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16
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 saturated air.

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 the pipes.

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

 
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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 again.

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 described.

16C3. Wet-bulb temperature. A wet-bulb

 
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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.

 
D. HUMIDITY
 
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

 
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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 degrees hotter.

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 the right.

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 measurement.

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 humidity lines.

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

 
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Figure 16-1. Psychrometric chart.
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 air.

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
 
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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 evaporation.

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 about.

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

 
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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.
Figure 16-2. Comfort chart.
 
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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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