Cool air to cabin Warm air from cabin Low-pressure liquid Blower High side Low side Evaporator Receiver dryer High-pressure/high-temperature vapor Low-pressure/low-temperature vapor Thermal expansion valve High-pressure liquid Condenser Compressor Return air to outside  of aircraft Ambient ram air from outside of aircraft Figure 16-74.  A basic vapor cycle air conditioning system. The compressor and the expansion valve are the two components that separate  the low side from the high side of the cycle. This figure illustrates this division. Refrigerant on the low side is characterized as having  low pressure and temperature. Refrigerant on the high side has high pressure and temperature. Vapor Cycle Air Conditioning System Components Refrigerant leaks and or malfunction. Use only the specified refrigerant  when servicing vapor cycle air conditioning systems.  [Figure 16-75] R12 and R134a behave so similarly that the  descriptions of the R134a vapor cycle air conditioning system  and components in the following paragraphs also apply to an  R12 system and its components.  By examining each component in the vapor cycle air  conditioning system, greater insight into its function can  be gained. For many years, dichlorodifluoromethane (R12) was  the standard refrigerant used in aircraft vapor cycle air  conditioning systems. Some of these systems remain in  use today. R12 was found to have a negative effect on  the environment; in particular, it degraded the earth’s  protective ozone layer. In most cases, it has been replaced by  tetrafluoroethane (R134a), which is safer for the environment.  R12 and R134a should not be mixed, nor should one be used  in a system designed for the other. Possible damage to soft  components, such as hoses and seals, could result causing  R134a is a halogen compound (CF3CFH2). As mentioned,  it has a boiling point of approximately –15 °F. It is not  poisonous to inhale in small quantities, but it does displace  oxygen. Suffocation is possible if breathed in mass quantity. Regardless of manufacturer, refrigerants are sometimes  called Freon$^{®}$, which is a trade name owned by the Dupont  Company. Caution should be used when handling any  refrigerant. Because of the low boiling points, liquid  16-46