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
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
What is the term for the amount of water vapor in
a given volume of air?
Q4. What instrument is used to measure relative