4 TC 5.03 Room Air Distribution TC 5.03 ActivitiesRP-1546 – ADPI Update (due 2014)RP-1629 – Energy Performance of Active Beam Systems (just started)SPC 200 – MOT Active Chilled Beams (public review)SPC 130 – MOT Terminal Units (public review)SPC 70 – MOT Air Inlets and Outlets (just formed)
5 TC 5.03 Room Air Distribution Join TC 5.03 – a large and active committeeChapters in (3) ASHRAE HandbooksFundamentalsApplicationsSystems and EquipmentSubcommitteesRoom Fan CoilsChilled BeamsUnderfloor Air DistributionAir Curtains
6 So Many ChoicesThere’s a Good, Better and Best System for Every BuildingOld and New TechnologyOverhead Air DistributionUnderfloor Air DistributionActive Chilled BeamsDisplacement VentilationLet me assure you that the purpose of this webcast is not to sell displacement ventilation as the best system for every building. We’ll discuss the reasons why certain applications are recommended while others are not, but it’s important to consider displacement ventilation as one more possible solution. I like to think of displacement as one more tool in my toolbox of air distribution systems.
7 GRD’s Grille Register Diffuser They are all outlets! Outlet similar in size to duct sizeRegisterGrille with an integral dampering deviceDiffuserOutlet that is often larger than duct sizeDesigned to create an air patternThey are all outlets!
8 The Occupied Zone Occupied Zone 6.0 ft above floor 3.3 ft from outside wall1.0 ft from interior wall3.3’6.0’1.0’
11 Mixed-Air System Concepts Supply air 38-55oFCold air supplied outside the occupied zone, thoroughly mixes with room airCreates an air pattern on the ceiling and/or wallsPicks up heat and pollutants at the ceiling levelCreates low velocity room air motionIdeally creates uniform temperature throughout the space and minimizes stratificationDisplacement systems typically supply 63-68°F air through specially-designed diffusers that are located on low side walls. This cool air discharges at less than 80 fpm from the face of the diffuser and quickly cascades down to the floor. The air continues to move slowly across the room in a layer about 4” deep until it locates a source of heat or an obstruction. In office applications the heat sources in the occupied zone are usually people and equipment. When this slow moving pool of air encounters a heat load, it quickly rises and carries the heat and pollutants towards the ceiling. Internal heat loads and contaminants are then carried away by the return air. We refer to this as a thermal plume.
12 Fully-Stratified Concepts Supply air oFCool air supply displaces warm room air at low velocitiesUses the natural buoyancy of warm air to provide improved ventilation and comfortCold air moves slowly across the floor until it reaches a heat source, then risesImproved IAQDisplacement systems typically supply 63-68°F air through specially-designed diffusers that are located on low side walls. This cool air discharges at less than 80 fpm from the face of the diffuser and quickly cascades down to the floor. The air continues to move slowly across the room in a layer about 4” deep until it locates a source of heat or an obstruction. In office applications the heat sources in the occupied zone are usually people and equipment. When this slow moving pool of air encounters a heat load, it quickly rises and carries the heat and pollutants towards the ceiling. Internal heat loads and contaminants are then carried away by the return air. We refer to this as a thermal plume.
13 Improved Contaminant Removal Stratification creates a single passUnlike mixed-air, contaminants are not redistributed throughout the roomDisplacement ventilation can be thought of a single pass system. Unlike a mixed-air system that tends to distribute contaminants more or less evenly throughout the entire room, displacement uses natural buoyancy to collect pollutants at the ceiling where they can exit the room. This action results in lower concentrations in the occupied zone.Displacement VentilationOverhead System
14 Improved VentilationASHRAE Standard Ventilation for Acceptable Indoor Air QualityZone Air Distribution Effectiveness, EzBest Overhead System (Ez = 1.0)Displacement Ventilation (Ez = 1.2)UFAD also qualifies if T50 is 4.5 ft or less16.7% Less Fresh Air RequiredDisplacement ventilation is known to be a way to increase comfort through improved ventilation. The goal of any air distribution system should be to efficiently deliver ventilation air to the breathing zone. For this reason, ASHRAE Standard 62.1 rates various types of systems with regard air change effectiveness. These ratings take into account the supply and return locations as well as the discharge pattern and temperature. According to this standard, the very best overhead systems qualify for a rating of 1.0 and other lesser systems could be as low as 0.5. On the other hand, displacement qualifies for the highest possible rating of 1.2. This means that displacement ventilation is 20% more effective than the best overhead system and the minimum fresh air requirement can therefore be reduced by nearly 17%.
15 Thermal ComfortASHRAE Standard 55 – Thermal Environmental Conditions for Human OccupancyMaximum recommended ∆Thf = 5.4°FASHRAE Standard 55 is often referred to as the thermal comfort standard. It recommends that for good thermal comfort the temperature difference between the head and foot level of a standing person should not exceed 5.4 °F. If we assume a constant floor-to-ceiling stratification gradient of no more than 1°F per foot, any design that provides comfort for a standing occupant should also work for a seated occupant because a seated occupant would experience no more than a 3.6°F differential. In addition, we need to pay attention to the difference between the supply and exhaust temperatures. As a rule-of-thumb, exhaust temperatures should not be more than 36°F higher than supply temperatures to avoid a sensation of draftiness.
16 What About Heating?Fully-stratified systems typically use a secondary system for heatingLow velocity warm air would short circuit to the ceilingFin tube perimeter heat is often used
17 Dual Plenum Diffusers Dual plenum diffusers provide Displacement outlet for coolingGrille for low sidewall heatingInternal diverting damperAllows a single system to cool and heat in mild climates
18 Outlet Performance Tested per ASHRAE 70 SP and TP Area factor, Ak Sound levelThrow, drop and spread
19 Outlet Performance Pressure drop (in wg) Area factor, Ak (ft2) SP measuredTP = SP + VPArea factor, Ak (ft2)cfm = Ak x fpmSound level (dB ref w)NC assumes 10 dB room effect
20 Outlet Performance Throw Drop Spread Terminal velocities T150, T100, T50Measured from centerlineIsothermal (unless specified)DropDistance below ceiling to center of discharge jetSpreadUnbounded jets spread at 11°angle (on each side)
21 Area Factor vs. Free Area Free area does not govern outlet performancePerformance is related to geometryHole size/shape/numberMaterial depthCurved/angled surfacesFree area may or may not be easy to determine, but it’s not really useful information
22 ADPI Air Diffusion Performance Index (ADPI) Statistically relates local temperatures and velocities to occupant comfortRatio of diffuser T50 to characteristic length of the room being servedADPI > 80 is acceptableCurrently only applies to cooling applicationsSoon may be expanded to include more diffuser types and add heating applications
23 ADPI ASHRAE RP-1546 Conducted at University of Texas at Austin Verify original researchExpand the types of outletsRun heating testsTesting will be completed by August, 2014
24 ADPI Example ADPI Example Results for 24x24 diffusers with 8” necks 200 cfm20° ∆T400 ft2Results for 24x24 diffusers with 8” necksPlaque Face = 93.0Multi-Cone = 93.0Perforated = 84.8It often makes sense to look at typical rather than 100% design conditions…
25 Overhead Heating Discharge temperature affects minimum ventilation In overhead heating applications, discharge temperatures should never be more than 15°F higher than the desired room temperature and T150 must be within 4.5 ft from the floor (Ez = 1.0)If ΔT > 15°F, then Ez = 0.8 and cfm increases by 25%
26 Split Pattern Linear50/50 throw pattern is the best compromise for both heating and coolingWorks best when splitting the diffuser length, rather than splitting slotsIn the cooling mode, 50% of the supply air is directed down the exterior walls – which also wastes energy and causes drafts near the floor…26
29 Air Patterns Swirl (floor) Displacement (sidewall) Linear (ceiling) Linear (air curtain)Laminar (OR, clean rooms)Hemispherical (lab, industrial)
30 Return GrillesContrary to popular belief – return grille locations generally do not affect room air motionReturn grilles merely provide an exit
31 Surface Effects Discharge jets attach themselves to surfaces CeilingsWallsGlassObstructions with an angle of incidence greater than 15° can kick the air pattern off the ceiling
32 Open Ceilings Unless otherwise specified assume Ceiling diffusers were tested with a ceilingSide wall grilles were tested near a ceilingInternal vs. external Coanda pocketMost diffusers need a ceiling for horizontal air patternSometimes a small lip can be added to create a ceiling effectFree jets result in a 30% throw reduction due to increased expansion
33 Temperature Effects T150 is temperature independent – velocity driven Horizontal ceiling throwCooling decreases throw by 1% per °FHeating inceases throw by 1% per °FExample – Catalog (isothermal) 4-7-9Cooling 4-6-7Heating
34 Active LengthLinear diffusers should not have active sections longer than 10 ftOverly long active sections cause problemsExtended and unpredictable throwUndulating air patternsSolutionsProvide 1-2 ft inactive breaks between sectionsAlternate throw direction
35 Acoustics Select diffusers such that they will not be heard Noisy diffusers create a poor communicationNC set by in octave bands 4-6 (500, 1000, 2000 Hz) – speech interference bands10 NC points lower than desired room level, and rarely higher than NC25 unless it’s an industrial application
36 What Type To Select? The choice can depend on many things Air pattern PerformanceAppearanceCostSpace limitationsInstallation/ceiling type
37 SummaryMany types of systems and outlets are available, but there’s always a best choiceSelecting the right air pattern is criticalBe aware of surfaces and ceilingsKeep overhead heating temperatures lowSelect diffusers to be inaudible