increases in temperature. The layer of air also absorbs
some of the water vapor given off by the body, thus
increasing its relative humidity. This means the body is
surrounded by an envelope of moist air that is at a higher
temperature and relative humidity than the ambient air.
Therefore, the amount of heat that the body can lose to
this envelope is less than the amount it can lose to the
ambient air. When the air is set in motion past the body,
the envelope is continuously being removed and
replaced by the ambient air. This movement increases
the rate of heat loss from the body. When the increased
heat loss improves the heat balance, the sensation of a
breeze is felt; when the increase is excessive, the rate of
heat loss makes the body feel cool and the sensation of
a draft is felt.
SENSATION OF COMFORT
From what you have just learned, you know that
three factors are closely interrelated in their effects upon
the comfort and health of personnel aboard ship. These
factors are temperature, humidity, and air motion. In
fact, a given combination of temperature, humidity, and
air motion produces the same feeling of warmth or
coolness as a higher or lower temperature along with a
compensating humidity and air motion. The term given
to the net effect of these three factors is known as the
EFFECTIVE TEMPERATURE. Effective temperature
cannot be measured by an instrument, but can be found
on a special psychometric chart when the dry-bulb
temperatures and air velocity are known.
The combinations of temperature, relative
humidity, and air motion of a particularly effective
temperature may produce the same feeling of warmth or
coolness. However, they are NOT all equally
comfortable. Relative humidity below 15 percent
produces a parched condition of the mucous membranes
of the mouth, nose, and lungs, and increases
susceptibility to disease germs. Relative humidity above
70 percent causes an accumulation of moisture in
clothing. For best health conditions, you need a relative
humidity ranging from 40 percent to 50 percent for cold
weather and from 50 percent to 60 percent for warm
weather. An overall range from 30 percent to 70 percent
Proper circulation is the basis for all ventilating and
air-conditioning systems and related processes.
Therefore, we must first consider methods used aboard
ship to circulate air. In the following sections, you will
find information on shipboard equipment used to
supply, circulate, and distribute fresh air and to remove
used, polluted, and overheated air.
In Navy ships, the fans used with supply and
exhaust systems are divided into two general
classes-axial flow and centrifugal.
Most fans induct systems are of the axial-flow type
because they generally require less space for
Centrifugal fans are generally preferred for exhaust
systems that handle explosive or hot gases. Because the
motors of these fans are outside the air stream, they
cannot ignite the explosive gases. The drive motors for
centrifugal fans are less subject to overheating to a lesser
degree than are motors of vane-axial fans.
Vane-axial fans (fig. 10-16) are high-pressure fans,
generally installed in duct systems. They have vanes at
the discharge end to straighten out rotational air motion
caused by the impeller. The motors for these fans are
cooled by the flow of air in the duct from the fan blades
across the motor. The motor will overheat if it is allowed
to operate while the supply air to the fan is shut off.
Tube-axial fans are low-pressure fans, usually
installed without duct work. However, they do have
sufficient pressure for a short length of duct.
Centrifugal fans (fig. 10-17, view A) are used
primarily to exhaust explosive or hot gases. However,
they may be used in lieu of axial-flow fans if they work
better with the arrangement or if their pressure-volume
characteristics suit the installation better than an
axial-flow fan. Centrifugal fans are also used in some
fan-coil assemblies, which are discussed later in this
Portable axial fans (fig. 10-17, view B) with flexible
air hoses are used aboard ship for ventilating holds and
cofferdams. They are also used in unventilated spaces
to clear out stale air or gases before personnel enter and
for emergency cooling of machinery.