Later research, especially by the American Society of Heating and Ventilating Engineers in co-operation with the U.S. Bureau of Mines, indicates that the introduction of so large a quantity of air from outside is unnecessary. It has been well proved for instance that odours can be controlled with as few as To cu.ft. of outside air per minute per occupant, and that with proper temper ature, moisture, and movement of the air, the comfort and well being of the occupants will be improved at a reduced cost for heat ing or cooling, humidifying or dehumidifying, and for filtering the outside air. It has also been proved that the movement of air over the bodies of the occupants cannot be reduced appreciably below that inherent to the old standard of 3o cu.ft. per person per minute, since with a slower movement the stratification of too warm, too-moist air against the skin will not be prevented. This is an interesting example of the truth so often shown that many an cient housewife remedies have a rational basis. It has been demon strated that excellent conditions of ventilation may be obtained in heavily populated enclosures such as school class rooms if at least 3o cu.ft. of air per minute per person is introduced; one-third be ing from out of doors and two-thirds being recirculated.
resent an optimum condition to be provided indoors as a result of the influence of a ventilating system. In winter, air from out of doors at, say, 20°F. may not exceed 16 grains of moisture per lb. of dry air, since this is the limit of vapour which air at this tern perature can carry. This air when heated to 70°F. for introduc tion into a room would have the same number of grains of mois ture per lb. of dry air, but its volume would be so expanded by the heat that the relative humidity would be around i s% instead of the desired so%.
In summer ventilation likewise, the outdoor air supply might be at 90°F. and 4o% relative humidity, containing 84 grains of mois ture per lb. of dry air. This air when cooled to 70°F. would con tract in volume and therefore, with the same number of grains of moisture, would be at about 77% relative humidity; altogether too damp for comfort. It is necessary, therefore, to control the rela tive humidity as well as the temperature of the air introduced into rooms for purposes of comfort ventilation. In winter the usual procedure is to supply water for evaporation into the air on the warm side of the heating devices, and in summer the excess mois ture may be condensed out of the air by chilling all or part of the latter below its dew point temperature before introducing it to the room. An alternative method of removal of excess moisture from a ventilation air supply is through use of adsorbers, which collect the moisture mechanically and which when saturated may be regenerated. In this process the latent heat in the water vapour is released as sensible heat when the vapour changes to a liquid, so that some refrigeration must be employed. The adsorption method of dehumidification of air, however, requires much less refrigera tion than the sub-dew point scheme and is in successful operation in many ventilating systems. The moisture content of the air in troduced into a room for ventilating purposes must be sufficiently different from that desired in the room to counteract the local additions or deductions and thus to maintain the desired happy medium.