low-pressure (0 to 125 psig), medium-pressure ( 126 to 399 psig), or high-pressure (400 to 6,000 psig) systems.

LOW-PRESSURE SYSTEMS

Low-pressure systems provide compressed air up to 125 psig pressure. When you are installing a low-pressure system, pressure is usually reduced at reducing stations for branches requiring lower pressures. Several air pressure requirements for low-pressure air consumers are listed below:

MEDIUM-PRESSURE SYSTEMS

Medium-pressure systems provide compressed air within the range of 126 to 399 psig pressure. These systems are not extensive and are generally provided with individual compressors located near the loads. Medium-pressure systems are mainly used for the starting of diesel engines, soot blowing of boilers and high-temperature water (HTW) generators, and hydraulic lifts.

HIGH-PRESSURE SYSTEMS

These systems provide compressed air within the range of 400 to 6,000 psig pressure. Hazards that increase with higher pressures and capacities can be minimized by the use of separate compressors for each required pressure. Systems operating at 3,000 psig may require small amounts of air at lower pressures, which is supplied through pressure-reducing stations.

Caution must be used with high-pressure systems because when high-pressure air enters suddenly into pockets or dead ends, the air temperature in the confined space increases dramatically. If there is any combustible material in the space and the air temperature increases to the ignition point of the material, an explosion may occur. This is known as auto ignition or diesel action. Explosions of this type may set up shock waves that travel through the compressed air system. This travel may cause explosions at remote points, Even a small amount of oil residue or a small cotton thread may be sufficient to cause ignition.

Some common pressure requirements for high-pressure systems may be as follows:

AIR QUALITY REQUIREMENTS

The quality of air supplied from a compressed air system will vary with application. The installer and maintenance personnel should consider the class of air entrapment and specific air quality requirements for each application,

CLASSES OF AIR ENTRAPMENT

The classes of air entrapment may be sub-divided into inert and chemical particulate, chemical gases, oil, and water. To prevent contamination of an air compression system by these types of entrapments, you should follow certain guidelines for each situation of possible contamination.

Particulate

Intake structures or openings should be free of shelves, pockets, or other surfaces that attract and accumulate particulate. Properly designed intakes are large enough to produce a low-velocity airflow. This limits the size of the particles that may be picked up by the intake suction.

Some particulate may contain active chemicals that may form acids or alkalines in the inevitable presence of water. These chemical particulate can accelerate damage to compressor surfaces.

Particulate are sized in microns or micrometers. This measurement is size, not weight. One micron is a unit of length equal to one millionth of a meter. Particles larger than 10 microns are visual to the naked eye. Filter systems are required for all air compressors. Generally, filters should be able to remet'e particles down to 1 to 3 microns in size.

Gases or Fumes

Cases or fumes are fully airborne and generally independent of air velocity. They can