Traditional Ducting ventilation and smoke extraction system vs Jet fan ventilation system in underground car parks

Underground closed garages become standard parking spaces in urban areas, and health and public safety are the number one priority. The ventilation system with Jet Fan is a relatively new ventilation system for underground garages, which aims to provide a clean and safe parking environment. Jet Fans are primarily intended for tunnel ventilation. Manufacturers of Jet Fans claim that their ventilation systems are very efficient compared to conventional ducting ventilation and smoke extraction systems in underground garages.

Several academic analyzes have been made on this topic, shows that ventilation systems with Jet Fans have serious shortcomings. Even the Jet Fan manufacturers themselves are indicating the problems of underground garage safety in case of a combination of a Jet Fan system with a Sprinkler system.

“Kašnjenje u pokretanju Jet Fan ventilatora od 5-7 min nakon detekcije požara bilo bi poželjno iz dva razloga:

• Delay of activation of the jet fans provides the best evacuation conditions upstream and downstream of the fire;

• Odlaganjem aktiviranja Jet Fan ventilatora može se izbeći izazivanje kašnjenja u aktiviranju prskalica u područjima u blizini sedišta vatre.”

Taken from the web site Jet Fan manufacturer:

Comparison of Ducting and Ductless Ventilation system for Car parks – Smoke Contours

1. As the streamline pattern analysis showed, a system with only extraction fans is inappropriate for the underground car parks with partition walls as well. Only conventional ductwork system does not depend on the partition walls. If requested, all day-to-day ventilation systems can be part of an active fire protection system. The main tasks of smoke extraction systems are life safety and reducing damage to the building in case of fire. Longer exposure of partition walls and ceiling to the hot gases can cause concrete spalling and dangerous conditions to the firemen. Therefore, the choice of the jet fan ventilation system as the mechanical system for ventilation in an underground car park with partition walls should be reconsidered.

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2. The results have shown that the fire ventilation system was not able to sufficiently ventilate the car park when the exhaust shafts were located in both parts of the car park and the jet fans were directed to the corresponding shafts (A1). Even 575 s after the fire had been put out the car park was still considerably polluted. Relocation of both exhaust shafts to the side opposite to the main air supply resulted in a massive amount of fresh air spreading through the whole car park (A2). Consequently, the car park was well ventilated, the escape routes were clear of smoke, and the pollution was exhausted in less than 600 seconds after the fire had been put out. The simulations have proven that proper location of the elements of the ventilation system is crucial to attaining high efficiency of fire ventilation systems with jet fans. Although the air change rate is indeed an important parameter to consider, diminishing the ventilation intensity from 15 down to 10 air change rates per hour in this study would have an only secondary effect on the efficiency of the fire ventilation system.

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3. There is always error in a CFD analysis. It is important to know the sources of these errors and take precautions accordingly. The major source of error for a CFD analysis is due to the selected numerical method to solve Navier-Stokes equations. Some of the numerical techniques employed in CFD are Finite Difference Methods, Finite Element Methods, and Finite Volume Methods. The one FLUENT uses is the Finite Volume Method and the source of error here arises when discretizing the transport equations. Interpolations are made to find values at the cell faces, whereas all the information is stored at the cell centres. This is the main approximation of the Finite Volume Method.

Taken from the document: