A special-purpose air system installed in many surface ships is the prairie-masker air system. This two-part system supplies a high volume of low-pressure air to a system of emitter rings or belts surrounding the hull and to the propeller blades through the hollow propulsion shafts.

The EMITTER RINGS, often referred to as MASKER BELTS, contain small holes which release the masker air into the sea, coating the hull with air bubbles. The bubbles disguise the shape of the ship so that it cannot be seen accurately by enemy sonar. The prairie air passing through the propulsion shafts is emitted to sea by small holes in the propeller blades.

The air supply for the prairie-masker system is provided by a turbocompressor. On ships with steam propulsion plants, the turbocompressor is composed of five major parts contained in one compact unit. They are the turbine-driven com-pressor, lube water tanks, air inlet silencer, lube water system, and control system.

The turbine-driven compressor consists of a single-stage centrifugal compressor driven by a single-stage impulse turbine. The compressor impeller and the turbine wheel are mounted at opposite ends of the same shaft. Two water-lubricated bearings support the rotor assembly.

The compressor runs at speeds approaching 40,000 rpm. A control system for the unit provides constant steam admission, overspeed trip, overspeed alarm, low lube pressure trip and alarm, and a high lube water temperature alarm. On ships with gas turbine propulsion plants, air from the prairie-masker system is taken from the bleed air system of an on-line gas turbine.

MOISTURE REMOVAL

The removal of moisture from compressed air is an important feature of compressed air systems. Some moisture is removed by the intercoolers and aftercoolers, as explained earlier in this chapter. Air flasks and receivers are provided with low point drains so that 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 problems which are potentially hazardous, such as the freezing of valves and controls. 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, which will 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, liquid water in any air system can cause serious water hammer within the system. For these reasons, air dryers or dehydrators are used to remove most of the water vapor from compressed air.

Figure 14-22.-High-pressure and low-pressure air systems.

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

Type I-Refrigeration

Type II-Heater, self-activating desiccant

Type III-Refrigeration, self-reactivating desiccant

Each of these types is designed to meet the requirements specified for the quality of the compressed air to be used in pneumatic control systems or for clean, dry air used for shipboard electronic systems. Specific requirements usually involve operating pressure, flow rate, dew point, and purity (percentage of aerosols and size of particles). We will briefly discuss each of the types of air dryers (dehydrators).