13A1. Air-conditioning. Air-conditioning is a field of engineering dealing with the design, construction, and operation of equipment used in establishing and maintaining desirable in door air conditions. These conditions vary according to the special requirements of the installation, which may be in a theater, factory, store, submarine, or any other enclosure occupied by human beings.

In the modern engineering practice of air conditioning, two phases are involved:

1. Actual conditioning of air, that is, the alteration under control of its temperature, humidity, purity, and oxygen content.

2. Ventilation, or replacement of stale air in an enclosure by conditioned air. These two aspects of air-conditioning are discussed separately in detail.

13A2. Need for air. Human beings are air breathing animals. As such, man's lungs work unceasingly, awake or asleep. No one can hold his breath longer than a minute or two. The mechanism of the body demands that air, regardless of whether it is fresh or stale, be pumped in and out of the lungs continuously. The muscles that do the pumping operate automatically and no effort of will can stop them for longer than a couple of minutes at most. As a result, every person

  inhales and exhales a large number of cubic feet of air per day.

13A3. Purposes of air-conditioning. Apart from this merely mechanical reaction, considerations of health, efficiency, and morale require that the air should be fit to breathe. This necessitates, among other things, the removal of fumes from a ship's galley, engine room, battery room, and water closets. Stale air must then be replaced by fresh air. More over, the body gives off excess heat and moisture by means of the air that is breathed and the air in contact with the surface of the body. It becomes obvious, therefore, that proper air-conditioning within the enclosed quarters of a ship is important, and that during the long dives of a submarine it is of even greater importance.

Air-conditioning is also needed for the protection of equipment, especially electrical apparatus. The large amount of moisture in the air given off daily from the bodies of the crew, from cooking, batteries, and bilges, would condense on any cool surface if it were not removed by air-conditioning. This moisture, is extracted from the air by the air conditioning equipment, and is run into a tank. It is not suitable for drinking, cooking, or bathing, but is suitable for the washing of clothes.

13B1. Oxygen content of the air. What we call air is not a single substance, but is a physical mixture of various gases. About 1/5 of ordinary outdoor air is oxygen; a little less than 4/5 is the inert gas nitrogen; about 0.03 percent is carbon dioxide; and the balance, less than 1 percent, is composed of the gases argon, helium, krypton, neon, xenon, and hydrogen. These are the components of dry air. Usually some water vapor is present also, varying greatly from day to day according to the weather.   It is oxygen, of course, that is used by the body. Measurements have shown that for each lungful of air breathed in by a person, only 4 percent of the oxygen in it is absorbed by the blood. It is therefore, evident that the air in a room can be circulated and breathed for a considerable period without ill effects.

13B2. Odors. Odors are always present, though the human sense of smell is not a keen one and usually is not aware of them. Almost everything gives off an odor, machinery, clothing, leather, books, food, the human


body, even when clean and newly bathed, flowers, and perfume. The odor in an enclosed space is a combination of all such odors and is carried by the air. However, it is well known that even if an odor is noticed on entering a room, a person rapidly becomes unaware of it because the sense of smell is easily fatigued.

13B3. The heat and moisture contributed by human presence. An adult, when engaged in light work, as in a submarine, gives off on an average of about 500 Btu per hour. In comparison, a 25-watt electric light bulb gives off 85 Btu per hour; a 60-watt bulb, 205 Btu per hour; and a 200-watt bulb, 682 Btu per hour. The heat from a bulb can be felt by the hand at a distance of several inches, because the heat-giving surface is concentrated in a small area. The heat from a human body cannot be

  felt because the surface of the body is large, measuring about 29 square feet for the aver age adult male. The body also gives off considerable moisture, but the amount varies greatly according to the activity.

Under normal conditions a person takes in somewhat more than 3 pounds of water per day, in beverages and food. Since the body is maintained at an average condition of equilibrium, this means that he gives off the same quantity per day, and much of this is evaporated directly into the air. If his activity or the air temperature is such as to cause a greater loss of water through perspiration, he feels thirsty and drinks more to maintain the balance. In a small enclosure, or room, the rise in temperature and moisture of the air caused by the presence of a number of people is considerable.


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