decrease in temperature. When the temperature reaches 35°F (0.2 in.Hg, absolute), dry air should be admitted through a chemical dehydrator into the system at a point farthest from the pump. Continue operating the pump so the  dry  air  will  mix  with  and  dilute  any  remaining moisture. Secure the opening that feeds the dry air into the  system.  Continue  evacuating  the  system  until  the indicator   again   shows   a   temperature   of   35°F.   The dehydration process is complete. Close the valves and disconnect the vacuum pump. Sometimes obtaining a temperature as low as 35°F in  the  vacuum  indicator  will  be  impossible.  The probable reasons for such a failure and the corrective procedures to take are as follows: l Excess moisture in the system. The dehydration procedure   should   be   conducted   for   longer periods. l   Absorbed   refrigerant   in   the   lubricating   oil contained in the compressor crankcase. Remove the  lubricating  oil  from  the  crankcase  before proceeding with the dehydration process. l   Leakage of air into the system. The leak must be found  and  stopped.  You  must  then  repeat  the procedure  required  for  detecting  leaks  in  the system. l   Inefficient   vacuum   or   defective   vacuum indicator.  The  defective  unit(s)  should  be repaired or replaced. Immediately  after  each  period  of  use  or  after  the system has been opened for repairs, replace the drying agent in the dehydrator. If a replacement cartridge is not available, reactivate the drying agent and use it until a replacement is available. You  can  reactivate  the  drying  agent  by  removing and heating it for 12 hours at a temperature of 300°F to bake out the moisture. Place the drying agent in an oven or  circulate  a  stream  of  hot  air  through  the  cartridge. Both   methods   are   satisfactory   for   reactivating commonly  used  dehydrating  agents  such  as  activated alumina or silica gel. The specific instructions furnished by  the  manufacturer  should  be  followed  to  reactivate special drying agents. After reactivation, replace the drying agent in the dehydrator shell and seal it as quickly as possible. This prevents absorption of atmospheric moisture. When the drying   agent   becomes   fouled   or   saturated   with lubricating  oil,  replace  it  with  a  fresh  charge,  or dehydrator cartridge, taken from a sealed container. Remember  that  the  dehydrators  permanently installed  in  refrigeration  systems  of  naval  ships  are designed   to   remove   only   the   minute   quantities   of moisture unavoidably introduced into the system. You must be careful to prevent moisture or moisture-laden air from entering the system. CLEANING  THE  SYSTEM Systems may accumulate dirt and scale as a result of improper techniques used during repair or installation of the system. If such dirt is excessive and a tank-type cleaner  is  available,  connect  the  cleaner  to  the compessor suction strainer. When such a cleaner is not available, a hard, wool felt filter about five-sixteenths inch thick should be inserted into the suction strainer screen. Run the plant with an operator in attendance for at least 36 hours or until the system is clean. The length of time required for a clean system depends upon the size and condition of the plant. AIR-CONDITIONING  SYSTEM Most of the information presented so far applies to the refrigeration side of a system, whether it is used for a  refrigeration  plant  or  for  air  conditioning.  The compressor controls for both types of systems are nearly identical;  however,  the  devices  used  to  control  space temperatures   differ,   The   two-position   dual   control, called 2PD, is used for the automatic control of most shipboard   air-conditioning   systems. TWO-POSITION DUAL CONTROL (2PD) This control is used on three types of systems: Type  1. Systems  employing  a  simple  thermo- statically  controlled  single-pole  switch to   control   flow   of   refrigerant   to   the cooling  coil Type  2. Systems  using  reheaters,  employing  a ther-   mostatic   element   actuating   two interlocked switches Type  3. Systems   using   reheaters   in   the   same manner as those in type 2, with control of humidity added where specified The type 1 system, because of its simplicity, requires little  explanation.  The  thermostat  consists  of  a temperature-sensing  element  actuating  a  single-pole, single-throw  switch.  It  opens  and  closes  a  magnetic valve  to  start  and  stop  the  flow  of  refrigerant-chilled water or commercial refrigerant. This type of control is 5-11