However, the best system for heating and ventilating a theatre is the one termed the plenum or forced-draught system, where the air is taken from the outside into an isolated chamber, there heated or cooled for use, and forced by blower fans into the auditorium.
This chamber may be established either at the top of the auditorium, or in the basement underneath as shown on the heating diagram on Page 145. Its adoption entails a modification of the direct radiation system, combined with other methods, and provides heating and ventilation simultaneously. It also admits of a mechanical provision for washing the air before it is blown into the auditorium.
In such a system the fresh air is drawn from the outer air through a fresh air" inlet, composed of louvers or shutters that may be closed by a damper, the opening itself being proportionate in size to that of the fan and the capacity of the theatre. It is better, if possible, to locate this inlet high enough above ground to prevent the outside dust from entering it. The air is then passed by suction through an inclosed dry screen air filter to specially designed radiators in winter, and in summer over an improvised ice box containing large cakes of ice. After being conditioned in this manner it is forced by the same paddlewheel fan into galvanized iron ducts that lead to the audience hall. In some systems the air is washed by a water spray, heated in winter and cooled in summer.
To prevent draughts, the air is admitted into the auditorium through tiny covered mushroom outlets installed beneath the seats. There are several large firms that install this air-conditioning outfit complete, but a home-devised arrangement of the description indicated on the before mentioned diagram will answer the same purpose.
To construct and equip this home-made plenum chamber, have a tinsmith make a correctly sized set of galvanized-iron shutters or louvers regulated by an ordinary damper, and install it, covered by a coarse wire netting, in the outer wall at the point indicated in the diagram. Then partition off a space for the installation of an air filter. This consists of two rows of circular wood uprights placed five feet apart and secured to the floor and ceiling. Fasten from top to bottom a tightly stretched chicken wire netting in zig-zag fashion, the purpose of this netting being to support a removable cheesecloth screen of the same size tacked to the circular uprights. These cloth filters should be made of wide strips of cheesecloth sewn together, and are intended to lie tightly against the wire netting. The total area of the filtering space thus exposed should be about ten times the sectional area of the louver inlet. These cloth filter sheets should be removed at intervals, and thoroughly cleansed and dried before being replaced.
Provide two openings, one above the other, leading into the heating and cooling spaces, with a non-heat-conducting partition between the openings regulated by a damper arrangement. In the upper space place bent cast-iron heating coils made up in sections for the purpose of heating the air by steam, and in the lower space place an ordinary zinc-lined wooden box with a drip pipe draining it. In the winter heat is admitted to any desired number of sections of heating coils, to give a temperature of from 60 to 70 degrees to the filtered air. In the summer the filtered air is passed directly through the lower space over cakes of ice deposited in the ice box Should heat at any time be desired, a portion of the filtered air may be admitted through the space above, heated or not, as the conditions require.
If the humidity of the air is to be increased. as is often desirable in cold weather, the heating engineer should attach to the top of a heater pipe, a long, shallow, open cast-iron receptacle filled with water to moisten the air by evaporation. The alternative is to install a standard humidifier, a costly fixture. At a temperature of 70 degrees humidity at from 40 to 50 degrees is most pleasant.
After the air is properly conditioned and blown into the auditorium through parallel ducts and the tiny mushroom outlets, it will circulate toward the stage, as shown on the diagram. Reaching the stage opening it will encounter a cooler current supplied for stage heating by direct radiation, and then will curve upward as indicated by the small arrows, and escape through ceiling ventilators or main outlets provided for that purpose.
Care should be taken to regulate the temperature introduced so that the air supplied on the stage will form a cooler air blanket than that furnished to the auditorium, thereby insuring a circulation of conditioned air in the audience hall. A proper distribution of mushroom floor vents on the main floor will prevent appreciable draughts, and horizontally-placed vents in the balcony risers will effect the same result in that section. No movement of air introduced into the auditorium should exceed two feet per second to be comfortable to the patrons.