THEORY OF AIR-CONDITIONING
A. AIR AND WATER VAPOR|
16A1. Definition. Psychrometry (sy-krometry) means literally, the measurement of cold,
from the Greek psychros, cold. It is the special name that has been given to the modern
science that deals with air and water vapor
mixtures. The amount of water vapor in the
air has a great influence on human comfort.
Such atmospheric moisture is called humidity,
and the common expression, "It isn't the heat,
it's the humidity," is an indication of the
popular recognition of the discomfort-producing effects of moisture-laden air in hot weather.
16A2. Air and humidity a physical mixture.
The water vapor in the air is not absorbed or
dissolved by the air. The mixture is a simple
physical one, just as sand and water are mixed.
The temperature of the water vapor is always
the same as that of the air.
16A3. Saturated air. If a tin can is filled
with sand to the top, there is still room into
which water can flow between the sand grains.
If the can of sand is then filled with water to
the top, that sand is holding all the water it is
able to hold. It is said that the sand is saturated with water.
In the same way, air can hold different
amounts of water vapor, and when it is holding all the vapor it is able to hold, it is called
The amount of moisture at the saturation
point varies with the temperature of the air;
the higher the temperature, the more moisture
the air can hold.
16A4. Dewpoint. The saturation point is
more usually called the dewpoint, for if the
temperature of the saturated air falls below its
dewpoint, some of the water vapor in it must
condense to liquid water, generally in drops.
The dew that appears early in the morning
on foliage when there is normally a drop in
temperature, if the air is moist, is such a
condensation, and is, as is readily recognized,
the source of the term dewpoint. The sweating of cold water pipes, with which almost
everyone is familiar, is also the condensation
of dew from moist air on the cold surface of
16A5. Condensation of saturated air. Condensation of water vapor from the air can take
place at any air temperature, providing the
temperature is below its dewpoint. In nature,
moisture is condensed on foliage and other
surfaces as dew if the air temperature is above
32 degrees F. If the temperature of the surface is
below freezing, the moisture condenses as
frost. Above the earth's surface it is mist.
and when the mist is very thick, it is called a
fog. If such condensation on dust particles is
high in the air, the fog is then called a cloud.
Under certain conditions of sudden cooling
with much condensation, the droplets grow so
large that they can no longer float in the air,
and then they fall as rain. Sometimes a layer
of air at a temperature below 32 degrees F exists
high in a storm area; through this cold layer,
raindrops may be carried down and up several
times by air currents until they freeze and fall
as hail. In cold weather when the temperature
is below 32 degrees F, condensation on the dust in
the air forms snowflakes.
16A6. Difference between water vapor and
water drops. The question is sometimes
asked If the air contains moisture, why does
the moisture not freeze when the temperature
is below 32 degrees F? The answer is that only a
liquid can freeze and a vapor is not a liquid.
Drops of water, however small they may be,
are merely small masses of liquid. In a mist
or fog, the drops are so small that they float
in the air, but they are nevertheless liquid. Air
moisture does indeed freeze sometimes, if that
moisture is in the state of liquid drops, and
then it takes the form either of hail, or of
sleet which is partially frozen moisture. Liquid
moisture in the air (for example, mist) may
exist in the form of drops subdivided so small
as to be imperceptible to the human eye as
individual drops; yet each single drop is
formed of a great multitude of molecules. In
a vapor or gas, the subdivision actually consists of single molecules.
16A7. Intermolecular distance determines
state. The fundamental difference between
the three states of matter-solid, liquid, and
gaseous-is the distance between the molecules. In a solid, they are close and hold to
one another, so that each has little or no freedom of motion. In a vapor or gas, the molecules are so far apart that all mutual attraction
is lost, and each has complete freedom of
motion, except as bounded by a container.
Solids and liquids are visible to the human
eye, but vapors and gases, with few exceptions, are invisible. Water vapor is invisible.
The visible white cloud arising from a tea
kettle or steam pipe is not really vapor or
steam although it is usually called steam, but
is formed of minute liquid droplets, that have
condensed on striking the cooler air. They
re-evaporate in a few minutes and are invisible
16A8. Formation of frost. When molecules
of water vapor come sufficiently close together,
they take on the liquid state and become
visible. If this is in the air, the finely sub
divided liquid appears as mist, fog, cloud, or
rain. If the molecules come still closer, they
finally get close enough to take on the solid
state, that is, the liquid water freezes. It is
apparent, therefore, that water vapor in the
air cannot freeze, for as the molecules get
closer, they first pass through the liquid state.
Thus, only a liquid can freeze.
Water vapor can, however, condense directly to the solid state under certain conditions.
This is not freezing, for the vapor does not
become a liquid. The required condition is a
surface at or below the freezing temperature,
on which the water vapor arranges itself in
solid geometrical forms or designs, called
crystals. If the cold surfaces are extended,
such as the ground, glass windows, and so
forth, the crystals are called frost. Frosting
always appears on the cooling coils (evaporators) of mechanical refrigerating systems. This
frost must be removed periodically since it has
some insulating quality and lessens the refrigerating capacity.
B. HEAT OF THE AIR|
16B1. Sensible heat of air. The heat of air
is considered from three standpoints. First,
sensible heat is that measured by household,
or dry-bulb, thermometers. This is the temperature of the air itself, without regard to
any humidity it may contain. It may be well
to emphasize this by stating that sensible heat
is the heat of dry air.
16B2. Latent heat in air. Second, air nearly
always contains more or less moisture. Conditions of complete absence of moisture rarely
occur, perhaps only in desert regions. Any
water vapor present, of course, contains the
latent heat which made it a vapor. Such latent
heat of the moisture in the air may be spoken
of as the latent heat in the air.
16B3. Total heat of air. Third, any mixture
of dry air and water vapor, that is, air as we
usually find it, does contain both sensible heat
and latent heat. The sum of the sensible heat
and latent heat in any sample of air is called
the total heat of the air. It is usually measured from zero degrees as a convenient starting point.
C. THE THREE AIR TEMPERATURES|
16C1. Need for three air temperatures. Inasmuch as air-conditioning deals with these
various heats of the air and the condensation
of the moisture in it as well three -different
temperatures are needed to understand and
control the operations. These are the dry
bulb, wet-bulb, and dewpoint temperatures.
16C2. Dry-bulb temperature. The dry-bulb
temperature is the temperature of the sensible
heat of the air, as measured by an ordinary
thermometer. Such a thermometer is called
in psychrometry, or air-conditioning engineering, a dry-bulb thermometer, because its bulb
is dry, in contrast to the wet-bulb type next
16C3. Wet-bulb temperature. A wet-bulb
thermometer is an ordinary thermometer with
a cloth sleeve, of wool or flannel, placed
around its bulb and then wet with water. The
cloth sleeve should be clean and free from oil
and thoroughly wet with clean fresh water.
The water in the cloth sleeve is caused to
evaporate by a current of air at high velocity,
and the evaporation, withdrawing heat from
the thermometer bulb, lowers the temperature,
as then measured, a certain number of degrees.
The difference between the dry-bulb and
wet-bulb temperatures is called the wet-bulb
depression. If the air is saturated, evaporation
cannot take place, and the wet-bulb temperature is the same as the dry-bulb. Complete
saturation, however, is not usual, and a wet
bulb depression is normally to be expected.
16C4. Wet-bulb temperature measures total
heat. The wet-bulb thermometer indicates
the total heat of the air being measured. If
air at several different times or different places
is measured and the wet-bulb temperatures
found to be the same for all, the total heat
would be the same in all, though their sensible
heats and respective latent heats might vary
considerably. Again, in any given sample of
air, if the wet-bulb temperature does not
change, the total heat present is the same,
even though some of the sensible heat might
be converted to latent heat, or vice versa.
16C5. Sling psychrometer. In air-conditioning work, the two thermometers, wet-bulb and
dry-bulb, are usually mounted side by side on
a frame, to which a handle or short chain is
attached so that the thermometers may be
whirled in the air, thus providing the high
velocity air current for evaporation. Such a
device is called a sling psychrometer. The
psychrometer must be whirled around rapidly,
at least four times per second. When the wet
bulb thermometer is examined at intervals, its
temperature reading will be found to be dropping; when no further drop is observed, that
reading gives the correct wet-bulb temperature.
16C6. Dewpoint temperature. The dewpoint
depends upon the amount of water vapor in
the air. If air at a certain temperature is not
saturated, that is, if it does not contain the
full quantity of water vapor it can hold at that
temperature, and the temperature of that air
falls, a point is finally reached at which the
air is saturated for the new, lower temperature
and condensation of the moisture then begins.
This point is the dewpoint temperature of the
air for the quantity of water vapor present,
16C7. Relation of dry-bulb, wet-bulb, and
dewpoint temperatures. The definite relation
ships between the three temperatures should
be clearly understood. These relationships are:
1. When the air contains some moisture but
is not saturated, the dewpoint temperature is
lower than the dry-bulb temperature, and the
wet-bulb temperature lies between them.
2. As the amount of moisture in the air
increases, the differences between the temperatures grow less.
3. When the air is saturated, all three
temperatures are the same.
16D1. Humidity. The word humidity is
often used in speaking generally of the moisture, or water vapor, in the air. It has, besides,
two technical meanings in the forms absolute
humidity and relative humidity.
16D2. Absolute humidity and specific humidity. Humidity in air is expressed according to
its weight. The weight of the moisture that
air can contain depends upon the temperature
of the air, and is independent of the pressure
of the air. This weight is usually given in
grains, there being 7,000 grains to the pound.
Absolute humidity is the weight of water
vapor in grains per cubic foot of air. Specific
humidity is the weight of water vapor in
grains per pound of dry air. This second form
is more generally used. It should be understood that the weight of moisture in grains
refers only to moisture in the actual vapor
state, and of in any way to any moisture that
may be present in the liquid state, such as fog,
rain, dew, or frost.
16D3. Relative humidity. Relative humidity
is the ratio of the weight of water vapor in a
sample of air to the weight of water vapor
that same sample of air contains when saturated. This ratio is usually stated as a percentage. For example, if the air were fully
saturated, its relative humidity would be 100
percent. If the air contained no moisture at
all, its relative humidity would be zero percent. If the air were half saturated, its relative
humidity would be 50 percent.
16D4. Importance of relative humidity. As
far as comfort and discomfort resulting from
humidity are concerned, it is the relative humidity and not the absolute or specific humidity that is the important factor. This can be
most easily understood by an example.
It should be understood that moisture
always travels from regions of greater wetness
to regions of lesser wetness, just as heat travels from regions of higher temperature to
regions of lower temperature. If the air above
a liquid is saturated, the two are in equilibrium
and no moisture can travel from the liquid to
the air, that is, the liquid cannot evaporate.
If the air is only partially saturated, some
moisture can travel to the air, that is, some
evaporation can take place.
Suppose the specific humidity of the air to
be 120 grains per pound of dry air. This is
the actual weight of the water vapor in that
air. If the dry-bulb temperature of the air is
76 degrees F, the relative humidity is nearly 90 percent, that is, the air is nearly saturated. The
body perspires but the perspiration does not
evaporate quickly because the air already contains nearly all the moisture it can hold. The
general feeling of discomfort is a warning that
the environment under such conditions is not
suitable for the best maintenance of health.
Nature has, however, given the human body
extraordinary powers of resistance, and the
body can take a great deal of punishment
without permanent harm, though its efficiency
drops for the time being.
But if the dry-bulb temperature is 86 degrees F,
the relative humidity is only 64 percent. That
is, although the absolute amount of moisture
in the air is the same, the relative amount is
less, because at 86 degrees F the air can hold more
water vapor than it can at 78 degrees F. The body
is now able to evaporate its excess moisture
and the general feeling is much more agreeable, even though the air temperature is ten
In both cases, the specific humidity is the
same, but the ability of the air to evaporate
liquid moisture is quite different at the two
temperatures. This ability to evaporate moisture is indirectly indicated by the relative
humidity. It is for this reason that extreme
importance is placed upon control of relative
humidity in air-conditioning.
16D5. Psychrometric chart. There is a relationship between dry-bulb, wet-bulb, and dew
point temperatures, and specific and relative
humidity. Given any two, the others can be
calculated. However, the relationship can be
shown on a chart, and in air-conditioning it is
customary to use the chart, since it is far easier
than calculating. Such a chart is called a
psychrometric chart, and a simple form of it
is given in Figure 16-1. In this chart, note
that the wet-bulb temperature scale and dew
point temperature scale lie along the same line;
which is, of course, the 100 percent relative
humidity line. But note that the dewpoint
temperature lines run horizontally. The wet
bulb temperature lines run obliquely down to
To use the chart, take the point of intersection of the lines of the two known factors,
interpolating if necessary. From this intersection point, follow the lines of the unknown
factors to their numbered scales and read the
Example 1. Given a dry-bulb temperature
of 70 degrees F and a wet-bulb temperature of 60 degrees F.
What are the dewpoint temperature and the
relative humidity? Note the intersection of the
two given lines. From this intersection, follow
horizontally along the dewpoint line (by interpolation) to the dewpoint scale. Answer. The
dewpoint temperature is 53.6 degrees F; the relative
humidity is 36 percent, read by interpolating
the intersection point between curved relative
Example 2. If the dewpoint remains at 53.6 degrees
F, what is the relative humidity if the air is
then raised to the dry-bulb temperature of 80 degrees
F? Answer. Follow the dewpoint line horizontally to the 80 degrees F dry-bulb temperature
line, where interpolation reads 40.5 percent.
Example 3. Given a dry-bulb temperature
of 80 degrees F and a dewpoint temperature of 70 degrees F.
What is the relative humidity if the dry-bulb
Figure 16-1. Psychrometric chart.
temperature of the air is then raised to 90 degrees F?
Answer. Note the intersection of the dewpoint
70 degrees F line running horizontally from the
dewpoint scale to the vertical 80 degrees F line.
Follow from the intersection horizontally to
the 90 degrees F dry-bulb line and the relative humidity is 52 percent.
The actual weight of any amount of water
vapor in air at any temperature can be read
on the chart from the scale at the right edge.
For example, take the 70 degrees F dry-bulb temperature line. The intersection on this line of the
various relative humidity percentage lines,
followed horizontally to the right, gives the
number of grains of water vapor per pound of
dry air. At the bottom is zero moisture, or
completely dry air. At the top is 100 percent
saturation, such air at 70 degrees F holding a maximum
of 110.5 grains per pound. The various
weights of water vapor that air at 70 degrees F holds
for any percentage of saturation can be found
by following horizontally to the right from
any relative humidity percentage point on the
70 degrees F dry-bulb line.
Example 4. What is the actual weight of
water vapor (specific humidity) in air at 85 degrees F
dry-bulb and 70 degrees F wet-bulb temperature?
Answer. About 85.5 grains per pound of dry
The various manufacturers of air-conditioning apparatus issue free large detailed
psychromatic charts that are convenient for
the accurate solution of problems. Such charts
are one of the most valuable tools an air
conditioning man can have.
E. FACTORS AFFECTING HUMAN COMFORT AND EFFICIENCY|
16E1. Comfort. In air-conditioning practice,
the term comfort is used to mean not comfort
in the sense of mere pleasure, such as relaxing
in a soft armchair, but rather comfort in the
sense of physiological well-being and general
efficiency of mind and body.
16E2. Humidity requirements for good
health. If air is too dry, the mucous membranes of the mouth, nose, and lungs are
adversely affected, and not only feel parched
and uncomfortable, but are also more susceptible to germs. If air is too moist, the body is
constantly in a state of perspiration, cannot
maintain a proper rate of evaporation, and
clothing stays damp. It has been found that
for best health conditions, a relative humidity
of from 40 to 60 percent is desirable. Even
within this range, a distinction can be made
between winter and summer conditions, for
the best possible results. In cold weather a
range of 40 to 50 percent of relative humidity,
and in hot weather a range of 50 to 60 percent
is best. However, these optimum ranges cannot always be maintained in practical working,
so that an overall range of 30 to 70 percent
relative humidity is acceptable, if not the best.
16E3. Temperature regulation of the human
body. Ordinarily, the body is at a fairly constant temperature of 98.6 degrees F. This, of course,
refers to the interior of the body and not to
the skin surfaces, which vary in temperature.
Nature has so evolved the human body that
any serious departure from this normal temperature of 98.6 degrees F is dangerous to health.
Even a change of one degree, up or down, is
noticeable. But since the body is continually
receiving a heat gain from surrounding and
interior processes, there must also be a continuous outgo of heat to keep a balance. Fortunately, the body is equipped to maintain this
balance automatically, and on the whole does,
an extraordinarily good job.
16E4. Body heat gains. The body gains heat
by 1) radiation, 2) convection, and 3) conduction, and 4) as a byproduct of physiological
processes that take place inside the body.
1. Heat radiation gain. The heat radiation
gain comes from our surroundings, but since
heat always travels from regions of higher
temperature to regions of lower temperature,
such surroundings must have a temperature
higher than 98.6 degrees F for the body to receive
heat from them. Indoor heat radiation is
gained from heating devices, stoves, operating
machinery, hot pipes, and electric light bulbs
(this latter in small or negligible amount).
The great source of heat radiation is the sun.
The sun's radiation has healthful properties
beyond mere heat, and submarine personnel
should take advantage of every opportunity
to stay in direct sunlight.
2. Heat convection gain. The heat convection gain comes from currents of heated air
only, and is usually found on shipboard only
near a galley stove or engine.
3. Heat conduction gain. The heat conduction gain comes from objects with which the
body may, from time to time, be in contact.
4. Body heat production. Most of the
body's heat comes from within the body itself.
Heat is being continuously produced inside the
body by the oxidation of foodstuffs and other
chemical processes, by friction and tension
within the muscle tissues, and by other causes
as yet not well known.
16E5. Body heat losses. The heat given off
by the body is of two kinds, sensible and
latent. Sensible heat is given off by the three
methods: 1) radiation, 2) convection, and 3)
conduction. Latent heat is given off 4) by
1. Heat radiation loss. The body is usually
at a higher temperature than that of its surroundings, and therefore radiates heat to walls,
floors, ceilings, and other objects. The temperature of the air does not influence this
radiation, except as it may alter the temperature of such surroundings.
2. Heat convection loss. Heat is carried
away from the body by convection currents,
both by the air coming out of the lungs, and
by exterior air currents. These may exist in
the air itself or be caused by a person's moving
3. Heat conduction loss. Since the body is
usually at a higher temperature than that of
its surroundings, it gives up heat by conduction through bodily contact with them.
4. Heat loss by evaporation. Under normal
air conditions, the body gets rid of much
excess heat by evaporation. When the body
perspires, liquid water comes through the
pores to the outer surface of the skin. There
it immediately begins to evaporate, and it does
so by withdrawing heat from the body. Inside
the body the heat is sensible heat; in the
process of evaporation, it becomes latent heat.
The rate of evaporation, and hence of heat
loss, depends upon the temperature, relative
humidity, and motion of the air.
Ordinarily, that is, with air at not too high
a temperature and relative humidity, and when
not too active, the body gets rid of its excess
heat by radiation, convection, and conduction.
When engaged in work or exercise, the body
develops much more internal heat, and perspiration begins. But perspiration rapidly
evaporates if the relative humidity is not high.
If, however, the relative humidity of the air is
high, the moisture cannot evaporate, or does
so only at a slow rate. In such cases, the
excess heat cannot be removed by evaporation,
Figure 16-2. Comfort chart.
and the body is dependent on radiation, convection, and conduction to eliminate its excess
heat; this, of course, it cannot do and discomfort follows.
16E6. Amount of body heat loss. The amount
of heat given off by the body varies according
to its activity. When seated at rest, the aver
age adult male gives off about 380 Btu per
hour. When working at fullest exertion, he
gives off 4,000 to 4,800 Btu per hour. On a
submarine, a man gives off from 500 to 600
Btu per hour as an average over a 24-hour day.
Research has shown that the total amount
of heat loss is divided as follows for light
work on a submarine: about 45 percent by
radiation, 30 percent by convection and conduction, and 25 percent by evaporation. Research has shown further that for normal body
comfort, it is important that the heat loss be
in these proportions.
Thus, if a person loses the same total of
heat in the proportions of 40 percent by radiation, 50 percent by convection and conduction,
and 10 percent by evaporation, he feels uncomfortable, damp, and chilly. This represents a
condition of high relative humidity and too
much air motion, as from a direct draft or fan
breeze. On the other hand, if the total heat
loss is the same, but divided in the proportions
of 30 percent by radiation, 25 percent by convection and conduction, and 45 percent by
evaporation, he feels uncomfortable, hot, and
parched. This represents a condition of low
relative humidity and no air motion.
It is apparent that while the total heat loss
may be a desirable amount in total, it may be
so given off as to produce distinct discomfort.
It is essential that the, air-conditioning be so
controlled as to enable these heat losses to
occur in the best proportions to produce comfort.
16E7. Comfort zones. Extensive research has
shown that a normal feeling of comfort is
experienced by most persons in air at different
temperatures, relative humidities, and air motion, within not too great a range. The
average temperature within a range in which
the greatest percentage of persons feel comfortable has been given the name comfort
line, and the range itself is called the comfort
zone. Since summer and winter weather conditions are markedly different, the summer
comfort zone varies from the winter comfort
zone. But the human body is able to adapt
itself automatically to summer and winter
conditions. Indoor air conditions that are
quite comfortable in summer are decidedly
uncomfortable in winter, and vice versa.
All the information gathered in the tests has
been assembled in a chart called the comfort
chart (Figure 16-2). This chart provides an
authentic guide for air-conditioning, and if the
air is maintained within the zones shown, it is
found that general comfort is experienced.
16E8. Heat and humidity as affected by air
motion. In this chapter it has been necessary
to explain individually the action of the various factors of heat and humidity. In reality,
they act simultaneously and, moreover, the
motion or lack of motion of the air itself
influences their effects considerably. This subject is discussed separately in Chapter 17.
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Version 1.10, 22 Oct 04