, ½-in. w.g.] or 39-L/s, 124.4-Pa [83-cfm, ½-in. w.g.]) are designed to snap into the factory-cut orifices. The nozzles also have an anti-condensation design to eliminate concerns of moisture formation on the material. In applications where aisles are not full, the nozzles can also be closed off to save energy.

This data center air dispersion is also factory-engineered to offer displacement delivery that does not entrain warm air from the hot aisle, essentially creating a configuration of separate cold/hot aisles without having physical partitions between the two. This results in a 1.6 to 2.8-C (3 to 5-F) colder aisle temperature, but with up to 20 to 40 percent less CRAC flow rates, especially when complemented with variable frequency drive (VFD) equipment. It also helps achieve lower power usage effectiveness (PUE) levels, which is a metric for measuring infrastructure energy efficiency for data centers.

Further, the fabric used in these systems has built-in anti-static qualities from its electrostatic dissipative (ESD) yarn, which is specifically designed for electric-sensitive environments such as data centers. It also disperses a small 12.7 L/s/m² (2.5-cfm/sf) of airflow through a traditional fabric portion of the product. This accounts for about one percent of its total porosity versus the aforementioned micro-pore strategy.

Thus, the push for better dispersion in the form of slower airflow and better directionality will drive the data center industry toward these more high-tech air distribution solutions in the future. A good example can be found with Involta, a provider of data centers, cloud services, and information technology (IT) outsourcing. The Cedar Rapids, Iowa-based firm has developed more than 23,780 m² (256,000 sf) of co-location data centers operating in Arizona, Pennsylvania, Ohio, Minnesota, Iowa, and Idaho. The facilities, which have either metal or traditional fabric ductwork not specifically designed for data equipment operations, are amid a roll-out conversion to combination fabric diffuser/spot cooling dispersion, according to the company’s chief security officer, Jeff Thorsteinson. Converted centers have demonstrated reduced air velocities, lower dB sound levels, and better efficiency as measured by PUE.

For example, Involta recently converted its Marion, Iowa, facility that previously used metal duct air distribution. The deployment, which also included mechanical modifications, reduced airflow by 40 percent, but maintained the same cooling temperatures due to better air distribution. Combined with some server and storage device change outs, the Marion facility HVAC retrofit reduced electric utility usage by 80,000-kWh/month, Thorsteinson said.

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Reducing UFAD temperature differential
Over the last decade, the trend in office ventilation has been toward UFAD where cabling, piping, utilities, and HVAC ventilation are concealed below raised floors, but also easily accessed through floor panels for reconfiguration, additions, or retrofits. A new fabric duct product developed especially for such assemblies helps improve the air distribution system’s temperature control to result in greater occupant air comfort and fan energy reduction.

The typical 305 to 460-mm (12 to 18-in.) high raised floor creates a convenient plenum to distribute air. Conditioned air typically pressurizes the plenum from building supply trunk line ductwork. The pressurized plenum forces conditioned air up into the occupied space through adjustable floor diffusers that can be incorporated into any 0.6 x 0.6-m (2 x 2-ft) floor panel. Office workers can adjust the floor diffuser closest to their workspace for personal temperature control.

UFAD’s greatest challenge is thermal decay (i.e. heat escape) at the perimeter, especially near solar gain sources, such as windows or exterior walls. Getting air to the perimeter is sometimes a pressurization challenge facility managers try to alleviate by raising the space temperature. This, in turn, makes inner spaces too cold near the building core. Increasing fan speeds on variable speed designs is a very inefficient and sometimes ineffective solution. Temperature differences from the perimeter to the inner areas near the building core can vary as much as 5.5 C (10 F) when UFAD does not operate efficiently.

The fabric duct solution to UFAD air distribution challenges consists of 305 to 460-mm (12 to 18-in.) diameter fabric duct lengths that are factory-modeled for every project. They connect to the plenum’s metal supply ductwork to deliver air where it is most needed. In perimeter thermal decay incidences, for example, air can be dispersed directly near floor diffusers where there are employee air comfort complaints. The duct flexibility easily makes it reconfigurable for routing around the inherent myriad of cabling, piping, vertical floor supports, and other concealed obstacles commonly found in UFAD systems.

A case in point is the new $47-million Mitchell Park Library & Community Center (Palo Alto, California), which recently opened with acclaim for its indoor air quality (IAQ) and controllable air comfort. The 3716-m² (40,000-sf) library, and its accompanying 1486-m² (16,000-sf) community center, is one of California’s most sustainable projects, according to Debra Jacobs, PE, PMP, LEED AP, a project engineer with the city’s Public Works Department.

While added ductwork is many times a UFAD retrofit remedy, the LEED Platinum-certified building’s consulting engineer Tunde Munz, PE, LEED AP
(a principal with San Francisco-based firm Guttmann & Blaevoet [G&B]), specified it in the library’s design phase to safeguard against potential temperature differential swings. G&B has designed dozens of buildings using UFAD and often incorporates fabric duct for better efficiency and indoor air comfort.

“As I walk through the building, the temperature is amazingly consistent, even when comparing the west and east portions of the building in the late afternoon’s setting sun,” said Jacobs.

Energy efficiency advantages over metal duct
The Mitchell Park Library and other buildings are demonstrating the fact the fabric duct industry continues to grow and innovate products that improve the delivery of HVAC systems and provide a more efficient, effective, and comfortable environment for commercial building occupants.

Indeed, fabric duct’s superior airflow makes it more energy-efficient than metal duct/register systems, according to a 10-month study performed by Iowa State University’s mechanical engineering department. “Thermal Comparison Between Ceiling Diffusers and Fabric Ductwork Diffusers for Green Buildings,” by Anthony Fontanini, M. Olsen, and B. Ganapathysubramanian, shows fabric duct has a 24.5 percent efficiency differential, because it heats rooms faster and more uniformly to satisfy temperature set points versus metal duct/diffusers. This results in reduced mechanical equipment runtime, saving energy in the process.

As the report outlines, most general enhancements to building efficiency have been a result of changes to the heating/cooling systems, improvements in construction materials, or building design code improvements. However, such approaches neglect the way in which air is dispersed into individual rooms or in a building—that is, the ducting system. This opens up the possibility of significant energy savings by making ductwork systems lighter and better insulating while ensuring cost-effectiveness.

The Iowa State study explores this idea by comparing the performance of conventional ductwork with recent advancements in fabric-based ductwork. Transient, fully three-dimensional computational flow dynamics (CFD) simulations were performed to compute flow patterns and thermal evolution in rooms containing either conventional or fabric ductwork. This analysis was used to construct metrics on comfort and efficiency. A number of different flow rates were examined to determine the performance over a range of operating conditions. Transient finite volume simulations consisted of more than 13 million degrees of freedom for over 10,000 time steps. The simulations utilized high-performance computing (HPC) for large-scale analysis.

The results conclusively showed fabric ducting systems to be superior to the conventional systems in terms of efficiency. Observations from the data illustrated fabric ducting systems heat the room faster,  more uniformly, and more efficiently. The increase in performance demonstrates the potential benefits of moving away from conventional systems to fabric systems for the construction of green buildings.

Kevin J. Gebke is the new product development engineer at DuctSox Corp., a Peosta, Iowa-based manufacturer of traditional overhead fabric HVAC duct products. He holds a degree in engineering from the University of Illinois and an MBA from the University of Dubuque. With more than 20 years of experience in HVAC air distribution, diffusion, and movement discipline, Gebke holds 16 patents. He is very active in the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE), and is the author of the “Textile Air Dispersion Systems” chapter in the soon-to-be published ASHRAE Duct Design Guide. Gebke can be reached at kgebke@ductsox.com[9].

 

Source URL: https://www.constructionspecifier.com/new-fabric-duct-innovations-solve-air-distribution-challenges/

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