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Why jet fan systems are the best choice

Enclosed or underground car parks are generally unable to vent toxic exhaust fumes naturally, and the same is true of harmful smoke if a fire breaks out. That’s why smoke control systems are required, but the technology and fire engineering may appear complex at first glance.

As a good starting point, the Building Regulations specify what is required to maintain safe condi-tions, in particular Approved Document B and Approved Document F (guidance is also given in BS 7346-7:2006).

In an ideal world, natural ventilation would be used as it simply requires openings to fresh air being provided – equal to a percentage of the footprint of the car park. However, 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 mech-anical extract systems.

These systems use powered fans to control the build-up of CO in the car park. They state that the system should limit the concentration of CO within the car park to below 30ppm (averaged over eight hours). For smoke clearance, 10 air changes/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 that 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.

Traditional mechanical extract systems require intrusive ducting to be installed throughout the car park, using up valuable headroom. Jet fans replace ductwork and drive fumes or smoke towards extract fans – keeping pollution within acceptable levels and clearing smoke.

Developed in the Netherlands about 15 years ago, they have become increasingly popular as they overcome many problems associated with sheet metal ductwork. They were originally developed for ventilating tunnels and work by propelling a small jet of air at extremely high velocity which causes surrounding air to be entrained; in a confined space like a tunnel or enclosed car park they can move a large volume of gas very quickly.

The benefits of a jet fan system are as follows:

  • Space saving – jet fan systems are more discreet and take up considerably less room than ducted ones.
  • Energy saving – they are often combined with CO detection to initiate CO control, so fans are precisely controlled to dilute pollution without running unnecessarily.
  • Low noise – because the main extract fans are relatively small and (for CO control) run at lower speeds than traditional systems, they make less noise.
  • Cost saving – jet fan systems cost less than ducted ones, are quicker to install and easier to maintain.

So meeting regulatory require-ments for a car park isn’t all that challenging; so long as the design of smoke zones within the car park is assessed correctly, any engineer should be able to specify and install a fume and smoke control system.

Christopher Jones is projects director at SCS Group

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