CENTRAL  DRY-AIR  SYSTEM and  liquid.  The  compressor  compresses  the  air  into  a The ship’s central dry-air system is usually located in one of the ship’s main engineering spaces and can be composed  of  a  low-pressure  (100  psig)  air  compressor, a  Type  I  dehydrator,  and  either  a  Type  II  or  III dehydrator. The air compressor compresses the air and then sends it to the Type I dehydrator (refrigerant). The Type I dehydrator is used to remove the majority of the water and oil in both liquid and gaseous vapor forms from the air. Next, the air is processed by a Type II (desiccant)  or  a  Type  III  (combination  of  refrigeration and  desiccant)  dehydrator  to  remove  the  last  traces  of moisture. This last bit of processing causes the air to become  electronic  dry. smaller volume causing the relative humidity of the air to increase. When the compressed air is saturated (100 percent humidity), it no longer holds any more vapor. Further  compression  causes  the  formation  of  liquid  to begin.  Also,  since  most  compressors  use  some  form  of lubricant, it too can get into the compressed air as a vapor or liquid. The ship’s dehydrators can remove all contamination,  both  liquid  and  gaseous  vapor.  In  the event of a failure in the central dry air system, the small air dryer (if installed with electronic equipment) can be placed  into  operation. EQUIPMENT DRY AIR DRY-AIR SYSTEMS If  you  are  involved  with  an  equipment  dry-air system,  it  is  essential  that  you  understand  how  to check  the  air  for  sufficient  dryness.  A  high-power waveguide system requires dry air at a pressure of 25 to 35  psig  with  a  dew  point  of  -40°F  at  atmospheric pressure. Dew point is the temperature at which water vapor begins  to  deposit  as  a  liquid  (at  atmospheric  pressure unless  otherwise  stated).  For  example,  when  the  dew point  is  given  as  40°F,  this  means  that  the  excess moisture  in  the  air  will  begin  to  condense  at  this temperature.  Condensation  appears  as  a  fog  or,  if enough moisture is condensed, as ice crystals. The dew point  is  affected  by  the  pressure  of  the  air  being measured.  Air  at  40°F  dew  point  (atmospheric pressure) contains approximately 120 parts of water per million parts of air (ppm). However, this same air at 30 psig has a dew point of about –21°F. As you can see, you have to take into account the effects of pressure when  you  measure  the  dew  point.  The  lower  the dew-point  reading  (more  negative/colder),  the  better  the air  quality. The equipment air dryers installed in electronic equipment  are  desiccant  dryers.  The  air  is  passed through  a  desiccant,  which  adsorbs  the  moisture.  The more moisture the desiccant adsorbs, the dryer (lower dew point) the air becomes. You should know the basic operation of a desiccant air dryer so that you can perform PM  and  trouble  isolation. SHIP’S  AIR  SUPPLY Air   from   the   ship’s   compressor   is   often contaminated  with  water  and  oil  in  the  form  of  vapor The purpose of the desiccant air dryer (fig. 2-1) is to remove oil and water in both forms, automatically and continuously,  and  to  deliver  clean,  very  dry  air  for pressurization of equipment waveguides. The air dryer is normally in the bypass mode. In this mode, input air is  routed  down  through  the  selection  valve  and  out  of the dryer by way of the flow limiter. When placing the selection valve so that the dryer will process the air, you must  first  follow  the  dryer’s  start-up  procedure. This procedure ensures that wet air doesn’t get into the  waveguide  systems  when  the  dryer  has  been shutdown  for  an  extended  period.  Basically,  the procedure  involves  turning  the  electrical  power  on  for the dryer and opening/closing of various valves to dump the air to the atmosphere. This is done until the dryer’s air monitor indicates that the dew point is below 40°F at atmospheric pressure. The operation of the air dryer is as follows: the input air  (fig.  2-1)  (selection  valve  is  indicated)  is  first checked on the pressure gauge for minimum air pressure (typically  80  psig).  Input  pressure  that  is  too  low inhibits the operation of the fluid separator. The  fluid  separator  extracts  most  of  the  free liquids  from  the  airstream.  Some  oil  and  water  mist, however, (extremely fine droplets) remain in the stream. The liquid, thus collected, drains into the dump trap below  the  separator. When a sufficient amount has collected,  the  trap  automatically  opens  to  discharge  the collected  liquids  under  pressure  into  a  waste  line  for disposal. 2-2