What are the regulations? (British standards)
Ventilation of car parks is important to prevent the build-up of toxic fumes and flammable gases from motor exhaust and also to clear smoke in the event of a fire.
The Building Regulations specify what is required to maintain safe conditions, in particular Approved Document B and Approved Document F. Guidance is also given in BS 7346-7:2006.
Why ventilate?
There are broadly two methods of complying with the Building Regulations regarding ventilation and these are natural or mechanical ventilation.
Whichever method is used, the required results are defined as
Smoke clearance:
To provide sufficient openings for enclosed car parks to allow any smoke being produced to leave the space naturally, or if this cannot be achieved then to remove smoke via a mechanical extract system.
Fume exhaust:
To provide sufficient openings arranged such that a through draught at low to mid-level is created to allow CO to leave the car park naturally, or if this cannot be achieved then to limit the concentration of CO within the space via a mechanical extract system.
1. Natural ventilation
In an ideal world, this is the preferred method of ventilating car parks as it simply requires openings to fresh air being provided to equal a percentage of the floor area of the car park. The areas to be provided are currently:
5% for ventilation of everyday vehicle pollution and smoke with at least 50% being provided between opposite faces of the building; the openings provided must be sufficient to create a through draught.
2.5% for smoke clearance only with at least 50% being provided in opposite faces of the building. If only 2.5% is provided a supplementary CO ventilation system will be required to limit the concentration of CO within the space.
This method obviously relies on a path to outside being freely available, which is not usually the case in underground car parks - hence the need for mechanical extract systems.
2. Mechanical extract
These systems use powered fans to control the build-up of CO in the car park. The regulations for fume exhaust state that the system should be capable of limiting the concentration of CO within the car park to below 30 parts per million (averaged over an eight hour period). For smoke clearance, ten air changes per hour (the number of times the volume within the car park is extracted within one hour) should be extracted.
The regulations for smoke clearance state that the system should have an extract facility which is split into two parts, each part capable of providing 50% of the required duty and extracting from both high and low level. Extract fans should be fire rated at 300°C for one hour.
Induction fan
A centrifugal fan with an air inlet positioned beneath the body of the fan and discharging through a reduced size opening, induction fans can provide up to 100N of thrust. Careful coordination is required if a sprinkler system is used. Suitable for medium to large car parks, benefit can be achieved from the greater coverage area. Attenuation is not provided on an induction fan.
Thrust fans have two main functions – mixing & accelerating air towards the extract point
Importance of delaying fan operation
Smoke clearance systems will usually spread smoke across a large area of car park. It is usual, therefore, for the system to shut down on detection of fire for the initial escape period.
The system is usually re-started in fire mode automatically after a pre-determined time delay or manually by fire-fighter’s override switch.
In fire mode extract fans and thrust fans run at full speed.
The operation of smoke control systems will depend on the design objectives of the system and will be specific to the actual fire location.
How to design for smoke control - Design steps
Determine design fire size according to whether or not there are sprinklers
Determine zone layout, at least one extract and one supply point per zone. Decide on general flow distribution and smoke travel distance
Calculate the velocity of smoke at required control distance (10 metres upstream)
Calculate the minimum design flow rate for smoke control
Calculate the mass-flow of smoke and smoke temperature at the fire
Calculate mass-flow towards the extract
Calculate density downstream
Calculate the extract flow rate required
Calculate Thrust fan quantity