The humidity is controlled by admitting saturated air to the warmer air of the rooms in such quantity as will produce the desired mixture. The humidostat, on the left-hand wall, regulates the quantity of moisture by opening or closing the steam valve V as occasion requires.
Another example of air-conditioning plant similar in principle to that just described is often called the dew-point system. It depends for its action on a definite dew-point temperature at which the air is saturated with moisture, before being heated to room temperature. The air to be conditioned is first warmed, by passing through a set of tempering coils, to a degree at which it will contain the necessary moisture when saturated. After saturation the temperature is raised by a second set of heating coils to the room temperature, the moisture contained being right to give the desired humidity.
To illustrate, suppose that it is desired to maintain a constant humidity of 50 per cent. saturation at 70°F. in the building. The temperature at which the air must be saturated, to contain 4 grains of moisture per cubic foot, is found in the table on [page 199] to be 48°F.
The entering air must first be raised to that temperature by the tempering coils. The air then enters the spray chamber where it absorbs moisture to saturation, by contact with a multitude of water particles. This saturated air now passes through a second set of heated coils and takes up heat sufficient to raise it to the finished temperature.
The dew-point temperature at which the air enters the spray chamber and the final temperature are kept constant by motor-operated valves which supply the heating coils with the necessary heat in the form of steam. The motors are controlled by thermostats, placed to measure the temperature of the air as it enters the saturator and the finished air as it enters the rooms. If these conditions are now kept constant, the finished air will be constantly 50 per cent. saturated.
Fig. 176.—School building section showing a complete air-conditioning plant.
A plant of this character is illustrated in Fig. 176. The figure shows the exterior of the casings which enclose the tempering coils and saturator at A, the eliminator at B, and the heating coils at C. This is another draw-through type of plant where a fan, enclosed in D, draws the air through the conditioning apparatus and forces it through the sheet-iron ducts E. The passages in the walls—as indicated by the arrows—conduct the air through the register R, into the room. The register S represents the discharge duct through which the vitiated air is forced from the room.
In this system of air conditioning, all of the ventilating air is to be saturated with moisture at a temperature such that when raised to room temperature will contain the desired percentage of humidity. The saturator occupies the space between A and B. A number of spray jets are arranged to fill the entire space with water drops that are moving in every direction. The air, as it passes, must come into contact with the drops again and again, until by repeated impact each particle is completely saturated and at the same time washed free from dust. It has already been explained that the movement of the saturated air through a mass of spray will carry with it a considerable amount of unabsorbed water that must be taken out by an eliminator. A section of the casing is broken out at B, showing the eliminator plates. The irregular surfaces of these plates repeatedly change the direction of the passing air, and the suspended water or remaining solid matter is thrown against the surfaces where they adhere. The moisture accumulates in drops of water that run down the plates to the bottom of the enclosure and finally into the sewer.