a vapor. For this reason, liquid Freon 12 or 22 is   used   in   most   vapor   cycle   refrigeration units  whether  used  in  aircraft  or  in  home  air conditioners and refrigerators. If liquid Freon 12 were poured into an open container   surrounded   by   standard   sea   level pressure, it would immediately begin to boil at temperatures   above   -22°F   (-30°C).   There would  be  a  continuous  flow  of  heat  from  the warm surrounding air through the walls of the container to the boiling Freon. Moisture from the air would condense and freeze on the exterior of the container. This  open  container  system  would  work satisfactorily  insofar  as  cooling  alone  is  con- cerned. A drum of Freon could be connected to a coil and the vaporized Freon piped outdoors. A system such as this would provide satisfactory refrigeration,   but   the   cost   of   continuously replacing the refrigerant would be prohibitive. Because of the cost involved, it is desirable to use the refrigerant over and over. To accomplish this, additional equipment, over and above that already mentioned, is required. Vapor  Cycle  Theory Refrigerant  used  in  the  vapor  cycle  re- frigeration system occurs as both a liquid and as a vapor. Conversion from a liquid to a vapor will occur at temperatures above – 21°F ( – 34°C) at sea level. If the refrigerant pressure is increased, conversion  to  a  vapor  will  occur  at  higher temperatures.  Maximum  heat  transfer  efficiency occurs when the refrigerant is at the boiling point (the point at which the liquid will vaporize). The  refrigerant  must  be  delivered  to  the evaporator  as  a  liquid  if  it  is  to  absorb  large quantities  of  heat.  Since  it  leaves  the  evaporator in the form of a vapor, some way of condensing the vapor is necessary. To condense the refrigerant vapor, the heat surrendered by the vapor during condensation must be transferred to some other medium.  For  this  purpose,  water  or  air  is ordinarily  used.  The  water  or  air  must  be at  a  temperature  lower  than  the  condensing temperature  of  the  refrigerant.  At  any  given pressure,   the   condensing   and   vaporizing temperature  of  a  fluid  are  the  same.  If  a refrigerant that vaporizes at 40°F (5°C) is to be condensed at the same temperature, water or air at a lower temperature is needed. Obviously, if water  or  air  at  this  lower  temperature  were available,  mechanical  refrigeration  would  not  be required. As the temperature of available water or  air  is  usually  always  higher  than  the temperature  of  the  boiling  refrigerant  in  the evaporator,  the  refrigerant  must  be  condensed after it leaves the evaporator. To condense the vapor, its pressure must be increased to a point that its condensing temperature will be above the temperature   of   the   water   or   air   available for  condensing  purposes.  For  this  purpose  a compressor is needed. After the pressure of the refrigerant vapor has been increased sufficiently, it  may  be  liquefied  in  the  condenser  with comparatively warm water or air. In a practical refrigeration circuit, liquid flows from the receiver to the expansion valve, which is essentially nothing more than a needle valve. The compressor maintains a difference in pressure between   the   evaporator   and   the   condenser. Without the expansion valve, this difference in pressure could not be maintained. The expansion valve  separates  the  high-pressure  part  of  the system from the low-pressure part. It acts as a pressure reducing valve because the pressure of the liquid flowing through it is lowered. Only a small trickle of refrigerant fluid flows through the valve into the evaporator. The valve is always adjusted so that only the amount of liquid that can be vaporized in the evaporator passes through it. The  liquid  that  flows  through  the  evaporator is entirely vaporized by the heat flowing through the walls of the evaporator. This heat has been removed from the air being cooled. After leaving the evaporator, the vaporized refrigerant  flows  to  the  compressor  where  its pressure  is  raised  to  a  point  where  it  can  be condensed by the condenser airflow available. After being compressed, the vapor flows to the condenser. Here, the walls of the condenser are cooled by the water or air; and as a result, the vapor is liquefied. Heat is transferred from the condensing vapor to the water or air through the walls of the condenser. From the condenser the liquid refrigerant flows back to the receiver, and the cycle is then repeated. Operations  and  Components The  Grumman  Aerospace  Corporation  chose a Freon 12 vapor cycle ACS to provide avionics equipment cooling in the E-2 “Hawkeye” aircraft. This  system,  the  VEA6-1,  is  described  in  this section. The basic difference between the basic vapor cycle system and the VEA6-1 system is the method of compensating for the variations in ram air temperature and the variation in the flow of 3-17