c. 1. 2. 3. 4. 5. 2-70. AIR CONDITIONING SYSTEM. Manual  heating  mode. Bleed  air  valve  switches  -  OPEN,  LEFT  and RIGHT. Cabin   temperature   mode   selector   switch   - MAN  HEAT. Vent  blower  switches  -  As  required. Manual  temperature  switch  -  As  required. Cabin,   cockpit   and   defrost   air   knobs   -   As required. a. Description.  Cabin  air  conditioning  is  pro- vided  by  a  refrigerant  gas  vapor  cycle  refrigeration system  consisting  of  a  belt  driven,  engine  mounted compressor,  installed  on  the  No.2  engine  accessory pad, refrigerant plumbing, N₁   speed  switch,  high  and low   pressure   protection   switches,   condenser   coil, condenser-under-pressure   switch,   condenser   blower, forward  and  aft  evaporator,  receiver-dryer,  expan- sion  valve  and  a  bypass  valve.  The  plumbing  from the  compressor  is  routed  through  the  right  inboard wing  leading  edge  to  the  fuselage  and  then  forward to  the  condenser  coil,  receiver-dryer,  expansion valve,  bypass  valve,  and  forward  evaporator,  which are  located  in  the  nose  of  the  aircraft.  A  7  1/2- ampere   circuit   breaker   placarded   AIR   COND CONTR,  located  on  the  overhead  control  panel  (fig. 2-12),  protects  the  compressor  clutch  circuit. (1.)   Forward   evaporator.   The   forward evaporator  and  blower  supplies  the  cockpit,  forward ceiling   outlets,   and   forward   floor   outlets.   The   for- ward  evaporator  blower  has  a  high  speed  which  can be  selected  by  setting  the  VENT  BLOWER  switch, located  on  the  overhead  control  panel  (fig.  2-12),  to the  HI  position.  The  forward  vent  blower  is  pro- tected  by  a  circuit  breaker  on  the  DC  power  distri- bution  panel,  located  below  the  aisleway  floor  for- ward  of  the  main  spar. (2.) Aft evaporator.  The aft evaporator and blower  are  located  in  the  fuselage  center  aisle  equip- ment  bay  aft  of  the  rear  spar.  Environmental  air  is circulated   through   the   evaporator   in   either   manual or  automatic  control  mode.  The  rear  evaporator supplies  the  aft  ceiling  outlets,  rear  floor  outlets,  and toilet   compartment.   Rear   evaporator   blower   is   pro- tected  by  a  circuit  breaker  on  the  DC  power  distri- bution   panel   located   below   the   aisleway   floor   for- ward  of  the  main  spar. (3.) High  and  low  pressure  limit  switches. High  and  low  pressure  limit  switches  are  provided  to prevent   compressor   operation   beyond   operational limits.  When  the  low  or  high  pressure  switches  are activated,   compressor   operation   will   be   terminated. TM   55-1510-221-10 When  compressor  operation  has  been  terminated  by limit   switch   activation,   the   system   should   be   thor- oughly  checked  before  returning  it  to  service. (4.) Thermal sense switch. A thermal sense switch  is  installed  on  the  forward  evaporator.  This sense  switch  actuates  a  hot  gas  bypass  valve  which bypasses  a  portion  of  the  refrigerant  from  the  for- ward   evaporator,   thereby   preventing   icing   of   the evaporator. (5.)   Condenser   blower.  A   vane-axial blower  draws  air  through  the  condenser  on  the ground  as  well  as  in  flight.  The  current  limiter  for the  blower  is  located  on  the  DC  distribution  panel below  the  aisleway  floor  forward  of  the  main  spar. When  the  cabin  temperature  mode  selector  switch  is set  to  the  A/C  COLD  OPN  position,  the  condenser blower  will  be  off,  and  will  remain  off  until  the  con- denser  blower  control  high  pressure  switch  senses  a compressor  discharge  pressure  equal  to  the  pressure it  is  set  to.  The  condenser  blower  will  then  remain in  operation  until  the  low  pressure  switch  senses  that the  system  pressure  has  dropped  to  the  pressure  it  is set  to. (6.) Air conditioning cold operation bypass valve.  Selecting  the  A/C  COLD  OPN  mode  on  the cabin  temperature  mode  selector  switch  permits  the operation  of  the  air  conditioning  system  by  overrid- ing  the  refrigerant  low  pressure  switch.  This  allows the  air  conditioning  system  to  operate  in  the  manual mode.  Starting  the  compressor  in  this  optional  mode at  low  ambient  temperature  will  decrease  the  opera- tional  life  of  the  compressor  by  five  hours  each  time the   air   conditioning   system   is   started   using   this mode  (A/C  COLD  OPN).  If  the  air  conditioning  sys- tem  has  been  operating  in  the  normal  mode  during flight,   and   due   to   decreasing   ambient   temperatures make  it  necessary  to  switch  to  the  A/C  cold  opera- tion  mode,  there  will  be  no  degradation  in  the  mean time  between  failure  for  the  compressor. (7.) Air  conditioner  cold  operation  advisory annunciator   light.   A   green   advisory   annunciator light   placarded   A/C   COLD   OPN,   located   on   the caution/advisory  annunciator  panel  (fig.  2.6),  illumi- nates  when  the  air  conditioning  system  is  operating in  cold  mode,  or  when  ambient  temperatures  require switching   to   cold   mode   if   air   conditioning   system operation  is  to  be  continued. b.   Normal   Operation. (1.)  Automatic  cooling  mode. 1. Bleed  air  valve  switches  -  OPEN, LEFT  and  RIGHT. 2.   Cabin   temperature   mode   selector switch  -  AUTO. 2-55