Basics of Room Air Distribution

8/17/2019 Basics of Room Air Distribution

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THE BASICS OF ROOM AIR

DISTRIBUTION

Dan Int-Hout

Chief Engineer / Krueger

Richardson Texas

3/22/2011


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Why Air Distribution?

What are we trying to accomplish with air

distribution system?

€ Provide Occupant Thermal Comfort€ Provide sufficient ventilation to meet codes

€ Control Latent loads (Humidity)


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What supplies the air we are

Distributing?

€ Most systems provide air from overheadand attempt to provide •well mixed‚spaces.

€ Sources include rooftops, VAV, WSHP,Chilled beams, etc

€ Some systems are partially or fully

stratified.€ Lets start by understanding basic

terminology:


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Understanding The Terminology

Primary Air Jets - Air jets from free round openings,

grilles, perforated panels, ceiling diffusers and other

outlets can be defined by three variables.

€ Throw

€ Drop

€ Spread


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THROWDROP


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50 fpm

Typical 100 fpm

150 fpm

22.5

22.5

Spread - is defined as the divergence of the airstream in a horizontal or vertical

plane after it leaves the outlet.


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Coanda Effect

Coanda Effect - a negative or low pressure area is created between the

moving air mass and the ceiling at or near the supply air outlet. This low

pressure area causes the moving air mass to cling to and flow close to the

ceiling surface and increases the throw.


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Understanding primary air jet variables enables

€ Accurate prediction of room air flow

€ Improvement of thermal comfort

€ Proper selection of grilles, registers and diffusers

€ Adherence with ASHRAE Ventilation Std 62.1 is a

Leed PREREQUISITE, and is required by many

codes.


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The Basis of Catalog Performance Data

€ Throw € The horizontal or vertical axial distance anairstream travels after leaving an air outlet, usually

assumes a surface adjacent to the air outlet

€ Pressure € Can be total pressure or static pressure

€ Sound € Can be either NC or Octave Band data


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Throw

€ Throws are cataloged for 150, 100 and 50 fpmterminal velocities.

€ Throws should be selected so that jets do not

collide, but have sufficient projection for the area to

be served.

7 € 8 € 12

150fpm

100fpm

50fpm


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Pressure

Pressure € Air outlet pressure data isrequired to properly size the air deliverysystem within a building.

€ Static Pressure € The outward force of air

within a duct, measured in inches of watercolumn.

€ Velocity Pressure € The forward movingforce of air within a duct, measured ininches of water column.

€ Total Pressure € The sum of the velocityand static pressures, expressed in inchesof water column and can be obtained byuse of a pitot tube.

PT = PV + PS

PT

PS

PS PT


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Sound

Sound levels reported for diffusers are conducted in

accordance with ASHRAE Standard 70.

€ Catalog sound data assumes 10 diameters of

straight duct.€ Room absorption is assumed to be 10dB in all

bands.

€ In practice however, room sound levels are

probably 5 NC higher than reported.


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NonNon- - Inductive Air DistributionInductive Air Distribution- -

Laminar and Radial FlowLaminar and Radial FlowOutletsOutlets

€ Hospital Operating Suites

€ Hi-Tech Electronics and other industrialapplications

€ Clean Rooms

€ Laboratories

Laminar Flow Radial Flow


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Pressurized plenum for core

Ducted perimeter with fan powered

boxes, or other techniques, depending on

climate, glass load, etc..

UFAD: Underfloor Air DistributionUFAD: Underfloor Air Distribution

€ A raised floor allowselectrical andcommunication

circuits to be easilyaccessed andchanged.

€ Air may be distributedwithin this space,without ductwork.


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Interior Outlet SelectionInterior Outlet Selection € €

1/workstation1/workstation


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UFAD Perimeter Outlet SelectionUFAD Perimeter Outlet Selection

€ Perimeter solutions vary considerably

€ Avoid condensing coils under the floor

€ Hydronic coils often leak€ Exhaust at the perimeter to draw heat

away

€ Best solutions seem to be heat and coolfrom overhead


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17

Displacement SystemsDisplacement Systems

€ To be added


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Select based on ‚adjacent zone‚


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ApplicationsClassroomsKitchen

Restaurant

Auditoriums

Atriums

GymsAnd more•


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Well Mixed-High Induction

Diffusers€ Commercial Office Spaces

€ High Velocity Jets

€ Long Throw

€ Designed to Mix in Zone PLQ-R

1400

Prism


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Air Distribution, Poor PatternAir Distribution, Poor Pattern

THERMOSTAT

POORLY ADJUSTED / SELECTED DIFFUSER


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PROPERLY ADJUSTED/SELECTED DIFFUSER

THERMOSTAT

Air Distribution, Good PatternAir Distribution, Good Pattern


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Thermal Comfort ASHRAE Standard 55-2010 mandates a maximum 5.4‚F vertical temperature

stratification in Occupied Zone

2ƒ 2ƒFloor to Occupantsƒ Head Level

(3.5 ft. for seated, 6 ft. for standing occupants)

Velocities within the occupied

zone shall be ƒ 50 FPM

Velocities within the occupied

zone shall be ƒ 50 FPM


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Understanding ADPI


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ADPI

€ ADPI is the percentage of points within the occupied zone

having a range of effective draft temperatures of -3„ to +2„ ofaverage room temperature at a coincident air velocity less

than 70 FPM.

€ ADPI is essentially a measure of the degree of mixing inzones served by overhead cooling systems.

€ When air distribution is designed with a minimum ADPI of

80% the probability of vertical temperature stratification or

horizontal temperature non-uniformity is low and

conformance with ASHRAE Standard 55 (Thermal Comfort)requirements is high.

€ ADPI does not apply to heating situations or ventilation-

related mixing.


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€ ADPI selection using T50 / L

was developed in the …60swhere L is the distance to the

nearest wall or halfway to the

nearest air outlet..

€ A relationship was found

between 50 FPM/minisothermal throw and cooling

throw,

€ Using this table engineers can

assure clients that diffuser

selections will provide

acceptable mixing and air

change effectiveness.

ASHRAE Fundamentals Chapter 20, Table 3


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Perforated 24X24, 10„ inlet, 4 way, 20‚ Delta-T

2

4

6

8

10

12

14

16

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

CFM/Sq.Ft.

1 / 2 U n i t S e p a r a t i o n D i s t a n c e

420

350

300

250

160

Range

Spacing for 80% ADPI

NC=35

CFM


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Prism 24x24

0

4

8

12

16

20

24

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

cfm/SqFt

1

/ 2

U

n i t S

e p a r a t i o n

D i s t a n c e , L

600

545

436

273

109

10in. Neck

NC=37

Prism, 24„x24„, 10„ inlet, 20‚…T

Spacing for 80% ADPI


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Room Air SpeedRoom Air Speed

Issues and FactorsIssues and Factors

€ Standard 55 says thermal comfort can be achievedwith 0 fpm air motion.

€ Uniform air temperatures indicate good mixing whenloads are present.

€ With conventional (well-mixed) systems, room airspeed is driven primarily by room loads when airsupply is below 0.9 CFM/sq. ft. and air diffusion isadequate per ASHRAE sponsored research

€ Partitions can provide excellent comfort with ceilingdiffusers when cooling.


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LEED & ASHRAE CodeLEED & ASHRAE Code

ComplianceCompliance

€ In LEED 2009, in order to get ANY LEED points, one must fully

meet the Ventilation Rate Procedure calculation in ASHRAE

Standards 62.1 (Ventilation)

€ ASHRAE 62.1-2010 VRP requires that if heating air supplied

from the ceiling is less than 15„ above room temperature butdoes not reach within 4.5 feet of the floor at 150FPM the

outdoor air supply must be increased by 25%.

€ ASHRAE 62.1-2010 VRP requires that if the heating air supplied

from the ceiling is greater than 15„ above room temperature the

outdoor air supply must also be increased by 25%.

€ LEED 2009 Ventilation points are gained by increased

ventilation 30% beyond 62.1 minimums.


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Non Typical Throw Analysis


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Diffuser Selection & Buoyancy

€ ADPI isnƒt always the best way to analyze, select and

place diffusers, especially with heating and high bay

applications.

€ One can estimate Throw as a function of ‡T andbuoyancy.

€ Simple rule: Distance to 75ft/min is affected by

1%/degree(F) ‡T.

Example:

1. 20„‡T Cooling, Vertical Down = +20% projection

2. 20„‡T Heating, Vertical Down = -20% projection

3. 20„‡T Heating, Along Ceiling = +20% projection


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Side Wall Register Selection &

Buoyancy€ Horizontal free jet:

€ Vertical change @ 75ft/min is affected by

1% of 75fpm throw/F0 ‡T.

T75

Example: 15oF Delta T heating

15% T75

Note: T150 is not affected by Delta-t


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Entrained vs. Free Jets

€ Most catalog throw data assumes jet is along a surface.

€ Exceptions include drum louvers, duct mounted grilles

and vertical linear diffusers.

€ A free jet will be shorter than an entrained jet because ithas more surface area to induce surrounding air, whichshortens throw.


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Special Applications

Continuous Duct Application Suggestions:

€ Use multiple drum louvers, duct mounted grilles and

continuous linear applications (longer than 10ƒ).€ Size duct as large a possible (Duct inlet velocity <1000fpm).

€ If inlet velocities are less than 1000fpm, maintainconstant duct size through entire length of run and

balancing will be minimal.


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ReturnsReturns

€ Typically, returns are located in the ceiling

in offices.

€ Returns have an almost immeasurableeffect on room air flows below 1.5 cfm/sf.

€ Suspended ceilings typically leak 1cfm/sf

at 0.1‚ differential pressure.

€ Spaces with high airflow rates can benefit

from low returns.


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Air Distribution SummaryAir Distribution Summary

€ LEED 2009 requires meeting Standard 62.1 forproject approval-No compliance=No Leed projectdesignation!

€ Documented use of ADPI is the ONLY way to assurecompliance to ASHRAE Std. 55 in the design phasefor cooling.

€ Reheat needs to be carefully considered in terms ofdischarge temperatures and velocities. High heatingsupply temperatures will void meeting Standard 55(and loose a potential LEED point).

€ Software is available to assist in selecting the best mixof products.

€ Iƒll discuss •the rules‚ in the next session!


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ANY QUESTIONS?


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ContactContact

dintdint--hout@krueger hout@krueger --hvac.comhvac.comwww.krueger www.krueger --hvac.comhvac.com

EXCELLENCE IN AIR DISTRIBUTION

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Basics of Room Air Distribution

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