indoor air conditions. It is used to maintain the
environment of an enclosure at any required
temperature, humidity, and purity. Simply stated, air
conditioning involves the cooling, heating,
dehumidifying, ventilating, and purifying of air.
One of the chief purposes of air conditioning aboard
ship is to keep the crew comfortable, alert, and
physically fit. None of us can long maintain a high level
of efficiency under adverse environmental conditions.
We have to maintain a variety of compartments at a
prescribed temperature with proper circulation. These
compartments must have the proper moisture content,
the correct proportion of oxygen, and an acceptable
level of air contamination (dust, airborne dirt, etc.). We
also have to provide mechanical cooling or ventilation
in ammunition spaces to prevent deterioration of
ammunition components. We have to provide them in
gas storage spaces to prevent excessive pressure buildup
in containers and contamination in the space caused by
gas leaks. Finally, we must provide cooling and
ventilation in electrical/electronic equipment spaces.
his is done to maintain the ambient temperature and
humidity, as specified for the equipment.
To properly air-condition a space, the humidity, heat
of the air, temperature, body heat balance, the effect of
air motion, and the sensation of comfort is considered.
HEAT OF AIR
The heat of air is considered from three standpoints
sensible, latent, and total heat.
SENSIBLE HEAT is the amount of heat, which,
when added to or removed from air, changes the
temperature of the air. Sensible heat changes can be
measured by the common (dry-bulb) thermometer,
Air always contains some water vapor. Any water
vapor in the air contains the LATENT HEAT OF
VAPORIZATION. (The amount of latent heat present
has no effect on temperature and it cannot be measured
with a dry-bulb thermometer.)
Any mixture of dry air and water vapor contains
both sensible and latent heat. The sum of the sensible
heat and the latent heat in any sample of air is called the
TOTAL HEAT of the air.
To test the effectiveness of air-conditioning
equipment and to check the humidity of a space, you
must consider two different temperatures-the dry-bulb
and wet-bulb temperature.
Measurement of Temperatures
The DRY-BULB TEMPERATURE is the
temperature of sensible heat of the air, as measured by
an ordinary thermometer. In air conditioning, such a
thermometer is known as a dry-bulb thermometer
because its sensing bulb is dry.
The WET-BULB TEMPERATURE is best
explained by a description of a wet-bulb thermometer.
It is an ordinary thermometer with a loosely woven cloth
sleeve or wick placed around its bulb and which is then
wet with distilled water. The water in the sleeve or wick
is evaporated by a current of air at high velocity (see
next paragraph). This evaporation withdraws heat from
the thermometer bulb, lowering the temperature by
several degrees. The difference between the dry-bulb
and the wet-bulb temperatures is called the wet-bulb
depression. when the wet-bulb temperature is the same
as the dry-bulb, the air is said to be saturated; that is,
evaporation cannot take place. The condition of
saturation is unusual, however, and a wet-bulb
depression is normally expected.
The wet-bulb and dry-bulb thermometers are
usually mounted side by side on a frame that has a
handle and a short chain attached. This allows the
thermometers to be whirled in the air, providing a
high-velocity air current to promote evaporation. Such
a device is known as a SLING PSYCHROMETER (fig.
10- 13). When using the sling psychrometer, whirl it
rapidly-at least four times per second. Observe the
wet-bulb temperature at intervals. The Point at which
there is no further drop in temperature is the wet-bulb
temperature for that space.
MOTORIZED PSYCHROMETERS (fig. 10- 14)
are provided with a small motor-driven fan and dry-cell
batteries. Motorized psychrometer are generally
preferred and are gradually replacing sling
Relationships Between Temperatures
You should clearly understand the definite
relationships of the three temperatures-dry-bulb,
wet-bulb, and dew-point.
When air contains some moisture but is not
saturated, the dewpoint temperature is lower than the
dry-bulb temperature; the wet-bulb temperature lies
between them. As the amount of moisture in the air
increases, the difference between the dry-bulb
temperature and the wet-bulb temperature becomes less.