The power-driven fan F draws in outdoor air from an opening A, through a set of heater coils H1, in which it is raised considerably in temperature. The heater in this case is a coil of steam pipes. The air—after being warmed—is taken into the fan and from it may be sent through a second set of coils H2, to receive additional heat, or if already sufficiently warmed the air from the fan may pass under the second set of coils and receive no heat from them. Under the first heater coil is also a bypass which may be opened by the motor N to admit cold air that is drawn directly into the fan. The movement of the air through these bypasses is under control of the thermostat, which causes the motor N to open or close the bypass to suit the temperature of the room. When the bypass is opened the steam is shut off from the heater coils.
Examination of the drawing will show that the air from the fan may pass through a second heater H2, to the place marked warm air, or it may pass under the heater to the compartment marked cold air. The amount of warm and cold air which enters the duct is regulated by the position of the dampers C.
The position of the dampers C, which is controlled by the motors M, is made to take amounts of cold or warm air to produce the desired temperature in the rooms. The motors C1, etc., are under control of the thermostat in each room. Any change of temperature in the room will immediately affect the thermostat. The effect on the thermostat will so change the air pressure in the motor that the valve C is moved to correct the difference in room temperature. If warm air is demanded, the motor changes the damper C to close the cold-air supply and take air that must pass through the heater coils H2. If only cold air is desired the damper will turn to shut off the course through the heaters and admit air directly from outdoors.
Humidifying Plants.
—Mechanical ventilation plants that are intended for washing the air may be made up of parts similar to that of Fig. 173, but in addition to the apparatus shown provision is made for the air to pass through a chamber filled with a spray of water. The air in passing through this spray is washed free of dust and at the same time absorbs water necessary for its desired humidity.
The humidity of air may be increased by the addition of moisture or decreased (dehumidified) by raising its temperature, thereby increasing its capacity for containing moisture. Suppose that air at 50° is saturated with moisture; it will contain practically 4 grains of water per cubic foot. If now the temperature of the air is raised to 70°, the same amount of air is capable of containing 8 grains of water and is, therefore, only 50 per cent. saturated.
Humidification is accomplished in air-conditioning plants through one of two general methods: by the evaporation type of apparatus, in which the passing air absorbs moisture from contact with a large area of water; or the spray method, in which the water is broken into a very fine spray by a specially devised nozzle and thus rendered easy of absorption by the air to be moistened. A third method is sometimes employed, in which steam is introduced into the air supply. Steam is already vaporized water and immediately becomes a part of the air without further change. The steam type of humidifying plant possesses features that limit its application, in that the steam in some cases may possess objectionable odor or includes the vapor of grease, either of which would materially effect its use. Further, the heat of vaporization liberated by the condensing steam is also a factor that influences the temperature of the air and in case of direct humidification must receive special attention.
Vaporization as a Cooling Agent.
—The evaporation of water has a distinct value aside from humidifying the air, in that the cooling effect is in direct proportion to the added moisture. In the process of evaporation the heat necessary to change the water into vapor is taken from the surrounding air and the temperature is thus materially lowered.