Ventilation as applied to a theatre is the process of supplying an adequate amount of fresh air, warmed or cooled to a proper temperature, in such a manner that the air circulation will be constant and thorough in all parts of the auditorium without the creation of draughts.
The most important elements of ventilation are motion, coolness and a proper degree of humidity and freshness. Cross ventilation, too, is essential whenever practicable. Air should never be allowed to become stagnant. Vitiated or overheated air produces drowsiness and dullness of the mental faculties. While our respiratory organs are naturally developed for a life in the open air, advanted civilization has reversed this condition, and we are forced to provide artificial means to correct the evil effects produced by confined areas.
The primary object of ventilation is the removal of vitiated air and the Substitution of fresh air, and this may be done by natural or mechanical means. The average person consumes in sixteen respirations about a cubic foot of air per minute. This air, at a temperature and humidity of 70 degrees, is composed of about one-fifth oxygen and four-fifths nitrogen. By the process of respiration about one-fifth, or twenty per cent, of the oxygen is lost in the formation of carbonic acid gas. Air thus vitiated and constantly diffused throughout the auditorium is wholly unfit for use.
Were it possible to expel the carbonic acid gas from the auditorium without taking large quantities of otherwise pure air with it the problem of ventilation would be simplified. Because of the rapid diffusion of carbonic acid gas it is necessary, in order to maintain a safe atmospheric value in the auditorium, to flood it with freshened air. Good country air contains about four parts of carbonic acid gas in every 10,000 parts. If a standard of double this amount, say about eight parts of carbonic acid gas in 10,000, could be maintained in a theatre it would be considered fairly satisfactory. The amount of fresh air required for a theatre is from 1500 to 2000 cubic feet per hour per person.
The manner in which fresh air is supplied to an auditorium is more important than the amount of supply, as air that traverses a room without reaching the breathing zone is of no practical value. One thousand cubic feet of air well distributed is worth ten times that amount introduced without mixing with the air in the breathing zone.
Few theatre patrons realize that they inhale for about three hours the vitiated exhalations of those seated about them. Many of them are fastidious persons who insist upon drinking filtered water from sanitary cups, and yet they do not object to paying for seats in a germ-laden atmosphere often so foul that it gives off an offensive odor.
The importance of coolness in temperature is usually as little appreciated as the importance of motion. A water spray to cleanse the air before it is introduced into the auditorium has a beneficial effect on the comfort of theatre patrons, as it serves to extract dust from the air that may irritate the mucous membrane of the respiratory organs, making it susceptible to disease germs. A water spray also increases the amount of humidity in the air, thereby rendering it cooler in the warm summer months. If the water of the spray also be artificially cooled, the air passing through it will naturally be rendered still cooler.
In theatres heated by direct radiation. where freshened air is not mechanically supplied, there will be a natural change of air amounting to from one to three complete renewals per hour, because of air infiltration. The quantity of air thus introduced depends largely on the arrangement, character and location of the various openings.
Small theatres may be reasonably well ventilated by means of exhaust ventilators. For additional ventilation in the winter months, a heavy galvanized-iron smoke flue, set like a core in the center of a large chimney, may be employed. The heat rising through the inner core will create an upward draught that will carry off the vitiated air through the outer space surrounding the core as rapidly as it is admitted through outlets from the auditorium. For larger theatres, or for per-feet ventilation in smaller ones, any so-called natural system of ventilation is about as good as no system at all.
To understand properly ventilation a knowledge of air circulation is necessary. The effect of heat on air is to increase its volume and diminish its density. Heated or vitiated air rises because of this lessened density, and the simplest method of exhausting such air is by means of mechanical exhaust ventilators installed in the ceiling. There are many forms of these automatic ventilators, but the best in common use is the siphon type. A reliable means also in exhaust ventilation is a propeller fan encased in a penthouse equipped with a shutter arrangement that closes and overlaps by gravity and opens with the force of the outgoing air current. If it be necessary to employ a duct to convey exhausted air from the blower or fan to the outside air the duct must be equal in diameter to the cross-section of the fan.
As vitiated air is expelled from the auditorium by blowers or fans, a lower pressure is created in the audience hall than that which exists outside, and fresh air will naturally rush in through the doors and fresh air openings to replace what is forced out. While this may be very satisfactory in the summer months, the draughts of cold air would be decidedly uncomfortable in the colder months. This in a measure may be corrected by the introduction of fresh air through a lobby heated by radiators or through fresh air inlets conveniently arranged behind wall radiators.