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 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.
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