Compressor

Compressor

Theory of Gas Compression

• Air has weight and it is the weight of the column of air over a particular location that determines the atmospheric pressure at that particular location. 
• At sea level and under average temperature and moisture conditions, a one square inch column of air extending up to the uppermost limit of the atmosphere weighs about 14.7 pounds. 
• Atmospheric pressure at sea level is, therefore, about 14.7 pounds per square inch at 60F and 36% relative humidity. 
• Consider a confined volume of gas. The gas molecules are distributed throughout the volume and are widely separated as compared to their size.
• They move at high velocity and collide frequently with each other and with the walls of the vessel. 
• The continuous bombardment of the enclosing walls produces pressure and the intensity of pressure depends on the number, mass, and velocity of the molecules. 
• Temperature is a measure of the kinetic energy of the molecules which, in turn, depends on their mass and velocity. 
• If the confined gas is heated, its stored energy will be increased and the molecules will move with increased speed. 
• Therefore both pressure and temperature will increase. 
• The rise in temperature is evidence of an increase in the amount of internal energy stored in the gas.
• If the enclosing vessel is fitted with a piston so that the air can be compressed into smaller volume, the moving piston delivers energy to the molecules, causing them to move with increased velocity. 
• As with heating, this results in a temperature increase. Thus, the work of compression is stored as internal energy in the air. 
• Further more, all of the molecules have been forced into smaller space which results in an increased number of collisions on a unit area of the wall. This, together with increased molecule velocity, results in increased pressure. 
• Compression may be thought of as forcing a confined volume (or weight) of gas into a smaller space to increase pressure, 
• It is accompanied by a rise in temperature (and an increase of stored internal energy).

Purpose of Gas Compressor

• Purpose of gas compressor is to compress a gas from an initial or suction pressure to a final or higher discharge pressure.
• Compression of gases to adequate higher pressure is necessary to perform operational functions.

Transmission – move gas from place to place or from one part of a process operation to another.

Recovery – mixture of gases remaining after separating condensable component are compressed for further liquification. 

Air compression - conveying, for power tools, process operations, etc

How compressors work

Individual gas always travel at high speed, at normal temperature, they strike against the walls of an enclosing vessel and produce what is known as pressure.

• When heat is added, the molecules even travel faster, so they hit the containing walls of the vessel harder and more often. This shows up as greater pressure.

• If the enclosing vessel is fitted with a piston so that the gas can be squeezed into a smaller space, molecule travel is restricted. The molecules hit the walls with greater frequency increasing the pressure. The moving piston also delivers energy to the molecules, causing them to move with increased velocity.

• As with heating, this results in temperature increase. Furthermore, all the molecules have been forced into a smaller space, which results in an increased number of collisions on a unit area of wall. This, together with increased molecules velocity results in increased pressure.

• The compression of gases to higher pressure can result in very high temperature creating problems in compressor design.

• All basic compressor elements, regardless of type, have certain limiting operating conditions.

• Basic elements are single stage.

• When a temperature limitation is involved it becomes necessary to multiple stages the compression process.