high rise building air pressure, infiltration & stack...

© 2012-ESD, Inc.

By: Mehdi Jalayerian, PE, LEED® AP BD+C

Executive Vice President Environmental Systems Design, Inc.

2012 Spring Seminar, June 14, 2012

High Rise Building Air Pressure, Infiltration & Stack Effect

Theory and Impact on Indoor Environment

© 2012-ESD, Inc.

Learning Objectives

• Basic Theory of Air Flow Around and In High Rise Building – Wind – Stack-Effect

• Vertical Compartmentation – Elevators – Stairs – Impact on Energy – Impact on Pressure

• Smoke Management – Pressurization (Stairs/Space) – Smoke Control Considerations

• Example Building Air Movement/Pressure Analysis – 16 Story Hotel – Mixed-use Tall Building

• Summary Guidelines

© 2012-ESD, Inc.

Air Flow in High Rise Buildings

• Forced ventilation – Building supply/exhaust air systems – Fully controlled and distributed

• Natural ventilation – Pressure from wind and/or stack effect – Strategically placed intentional openings

• Infiltration – Uncontrolled air leakage – Unintentional openings in building envelope – Air leakage through floor/wall openings – Driven by wind force, temperature difference (buoyancy), and/or

system/appliance induced • Space Pressurization

– Normal mode pressurization – Fire mode pressurization (fire floor and stairs) for smoke

management

© 2012-ESD, Inc.

• Air movement into/out of a building is caused by wind pressure and stack effect pressure

• Wind pressure causes air to infiltrates into the building, or ex-filtrate out of the building

• Stack effect (air buoyancy) pressure causes air to rise or drop

Cold Weather • Air infiltrates into the lower half of the

building, rises to upper floors due to stack effect and exfiltrates in the upper half of the building.

Hot Weather • Air infiltrates into the upper half of the

building, drops to lower floors due to stack effect and exfiltrates in the lower half of the building.

Basic Theory WIND VELOCITY PROFILE

EXFILTRATION

INFILTRATION

STAC

K E

FFEC

T

NUTRAL PLANE

AVERAGE SURROUNDING ROOF HEIGHT OR 10M ABOVE GRADE

© 2012-ESD, Inc.

Basic Theory – Air Flow Around Building

© 2012-ESD, Inc.

Basic Theory - Wind Pressure

Wind creates a distribution of static pressure on the building envelope, which is dependent on wind direction and location on building envelope. ∆ Pw = C Cp ρ v2/2

∆ Pw = wind pressure difference, inches of water C = unit conversion, 0.0129 Cp = surface pressure coefficient, dimensionless ρ = air density, lbm/ft3 (about 0.075) v = wind speed, mph Note: Wind Pressure at top of 60 Story: Upwind = +1.32 in. of Water Sides = +0.61 in. of Water Downwind = -0.61 in. of Water

2X

X

Wind 15 mph

+0.33 in WG

© 2012-ESD, Inc.

Basic Theory - Stack Effect Pressure

Air density varies with temperature. In cold weather, low density air infiltrated into a building rises and creates stack effect pressure. ∆ Ps = C2 ρi g (h-hNPL)(Ti -To)/To

∆ Ps = pressure difference, inches of water

C2 ρi g = density and gravity constant, 0.01444 h = height of observation, ft NPL = building neutral pressure level T = absolute temperature, oR i & o = inside & outside Note: Stack Pressure is approximately 1 in. of water for a 60-story building and -10oF outside T

+0.95 in WG

© 2012-ESD, Inc.

Basic Theory – Overall Pressure

Natural air movement in a building is due to pressure difference caused by wind and temperature difference between indoor and outdoor air (stack effect). ∆P = (Po - Pi )+ ∆Pw + ∆Ps

∆P = pressure difference Po = outside static pressure Pi = interior static pressure ∆Pw= pressure difference due to wind (depends on orientation) ∆Ps = pressure difference due to stack effect

Wind

Stack Effect

Overall

Adjusted Neutral

© 2012-ESD, Inc.

Basic Theory – Air Flow Through Opening

Dynamic air flow through an opening is proportional to square root of the pressure difference across the flow path.

Q = C x Cf x A x √(2∆P/ρ)

Q = Air flow, CFM C = Unit Conversion, 776 Cf = Flow Coefficient, typically = 0.6-0.7 A = Opening Area, ft2

∆ P = pressure difference, inches of water ρ = air density, lbm/ft3 (about 0.075 at standard conditions)

Operation of building appliances and mechanical ventilation systems impact natural air movement. The impact is included with outside/inside pressure difference

© 2012-ESD, Inc.

Basic Theory - Air Flow Network

AMBIENT NODES

SPACE NODES

STAIR NODES

ELEVATOR NODES

Flow equations are developed between nodes (pressure & mass balance) Q = C x A x √(2∆P)/ρ System of equations are solved for each calculation time period Software: NIST CONTAM LBNL COMIS e-Quest

STAIR/ELEVATOR INTERCONNECTION

SPACE NODES INTERCONNECTION

© 2012-ESD, Inc.

Example High Rise Building Air Infiltration

Assumptions • 60-Story Office • 200 feet x 100 feet floor plate • Average leakage curtain wall • Weather-stripped exterior exist

doors • Revolving doors + swing exit

doors at main lobby • Standard door on typical floor stair • Average leakage elevator doors • Exterior condition at -10oF • Interior condition at 73oF • Wind direction from south (longer

wall) • Wind speed 15 mph (33 feet

above grade)

GROUND FLOOR

LOBBY

TENANT TENANT

N

TYPICAL FLOOR

TENANT

© 2012-ESD, Inc.

Building Air Infiltration – Effect of Elevators

Single Zone Elevators

Two Zones Elevators

Three Zones Elevators

Total Infiltration Heat Load 6,357 KW 2,903 KW 1,652 KW 100% 45.7% 25.9%

60-Story Office BuildingEffect of Elevator Zones

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Infiltration, scfm

Single Zone Two Zones Three Zones

~74% reduction

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#1 #2ELEVATORS STAIRS

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LR HR #1 #2ELEVATORS STAIRS

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LR MR HR #1 #2ELEVATORS STAIRS

© 2012-ESD, Inc.

Elevator Shaft Air Flow

60 Story Office BuildingElevator Shaft Air Flow (Winter)

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Air Flow, CFM

Single Zone Two Zones Three Zones

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#1 #2ELEVATORS STAIRS

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LR HR #1 #2ELEVATORS STAIRS

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Single Zone Elevators

Two Zones Elevators

Three Zones Elevators

© 2012-ESD, Inc.

Building Air Infiltration – Effect of Stairs

Single Zones Stairs

Three Zones Stairs

Total Infiltration Heat Load 1,652 KW 909 KW

100% 55%

60-Story Office BuildingEffect of Stair Zones (Three Elevator Zones)

-6000 -4000 -2000 0 2000 4000 6000 8000 10000 12000 14000

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Infiltration, scfm

Three Zones Elevators/Single Zone Stairs Three Zones Elevators/Three Zones Stairs

~45% reduction

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© 2012-ESD, Inc.

60 Story Building – Space Pressure

© 2012-ESD, Inc.

Typical Stair Air Separation/Lock

Up

Up

Down

AIR LOCK

2'-6"

2'-6"

2'-6"

Down

2'-6"

TYPICAL FLOOR

TENANT

Alternate B

Alternate A

Up

UpDown

Down

Air S

epar

atio

n

2'-6

"

2'-6

"

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© 2012-ESD, Inc.

60 Story Building – Infiltration Load (Energy)

© 2012-ESD, Inc.

High Rise Building - Smoke Management

System Objectives • Reduce occupant death and

injuries – Delay smoke accumulation – Reduce smoke migration – Provide safe escape route – Provide safe refuge area

• Reduce property loss – Purge smoke – Manage fire impact

System Approach • Design/operate system to

prevent smoke entering the unaffected areas

• Integrate Smoke Control with Fire Protection System

• Allow for operational flexibility • Use HVAC system for smoke

control to improve system reliability

• Utilize compartmentation • Pressurization

– Horizontal – Vertical – Stairs – Lobbies

© 2012-ESD, Inc.

Pressurization – Normal Mode of Operation

Floor Pressurization • Main building lobby is pressurized • Each floor is provided with required

supply and exhaust • Each zone of building is pressurized

by maintaining differential air flow between supply/exhaust to minimize infiltration

• Stair pressurization systems are off GROUND FLOOR

TYPICAL FLOOR

LOBBY

TENANT TENANT

100% SUPPLY and 80-90% EXHAUST

© 2012-ESD, Inc.

Pressurization – Normal Mode of Operation

Three Zones Central AHU

Three Zones Elevator Single Zone Stairs

60-Story Office Building InfiltrationEffect of AHU Pressurization (Normal Mode)

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Infiltration, scfm

AHU - OFF AHU - ON

Building Infiltration Load Reduces by 70% (1652 kW reduced to 506 kW)

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© 2012-ESD, Inc.

Pressurization – Fire Mode of Operation

Floor Pressurization • Main building lobby is pressurized • Fire Floor is provided with full

exhaust • Floors above and below fire floor is

provided with full supply air • All other floors operate normal mode • Stairs not pressurized

GROUND FLOOR

FIRE FLOOR

LOBBY

TENANT TENANT

NO SUPPLY and 100% EXHAUST

ABOVE OR BELOW FIRE FLOOR

100% SUPPLY and NO EXHAUST

© 2012-ESD, Inc.

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Pressurization – Fire Mode of Operation

Exhaust on Fire Floor & Supply Above/Below

Single Zone Stairs Non-pressurized

60-Story Office Building InfiltrationSmoke Condition (Level 28) - Fire Mode Pressurization

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Infiltration, scfm

AHU - ON AHU - ON - Smoke Mode

Negative Press. (Exhaust) Positive Press. (Supply)

No Stair Pressurization Provided

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© 2012-ESD, Inc.

Pressurization – Fire Mode/Stair Impact

Floor and Stair Pressurization • Main building lobby is pressurized • Fire Floor is provided with full

exhaust • Floors above and below fire floor is

provided with full supply air • All other floors operate normal mode • Stairs are pressurized

GROUND FLOOR

FIRE FLOOR

LOBBY

TENANT TENANT

ABOVE OR BELOW FIRE FLOOR

NO SUPPLY and 100% EXHAUST STAIRS PRESSURIZED

100% SUPPLY and NO EXHAUST STAIRS PRESSURIZED

STAIRS PRESSURIZED

© 2012-ESD, Inc.

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Pressurization – Fire Mode/Stair Impact

Exhaust on Fire Floor & Supply Above/Below

Single Zone Stairs Pressurized – doors open

Open Exit Door

Open Exit Doors

Stairs Are Pressurized

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© 2012-ESD, Inc.

Elevator Shaft Air Flow – Fire Mode

Open Exit Door

Open Exit Doors

Exhaust on Fire Floor & Supply Above/Below Doors open on fire, above/below and ground floors

60 Story Office Building - Elevator Shaft Air Flow (Winter)Smoke Condition on Level 28 (Building and Stairs Pressurized)

-4000 -2000 0 2000 4000 6000 8000 10000 12000 14000 16000

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Air Flow, CFM

Three Zones Three Zones - FireMode/Pressurized

© 2012-ESD, Inc.

Stair Air Flow – Fire Mode

Open Exit Door

Open Exit Doors

60 Story Ofice BuildingEffect of Stair Compartment on Pressurized Stair Air Flow - Smoke Condition (Level 28)

-15000 -12500 -10000 -7500 -5000 -2500 0 2500 5000 7500

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Air Flow, CFM

Single Compartment Stair Three Compartment Stair

Exhaust on Fire Floor & Supply Above/Below Doors open on fire, above/below and ground floors

© 2012-ESD, Inc.

Stair Pressure – Fire Mode

Open Exit Door

Open Exit Doors

60 Story Ofice BuildingEffect of Stair Compartment on Stair Pressure - Smoke Condition (Level 28)

-0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

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Pressure, in WG

Single Compartment Stair Three Compartment Stair

MA

XIM

UM

STA

IR P

RES

SUR

E TO

MA

INTA

IN D

OO

R F

OR

CE

Exhaust on Fire Floor Supply Above/Below

Doors open on fire floor, floors above/below fire floor and

ground floors

© 2012-ESD, Inc.

Smoke Control Considerations

• Building envelope design • Compartmentation

– Building occupancy types – Zones

• Elevator shafts – design/construction

• Stair shafts – design/construction

• Air leakage paths (smoke movement)

• Local design temperatures • Local wind velocities:

– Site – Envelope (wind tunnel test)

• HVAC system: – System zoning – Components design – System Activation/Controls

• Fire protection, detection and alarm system – Integration/coordination

• Firefighters Access: – To site – To manual/remote control of

system • Exiting plan

– Refuge areas – Exit duration

• Security and access control (door status/control)

© 2012-ESD, Inc.

Tall Building Example – Space Pressure Hot

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huttl

e

Pub

lic P

arki

ng S

huttl

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Res

iden

tial P

arki

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huttl

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Res

iden

tial L

ow Z

one

Res

iden

tial -

Hig

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ne

Res

iden

tial F

riegh

t

Bui

ldin

Frie

ght

Hot

el F

riegh

t

Am

eniti

es F

riegh

t

Hot

el

Am

eniti

es -

1

Am

eniti

es -

2

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98765432

LobbyLL-1LL-2LL-3LL-4LL-5

© 2012-ESD, Inc.

Summary

• High rise building infiltration can be significantly reduced by vertical compartmentation of building shafts (elevators, stairs, HVAC risers)

• Vertical compartmentation of building shafts creates more uniform pressure and air flow characteristics

• High rise building’s HVAC system will perform more effectively and more efficiently through a managed plan for stack effect

• Smoke control system configuration and performance must be carefully analyzed for various fire conditions and plan designs

• Integrated design process between architecture, structure and MEP assures an optimized building natural air flow

• High rise building’s natural air flow, air pressures characteristics and stack effect must be reviewed early in the design stage

• Minimum of two air barriers (walls, doors & vestibules) shall separate the internal building shafts (elevator, stairs) from the outside environment on each floor – main lobby, sky lobbies and loading dock levels may require additional layer of separation)

high rise building air pressure, infiltration & stack...

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