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318 HANDBOOK OF PROJECTION FOR
If the vent flue or pipe be of the open type it should have an area of not less than 288 square inches, regardless of the size of the room.
Where a vent flue depending upon a fan for its action is used, the fan should not be less than 24 inches in diameter. Where fans are used it is an exceedingly good practice to install two vent pipes and two fans instead of one, so that in case one of the fans gets out of order there will still be the second one to fall back on. This may seem like rather an expensive precaution, but since the pipe containing the additional fan may join the pipe of the other fan, the added expense will be largely that of the second fan, and where the safety of the audience is concerned expense should not be considered.
It is essential that the vent flue, if made of metal, be thoroughly and completely insulated from any inflammable substance throughout its entire length, since it is likely to get very hot if there is a serious fire in the projection room
With proper means provided for the egress of smoke and gas, when a film fire occurs, the projection room with fireproof walls will be nothing more nor less than a hug-? stove, the draft being inward around the cracks of the port shutters and door, and outward through the vent flue; s* that no smoke or gas will in any event show in the auditorium. Hence the audience will never know there is a fir* in progress, even though their attention be attracted direct% to the room by the stoppage of the show.
In addition to the vent flue, there must, for the sake of establishing healthful conditions through proper ventilation, be means provided for the ingress of fresh air. The projection room is often (we might almost say usually) located immediately under the roof of the building, and in consequence is, in summer time, very hot with "natural" heat. Add to this the heat generated by the powerful arc lamps, as well as perhaps one or two rheostats, and you have a condition which makes good ventilation absolutely imperative.
In connection with this it must also be remembered that air taken from the auditorium will not be pure air, and where the projection room is near the ceiling will be the warmest air in the theatre.
It must also be remembered that if it is taken in through the lens and observation ports a draft is created through them which blows directly upon the projectionist, a fact