Humidity and air-motion

There is no such condition as absolutely motionless air in any space being heated or cooled. In such spaces there are always differences between the temperatures of heating or cooling elements and the air around them and these temperature differences always cause air -motion. In a space heated by radiators, where there is no fan, the air which comes into contact with the radiators is heated and made lighter.

The cooler and heavier air falls, while the warmer and lighter air rises. Thus we have air - motion to distribute the heated air within the room, but it is not sufficiently rapid to give a feeling of comfort.

Air motion which is sufficiently rapid to have an effect of comfort is caused by a heating or cooling apparatus with a fan, then air from such ap­paratus comes into the room, the motion of the air must be controlled and directed so that the heated air or cooled one is mixed with the air in the room.

A moderate humidity is desirable - neither so high as to be unpleas­ant, nor so low as to cause excessive dryness. Although, as rightly pointed out various investigators, there are many physicians who hold that excessive dryness is harmful and increases the possibility of infection. There­fore, from the standpoint of comfort and health, is a rather widely accepted view, that a moderate humidity (with a minimum of 25 to 30 percent) should be maintained. A certain amount of air motion is also to be desired. In still air the body is enveloped by a layer of warm and humid air which a moderate air movement helps to blow away.

The following precautions regarding air motion must be observed in the design of an air - conditioning system if satisfactory results are to be obtained:

1.the maximum air velocity which can be tolerated with comfort by human beings at rest is approximately 50 ft per minute.

2.there should be no high velocity wets of air directed into the occupied zone.

3. air should nut be introduced at a temperature too far below that of the room, especially in winter.

Satisfactory air - conditioning depends upon the number of factors, such as air supply, temperature, humidity, air motion, odour, bacteria, harmful substances. No complete study of the conditions in a room can overlook any of these factors and no system of air - conditioning should be designed without considering them.

7. Read the text «HEATING AND AIR - CONDITIONING» and pick out information referring to humidity and air- conditioning.