The removal of moisture from compressed air is an important feature of compressed air systems. As you have learned, some moisture is removed by the intercoolers and aftercoolers. Air flasks and receivers are provided with low-point drains so any collected moisture may drain periodically. However, many shipboard uses for compressed air require air with an even smaller moisture content than is obtained through these methods. Water vapor in air lines can create other potentially hazardous problems. Water vapor can cause control valves and controls to freeze. These conditions can occur when air at very high pressure is throttled to a low-pressure area at a high-flow rate. The venturi effect of the throttled air produces very low temperatures that cause any moisture in the air to freeze into ice. Under these conditions, a valve (especially an automatic valve) may become very difficult or impossible to operate. Also, moisture in any air system can cause serious water hammering (a banging sound) within the system. For these reasons, air dehydrators or dryers are used to remove most of the water vapor from compressed air.

The Navy uses two basic types of air dehydrators and a combination of the two. These air dehydrators are classified as follows:

1. Type I-refrigeration

2. Type II-heater, desiccant

3. Type III-refrigeration, desiccant

Each of these types meets the specified requirements for the quality of the compressed air used in pneumatic control systems or for clean, dry air used for shipboard electronic systems. Usually, specific requirements involve operating pressure, flow rate, dew point, and purity (percent of aerosols and size of particles). We will briefly discuss each of the types of air dehydrators.

REFRIGERATION AIR DEHYDRATOR (TYPE I)

Refrigeration is one method of removing moisture from compressed air. The dehydrator shown in figure 10-29 is a REFRIGERATION DEHYDRATOR or REFRIGERATED AIR DRYER. This unit removes water vapor entrained in the stream of compressed air by condensing the water into a liquid that is heavier than air. Air flowing from the separator/holding tank first passes through the air-to-air heat exchanger, where some of the heat of compression is removed from the airstream. The air then moves through the evaporator section of the dehydrator, where the air is chilled by circulating refrigerant. In this unit, the airstream is cooled to a temperature that is below the dew point. This will cause the water vapor in the air to condense so the condensate drain system can remove it. After leaving the evaporator section, the dehydrated air moves upward through the cold air side of the air-to-air heat exchanger.

Figure 10-29.-Dehydrator (type 1).

In the air-to-air heat exchanger, the dehydrated air is raised in temperature by the warm air entering the dehydrator. Heating the air serves to reduce thermal shock as the air enters the system. The exiting dry air flows into the receiver for availability to the ship's air system.

DESICCANT AIR DEHYDRATOR (TYPE II)

The desiccant is a drying unit. More practically, desiccant is a substance with a high capacity to remove (adsorb) water or moisture. It also has a high capacity to give off that moisture so the desiccant can be reused. DESICCANT-TYPE DEHYDRATORS fig. 10-30 are basically composed of cylindrical flasks filled with desiccant.

Compressed air system dehydrators use a pair of desiccant towers. One tower is in service dehydrating the compressed air, while the other is being reactivated. A desiccant tower is normally reactivated when dry, heated air is routed through the tower in the direction opposite to that of the normal dehydration airflow. The hot air evaporates the collected moisture and carries it out of the tower to the atmosphere. The air for the purge cycle is heated by electrical heaters. When the reactivating tower completes the reactivation cycle, it is placed in service to dehydrate air, and the other tower is reactivated.