A size of 24 square inches per head for inlet and the same for outlet meets common conditions.
It is desirable to make each individual inlet not larger than 48 to 60 square inches in area, i.e. large enough for two or three men; and each outlet not larger than one square foot, or enough for six men (Parkes). This ensures more uniform diffusion of the air throughout a room. On the other hand, the loss by friction is greatly increased by having a number of small openings instead of one large opening. This loss is inversely to the square roots of the respective areas. Thus the square root of 100 is 10; the sum of the square roots of the four apertures of 25 square inches each is 20. The loss by friction is double in the second case what it was in the undivided opening. It is evident, therefore, that in order to get as much air through the four openings as through the original large opening, each must be equal in size to half the original opening.
Why is ventilation more difficult in upper rooms of large houses and in single-storied houses than in the lower storeys of large houses?
Cold external air being heavier than the internal warm air presses downwards to the lowest point, and pushes up the warmer air. If there were a vacuum in the room, air would rush into it with a velocity which, as seen before, is represented by the formula—
v = √(gs).
Where g = 32, s = height of column of air, which we may take as roughly 5 miles.
From this formula we obtain v = 1,306 feet per second.
It is evident that in such a case the velocity of entry of air into a vacuum on the ground floor would be greater than into a vacuum on any of the higher storeys, owing to the greater velocity acquired through the increased action of gravity.
And the same increased facility of entry of air into lower rooms must hold good under ordinary circumstances, inasmuch as by Montgolfier’s formula (which is founded on the fundamental formula v = √2(gs))
v = √2(gh(t-t1) ∕ 492)