Figure 16-61.  A duct hose installed on this airliner distributes hot  or cold air from a ground-based source throughout the cabin using  the aircraft’s own air distribution system ducting.  Figure 16-62.  Boeing 737 air cycle system. The photo is taken  looking up into the air conditioning bay located in the lower fuselage  on each side of the aircraft. Refrigeration bypass valve sensor Refrigeration bypass 350° valve Primary heat exchanger Water separator drain Water separator ACM turbine expansion outlet Turbine Compressor Mixing valve Bypass bleed air Air cycle machine Primary heat exchanger Secondary heat exchanger Refrigeation byp ass air FWD Component Operation Pack Valve  Bleed Air Bypass Primary Heat Exchanger manually with the cabin temperature selector in manual  mode. Other air cycle systems may refer to the valve that  controls the air bypassed around the air cycle cooling system  as a temperature control valve, trim air pressure regulating  valve, or something similar. The pack valve is the valve that regulates bleed air from the  pneumatic manifold into the air cycle air conditioning system.  It is controlled with a switch from the air conditioning panel in  the cockpit. Many pack valves are electrically controlled and  pneumatically operated. Also known as the supply shutoff  valve, the pack valve opens, closes, and modulates to allow  the air cycle air conditioning system to be supplied with  a designed volume of hot, pressurized air.  [Figure 16-64] When an overheat or other abnormal condition requires that  the air conditioning package be shut down, a signal is sent  to the pack valve to close. A means for bypassing some of the pneumatic air supplied  to the air cycle air conditioning system around the system  is present on all aircraft. This warm bypassed air must be  mixed with the cold air produced by the air cycle system so  the air delivered to the cabin is a comfortable temperature.  In the system shown in  Figure 16-58,  this is accomplished  by the mixing valve. It simultaneously controls the flow of  bypassed air and air to be cooled to meet the requirements  of the auto temperature controller. It can also be controlled  Generally, the warm air dedicated to pass through the air  cycle system first passes through a primary heat exchanger.  It acts similarly to the radiator in an automobile. A controlled  flow of ram air is ducted over and through the exchanger,  which reduces the temperature of the air inside the system.  [Figure 16-65]  A fan draws air through the ram air duct  when the aircraft is on the ground so that the heat exchange  is possible when the aircraft is stationary. In flight, ram air  doors are modulated to increase or decrease ram air flow to  the exchanger according to the position of the wing flaps.  During slow flight, when the flaps are extended, the doors  are open. At higher speeds, with the flaps retracted, the doors  move toward the closed position reducing the amount of ram  air to the exchanger. Similar operation is accomplished with  a valve on smaller aircraft.  [Figure 16-66] 16-37