NSHVS – The silent life

saver in factories and warehouses

Facts & figures - Smoke

Although, apparently flames are most dangerous element,

smoke causes most deaths

The main hazard from fire is smoke

Facts & figures:

https://www.khaleejtimes.com/nation/general/70-of-fire-deaths-cause-due-to-suffocation

70% of fire deaths are caused

due to suffocation.

According to the National Fire

Protection Association (NFPA),

most fire deaths are the result

of smoke inhalation rather than

burns. NFPA notes that every

20 seconds somewhere in the

United States a fire department

is responding to a fire.

Facts & figures:

Recent research shows that there

are around 4,000 warehouse fires

per year in UK

The British economy has lost £1

billion in GDP and 5,000 full-time

jobs through preventable fires in

commercial warehouses over the

last five years.

https://www.business-sprinkler-alliance.org/news/1-billion-cost-of-warehouse-fires-to-uk-plc/.

Why is smoke venting necessary

Smoke created by fire can fill whole room & areas of

buildings and quickly becomes

death trap

Accumulated smoke will pressurize and

spread through opening, semi-open

windows/doors

Smoke causes impairment of

senses making it more difficult to

escape from danger zone.

Smoke entering stairs and escape routes can

delay the evacuation/escape

High temperature smoke can cause

flashover fires and easily lead to collapsing of roofs and structures

Presence of smoke can halt production and even

hours of inactivity can lead to loss of millions

in revenue.

Smoke behavior without venting

Goals of smoke control system

One of the main objectives of smoke control system is to

keep fire escape routes free from

smoke

They channel out smoke before it can sink to bottom and

helps in depressurization of

buildings

They also aid fire fighters by

increasing visibility and preventing

spread of smoke

They play crucial role in preventing

flashover fires, which cause fires at location away from

initial source

Reduce thermal damage to structure

and provide protection to

property

They help in increasing the ventilation of

buildings and hence helps to reduce the internal heat gain.

Smoke behavior with venting

Formation of controlled smoke layer

Key elements Influencing NSHEVS Design:

Smoke vent Design

Temperature of Smoke

Fire Size

Aerodynamic free area of ventilators

Wind Effect

Inlet area of openings and their location

Structure and dimensions of building

Effect of sprinklers

Effect of Powered ventilators

Temperature of smoke

Temperature determines the buoyancy of the smoke layer and hence the flow rate of smoke

Smoke will rise due to having a lower density than the surrounding air.

Smoke moving away from source will reduce in temperature and entrain cool air, thus reducing its buoyancy

Fire Size

The fire sized should be optimized as per the building type and inventory as wrong fire size can lead to either over sizing or downsizing the system.

Fire sizes as per client requirements can be used but engineering analysis like CFD should be done to verify the effectiveness.

Fire Size

Ref STD- BS 7346-4

Aerodynamic free area of ventilators (Cv)

This is defined as the actual area available for smoke ventilation. (considering obstructions along opening and boundary layer effect)

Models with same geometrical area can have different Cv value depending on construction

This area can be calculated by 3rd

parties, in test standards such as EN 12101-2 and is expressed as a ratio between theoretical and actual area.

Vents should be positioned so that they are not negatively affected by external wind conditions.

Effective operation of natural ventilators depends on the following factors:

• Location of vent

• Surrounding terrain

• Prevailing wind direction

Wind Effect:

Inlet Area of openings & location

Any smoke ventilation system should have sufficient supply of replacement air

When inlet air is supplied by powered means, it should be ensured to keep the inlet velocity below 5 m/s.

Upper edge of inlet opening should be at least 1m below smoke layer to prevent mixing with smoke. (venturi effect)

Structure of Building

If the false ceilings are present then effective height of building should be considered till false ceiling only.

Locating the smoke vents close to ridge or apex points of warehouse maximizes their effectiveness

No need to consider mezzanine in design if it occupies an area less than or up to 25% of total floor area.

Ventilators should not be placed at slope of more than 30 deg.

Effect of sprinklers

Where sprinklers and SHEVS are employed together the overall effectiveness of the fire protection is increased and not reduced.

The system using natural smoke ventilators would be under-designed if the sprinkler cooling were underestimated.

The smoke vent area required increases with decrease in buoyant layer temperature up on activation of sprinklers.

Effect of powered ventilators (fans) on NSHEVs

Natural and powered smoke vents should never be combined for smoke extract

If natural and powered vents are located close to each other, then counterflow of fresh air is possible by vent serving as inlet and powered extract as discharge.

If fans are used for normal ventilation, then provision should be made in the design for switching off the ventilator during fire scenarios

Myths & facts about natural smokevents

Sr. No. Myths Facts

1 Natural Smoke vents on their own does help to extinguish fires.

Smoke vents does not help in extinguishing fire and can even aid fire by increasing supply of air

2 Combination of natural and powered vents will make system more effective

Combination of natural and powered vents can led to counterflow and decrease effectiveness of smoke vents.

3 Lesser number of smoke vents are needed if system employs sprinklers

Sprinklers are essential to control the fire and presence of sprinkler will only increase the number of smoke vents as they decrease the smoke temperature.

4 Natural smoke vents can be only used for smoke ventilation

Smoke vents tested for dual purpose can be used for natural ventilation as well as smoke ventilation.

5 Smoke vents and smoke dampers are the same. Both of them have separate test standards and application is also different

Natural ventilator vs Powered ventilators (fans)

Features Natural Smoke vent Powered smoke extract (fan)

Fail safe operationFailsafe operation can be enabled by use of Fusible links or

battery back upOperation depends on reliable power supplies

Noise during operationSilent operation as no high-speed rotating components are

involved

Too noisy when run under fire conditions due to

higher speed operation

Self-regulatingCan accommodate bigger than design fires as they cause

higher buoyancy and higher extract rates

The airflow rate is fixed and cannot accommodate any

additional requirements than design

Duration of operationNo time limit to operation and can be operated up to 72hrs

from power loss

Can operate only till rated operation limits eg- 2 hrs

for F400 or F300 rating

Weight of unit Light weight and hence easy to provide support structuresComparatively much heavier and hence requires more

robust support and vibration isolation

Operation costs/energy

consumptionEither not required or very minimal. High energy consumption and costlier cabling

Maintenance requirements Easy maintenance or maintenance free operationRequires periodic maintenance and re-greasing of

bearings

Ducted applications Not suited for complex ducting. Suitable for complex and branched ducting

Wind sensitivitySensitive to winds effects and has to be considered during

designNot affected by wind effects

Comparison btw Smoke vent and Smoke Damper

Specification/ Principle Smoke Damper Smoke Vent

FunctionClose when smoke is detected and prevent the spread of smoke

Open when smoke is detected and allow the smoke to

escape

Applications

Along fresh air supply lines and channels/ branches. This damper will close the branch and prevent the mixing of smoke with circulated air.

Mostly located on roofs and walls to allow the escape

of smoke‐ eg, warehouses, airports, shopping malls,

atriums.

Operation during emergency

Can only be closed during fire. (mostly by fusible links). The damper cannot be opened automatically once closed during fire scenario

The smoke vent can be opened and closed by manual

override if required to regulate the fresh air intake.

This will aid fire fighter to prevent growth of fire and

protect valuables.

Fire ratingShould have the same fire rating as the duct or the wall

Not required to have same fire rating as attached

structure/ duct. Fire rating required is only 300 Deg C

for 30 min.

Standards applicable UL 555S

EN 12101‐2 or FM 4430 or UL 793

Accepted global test standards

EN12101-2- European standard

UL 793- American standard FM 4430- American standard

Brief comparison between standards

TEST CRITERIAS UL 793 FM4430 EN12101-2 REMARKS

No of test cycles for

Reliability

250 Full strokes under static,

50 strokes for manual

operating device.

60 Cycles

Re50 or Re1000 cycles for normal

smoke ventilators and 10,000

cycles for dual purpose ventilator

10,000 cycles by EN is the most no of

test cycles by any standard. Only EN has

test method for dual purpose ventilators

Speed of opening testsOn an average less than 2.5

sec (5 cycles). Less than 10 sec Less than 60 Seconds

Better to test and comply the 60

seconds rule as it gives better reaction

time to people nearby

Aerodynamic Free Area No test to determine the

effective area

No test to determine

effective area

Do have an experimental method

which specifies the Cv value of

product.

Cv value is important to determine the

effective area available for ventilation.

Fire resistance testComply with burning band

test of class A, UL 790

Not applicable for

metallic lids300 Deg C for 30 min

Wind uplift force 30 lbs/Sqfeet 60 lbs/Sq feet WL 1500 PaBoth UL and FM have higher test

requirements for windload.

Impact resistance test45.4 Kg from 1.2m

90.8kg from 0.61m

45.4 Kg from 1.2mNot Applicable

EN does not have a special test for

impact resistance.

Use of actuatorsNot permitted to use

electrical actuators

Should be coupled

with Thermal deviceAllows electric actuator

Warehouse

Workshop

Factories

Shopping Malls

Supermarkets

Major Application Areas