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head system the air must necessarily be blown in at the side walls at a comparatively high velocity to insure any fresh air reaching the central part of the building. Furthermore, it is blown in 7 or 8 ft. above the floor line and must be directed downward in order to reach the breathing zone, and unless the system is scientifically laid out, a large percentage of this fresh air passes out without serving its purpose and there are apt to be stagnant areas in some parts of the building. With this system the foul air is usually removed through vent registers in the walls at the floor line. From there it passes to the attic through ducts in the walls, and discharged outside the building, usually by an exhaust fan.
One point which is very much overlooked is the importance of positive removal of the foul air as the fresh air is introduced. If it is not promptly removed it will mix with the incoming fresh air and again be thrown into the breathing zone, and in a large theatre an exhaust fan placed on the roof or in the attic is recommended. In smaller theatres natural draft ventilators are used. The gallery is usually taken care of by a separate fan or ventilator placed on the roof.
There is no building which should receive more careful attention with reference to proper heating and ventilation than the theatre. It represents one of the most interesting problems for the heating and ventilating engineer to solve. The immense open area, and the large number of people combine the extremes in both heating and ventilating conditions. The rapidly increasing number of motionpicture theatres has caused the fan engineer to give this subject more consideration than in the past. The majority of the patrons at a theatre attend to be entertained, and if atmospheric conditions aren't such as to insure comfort, the patron is apt to become impatient, dissatisfied, and will seek pleasure elsewhere.
Sufficient air at the proper temperature is not the only essential. Humidity is of equal importance with reference to both comfort and health. By humidity we mean, of course, the amount of moisture in the air.
You must appreciate that dryness of air is a relative thing. It has nothing to do with the amount of moisture actually contained as you might suppose, but it depends upon the temperature of the air, as that temperature is related to the actual moisture contents. Remember the quantity of water vapor which the air can hold depends upon its temperature. As an example of what is meant: Suppose the outside temperature is zero. Even if the air is saturated at zero it can hold but a minute quantity of water vapor {}4 grain per cu. ft.). If that same air is taken into a building and heated to 70°, its capacity to hold water vapor is increased 16 times.
The outside air at zero, and the inside air at 70° may both contain the same actual quantity of water vapor, but the outside air was saturated, that is, as wet as it could be, while the heated inside air contains but one-sixteenth of the water vapor which it is capable of holding, therefore the inside air is very dry; thus you
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