Air is a gas consisting of a mechanical mixture of 23.2% oxygen (by weight), 75.5% nitrogen, and 1.3% argon with small amounts of other gases. It functions as the heat-conveying medium for warm-air heating systems.
Atmospheric pressure may be defined as the force exerted by the weight of the atmosphere in every point with which it is in contact (Figure 2-8), and is measured in inches of mercury or the corresponding pressure in pounds per square-inch (psi).
The pressure of the atmosphere is approximately 14.7 psi at sea level. The standard atmosphere is 29.921 inches of mercury (in Hg) at 14.696 psi. “Inches of mercury” refers to the height to which the column of mercury in a barometer will remain suspended to balance the pressure caused by the weight of the atmosphere.
Atmospheric pressure varies due to elevation by decreasing approximately */2 lb for every 1000 ft ascent above sea level. Atmospheric pressure in pounds per square-inch is obtained from a barometer reading by multiplying the barometer reading in inches by 0.49116. Examples are given in Table 2-1.
Gauge pressure is pressure whose scale starts at atmospheric pressure. Absolute pressure, on the other hand, is pressure measured from true zero or the point of no pressure. When the hand of a steam gauge is at zero, the absolute pressure existing in the boiler is approximately 14.7 psi. Thus, by way of example, 5 lb pressure measured by a steam gauge (i. e., gauge pressure) is equal to 5 lb plus
14.7 lb, or 19.7 psi of absolute pressure.
When air is compressed, both its pressure and temperature are changed in accordance with Boyle’s and Charles’ laws. According to Robert Boyle (1627-1691), the English philosopher and founder of
Modern chemistry, the absolute pressure of a gas at constant temperature varies inversely as its volume. Jacques Charles (1746-1823) established that the volume of a gas is proportional to its absolute temperature when the volume is kept at constant pressure.
Table 2-1 Atmospheric Pressure per Square-Inch for Various Barometer Readings
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UNITY VOLUME —
Figure 2-9 Elementary air compressor illustrating the phenomenon of compression as stated in Boyle’s and Charles’ laws.
If the cylinder in Figure 2-9 is filled with air at atmospheric pressure (14.7 psi absolute), represented by volume A, and the piston B moved to reduce the volume to, say, V3 A, as represented by B, then according to Boyle’s law, the pressure will be tripled (14.7 X 3 = 44.1 lb absolute, or 44.1 — 14.7 = 29.4 gauge pressure). According to Charles’ law, a pressure gauge on the cylinder would at this point indicate a higher pressure than 29.4 gauge pressure because of the increase in temperature produced by compressing the air. This is called adiabatic compression if no heat is lost or is received externally.