In several types of automated industries, compressed air is used for various purposes such as spray painting or pneumatic tools (compressed air equipment). A common problem in compressed air systems is the condensation of humidity. It appears along distribution pipes, blocking filters and machinery, and causes malfunction of the system. The water can cause corrosion and considerable damage to the equipment using the air.
The condensation is due to the water vapor in the compressed air. This vapor is cooled along the pipes, and thus transformed to water droplets.
To avoid water condensation in a pneumatic system, an air dryer can be installed in the system (see Figure 10.21). The basic function of the air dryer is to remove moisture from the air by cooling it with a refrigerant. Thus, the water vapor is condensed, and the air can be compressed. The result is dry compressed air, which can be used in compressed air equipment without causing any damage.
In a pneumatic system, the air leaving the compressor (point 1 in Figure 10.21) contains a significant amount of water. In the after-cooler (2), up to 70% of the moisture can be removed. After the separator (3), the compressed air is still saturated with moisture. The air is then stored in a receiver (4), which can be installed before or after the air dryer (6). The goal of the air dryer is to eliminate the remaining 30% of the moisture.
The system design with the air dryer after the receiver, as shown in Figure 10.21, is recommended when the compressor operates intermittently and the air demand is not more than the maximum compressor capacity. If the air demand can exceed the maximum capacity of the compressor, the installation of the air dryer before the receiver is recommended.
The cooling conditions in a pneumatic system can be demonstrated by monitoring the pressure, temperature and humidity during the compressing and cooling stages, as shown in Figure 10.22.
Figure 10.22 Pressure, temperature and humidity changes along a pneumatic system.
In the compressor, free air at 20°C, 1 bar and 50% relative humidity is compressed to typically 7 bar g.
During compression, the temperature increases (from 20 to 80°C), and the relative humidity decreases. Because no vapor condensation takes place, the absolute humidity is constant. When the air is cooled in the after-cooler, the relative humidity increases to 100%. After the "dew point" (where the air is saturated), the vapor condenses with a reduced absolute humidity as a result. Most of the remaining humidity is condensed in the air dryer. During this condensing process, the air is in saturated conditions, which correspond to a relative humidity of 100%. When re-heating the air in the air dryer, the relative humidity will again decrease, as shown in Figure 10.22.