Copyright Goodheart-Willcox Co., Inc. 400 Fluid Power water vapor in air is an important and constant factor that must be considered when compressing air for use in a pneumatic system. Water in vapor form does no harm in a pneumatic system. The problem develops when this vapor condenses into liquid water. When working with a pneumatic system, it is important to understand the terminology and relationships involv- ing humidity, vapor condensation, and compressed air. Water vapor and compressed air The amount of water vapor that can be retained in air depends on the temperature and volume of the air. When air at a given temperature is holding the maximum amount of water vapor, it has reached a level known as the saturation point. Reducing the temperature of this saturated air results in the formation of liquid water. The temperature at which this water vapor begins to condense to form liquid water is known as dew point. The importance of this principle to the operation of a pneumatic system can be illustrated by using an example of free air at 70°F. At this temperature, air can hold 1.14 pounds of water vapor per 1,000 cubic feet at the saturation point. This is equal to approximately 31.5 cubic inches of liquid water, which is slightly more than 1 pint. Compressing this saturated air to system operating pressures considerably reduces the volume of the air. However, the ability of air to hold water vapor is dependent on the temperature and pressure of the air. Increasing temperature increases the ability of Air can be cooled either before, during, or after compression. In industrial pneumatic systems, control of the air temperature before compression is typically restricted to locating the compressor intake in the cool- est atmospheric air available. Cooling of the air during compression involves the general cooling system used for the protection of the compressor. In multiple-stage compressor designs, a separate intercooler unit is used to cool the air between stages. An aftercooler is a sepa- rate unit used to cool the air after compression is com- plete, Figure 16-6. In industrial pneumatic systems, intercoolers and aftercoolers are typically surface coolers using air or water as the cooling medium. Refrigeration equipment may also be used in some situations. When air is used for cooling, the system may be as simple as external fi ns on a tube or pipe exposed to air circulated by the compressor cooling fan. Double-pipe, shell-and-tube coolers are used in more complex air systems or in water-cooled applications. 16.1.3 Controlling Moisture in Compressed Air Atmospheric air contains moisture that is essential to the operation of our environment. This moisture, which is in vapor form, is commonly referred to as humidity. Generally, humidity goes unnoticed until the moisture content becomes excessive, which causes us to become physically uncomfortable. However, the Aftercooler Intake-line filter Prime mover Compressor Atlas Copco Figure 16-6. An air-cooled aftercooler is often used to reduce the temperature of the compressed air as it moves from the compressor outlet to the receiver.