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. Flow Modulating System Pressure Regulator Valve The flow modulating system pressure regula- tor valve is a combination butterfly, modulating valve, and solenoid shutoff valve. The modulating valve is normally open and pneumatically actuated. The valve is in the distribution ducting between the primary heat exchanger and the refrigeration turbine/compressor assembly. The valve uses electrical signals from the avionics temperature/flow sensor by way of the ACS temperature/flow controller to modulate down- stream pressure. The valve is also connected pneumatically to the turbine and compressor protective temperature sensors, which override all other valve functions to close the valve to protect the refrigeration turbine/compressor assembly from heat damage during overtemper- ature conditions. The solenoid shutoff is con- trolled by the environmental control system (ECS) mode switch, which allows the valve to be opened or closed by dumping control air from the butter- fly diaphragm chamber. The valve also includes a visual position indicator. The indicator is built into the valve housing and shows the position of the butterfly. Avionics Ram Air Servo The avionics ram air servo monitors the differential pressure of bleed air upstream and downstream of the flow modulating system pressure regulator valve. If the upstream^-pre^s_sure is less than 35 psi and the differential pressure is less than 10 psi, the avionics ram air servo drives the avionics ram air valve open. If downstream pressure is less than 4 psi, the avionics ram air servo drives the avionics ram air valve open. The operation of this valve, though not a part of the ACS, ensures that avionics will receive sufficient cooling air in the event of a low or no airflow condition from the ACS. 3-5