Under Floor Air Distribution (UFAD) - CED …

1 Under Floor Air Distribution ( ufad ) Course No: M01-011 Credit: 1 PDH Elie Tawil, , LEED AP Continuing Education and Development, Inc. 9 Greyridge Farm Court Stony Point, NY 10980 P: (877) 322-5800 F: (877) 322-4774 Leading by example,saving energy andtaxpayer dollarsin federal facilitiesDOE/EE-0295 Internet: portion of this publication may be altered in any form withoutprior written consent from the Department of Energy, EnergyEfficiency and Renewable Energy, and the authoring national Air Conditioning Technologies:Underfloor Air Distribution ( ufad )An overview of ufad technology and its potential in federal facilitiesIntroduction and BackgroundRecent trends in today s office environment make it increasingly more difficult for conventional central-ized HVAC systems to satisfy the environmental preferences of individual office workers using thestandardized approach of providing a single uniform thermal and ventilation environment.

2 Since itsoriginal introduction in West Germany during the 1950s, the open plan office containing modularworkstation furniture and partitions is now the norm. Thermostatically controlled zones in openplan offices typically encompass relatively large numbers of workstations in which a diverse workpopulation having a wide range of preferred temperatures must be accommodated. Modern officebuildings are also being impacted by a large influx of heat-generating equipment (computers, printers, etc.) whoseloads may vary considerably from workstation to worksta-tion. Offices are often reconfigured during the building slifetime to respond to changing tenant needs, affectingthe Distribution of within-space loads and the ventilationpathways among and over office partitions. Compound-ing this problem, there has been a growing awareness ofthe importance of the comfort, health, and productivityof individual office workers, giving rise to an increaseddemand among employers and employees for a high-qualitywork Ambient ConditioningDuring recent years an increasing amount of attention has been paid to air Distribution systems thatindividually condition the immediate environments of office workers within their workstations toaddress the issues outlined above.

3 As with task/ambient lighting systems, the controls for the task components of these systems are partially or entirely decentralized and Under the control of the occu-pants. Typically, the occupant has control over the speed and direction, and in some cases the tem-perature, of the incoming air supply. Variously called task/ambient conditioning, localized thermaldistribution, and personalized air conditioning systems, these systems have been most commonlyinstalled in open-plan office buildings in which they provide supply air and (in some cases) radiantheating directly into workstations. TAC systems can be classified into the following two major categories:1) furniture-based, and 2) Floor -based, underfloor air Distribution ( ufad ). A majority of these sys-tems include a raised Floor system with which underfloor plenums are used to deliver conditioned airto the space through Floor grills, or in conjunction with the workstation furniture and of typicalswirl diffuser used inUFAD Air DistributionIn the 1970s, underfloor air distributionwas introduced into office buildings inWest Germany as a solution to cablemanagement and heat load removalissues caused by the proliferation ofelectronic equipment throughout theoffice [1].

4 In these buildings, the com-fort of the office workers had to be con-sidered, giving rise to the developmentof occupant-controlled localized supplydiffusers to provide task condition-ing. Some of the first ufad systemsin Europe used a combination of desk-top outlets for personal comfort con-trol and Floor diffusers for ambientspace control [2].Prior to the 1990s, office installationsusing underfloor systems had beenfound primarily in South Africa,Germany, and other parts of technology was not commonlyused in North America prior to about1995, in part due to the downturn inoffice building construction beginningin the mid-1980s. Japan did not experi-ence this same downturn, and as a result,significant growth in ufad technologywas observed during this period. Between1987 and 1995, more than 250,000 m2( million ft2) of office space in morethan 90 buildings were installed withUFAD systems in Japan [3].

5 However, in the late 1990s growthfor raised Floor installations in theUS was dramatic and designers andmanufacturers predicted that 35%of new offices would use raised floorsby 2004. Half of these installationswere expected to incorporate ufad technology. This rate of increase hasslowed now (2003) due to the eco-nomic downturn and much reducedoffice purpose of this technology reviewis to provide federal facilities managersan overview of the principles, features,benefits, and limitations of this impor-tant innovation in building condition-ing technology. Technology DescriptionA task/ambient conditioning system isdefined as any space conditioning sys-tem that allows thermal conditions insmall, localized zones ( , regularlyoccupied work locations) to be individu-ally controlled by building occupants,while still automatically maintainingacceptable environmental conditionsin the ambient space of the building( , corridors, open-use space, andother areas outside of regularly occu-pied work space).

6 ufad systems are uniquely character-ized by their ability to allow individualsto have some degree of control over theirlocal environment, without adverselyaffecting that of other nearby on the ufad system design,ambient environmental control innon-work areas may be provided byadditional local supply outlets, or by aseparate space conditioning system, butin either case Under automatic types of diffusers supported,active or passive, further distinguishUFAD systems from furniture-basedTAC systems. Active diffusers (forpurposes of this report) are definedas those with local means of volumeadjustment (such as an integral variablespeed fan or damper) that is amenableto automatic zone control (in additionto means for occupant control). Passivediffusers, although they may have meansfor occupant adjustment, are combinedwith terminal or system elements toachieve zone control.

7 Systems designedwith all fan-assisted active diffuserstypically utilize zero-pressure diffusers require pressurizedplenums. The majority of ufad systems currently being deployedhave pressurized plenums with eitheractive or passive of OperationFigures 1 and 2 illustrate the funda-mental differences between traditionaloverhead and furniture-based TAC andUFAD systems, respectively. As shownin Figure 1, overhead systems (in officebuildings, these are predominately vari-able air volume [VAV] all-air distribu-tion systems) employ an extensive arrayof ductwork and terminal devices toprovide supply air through the ceiling-mounted diffusers. Often referred to asmixing ventilation systems, these sys-tems are designed to promote completemixing of supply air with room air,thereby maintaining the entire volumeof air in the space at the desired tem-perature setpoint.

8 Space air is typicallyreturned to the AHU via an open ceil-ing plenum that also contains variousother systems for lighting, electrical,communications, and fire systems turn this conceptupside down and have in commonthe following characteristics: Supply air, including at least theminimum required volume of out-side air, is filtered and conditionedto the required temperature andhumidity by a conventional AHUand passed through a minimumamount of ductwork to an Under - Floor plenum. The underfloor ple-num is formed by installation of araised Floor system, typically con-sisting of m x m (2 ft x 2 ft)concrete-filled steel Floor panelspositioned m (12 18 in.)above the concrete structural slabof the building. The raised floorsystem also allows all cable services,such as power and communication,to be located in the plenum andprovides easy access for modifica-tions and In open plan settings there is less control due to airflow rate change associated with opening and closing Floor diffusers than in private offices.

9 Betteroccupant control is achieved by designs that allow the airflow to be directed toward the FEDERAL ENERGY MANAGEMENT PROGRAM personal comfort preferences. Differ-ent supply outlet configurationsmay be used depending on theconditioning requirements for aparticular zone of the building, asdiscussed below. ufad systems benefit from a Floor -to-ceiling airflow pattern that takesadvantage of the natural buoyancyproduced by heat sources in theoffice to efficiently remove heatloads and contaminants from thespace. Air is returned from the roomat ceiling level through recessedlighting fixtures and return grillesvia a conventional ceiling returnplenum or return grilles locatedhigh in the space when a returnplenum is not used. Because the air is supplied directlyinto the occupied zone (up to 6 ft( m) height), supply outlet tem-peratures are generally maintainedabove 17 to 18 C (63 to 64 F) toavoid uncomfortably cool condi-tions for the nearby occupants andto minimize cool temperatures nearthe are a wide variety of approachesbeing used to provide a combination ofindividual and automatic zone controlfor ufad (See reference [4]).

10 Typically, these systems use variable -air- volume (VAV) or constant-air- volume (CAV) methods for general zone con-trol ( , overall zone control otherthan local occupant control). Six types of diffusers are currentlybeing offered;1. Fan-assisted, active2. variable area, active3. Swirl, passive4. Swirl, active5. Linear bar grille, passive6. Linear bar grille, activeFigure 1. Overhead 2. TAC The author has identified over a dozen variations of these systems. Individual office workers can controltheir local thermal environment overa relatively wide range (typicallyby adjusting the volume and/ortrajectory of the supply air enteringthe space), giving them the oppor-tunity to fine-tune the thermal con-ditions in their workstation to theirFEDERAL ENERGY MANAGEMENT PROGRAM 3 The heating and cooling loads ofperimeter zones are handled by fan-powered constant volume or variablevolume terminal reheat units locatedin the underfloor plenum (similarto those shown in Figure 3).