The other type of system in current use has its own compressor and other equipment necessary for maintaining  an  adequate  supply  of  compressed  air during  flight.  Provision  for  ground  charging  this  type of  system  is  also  provided.  In  addition  to  a compressor,  the  components  in  this  type  of  system usually   include   a   filter,   a   pressure   regulator,   a moisture separator, a relief valve, a chemical drier, and  storage  cylinder(s). AIR   COMPRESSORS.—A   typical   air   com- pressor is shown in figure 7-43. An installation of this type  receives  its  supply  of  air  from  the  compressor section   of   the   aircraft   engine. This  air  is  then compressed   further   to   the   required   pressure   for operating  the  system.    Compressors of this type are capable  of  maintaining  up  to  and  above  3,000  psi pressure during flight. On some aircraft, the compressor is operated by an  electric  motor.  On  others,  a  hydraulic  motor  is used to drive the compressor. Compressors must be serviced  with  oil  periodically,  as  outlined  in  the aircraft MIM. An oil level sight gauge is provided on the compressor (fig. 7-43). AIR FILTERS.—An air filter is usually located in  the  line  leading  into  the  system  compressor. Additional filters may be located at various points in the system lines to remove any foreign matter that may enter the system. Figure 7-43.—Air compressor. Like hydraulic filters, air filters have a removable element  and  a  built-in  relief  valve.  The  relief  valve  is designed to open and bypass the air supply around the filter  element  should  the  element  become  clogged. Some air filters are equipped with the micronic-type element,  which  must  be  replaced  periodically.  Others have the screen mesh type, which requires periodic cleaning.  The  latter  type  may  be  reinstalled  after cleaning and drying. AIR  PRESSURE  REGULATORS.—A  pressure regulator  is  generally  located  in  the  line  between  the engine   compressor   and   the   pneumatic   system compressor; however, it may be incorporated within the  system  moisture  separator.  Its  purpose  is  to regulate the pressure of the supply air before it enters the  system  compressor.  The  pressure  regulator maintains a stable outlet pressure regardless of the inlet  pressure. MOISTURE   SEPARATORS.—The   moisture separator  in  a  pneumatic  system  is  always  located downstream  of  the  compressor.  Its  purpose  is  to remove  any  moisture  caused  by  the  compressor.  A complete  moisture  separator  consists  of  a  reservoir,  a pressure switch, a dump valve, and a check valve, and it may also include a regulator and a relief valve. The dump  valve  is  energized  and  de-energized  by  the pressure  switch.  When  de-energized,  it  completely purges  the  separator  reservoir  and  lines  up  to  the compressor. The  check  valve  protects  the  system against pressure loss during the dumping cycle and prevents reverse flow through the separator. RELIEF     VALVES.—A     relief     valve     is incorporated  in  a  pneumatic  system  to  protect  the system from overpressurization. Overpressurization is  generally  caused  by  thermal  expansion  (heat). Relief  valves  are  generally  adjusted  to  open  and  close at pressures slightly above normal system operating pressure. For  example,  in  a  system  designed  to operate at 3,000 psi, the relief valve might be set to open at 3,750 psi and reseat at 3,250 psi. CHEMICAL  DRIERS.—Chemical  driers  are incorporated  at  various  locations  in  a  pneumatic system.  Their  purpose  is  to  absorb  any  moisture  that may collect in the lines and other parts of the system. Each drier contains a cartridge, which should be blue in color. If otherwise noted, the cartridge is to be considered contaminated with moisture and should be replaced. STORAGE CYLINDERS.—Pneumatic storage cylinders (bottles) are made of steel and maybe either 7-44