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 Steam Heating and Ventilation - Figure 26.png

Figure 26.—Diagram of Indirect Apparatus.

foot of a gas of any temperature is inversely proportional to its absolute temperature, which is the temperature in degrees Fahrenheit + 460, or $$W = c \div (t + 460)\,$$, where $$W\,$$ is the weight per cubic foot, $$t\,$$ the temperature in degrees Fahrenheit, and $$c\,$$ a constant, different for each gas and which, for air, equals approximately 40. If t be the temperature of the cold air and $$T\,$$ be that of the air in $$D\,$$, then the pressure per square foot due to a column of cold air of height $$H\,$$ feet would be $$p = 40 H \div (t + 460)\,$$ and the pressure due to a column of hot air of the same height would be $$P = 40 H \div (T + 460)\,$$. The difference of pressure which creates the flow of air then is

$p = \dfrac{40 H}{t + 460} - \dfrac{40 H}{T + 460} = \dfrac{40 H (T - t)}{(t + 460)(T + 460)}$

The head, $$h\,$$, which creates the velocity of flow, is equal to the height of a column of air of temperature $$T\,$$ which would give the pressure $$p\,$$, or

$h = \dfrac{p (T + 460)}{40} = \dfrac{H (T - t)}{t + 460}$

By the laws governing the flow of fluids the theoretical velocity