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!