within a few degrees of ambient air temperature. Further   temperature   reduction   results   from expansion  at  the  turbine.  Condensation  is removed  in  the  water  separator. System Operation Regulated  hot  bleed  air  from  the  bleed-air supply subsystem enters the refrigeration unit heat exchanger,  where  it  is  cooled  to  within  a  few degrees of ram-air temperature. The cooled high- pressure  bleed  air  enters  a  radial  flow  turbine, where  it  expands  to  approximately  cabin  pressure. The power output of the expansion turbine drives an  axial-flow  cooling  air  fan.  A  substantial temperature  drop  occurs  in  the  expansion  of  high- pressure air to cabin pressure (165 psi bleed-air to   15   psi   cabin   air),   which   results   in   air temperatures  well  below  ram-air  temperature. Depending upon the cool air temperature and dew point, a portion of the water vapor in the air condences  as  small  droplets.  A  water  separator is installed downstream from the turbine discharge to  remove  between  50  and  70  percent  of  the moisture in the cooled air. If the turbine discharge air is also below 32°F, the water vapor condenses as  ice  crystals.  Potential  icing  and  blockage  are eliminated  by  the  nonice  and  low-limit  control valve,  the  ice  screen,  and  the  mixing  muff.  The nonice  and  low-limit  control  valve  senses  any pressure  drop  through  the  ice  screen.  If  ice accumulates,  the  nonice  and  low-limit  control valve admits turbine bypass air into the mixing muff  to  increase  air  temperature  above  icing conditions. Ram cooling air for the heat exchanger flows through  the  heat  exchanger  core.  The  turbine shaft  drives  the  fan,  which  pulls  the  ram  air through  the  heat  exchanger  and  discharges  it overboard  through  the  heat  exchanger  exhaust duct. Components There  are  nine  basic  components  in  the refrigeration   subsystem.   Each   of   these components   is   discussed   in   the   following paragraphs. The relationship of the items is shown in  figure  4-6. TURBINE  AND  FAN  ASSEMBLY.—  The turbine  and  fan  assembly  (fig.  4-7),  which  is mounted  in  the  heat  exchanger  upper  plenum  (7), is  a  removable  component  of  the  refrigeration package.  High-pressure,  partially  cooled  bleed  air drives the turbine, which is mechanically coupled to an axial flow fan. The fan is used to impel ram air through the heat exchanger and an overboard exhaust duct. Pressure reduction and final heat loss occur as a result of energy loss and expan- sion   of   bleed   air   as   it   passes   through   the turbine. Wool wicks, with one end submerged in MIL- L-23699  oil,  transmit  lubricant  to  the  bear- ings supporting the common shaft of the turbine and  fan  assembly.  A  sight  gauge  on  the turbine   housing   is   used   to   check   the   oil level. Two overtemperature indicators are installed on  the  turbine.  Each  sensor  probe  head  holds down a spring-loaded pop-up stem with an eutec- tic solder alloy. If the air in the passage reaches the melting point for the solder alloy, the indicator head   pops   up   and   stays   exposed   to   alert maintenance  personnel  that  the  cooling  turbine has been exposed to an excessive temperature level and needs to be replaced. The probe in the turbine inlet  is  set  to  trip  at  217°±10°F,  and  the  probe in    the    fan    inlet    will    trip    at    450°±10°F. Obstructions  or  collapse  of  the  ram  air  inlet duct  is  the  most  likely  cause  of  actuating  this indicator. HEAT EXCHANGER CORE, UPPER AND LOWER PLENUMS.— The heat exchanger lower plenum (2) contains the ducting for the ram air inlet  and  outlet.  Cooling  ram  air  (4)  flows  into the lower plenum and through the heat exchanger core (3) and out through the overboard exhaust duct (6) to the upper plenum, or to the ram air augmentation  subsystem  (5). The  heat  exchanger  upper  plenum,  which  sup- ports the turbine and fan assembly (1), is mounted on the opposite side of the heater core. Ram air drawn  through  the  heater  core  for  cooling  pur- poses is diverted to the heat exchanger exhaust duct through the heat exchanger upper plenum. The  heat  exchanger  core  is  the  air-to-air  heat sink,  and  it  uses  ram  air  to  cool  the  bleed-air supply. NONICING AND LOW-LIMIT CONTROL MODULATING VALVE.— The nonice and low- limit control valve maintains conditioned airflow through the water separator by adding bleed air at  the  mixing  muff  to  prevent  water  separator 4-10