The bleed-air leak detection system warns the pilot of a leak in the bleed-air distribution lines or shuts down the system, as necessary. The leak detection system consists of a control unit and nine detectors. When one of the detectors senses an overheat condition, it sends a signal through the control unit. The control unit signals the respective bleed-air pressure regulator to close and lights a warning light on the advisory panel in the cockpit, giving the location of the detector sensing the overheat condition.

AIR CYCLE AIR-CONDITIONING SYSTEMS

Learning Objective: Recognize the operating principles and components of air cycle air-conditioning systems (ACS). Most naval aircraft are designed with an air cycle ACS because it is efficient for the weight and space required and is relatively trouble free. The name air cycle or air-to-air comes from the principle of cooling the air without the use of refrigerants by compression and expansion of hot bleed air. The F-18 air cycle ACS is an example of this type system (fig. 3-2, a foldout at the end of this chapter).

SYSTEM OPERATION The air cycle ACS was designed to operate by passing hot engine bleed air through the primary heat exchanger where ram air, forced across the heat exchanger by the aircrafts forward motion, absorbs heat from the bleed air, reducing the air temperature. On the ground and during low-speed operation, ram air is pulled across the heat exchangers by hot air ejected into the heat exchanger exit ducts by the primary and secondary heat exchanger ejectors. The cooled bleed air then passes to the flow modulating system pressure regulator valve where, controlled by electrical and pneumatic sensors, the downstream pressure is maintained by regulator modulation. The air enters the compressor end of the refrigeration turbine/ compressor assembly where it is com-pressed to approximately twice its inlet temper-ature. The compressed air enters the secondary heat exchanger where ram air absorbs the heat acquired through compression. From the secondary heat exchanger, air enters the refrigera-tion cycle. The cycle is made up of a reheater heat exchanger and a condenser/ vent suit heat exchanger and water extractor, which removes 90 percent of the moisture content through repeated heating and cooling. The conditioned dry air is transported to the turbine end of the refrigeration turbine/ compressor assembly where it is cooled by rapid expansion. Both the turbine and compressor are protected from overheat damage by the turbine and compressor protective temper-ature sensors. As the cold dry air leaves the turbine, it is mixed with warm air from the environmental control and related systems, and then it is routed to cockpit and avionics compartments to satisfy environmental control requirements. Air cycle ACS components are discussed in the following paragraphs.

Primary Heat Exchanger

The primary heat exchanger (fig. 3-2) is a cross-flow, air-to-air heat exchanger that uses ram air to initially cool hot engine bleed air. It operates on the same principle as the radiator in an automobile with the bleed air replacing the liquid. Hot bleed air is transported through the heat exchanger core where ram air, forced across the core by aircraft forward motion, absorbs the heat from the bleed air, reducing the air temperature. During ground and low-speed operation, cooling air is pulled across the heat exchanger core by ejecting hot air into the heat exchanger exit duct. The ejected air is controlled by the primary ejector valve in response to signals from the Air Data Computer (ADC). The cooled air exiting the heat exchanger is divided into two ducts that provide air at varying temperatures for use in related systems. The temperature difference occurs because of the distance the bleed air travels through the heat exchanger core.

Primary Ejector Valve

The primary ejector valve is a normally open, in-line poppet, pneumatically actuated, solenoid-controlled shutoff valve. The valve controls the flow of bleed air to the primary heat exchanger ejector in the primary heat exchanger exit duct. The hot bleed air flowing through the ejector nozzles causes an area of low pressure to form at that point. This causes ambient air to flow across the core of the heat exchanger from the high pressure side to the area of low pressure. The valve is controlled by an electrical signal from the ADC. At airspeeds below 100 knots, the ADC provides an electrical ground that energizes the primary ejector control relay to remove power from the valve solenoid, allowing differential pressure to hold the valve open. At airspeeds above 100 knots, the ADC ground is lost. This de-energizes the primary ejector control relay to apply power to the valve solenoid, allowing spring tension to close the valve.