The dry bulb temperature of the air introduced into a room for ventilating purposes similarly must be somewhat warmer or cooler than the desired temperature in the room as may be needed to change the latter, and of course must be in sufficient volume to de liver or carry away the necessary heat. In general it has been found, for intensively occupied rooms such as class rooms and assembly halls, that if the temperature of the entering air is more than about 8°F. cooler than the room temperature at the level where the air enters, there will be noticeable draughts. It is neces sary to control accurately the temperature of air introduced for counteracting excess heat from crowded rooms, and this applies as well in winter as in summer.
Many older theatre ventilating systems have air introduction through' hooded nozzles in the floor under the seats, with exhaust openings overhead. They are acceptable if the entering air tern perature is controlled very accurately and if nearly all seats are occupied and so have human air-heating elements. When there are many vacant seats the entering air, usually cooler than the de sired room temperature, causes objectionable draughts. Most theatres use the floor nozzles for exhaust purposes with air sup ply overhead, but floor nozzles, for either supply or exhaust, are practicable only in theatres and similar assembly rooms hav ing fixed seats. Ventilating systems, particularly for office rooms and restaurants of moderate size, have employed side-wall air supply openings, almost always delivering the air above head level, with exhaust openings preferably on the same side of the room as the supply openings, usually near the floor but often above or at the same level as the supply openings. For rooms of very large area. a successful distributing arrangement involves both supply and exhaust at the ceiling, preferably near the centre of each construction bay. or say one inlet-outlet for every 400 sq.f t. of floor area. The air supply is deflected horizontally in all directions close to the ceiling, while the exhaust air rises vertically in the centre of each bay. One objection to this scheme, which
otherwise gives satisfaction, is that dust carried upward by the recirculated and departing air tends to be deposited against the ceiling.
An improvement in air distribution from overhead is obtained by use of an anemostat made usually of spun metal and capable of be ing combined with an electric lighting fixture or with a central exhaust grille. The anemostat delivers the air in a diagonally downward uniformly distributed manner and has solved satisfac torily many problems of ventilation without adding grilles of ob jectionable appearance. An axiom well proved by experience is that where mechanical ventilation is employed the distribution of the air should be accomplished by designed and positive delivery of the supply rather than by the exhaust. In other words the removal of a given volume of air from a room at a certain location cannot be depended upon to bring about the satisfactory distribu tion of the air supply to that area. Adjustable vanes behind the face plates of air supply openings are of great value, and are available to give directional air flow in both horizontal and verti cal planes. In some cases the vanes are at right angles each to the other in close juxtaposition. The location of inlets to electric lights of heavy intensity is of importance, especially when cooling a room, since the otherwise satisfactory entering air can be over heated by contact with the lights. This effect can be avoided by arranging for the exhaust air to pass the lights on its way out, after having cooled the occupants of the room.