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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Understanding The Terminology
THROWDROP
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Understanding The Terminology
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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Understanding The Terminology
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 The Terminology
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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Understanding The Terminology
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
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