This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
266. It may easily happen that the ventilation of a room will be very unsatisfactory, notwithstanding that a current of fresh air of sufficient quantity is constantly passing into and out of the apartment. Unless the incoming air is introduced in a proper manner, it may pass in a nearly unbroken current from the inlet to the outlet, and practically fail to disturb and renew the main body of air in the room.
Good ventilation requires that the foul air shall be well mixed and diluted with that which is pure; but whether the desired mixing will take place or not depends mainly upon the care and skill employed to insure the conditions necessary for proper diffusion. Neglect at this point has led to many serious failures in ventilating buildings. Diffusion proceeds best when the whole body of air is at a uniform temperature.
In still, cold air, the products of respiration, being warm, ascend at a rate slightly greater than the rate of diffusion; consequently, there is sometimes a little more carbonic acid, etc., to be found near the ceiling than near the floor, but, as a rule, it is uniformly distributed throughout the space.
267. If warm, fresh air is introduced near the top of a room it will lie against the ceiling in a body, and will not diffuse to any satisfactory extent into the colder air below. To get it down to the breathing level, it must be driven down by force; there is no other way.
The tendency of ascending currents of hot air is to flow in well defined streaks, the separation becoming more marked as the difference in temperature increases. The diffusion may be greatly improved by introducing the hot air in a large number of small streams, but this is impracticable in many cases on account of the expense involved.
Adhesion Of Currents To Surfaces. Another important property of air-currents, both hot and cold, which is of great importance in the art of ventilation, is their tendency to adhere to surfaces along which they happen to be moving. For instance, if a current flows horizontally through an opening at the level of the floor, it will keep close to the floor and be plainly perceptible a long distance away; but a similar current issuing from an opening in the wall, midway between the floor and ceiling, would be quite imperceptible at only a few feet in front of the register.
In a similar manner, a current of cold air formed by the cooling action of a large window will, in many cases, descend and creep along the floor almost to the opposite side of the room before it diffuses, making it very uncomfortable around the feet and ankles of the occupants. In this way, annoying drafts may occur at points where they are least expected.