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Room Air DistributionPresented 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
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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
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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)
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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
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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
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
Mixed-Air System 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
Fully-Stratified Concepts
Improved Contaminant Removal
Stratification creates a single pass Unlike mixed-air, contaminants are not redistributed
throughout the room
Displacement Ventilation Overhead System
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
Improved Ventilation
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!