9. purpose of ventilation is to maintain the air of enclosed spaces in a condition that will result in the comfort of those using it. It does not necessarily imply the displacement of air, but should provide for its motion. It is impossible to make estimates of the proper allowance or condi tion of air by any cubic foot per capita allowance. If the air in an enclosed space is maintained at not over 68 degrees F., without excess humidity and in slight motion a person will be confortable and the air can be rebreathed many times without discomfort.
In a consideration of the air in enclosed spaces attention should be called to both the breathing zone and the breathing cone. The breathing zone in a room includes the air lying within those limits from which a person draws air while either sitting or standing. If the air is stagnant and contains much smoke its limits will be visibly defined. The breathing cone consists of a more or less conical zone of rebreathed air surround ing a sedentary person, whose apex is at his nostrils. The circulation of the air of an enclosed space breaks up these local concentrations of respired air.
Ventilation is either natural or artificial. Natural ventilation is due to either: (a) gravity and diffusion, or (b) perflation and aspiration. The former represents the diffusion of air into a room through porous walls and the spaces around doors and windows. The warm air escapes and is replaced by cool air. It is important in the ventilation of small rooms. Perfla tion is the ventilation secured by the removal of obstacles to wind pressure, such as that accomplished by the opening of windows. The effect of aspiration is shown by the air currents in flues and chimneys. It is assisted by the provision of movable chimney cowls which turn the vent away from the wind.
Mechanical ventilation is the propulsion or extraction of air by means of fans or blowers whose operation is continuous. It is of two types: (a) the plenum system and (b) the vacuum system. In the first the air is forced in by a blower, having an intake at the ground level. In the second the air is withdrawn by an exhaust fan and the intake is secured by the seepage of air through walls and around windows. It gives rise to drafts. The plenum system is used in connection with heating systems and automatic thermo-regulation. As a rule it does not work satisfactorily if influenced by factors of natural ventilation. The air at the intake is washed to remove dust and also to humid ify it, warmed by passage over steam coils and distributed through ducts. The best diffusion is secured where the discharge of warm air is near the ceiling, and the outlet is on the same wall near the floor. Both systems so far are only practicable for large buildings.
to. The Management of Halls and Large is possible to so manage these enclosures during winter months that a large audience will be comfortable and alert during the period of their congregation. Good results will be secured if the hall is filled with fresh air at about 6o degrees before the audience congregates. The intakes are then closed. The audience should be watched closely and if a slight general coughing starts or if a scattering of women clear their throats, a little heat should be turned on.
In hot weather every endeavor to secure the beneficial effects of perflation by removing all obstacles to wind pressure should be made. In enclosed halls and theatres where such is not possible the installation of artificial cooling systems based upon the circulation of cold brine, is of material value.