heated water surface over which conditioned air passes and picks up a certain amount of water vapor by evaporation, depending upon the degree of humidifying required. A second arrangement to humidify air is to spray or wash the air as it passes through the air-conditioning unit.

During the heat of the day, the air usually absorbs moisture. As the air cools at night, it may reach the dew point and give up moisture, which is deposited on objects. This principle is used in dehumidifying air by mechanical means.

Dehumidifying equipment for air conditioning usually consists of cooling coils within the air conditioner. As warm, humid air passes over the cooling coils, its temperature drops below the dew point and some of its moisture condenses into water on the surface of the coils. The condensing moisture gives up latent heat that creates a part of the cooling load that must be overcome by the air-conditioning unit. For this reason, the relative humidity of the air entering the air conditioner has a definite bearing on the total cooling load. The amount of water vapor that can be removed from the air depends upon the air over the coils and the temperature of the coils.

PURITY OF AIR

The air should be free from all foreign materials, such as ordinary dust, rust, animal and vegetable matter, pollen, carbon (soot) from poor combustion, fumes, smoke, and gases. These types of pollution are harmful to the human body alone; however, they include an additional danger because they also carry bacteria and harmful germs. So, the outside air brought into a space or the recirculating air within a space should be filtered during air conditioning.

Air in an air conditioner may be purified or cleaned by filters, air washing, or electricity.

Filters may be designed as permanent or throwaway types. They are usually made of fibrous material, which collects the particles of dust and other foreign matter from the air as it passes through the filter. In some cases, the fibers are dry, while in others they have a viscous (sticky) coating. Filters usually have a large dust-holding capacity. When filters become dust-laden, they are either discarded or cleaned. Permanent filters are usually cleaned. Throwaway filters are only one-time filters and are discarded when they become dust-laden.

Often water sprays are used to recondition the air by washing and cleaning it. These sprays may also serve to humidify or dehumidify the air to some extent.

In some large air-conditioning systems, air is cleaned by electricity. In this type of system, electrical precipitators remove the dust particles from the air. The air is first passed between plates where the dust particles are charged with electricity; then the air is passed through a second set of oppositely charged plates that attract and remove the dust particles (fig. 7-4). This method is by far the best method of air cleaning, but the most expensive.

CIRCULATION OF AIR

The velocity of the air is the primary factor that determines what temperature and humidity are required to produce comfort. (The chart in figure 7-3 is based on an air movement of 15 to 25 feet per minute.) We know from experience that a high velocity of air produces a cooling effect on human beings. However, air velocity does not produce a cooling effect on a surface that does not have exposed moisture. A fan does not cool the air, but merely increases its velocity. The increased velocity of air passing over the skin surfaces evaporates moisture at a greater rate; thereby, cooling the individual. For this reason, circulation of air has a decided influence on comfort conditions. Air can be circulated by gravity or mechanical means.

When air is circulated by gravity, the cold, and therefore heavier, air tends to settle to the floor, forcing the warm and lighter air to the ceiling. When the air at the ceiling is cooled by some sort of refrigeration, it will settle to the floor and cause the warm air to rise. The circulation of the air by this method will eventually stop when the temperature of the air at the ceiling is the same as the temperature on the floor.

Air may be circulated by mechanical means by axial or radial fans. When either the axial or radial fan is mounted in an enclosure, it is often called a blower.

Q1. What is the term given to the net effects of temperature, humidity, and air motion?

Q2. The comfort line is the specific effective temperature at which most people feel comfortable. True /False

Q3. What is the term for the amount of water vapor in a given volume of air?

Q4. What instrument is used to measure relative humidity?