According to regulation 20 in the “Protection of vehicle, special category and roll on-roll off spaces” part G, as a precaution against ignition of flammable vapours in closed vehicle spaces, closed ro-ro spaces and special category spaces, there should be an effective mechanical ventilation system sufficient to give the required number of air changes per hours.
The term ‘air changes per hour’ refers to how many times the industrial fans extract and replace the total volume of air in the empty cargo or passenger compartment in one hour. A fan-rating of ’20 air changes per hour’ for a hold of 1,300 cubic metres air capacity, means that, with the fans operating at full speed on full power, all the air in that hold could be continuously changed with fresh air 20 times in any one hour in an ‘open’ system – or that the original air could be re-circulated within the hold 20 times in any one-hour period in a ‘closed’ system.
A general cargo vessel with a hold ventilation system which is not fan-assisted, can only change the air in the cargo compartments to the extent that the ventilator cowls, of whatever type, can be trimmed/opened to the external atmosphere. This will allow inward passage of outside air and outgoing exhaust of stale air. This type of air change, being unmonitorable, falls outside of the scope of the term air changes per hour used in this article.
Again according to regulation 20, air changes per hour for passenger ships should be set as a minimum. Within closed ro-ro and vehicle spaces, when carrying more than 36 passengers, there should be a minimum of 10 air changes per hour. This can be reduced to a minimum of 6 air changes per hour when the passenger count is less than 36. An increase in the number of air changes per hour may be required when vehicles are being loaded and unloaded due to the concentration of vapours and noxious gases which may become a health hazard to passengers and employees in the vicinity. In passenger ships, ventilation systems should be in operation at all times when vehicles are present during any loaded voyage. In cargo ships, ventilation fans should normally be run continuously whenever vehicles are on board.