IT 264 INDUSTRIAL FLUID POWER Chapter 13 Pneumatics: Air Preparation and Components

PROPERTIES OF AIR There is actually a mixture of gases containing about 21% oxygen, 78% nitrogen and 1% other gases such as argon and carbon dioxide. These values are based on volume. There also contains up to 4% water vapor depending on the humidity. The percent of water vapor in atmospheric air can vary constantly from our our even the same location. Earth is surrounded by a blanket of air-the atmosphere. Because air has weight, the atmosphere exerts pressure at any point do the column of air above that point. The reference point at sea level, were atmospheric pressure exerts a pressure of 14.7 psia. Atmospheric pressure decreases with altitude, further region of altitude of 20,000 feet, the relationship is nearly linear, with a drop in pressure of about.5 psi per 1000 foot change in altitude. When making pneumatic circuit calculations, atmospheric pressure of 14.7 psia is used as the standard. Air is not only readily compressible, but it's volume will vary to fill the vessel containing it because the air molecules have substantial internal energy and are at a considerable distance from each other. This accounts for the sensitivity of density changes with respect to the changes in pressure and temperature free air is considered to be error at actual atmospheric conditions. Since atmospheric pressure and temperature vary from day to day, the characteristics vary accordingly. This would making pneumatic circuit calculations the term standard air is used. Standard air is Sea-level air having a temperature of 68°F, and a pressure of 14.7 psia, and a relative humidity of 36%.

ABSOLUTE PRESSURES AND TEMPERATURES Circuit calculations dealing with volume and pressure changes of air must be performed using absolute pressure an absolute temperature values. The units of absolute temperature in the English system are degrees Rankin 0°R (-460°F) is the temperature at which all molecular motion ceases to exist in the volume and pressure the gas theoretically become zero. The absolute temperature in the metric system are degrees Kelvin.

THE PERFECT GAS LAWS During the 16th century, scientists discovered the law to determine interactions of pressure, volume, and temperature of a gas. These laws are called "perfect gas laws" because they were derived on the basis of a perfect gas. Even though perfect gases do not exist, air behaves very closely to that predicted by Boyle's law, Charles is law, Gay-Lussac's law, and the general gas law for the pressure and temperature ranges experienced by pneumatic systems.

BOYLE’S LAW

CHARLES' LAW

GAY-LUSSAC'S LAW

GENERAL GAS LAW