smoke extraction in buildings

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SMOKE EXTRACTION IN BUILDINGS

Presented by

Vysakh Manohar

GATE-Chennai

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INTRODUCTION

• Effective smoke extraction in buildings has been one of the inevitable considerations in the building design as the number of buildings and the concern of people about the regular fire accidents in buildings are increasing much.

• This presentation includes a brief description about the different design consideration principles and methods to be taken care for the effective heat extraction in different types of buildings .

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What is smoke?

• By product of combustion or pyrolysis .• Mixture of liquid , solid particulate matter and

gaseous products.• It is a cocktail of carbon monoxide, carbon

dioxide ,HCL and HCN

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FIRE TIME LINE• 00.00- Household is asleep• 00.30- Fire ignites and grows in downstairs• 1.04- Fire spreads and smoke begins to fill

In the room.• 1.35- Smoke layer descends rapidly and temperature exceeds 88 deg• 1.50 – Smoke detector sounds an alarm.• 2.30- Temperature becomes 205 deg• 2.48- smoke pours into other rooms

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• 3.03-Temperature in the room becomes more than 260 deg

• 3.20-upstairs,corridors,all the rooms are filled with smoke

• 3.41- flashover occurs , temperature becomes 760 deg

• 3.50 –Two minutes after the alarm sound, only the second exit will be the way out

• 4.33- Flames will be apparent from the outside , the first evidence of fire visible from outside.

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• Will follow similar scenario in other location• The variations will be based on 1-fire size 2-size of building 3-flammable materials in the building 4-alarm systems 5-safety measures available

Factors affected

1. Design fire size2. Stack and wind effects3. Temperature effect of fire4. HVAC and ventilation systems5. Climate6. Expansion of ducts

GETTING THE DESIGN RIGHT FROM THE BEGINNING

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COMMON SAFETY METHODS

• Use sprinklers• Smoke curtains• HVAC system incorporating F & S dampers.• Extraction fans• Alarm systems• Pressurisation

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HAZARDS OF DANGEROUS SMOKE• Impair visibility• Coughing,nausea,vomiting• Generation of high temperature• Irritation and respiratory problems• Bronchospasms and hypoxia• Inflict death if it contains dangerous gases• Even low concentration of CO can cause diminished

coordination and increased disorientation• Amount of oxygen is reduced- suffocation• Exacerbate corrosive nature of HCL

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Smoke Pathways

• Air and service ducts connecting floors

• HVAC/ventilation systems • Extract duct systems

• Gaps between wall and floor constructions

• Stairwells

• Lift shafts

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Need of smoke extraction

• for life safety and reducing property loss

• for fire fighter access

• To clear smoke after a fire –smoke purging

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Smoke extraction design objectives

• Effective protection of building occupants by maintaining tenable conditions on egress systems.

• Reliable and functional fire fighting controls used by firefighters to increase visibility and reduce heat exposure.

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Make up air routes

• External doors• Louvers in wall or roof• Central air plant configured to supply air• From adjacent unaffected areas

→avoid mixing of smoke and make up air→extraction points-avoid creation of “hole”

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Smoke Production and movement

• Amount of smoke produced is dependent on size of fire

• Path through which it goes• Plume height

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Fundamental design factors

1. Design fire size2. Smoke layer depth3. Identify smoke reservoirs4. Calculate smoke volume and temperature5. Minimum number of extract points6. Vent or fan size and fan temperature rating7. Inlet air8. Duct work

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Design fire size

• Fire starts small and becomes large• Vent area=2.5 % of floor area-otherwise

mechanical• 6 ac/hour is used often• Design fire size is greatly dependent on building

contents.• Based on the largest fire source• Cannot surely withstand post flash over

conditions

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Heat output of fire

• National bureau of standards in USA• Based on time-• safe escape time• brigade response time

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Acceptable smoke layer depth

• Smoke layer base should be above the people• 2.5 m-single storey• 3 m - upper storey of 2 storey

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Smoke reservoirs

• If smoke is held in a too large reservoir-lose buoyancy

• 2000 m^2 - natural extraction• 2600 m^2 -powered extraction• Smoke curtains can be used to reduce

horizontal spread

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Smoke- volume, mass flow rate and temperature

• T=Q/m cp

• 1/3 is gained by building structure

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Minimum no of extract points

• M cr is not exceeded• No smoke need to travel more than 30 m to

extraction point

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Inlet air

• To make up the smoky gases• At least 1.5m beneath smoke layer• If natural inlet is given• Inlets should be away from the outlets• Inlet velocity should be less than 3 m/s

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Ductwork

• Must be able to withstand anticipated temp- From the formula

• HVCA specification DW144• For sprinkled buildings• To limit fire spread -fire protection of steel ducts -construct from proprietary material - use fire and smoke dampers

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Fire resisting requirement of ductwork

• To prevent fire inside one duct to breaking out to other

compartment.

• To prevent fire from outside enter into duct and then enter another fire compartment

• To prevent fire from outside enter into duct and the break out into other compartment

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Performance criteria for fire resisting duct work

• Stability

• Integrity • Insulation

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Other things to be noted

• Duct must be able to retain at least 75% of its size after the smoke removal.

• Sound or thermal insulation should not be raised to ignition temperature.

• Fire stopping seals should be used• Buckling or collapse.

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Construction of fire resisting smoke extract ducts

a) galvanized steel duct with additional protection b) fire resisting boards(eg . calcium silicate) c)proprietary fire resistant ductwork eg : PROMATECT-H

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Protection of duct with fire resisting board

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Ductwork construction from fire resisting board

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Spreading of smoke

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Design guide for different building types

• Multi-storey office and similar buildings• Warehouse• Underground car parks/basement• Atrium buildings• Shopping malls

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Multi storey office and similar buildings

• Phased evacuation• Evacuation time will be varying• Smoke should not spread• 1, smoke extraction• 2, pressurisation• The recirculation of air in normal hvac system

is prevented by dampers

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Important design considerations

• The system is not for providing a smoke free layer

• Both supply and extract ducts should be having fire resistance equal to that of the floors /walls

• Exhaust fans should be able to run at the highest expected temperature

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• The power supply to the system must be duplicated

• The power and instrumentation cabling should be of fire resistant type

• Smoke and fire dampers should be having suitable specification-UL555s

class-I,II,III,IV

• in high storey buildings-wind pressure

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Ware houses

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Underground car parks/basement

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ATRIUM CONSTRUCTIONS

• Fully enclosed atrium

• Partially opened atrium

• Fully opened atrium

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Fully enclosed atrium

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• Enclosures are 2 types

1- fire resisting enclosure-fire resisting glazing

2- enclosure of limited combustibility but not fire resisting

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Partially enclosed atrium

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Fully open atrium

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Shopping malls

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Extract from single shops

• Smoke is prevented to enter to mall area• Very large fans are not required• But complex and costly• Shops must be sprinkled• Combined duct work is used to reduce cost

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Extract from mall

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Channeling screens

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Limitations

• With high atriums there is a cut off point of mass flow rate of 150 to 250 kg/s-economically impractical.

• Temperature stratification

• Cooling effect

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Fire,smoke and building code agencies

• Industrial risk insurers (IRI)

• Southern building code congress inc(SBCCI)

• National fire protection association(NFPA)

• Underwriters Laboratories(UL)

• British standards(BS)

• National Building Standards(NBS)

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CONCLUSION

• By following discriminative approaches and methods needed for different types of buildings and constructions provided by the different approved agencies ,the threat from fire and smoke can be reduced to a considerable level.

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References

• Promat International Ltd• National Fire protection Association,Guide for

smoke and heat venting,NFPA-204 M• BS 5839: Fire Protection and alarm systems in

buildings,Part1 2002• Underwriters Laboratories,Leakage rated

dampers for use in smoke in control systems,UL555 S

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THANKING YOU ALL……..