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Download Template Samples - Central Pennsylvania Chapter of ASHRAE
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Room Air Distribution Presented by Randy Zimmerman Introduction TC 5.03 update Mixed air systems vs. stratified systems Thermal comfort Ventilation effectiveness Diffuser performance Overhead heating Product selection Questions and answers 2 TC 5.03 Room Air Distribution TC 5.03 Officers – – – – – – Jerry Sipes – Chair Randy Zimmerman – Vice Chair/Research Chair Kevin Gebke – Secretary Fred Lorch - Membership Curtis Peters – Handbook Andrey Livchak - Programs 3 TC 5.03 Room Air Distribution TC 5.03 Activities – RP-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) 4 TC 5.03 Room Air Distribution Join TC 5.03 – a large and active committee – Chapters in (3) ASHRAE Handbooks • Fundamentals • Applications • Systems and Equipment – Subcommittees • Room Fan Coils • Chilled Beams • Underfloor Air Distribution • Air Curtains 5 So Many Choices There’s a Good, Better and Best System for Every Building Old and New Technology – Overhead Air Distribution – Underfloor Air Distribution – Active Chilled Beams – Displacement Ventilation GRD’s Grille – Outlet similar in size to duct size Register – Grille with an integral dampering device Diffuser – Outlet that is often larger than duct size – Designed to create an air pattern They are all outlets! The Occupied Zone Occupied Zone – 6.0 ft above floor – 3.3 ft from outside wall – 1.0 ft from interior wall 3.3’ 6.0’ 1.0’ Conventional Mixed-Air System Fully-Stratified System Mixed-Air System Concepts Supply air 38-55oF Cold air supplied outside the occupied zone, thoroughly mixes with room air Creates an air pattern on the ceiling and/or walls Picks up heat and pollutants at the ceiling level Creates low velocity room air motion Ideally creates uniform temperature throughout the space and minimizes stratification Fully-Stratified Concepts Supply air 63 - 68oF Cool air supply displaces warm room air at low velocities Uses the natural buoyancy of warm air to provide improved ventilation and comfort Cold air moves slowly across the floor until it reaches a heat source, then rises Improved IAQ Improved Contaminant Removal Stratification creates a single pass Unlike mixed-air, contaminants are not redistributed throughout the room Displacement Ventilation Overhead System Improved Ventilation ASHRAE Standard 62.1 - Ventilation for Acceptable Indoor Air Quality Zone Air Distribution Effectiveness, Ez Best Overhead System (Ez = 1.0) Displacement Ventilation (Ez = 1.2) – UFAD also qualifies if T50 is 4.5 ft or less – 16.7% Less Fresh Air Required Thermal Comfort ASHRAE Standard 55 – Thermal Environmental Conditions for Human Occupancy Maximum recommended ∆Thf = 5.4°F What About Heating? Fully-stratified systems typically use a secondary system for heating – Low velocity warm air would short circuit to the ceiling – Fin tube perimeter heat is often used Dual Plenum Diffusers Dual plenum diffusers provide – – – – Displacement outlet for cooling Grille for low sidewall heating Internal diverting damper Allows a single system to cool and heat in mild climates Outlet Performance Tested per ASHRAE 70 – – – – SP and TP Area factor, Ak Sound level Throw, drop and spread Outlet Performance Pressure drop (in wg) – SP measured – TP = SP + VP Area factor, Ak (ft2) – cfm = Ak x fpm Sound level (dB ref 10-12 w) – NC assumes 10 dB room effect Outlet Performance Throw – – – – Terminal velocities T150, T100, T50 Measured from centerline Isothermal (unless specified) Drop – Distance below ceiling to center of discharge jet Spread – Unbounded jets spread at 11°angle (on each side) Area Factor vs. Free Area Free area does not govern outlet performance Performance is related to geometry – Hole size/shape/number – Material depth – Curved/angled surfaces Free area may or may not be easy to determine, but it’s not really useful information ADPI Air Diffusion Performance Index (ADPI) – Statistically relates local temperatures and velocities to occupant comfort – Ratio of diffuser T50 to characteristic length of the room being served – ADPI > 80 is acceptable – Currently only applies to cooling applications – Soon may be expanded to include more diffuser types and add heating applications ADPI ASHRAE RP-1546 – – – – – Conducted at University of Texas at Austin Verify original research Expand the types of outlets Run heating tests Testing will be completed by August, 2014 ADPI Example ADPI Example – 200 cfm – 20° ∆T – 400 ft2 Results for 24x24 diffusers with 8” necks – Plaque Face = 93.0 – Multi-Cone = 93.0 – Perforated = 84.8 It often makes sense to look at typical rather than 100% design conditions… 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% Split Pattern Linear 50/50 throw pattern is the best compromise for both heating and cooling Works best when splitting the diffuser length, rather than splitting slots Air Patterns Cross flow – Ceiling – Longer throw Air Patterns Round – Ceiling – Shorter throw Air Patterns Swirl (floor) Displacement (sidewall) Linear (ceiling) Linear (air curtain) Laminar (OR, clean rooms) Hemispherical (lab, industrial) Return Grilles Contrary to popular belief – return grille locations generally do not affect room air motion Return grilles merely provide an exit Surface Effects Discharge jets attach themselves to surfaces – Ceilings – Walls – Glass Obstructions with an angle of incidence greater than 15° can kick the air pattern off the ceiling Open Ceilings Unless otherwise specified assume – Ceiling diffusers were tested with a ceiling – Side wall grilles were tested near a ceiling Internal vs. external Coanda pocket Most diffusers need a ceiling for horizontal air pattern Sometimes a small lip can be added to create a ceiling effect Free jets result in a 30% throw reduction due to increased expansion Temperature Effects T150 is temperature independent – velocity driven Horizontal ceiling throw – Cooling decreases throw by 1% per °F – Heating inceases throw by 1% per °F Example – Catalog (isothermal) 4-7-9 – Cooling 4-6-7 – Heating 4-8-11 Active Length Linear diffusers should not have active sections longer than 10 ft Overly long active sections cause problems – Extended and unpredictable throw – Undulating air patterns Solutions – Provide 1-2 ft inactive breaks between sections – Alternate throw direction Acoustics Select diffusers such that they will not be heard Noisy diffusers create a poor communication NC set by in octave bands 4-6 (500, 1000, 2000 Hz) – speech interference bands 10 NC points lower than desired room level, and rarely higher than NC25 unless it’s an industrial application What Type To Select? The choice can depend on many things – – – – – – Air pattern Performance Appearance Cost Space limitations Installation/ceiling type Summary Many types of systems and outlets are available, but there’s always a best choice Selecting the right air pattern is critical Be aware of surfaces and ceilings Keep overhead heating temperatures low Select diffusers to be inaudible Questions and Answers Questions? Thank-you!
