Enclosed car park ventilation is usually discussed in terms of the fire scenario. But almost all of the system's running time is spent in normal operation, where the criterion is different: keeping the concentration of pollutants from vehicle exhaust below the limits.
The criterion is concentration, not flow rate
Regulations usually specify an air change rate, but the real aim is concentration control. The two diverge markedly in a car park, because:
- The source is not fixed; vehicles move, and most of the emission occurs during manoeuvring and cold start.
- The pollutant does not distribute evenly through the geometry. Ramp bottoms, corners and the space behind columns form dead zones.
- Short-circuiting can occur between the supply and extract positions; the air change rate is achieved on paper while the dirty region is never cleared.
All three are visible only in a field solution. See the cleanroom note — the same logic applies there.
Jet fan systems
Ductless jet fan systems entrain the air towards the extract points. The design questions:
- Fan spacing and orientation. Fans placed too close disturb each other's jets; the total thrust is less than the sum of the individual thrusts.
- Column and beam obstructions. They shorten the jet's reach, which is governing in car parks with low ceilings.
- No dead zones. The measure of success is not the mean velocity but that no corner is left with stagnant air.
Two scenarios, one system
The same fans are used to drive smoke in the fire scenario, where the criterion is entirely different. See the smoke extraction note and the critical velocity note.
The design must satisfy both, and they call for different optima: normal operation wants low-energy, staged running; the fire scenario wants full capacity. The analysis should run them separately.
References
- National Fire Protection Association. NFPA 502 — Standard for Road Tunnels, Bridges, and Other Limited Access Highways. NFPA, Quincy, MA.
- ISO 14644-1:2015. Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by particle concentration. ISO, Geneva.