Copyright Goodheart-Willcox Co., Inc. Chapter 16 Conditioning and Distribution of Compressed Air 401 Other locations for moisture removal in a com- pressed-air system are the receiver and the system distribution lines. Both of these areas allow further air cooling. Liquid water is formed in these areas when- ever the temperature of the compressed air drops below the dew point. Those aspects of moisture control are discussed in those specifi c sections of this chapter. Moisture can also be removed from the compressed air using air-drying equipment especially designed to remove water vapor. These units are typically located between the receiver and the main distribution line or at specifi c locations in a system where dry air is essen- tial. These driers use one of three methods: • Chemical desiccant to absorb moisture. • Refrigeration unit to condense water vapor. • Membranes made from a material that allows water vapor to pass directly from the compressed air to the atmosphere. A variety of designs and capacities are available in these units. 16.1.4 Compressed Air Storage Compressed air in a pneumatic system is stored in a specialized tank called a receiver. This tank not only stores compressed air for periods of heavy demand that exceed compressor capacity, but also serves a number of other functions. One of these functions is dampening air pulsations resulting from compressor operation. These pulsations result from the varying air air to hold vapor. Increasing pressure raises the dew point temperature of the air, essentially decreasing the ability to hold water vapor. During the compression process in a pneumatic system, as air pressure increases, the air temperature may also increase depending on how much heat is retained in the system. The change in water vapor content with compression can be diffi cult to predict, but typically, as the air cools as it moves through the system, the saturation point or dew point will be reached and liquid water will form. The total volume of liquid water that forms depends on the volume of air that moves through the system, the ini- tial water vapor content of the air, and the changes in pressure and temperature that occur during system operation. The fi gures in this example are relatively small. If these fi gures are multiplied by the large volume of air that passes through even a small pneumatic system, you can see the signifi cant quantity of liquid water that can be produced in a system. It must be kept in mind that whenever the temperature of compressed air drops below the dew point, liquid water is produced within the system components and lines. Water removal Water in a compressed-air system can cause a number of serious problems: • Washing away of lubricants in system tools. • Increased component wear and maintenance. • Inconsistent operation of system control valves and actuators. • Freezing of water that accumulates in a distribution line. • Lowering of the fi nished quality of processes directly using the air (such as painting). These problems warrant the lowering of the water vapor level in the system air as early as possible during air compression and distribution. Reducing moisture problems should begin with the location of the atmospheric air intake for the com- pressor. A protected location that provides the driest- available air to the compressor helps reduce problems even before the air is compressed. The next step is an intercooler located between the stages of a multistage compressor. This component cools the compressed air to increase compressor effi ciency, but, on larger units, may also be used to remove some moisture. Typically, the aftercooler is a more critical component for mois- ture control. This unit lowers the air temperature below the dew point. Part of the water vapor condenses in the unit where it can be removed. A moisture separator is often teamed with the aftercooler to increase the effec- tiveness of the moisture removal process. The liquid water removed by these separators is often drained by automatic traps. See Figure 16-7. Centrifugal- flow chamber Outlet Inlet To water trap with drain Goodheart-Willcox Publisher Figure 16-7. The air entering a moisture separator forms a centrifugal flow that forces liquid and dirt to the outer wall of the unit. These materials collect in a trap for easy removal.