The F-18 aircraft waveguide pressurization system (fig. l-16) provides regulated, filtered, dry air to the radar and electronic countermeasures waveguide cavities. The air is supplied by either the ACS or ground air cooling through a waveguide test port. The system is made up of a filter, two pressure regulating valves, two air desiccators, two ground test ports, and tubing to contain and transport the regulated air. Filter System The filter assembly is made up of a filter element and pressure relief valve. During operation, air passes through the filter ele-ment, which removes 98% of particles larger than 10 microns and 100% of particles larger than 25 microns. When the filter element is contaminated, a pressure differential of 4 to 5 psi will open a relief valve and bypass air around the filter element without interrupting the system operation.

Pressure Regulating Valves The waveguide pressurization system contains two pressure regulating valves, one for electronic countermeasures and one for radar waveguide. Each valve regulates air to 19 psia. The valves use two aneroids to regulate air pressure. Primary aneroid force is balanced against spring force through an opening on which a ball is seated. An auxiliary aneroid functions as an added adjust-ment and as a trigger assembly to allow internal pressure relief to atmosphere when regulated pressure exceeds 19 psia. A resettable indicator is visible when an overpressure condition has occurred. A check valve next to the outlet port of the regulating valve closes to maintain

Figure 1-16.- Waveguide pressurization system.

waveguide pressure during an overpressure condition. Air Desiccators The waveguide pressurization system contains two air desiccators. The desiccators remove

moisture from the regulated air. The desiccators contain desiccant to absorb moisture and a silica gel for an indicating agent. During normal operation, the silica gel is blue. When moisture saturation occurs, the gel turns pink. The color change is visible through an inspection window on the housing of the desiccator. Ground Test Ports The waveguide pressurization system contains two test ports, one for electronic countermeasures and one for the radar waveguide. The two test ports provide a method of using a ground air source to pressure test the system.