CHAPTER 2 DRY AIR SYSTEMS For  optimum  performance  of  today’s  transmitting equipment, especially high-power radar systems and low-power satellite systems, some rigid coaxial cable and waveguides are required to be pressurized by air. In some  waveguide  systems,  dry  air  is  used  primarily  to increase  the  dielectric  constant  inside  the  waveguide  to prevent  rf  energy  from  arcing  inside  it.  Arcing  causes damage to the inside of the waveguide, and it also reflects a short circuit back to the power amplifier tube. As a result, the power tube could sustain major damage. Also with the use of pressurized dry air, the problems of corrosion,  contamination,  collection  of  moisture,  and  oil droplets  (which  affect  preservation)  are  decreased.  At the same time, the overall reliability of the waveguide system  is  increased. In high-power waveguide runs, the dry air pressure is  approximately  20  to  35  psig.  The  increased  air pressure  increases  the  dielectric  (resistance)  strength  of the air. Arcing is then less likely to take place. In  low-power  waveguide  applications,  the  dry  air  is approximately 1 to 8 psig. The dry air is used primarily to  prevent  corrosion  and  contamination  inside  the waveguide.  These  effects  are  caused  mainly  by moisture  in  the  waveguide. The number of equipments requiring dry air for operation has increased drastically in recent years. Central dry air systems have been installed in many ships   to   overcome   the   problems   of   individual maintenance,  repair,  and  supply  support  required  by individual air dehydrators. There are, however, a large number of individual equipment dehydrators still in use on many ships. They are now being used as a back-up systems should there be a failure in the ship’s central air system. ELECTRONICS DRY AIR On ships having multiple dry air users, a dedicated dry air main is installed to support clean, oil free, dry air to pressurized coaxial cables, waveguides, and other electronic  equipment.  Supply  to  this  main  is  from  the vital main by way of type II or type III dehydrators installed in parallel so that one serves as a 100 percent stand-by for the other. In large ships with extensive air demands, four dehydrators are installed and the air main can  be  split  for  casualty  control.  The  dry  air  main terminates at air control panels, which control and regulate pressure to the electronic user equipment. Four types  of  NAVSEA  air  control  panels  (type  I  [user pressure to 30 type II [user pressure to 60 type III [user pressure of 75 and special [usually used  where  pressure  flowrate  is  unused])  are  available. Type I Panels: Typical users URA-38, WRT-1 and WRT-2, SPS-39, ULQ-6, WRL-1, SPS-40 waveguides. Type II Panels: Typical users SPS-40 cavity, SPG-51  and  SPG-60. Type III Panels: Typical users require air at 75 to equipment contained regulators such as SPG-55. Special:  Typical  users  SPS-32  and  SPS-33, SMQ-10. In  addition,  equipment,  such  as  SPS-48  and SPS-49, are supplied with panels designed for 80 to 125 Each  panel  is  equipped  with  a  sampling connection, humidity indicator, flow meter, pressure gauges, and associated valves, to permit the user to monitor  the  equipment. To ensure the reliability of the dry air supply to the electronic   equipment,   local   dehydrators   or   local compressor-dehydrators  may  be  provided.  These  local dehydrators are intended for emergency use when battle damage or casualties result in failure of the central supply  system. Several  methods  can  be  employed  to  remove  excess moisture from the air. One method is by freezing the air by means of a refrigerant to remove the moisture. A second  method  is  to  pass  the  air  through  a  desiccant, which  absorbs  the  moisture.  Some  dehydrators  use  a combination of both methods to remove moisture from the air. 2-1