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Colt International Ltd
Car Park Ventilation - Impulse Ventilation Systems
“Impulse Ventilation systems are the most significant new weapon made
available to the industry in the last 25 years. The benefits of the
system can be reaped not only by the car park owners, but also the
users, designers and in the event of fire, the fire service.”
So said the BPI recently and it is clear to see why. For years car parks
have been ventilated using the traditional method of miles of ducting
taking up valuable car parking space. There is now an innovative new
approach known as impulse ventilation that removes the need for the
endless ductwork and uses slim, efficient and powerful fans. Paul
Compton of Colt International, who has extensive experience in Smoke
Control, looks at the old methods and demonstrates the considerable
benefits of the new system.
Traditional Car Park Ventilation & Smoke Extract
Approved Document B – Fire Safety (ADB) and Approved Document F –
Ventilation (ADF) provide recommended methods of complying with Building
Regulations. They describe three traditional methods of ventilation.
1. Open Sided Car Parks. These are fully above ground car parks with
permanent wall openings on each level equal to 5% of the plan area
arranged to provide cross ventilation. These openings provide sufficient
ventilation for both smoke clearance and to remove exhaust fumes. The
2006 edition of ADF now defines the area of wall openings as an
"equivalent area", meaning designers have to take full account of the
aerodynamic effect of any louvres, screens, barriers, etc across the
ventilation openings.
2. Natural Ventilated Car Parks. These have permanent wall openings on
each level equal to 2.5% of the plan area arranged to provide cross flow
ventilation. This is sufficient ventilation for smoke clearance, but in
addition mechanical extract providing three air changes per hour is
needed to remove exhaust fumes.
3. Mechanically Ventilated Car Parks. Where natural ventilation is not
possible, a mechanical extract system can be used. This needs to achieve
six air changes per hour for exhaust fume extract and ten air changes
per hour for smoke clearance. This system is designed to run in two
parts, each one capable of extracting 50% of the required extract rate.
A standby power supply to operate in the event of mains power failure is
required. The fans must be rated to run at 300 degrees centigrade for a
minimum of sixty minutes and the ductwork must have a melting point of
not less than 800 degrees centigrade. The ductwork distribution must be
planned to the satisfaction of the Local Statutory Authority and the
extract points should be designed to extract 50% at high level and the
remainder at low level.
These systems are not intended to assist means of escape in case of
fire, but to assist fire fighters by providing smoke clearance.
There are a number of issues relating to traditional mechanical car park
ventilation that have encouraged designers to progress to a more
innovative system :-
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The ductwork runs underneath the ceiling, reducing the already
restricted available height.
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Downstand beams require the ducting to set down below them, thus
diminishing the height even further.
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Low level extract grilles are required, often with protective barriers
surrounding them, and these take up valuable floor space.
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The ductwork gives the car park a cluttered look and can interfere
with CCTV coverage.
The Modern Approach - Impulse Ventilation
The system of impulse ventilation has increased in popularity in the UK
over the past four or five years. This system uses the principle of
adding momentum to the air to thrust it towards a pre-designated extract
point and to ensure that there are no dead spots for fumes and smoke to
stagnate and collect. Although the system is relatively new for car
parks, it has been developed from well proven tunnel ventilation
systems. The major benefit of the system is that it removes the need for
all the distribution ductwork in the car park. This provides more space
for parking bays. It also provides a safer, lighter environment, and
leads to better security from CCTV systems. The installation of the
equipment is much easier and significantly quicker. Power consumption
can also be significantly reduced. In place of distribution ducting, a
number of small fans should be carefully located to ensure air movement
throughout the car park. The 2006 edition of ADB references impulse
ventilation to BS7346-7 as an alternative method of ventilation for car
parks.
With this in mind there are two design approaches that can be adopted:-
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For a fully mechanically ventilated car park, the extraction system
should be sized to provide six and ten air changes per hour as
recommended in the Approved Documents, ADB and ADF. Impulse fans are
then located over the roadways and the layout engineered to ensure there
are no areas where it would be possible for fumes to build up due to
lack of air movement. Using this method satisfies the requirements of
both Approved Documents.
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As a second option, a fire-engineered solution can be designed to
limit the spread of smoke to a pre-defined area in the event of fire.
This more expensive option incorporates complex controls and additional
extract and is not a requirement under the guidelines within ADB and ADF.
It is normally used as a compensating feature for relaxation of other
fire protection, typically removal of sprinkler systems or extended
travel distances. The site of the fire must be pinpointed by an
addressable fire detection system. The control system then starts the
selected impulse and extract fans to control the direction of the smoke
and to provide clear air upstream of the fire.
Since there are currently no standards or published methods for design
of impulse ventilation systems, the design is usually proven by use of
CFD analysis.
The Fans
Impulse fans developed from tunnel fans and comprise an axial fan,
attenuators and deflector. Typical units are 400mm to 500mm diameter.
The Colt Jetstream is an example of an impulse fan. More recently a
newer design using a centrifugal fan in a streamline casing has been
developed to minimise the depth and increase the performance. These fans
are more efficient and powerful, thus radically reducing the number
required and, with a lower profile, they are ideal where headroom is at
a premium. The Colt Cyclone is an example of this new breed.
CFD in Car Parks
CFD (Computational Fluid Dynamics) was first introduced in the UK in
1970s and becomes more popular as computer technology advances. It is
now commonly used in the design of car parks. In the absence of robust
design methodology, CFD analysis allows detailed computation of airflow
in car parks, taking into account the often complex geometry of
individual buildings. This is often essential to demonstrate to Building
Control that designs will perform satisfactorily.
Controls
There is no single standard system. Currently there are two options
commonly used, although the second option with CO control is rapidly
becoming the norm: -
Option One
For general ventilation purposes the impulse and extract fans run at
slow speeds. For smoke clearance both sets of fans run at full speed.
This option is simple and inexpensive, easy to commission and also to
maintain. However, using this method all fans need to run continuously,
wasting energy and money during quiet periods.
Option Two
In addition to the above control, detection is used to monitor CO levels
in the car park. When in general ventilation mode, the ventilation rate
can be reduced below the normal Building Regulations recommendations
provided that the CO level is kept below 30ppm. This can be on the basis
of providing 2 (or more) stage control or a more complex variable speed
system.
It has become common practice to provide a delay in operation of the
impulse fans (but not the extract fans) on switching from normal to fire
mode. The reason for this is that the impulse fans are designed to mix
the air in the car park and it is therefore considered sensible in the
early stages of the fire to avoid this mixing and keep smoke in a
discrete high level layer while occupants escape. This delay is
typically between 2 and 5 minutes.
Legislation and Standards
A new British Standard Car Park Ventilation, BS7346-7, was published in
the Autumn of 2006, which covers all methods and sets basic functional
requirements.
Inevitably, with all the significant benefits, impulse ventilation
systems for car parks are here to stay.
Request information above
Colt International Car Park Ventilation Smoke
extract Impulse Ventilation Systems
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Car Park Ventilation
Smoke Extract Systems
Impulse Ventilation Systems
Colt International Limited
New Lane
Havant
Hampshire
PO9 2LY
Email this company
Tel : +44(0)23 9245 1111
Fax : +44(0)23 9245 4220
Email : info@coltgroup.com
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