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 :-

• The ductwork runs underneath the ceiling, reducing the already restricted available height.
  
• Downstand beams require the ducting to set down below them, thus diminishing the height even further.
  
• Low level extract grilles are required, often with protective barriers surrounding them, and these take up valuable floor space.
  
• 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:-

• 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.
   
  
• 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.