17A1. Definition. The term ventilation is an
old one, certainly long in use before air-conditioning was developed. It is derived from the
Latin ventilare, meaning to whip up a breeze
with wings or a fan. In common usage it
means the supplying of fresh air to enclosed
areas. In the old days, the only way of doing
this was by opening doors and windows to
permit the fresh outdoor air to blow in. That
method is still in widespread use, of course.
It is simple, costs nothing, and is reasonably
efficient, provided that a sufficient cross
current exists to siphon out the old air and
bring in the new.
In modern air-conditioning, however, the
term ventilation is restricted and means the
motion of conditioned air inside an enclosure,
fresh air being blown by electric fans through
ducts or mains to the locations where it is
needed, while the old air is removed by similar
ducts and fans.
17A2. Importance of air motion for comfort.
It is a well-known fact that when the air in a
room is motionless, it soon feels stuffy to its
occupants, even though the air may be quite
fresh. On the other hand, air that is kept
stirred, even if it is somewhat stale, at least
does not feel stuffy, and though perhaps too
warm, it is nevertheless bearable. It is chiefly
to keep the air in motion that electric fans are
used during hot weather in subways, street
cars, offices, household rooms, and other in
B. EFFECTS OF AIR MOTION
17B1. Three effects of air motion. When the
air in a room is stirred, three effects on the
human body result, all adding up to a feeling
of greater comfort. One is a purely sensory
effect, another affects humidity, and the third
affects the room temperature. The three are
closely interrelated and depend upon the velocity of the air motion.
17B2. Sensory effect of air motion. Air in
motion has a definite action on the sensory
organs in the skin. When the air has a gentle
motion, a velocity of 20 to 50 feet per minute,
the tactile sensory nerves in the skin are stimulated, and a feeling of greater comfort is experienced than when the air is completely still.
17B3. Effect of air motion on temperature.
The body is always giving off heat to the air
around it by conduction. If the air is still, the
air close to the body gradually becomes more
heated, and this heat is not carried away by
convection currents in the air. Thus, although
the average temperature of the air in a room
may remain nearly constant, the body itself is
in air of higher temperature. If the air is in
motion, however, the heat coming from the
body is carried away by convection and not
permitted to build up.
17B4. Effect of air motion on humidity. The
body is always evaporating moisture, even
though the evaporation may be at such a slow
rate that it is not perceptible as perspiration.
If the air is still, this evaporated moisture
stays close, forming with the heat also given
off, a damp hot blanket around the body.
Within such a blanket as the relative humidity
rises, air is less able to absorb the evaporation
from the body; hence a feeling of discomfort
ensues. But if the air is stirred, the convection
currents thus formed carry away the moisture
as rapidly as it is given off, and a normal rate
of evaporation is restored.
17B5. Interrelationship of the three effects.
Up to an air motion of about 60 feet per minute,
a person is conscious only of the stimulating
effect, that is, the air feels alive. Above this
velocity, an average person feels comfortable
up to an air velocity of about 100 feet per
minute. This is the limit of definite comfort.
At air velocities above 120 feet per minute,
the air motion does more than the mere
removal of moist air from the vicinity of the
body; it causes evaporation at a rate greater
than normal. Such evaporation can take place
only by using up additional body heat; as a
result, the body feels cold and uncomfortable.
17B6. Limits of air motion. Thus, while the
air needs to be kept in motion, there are
necessary limits, from about 15 or 20 feet per
minute to about 100 feet per minute. In general, if an air current is definitely perceptible,
that is, if it attracts attention, then it is too
much for comfort and may be a hazard to
In air-conditioning, air with a motion of 15
to 25 feet per minute is called relatively still
air, because though it is not completely still,
it does not readily attract attention.
The upper limit of 100 feet per minute is
suitable for persons at rest or doing light
work, as is normally the case in a submarine.
When engaged in heavier work or exercise, a
somewhat higher velocity of air motion may
be accepted with comfort; but any substantial
increase, while it may be momentarily cooling,
is likely to be hazardous to health.
17B7. Direction of air current for comfort.
When indoors, the body can stand a consider
ably greater air current from the front than
from the rear or above. The higher air velocity
limit should, for comfort, be avoided on the
back of the body, on the head, and also on the
feet, and still higher velocities indoors should
be avoided for reasons of health.
17B8. Difference between indoor and outdoor air motion. It should be noted that the
conditions described above apply only indoors.
There is a surprising difference between the
effects of air motion indoors and outdoors. A
person can stand much greater air motion
outdoors than indoors, without feeling discomfort. A strong draft indoors would be a
mere pleasant breeze outside.
17B9. Location of air motion. It should be
noted that the air motion or comfort lies
within the layer occupied by a person, from
the floor to a little over his head. Naturally
the air coming out of the air-conditioning
ducts into the rooms and quarters is at a
considerably higher velocity, but the direction
of these inlet currents should be and usually
is so arranged that they do not strike directly
on the occupants.
C. AIR CURRENTS IN VENTILATION
17C1. Natural convection. When air is
warmed, it expands. Therefore, a unit volume
of warm air becomes lighter and rises, or tends
to rise. When air is cooled, it contracts, and
a unit volume of it becomes heavier and sinks,
or tends to sink. In an enclosed space, if air
masses of different temperatures are present,
and if no extraneous forces such as fans are
present to move them about, the mass of warm
air rises and the mass of cool air drops. A
current thus created by masses of air moving
in opposite directions because of differences in
their relative weights is called a natural
If a heater is near the floor on one side of a
room, warm air rising from it draws cooler air
along the floor to take its place. A continuous
circuit of air around the room, created by
natural convection currents, is set up.
17C2. Forced convection. If, on the other
hand, forces such as fans act on the air and
actually move the air, currents are set up.
Such currents are caused, not by the relative
weights of the air, but by extraneous forces,
and hence are called forced convection currents. Air-conditioning operates by forced
17C3. Natural ventilation. When ventilation
occurs by natural means, that is, by fresh air
blowing in through open windows, doors,
portholes, ordinary ventilators, or air ducts
not containing a fan, and the used air finds an
outlet, the method is called natural ventilation. Natural ventilation depends largely upon
natural convection. Unfortunately, natural
convection currents and drafts always take
the most direct paths possible, and many
places in a group of rooms such as in a
building or in a ship, are bypassed and left as
A ship is a difficult structure to ventilate
by natural means. A submarine in particular,
because of its shape, construction, and purpose, is impossible to ventilate by natural ventilation.
17C4. Forced ventilation. Fortunately, it is
no longer necessary to rely on natural ventilation. Forces can be brought to bear on the
air to move it wherever desired. Fresh or
newly conditioned air is pumped by fans
through ducts to interior enclosed spaces, and
used air and fumes are pumped out through
separate ducts. Since such ducts can be led
wherever necessary, thorough ventilation is
thus assured. Such ventilation is known as