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17
VENTILATION
 
A. NEED FOR VENTILATION
 
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 door quarters.

 
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

 
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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 health.

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 convection current.

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 convection.

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 dead-air pockets.

A ship is a difficult structure to ventilate by natural means. A submarine in particular,

 
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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 forced ventilation.
 
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