Manual override Valve actuator Cabin pressure sensor To A/C pack APU supply  solenoid to 13th stage valve reg Shutoff solenoid From aircraft  pneumatic manifold 16-39 Figure 16-66.  A ram air door controls the flow of air through the  primary and secondary heat exchangers. Figure 16-65.  The primary and secondary heat exchangers in an air  cycle air conditioning system are of similar construction. They both  cool bleed air when ram air passes over the exchanger coils and fins.  Figure 16-64.  This pack valve drawing illustrates the complexity of the valve, which opens, closes, and modulates. It is manually actuated  from the cockpit and automatically responds to supply and air cycle system parameter inputs.  The heart of the air cycle air conditioning system is the  refrigeration turbine unit, also known as the air cycle machine  (ACM). It is comprised of a compressor that is driven by  a turbine on a common shaft. System air flows from the  primary heat exchanger into the compressor side of the ACM.  As the air is compressed, its temperature rises. It is then  sent to a secondary heat exchanger, similar to the primary  heat exchanger located in the ram air duct. The elevated  temperature of the ACM compressed air facilitates an easy  exchange of heat energy to the ram air. The cooled system air,  still under pressure from the continuous system air flow and  the ACM compressor, exits the secondary heat exchanger. It  is directed into the turbine side of the ACM. The steep blade  pitch angle of the ACM turbine extracts more energy from the  air as it passes through and drives the turbine. Once through,  Refrigeration Turbine Unit or Air Cycle Machine and  Secondary Heat Exchanger