Humidity Control.
—The method of regulating atmospheric humidity in a humidifying plant will be determined by the conditions under which it is intended to work. There are a variety of means employed that may be used to bring about the same effects, each of which is particularly suited to certain requirements. The present object is to describe the essential features of airconditioning plants, by use of illustrations representing each of the three methods mentioned above. That of the ventilation of a school building under winter conditions will be taken as an example.
In Fig. 174 is shown a heating and ventilating system in which the air conditioning is accomplished by automatic regulators for both temperature and humidity. The plant occupies a room in the basement, and a room directly above illustrates the conditions that prevail in all of the other rooms of the building. The principal features of the plant are the fan G, which supplies the air; the hot-air furnace H, which furnishes the heat; and the water spray S, which provides the moisture with which the air is humidified.
Fig. 174.—Furnace blast system of heating, with temperature regulation and humidity control.
The air is drawn in at A to a room in which a motor-driven fan G forces the supply through the heating apparatus into the building. The air after leaving the fan passes through a cold-air duct C to the heating surfaces H to be warmed. The air in passing over the heating surfaces is raised to a degree considerably above the temperature of the rooms. The hot air leaving the heater H enters the tempered air chamber T through the passage K. A damper M provides means for also admitting cold air to the chamber T directly from the fan. The thermostat, located at O, is connected with a pneumatic motor V (similar to Fig. 170) which regulates the supply of cold and hot air from K and M to suit the desired temperature of the air supply for the rooms above. The arm of the motor V is so arranged that an upward movement opens the cold-air and closes the hot-air passages; the downward movement produces the opposite effect. The motor V thus controls the temperature of the air.
In this system the air is humidified by a direct water spray marked S in the drawing. A part of the hot air from the heater H may escape through the damper W and absorb water on its way to the duct D, which takes the air to the room above, where it enters through the register E. This air as it comes from the heater, being hot, will absorb a larger amount of water than the air could hold when cooled to room temperature; for this reason only a part of the air supply is humidified. The supply of the hot humid air is admitted to the duct D in such quantity as will produce the desired degree of humidity in the rooms.
The degree of room temperature is governed by the thermostat, in the room, which, by means of the motor N, controls the damper F. This damper admits hot humid air and the tempered air from the chamber T in proper proportion. At any time the humidity of the air in the room reaches the maximum amount for which it is set, the humidostat, through its motor, closes the valve R, which controls the water supply to the spray nozzle, and the moisture in the air is reduced until a further amount is demanded. With apparatus of this kind the temperature and humidity may be kept practically constant.
Fig. 175.—Direct steam heating system with mechanical fan-blast ventilation, temperature regulation and humidity control.
Fig. 175 shows another arrangement of a similarly controlled plant in which steam is used for humidifying the air. The air is admitted at A, from whence it passes through a steam-heating coil S, which raises it to a predetermined temperature. The steam jets are arranged at H, for providing the necessary moisture. The humidostat through a motor valve V governs the amount of steam that is permitted to enter the humidifying chamber. A thermostat located in the air duct at B controls the temperature of the air sent to the rooms by regulating the amount of heat given out by the steam coils S. This control is made still more sensitive by use of a cold-air bypass. The damper D is opened by a motor valve to admit cold air at the same time the steam is shut off from the heater coils.
In this plant the ventilating air is not intended to supply all of the heat to the rooms. A thermostat on the wall controls the room temperature by regulating the amount of steam admitted to the radiators. In the ventilating plant previously described, all of the heat for the building is supplied through the ventilating system; in the plant shown in Fig. 175, the heating apparatus which warms the building is entirely separate and may be used when the ventilating system is inoperative.
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.
From the eliminator the air passes through the heating coils enclosed in C, where it is heated to the necessary temperature for admission to the rooms.
The regulation of the temperature of the tempering coils and heating coils is accomplished as in the other plants described. The thermostats with their motors operate the valves of the heaters to admit steam sufficient to keep constant temperatures at the different parts. The humidity is maintained at a constant amount by saturating the air at a constant temperature and therefore no humidostat is required.