TM 1-1510-218-10 2-91 2-78.  AIR CONDITIONING. a.   Cabin    air    conditioning    is    provided    by    a refrigerant     gas     vapor     cycle     refrigeration     system consisting of a belt driven engine mounted compressor, installed on the #2 engine accessory pad, refrigerant  plumbing,  N₁  speed  switch,  high  and  low pressure protection switches, condenser coil, condenser blower, forward and aft evaporator, receiver dryer,   expansion   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.  The high and low-pressure limit switches, and N₁   engine   speed   switch,   are   provided   to   prevent compressor    operation    beyond    required    operational limits.      The   N₁   speed   switch   will   prevent   electrical power  from  being  delivered  to  the  compressor  clutch when  engine  RPM  is  below  65%  RPM.    When  the  N₁ speed   switch   is   open   and   there   is   a   demand   for refrigeration,   a   green   light   on   the   caution/advisory annunciator panel, AIR COND N₁ LOW, will illuminate. b.  The  system  incorporates  over-pressure  and under-pressure  protect  switches.    Activation  of  the over-pressure  or  under-pressure protect  switches  will discontinue  compressor  clutch  and  condenser  blower operation,  illuminate  its  respective  lockout  light/reset switch   located   in   the   nose   wheel   well,   and   trip   a 7.5-ampere   circuit   breaker,   placarded    AIR    COND CONTR,    located    in    the    overhead    circuit    breaker control panel.  When a system shutdown occurs due to overpressure or underpressure protect switch actuation,  the  system  should  be  thoroughly  checked before returning it to operation. c.   A  second  evaporator  and  blower  installation is  located  in  the  fuselage center aisle equipment bay aft  of  the  rear  spar.    Environmental  air  is  circulated through     each     evaporator     automatically     in     either manual   or   automatic   control   mode.      The   forward evaporator  blower  has  a  high  speed,  which  can  be selected    by    the    VENT    BLOWER    switch    on    the overhead control panel.  A 33 °F thermal sense switch is  installed  on  the  forward  evaporator.    This  sense switch  actuates  a  hot  gas  bypass  valve,  which  bleeds off    a    portion    of    the    refrigerant    from    the    forward evaporator, thereby preventing icing of the evaporator.   The   forward   evaporator   and   blower   will   supply   the cockpit,    forward    ceiling    outlets,    and    forward    floor outlets;  while  the  rear  evaporator  and  blower  supply the   aft   ceiling   outlets,   rear   floor   outlets   and   toilet compartment.  Both blower circuit breakers are located in    the    dc    power    distribution    panel    in    the    lower equipment bay.  A vane-axial blower draws air through the  condenser  on  the  ground.    A  50-ampere  circuit breaker for this blower is located on the dc distribution panel in the lower equipment bay. d.  When     operating     under     very     hot,     humid conditions, where maximum air conditioning is required, the bleed air valves may be left in   ENVIRO OFF until airborne and sufficient altitude is gained. 2-79.  UNPRESSURIZED VENTILATION. Ventilation   is   provided   by   two   sources.      One source is through the bleed air heating system in both the pressurized and unpressurized mode.  The second source of ventilation is obtained from RAM air through the  condenser  section  in  the  nose  through  a  check valve in the  vent  blower  plenum.    Ventilation  from  this source is in the unpressurized mode only with   CABIN PRESS switch in the DUMP position.  The check valve closes     during     pressurized     operation.          Ram     air ventilation   is   distributed   through   the   main   dueling system  to  all  outlets.    Ventilation  air  ducted  to  each individual  eyeball  cold  air  outlet  can  be  directionally controlled by moving the ball in the socket.  Volume is regulated  by  twisting  the  outlet  to  open  or  close  the valve. 2-80.  ENVIRONMENTAL CONTROLS. An environmental control section on the overhead    control    panel    provides    for    automatic    or manual control of the system.  This section contains all the    major    controls    of    the    environmental    function including   bleed   air   valve   switches,   a   vent   blower control   switch,   an   aft  vent  blower  switch,  a  manual temperature  switch  for  control  of  the  heat  exchanger valves,  a  cabin  temperature  level  control,  and  the cabin     temp     mode     selector     switch     for     selecting automatic   heating   or   cooling   or   manual   heating   or cooling.  Four additional manual controls on the main instrument    subpanels    may    be    utilized    for    partial regulation of cockpit comfort when the cockpit partition door    is    closed    and    the    cabin    comfort    level    is satisfactory. a.   Heating Mode. (1)   If the cockpit is too cold: 1.   PILOT and  COPILOT AIR knobs – As required. 2.   DEFROST AIR knob – As required. 3.   CABIN     AIR    –    Pull    out    in    small increments.    Allow  3–5  minutes  after each adjustment for system to stabilize.