earth’s atmosphere also contains varying amounts of water. Depending on weather conditions, water will appear in a variety of forms, such as rain (liquid water), snow crystals, ice (solid water), and vapor. Vapor is composed of tiny drops of water that are light enough to stay airborne. Clouds are an example of the existence of water vapor. Since  air  is  a  gas,  it  expands  when  heated. Consequently, heating air causes a given amount of air2routed  through  to  expand,  take  up  more  space (volume), and hold more water vapor. When a given amount of air at a given temperature and pressure is no longer able to soak up water vapor, the air is saturated, and the humidity is 100 percent. When  air  cools,  its  density  increases;  however,  its volume  and  ability  to  hold  water  decrease,  When temperature and pressure conditions cause the air to cool and to reach the dew point, any water vapor in the air condenses into a liquid state (water). In other words, one method of drying air out is to cool it until it reaches the dew  point. Besides  nitrogen,  oxygen,  and  water  vapor,  air contains particles of dust and dirt that are so tiny and lightweight that they remain suspended in the air. You may wonder how the composition of air directly affects the work of an air compressor. Although one cubic foot of air will not hold a tremendous amount of water or dirt, you should realize that air compressors have capacities that are rated in hundreds of standard cubic feet per minute (cfm). This is a very high rate of flow. When a high flow rate of dirty, moisture-laden air is allowed to enter and pass through an air compressor, the result is rapid  wear  of  the  seals  and  load-bearing  parts,  internal corrosion  (rust),  and  early  failure  of  the  unit.  The reliability  and  useful  life  of  any  air  compressor  is extended by the installation of filters. Filters remove most of the dirt and dust from the air before it enters the equipment. On the other hand, most of the water vapor in the air at the intake passes directly through the filter material and is compressed with the air. When air is compressed, it becomes very hot. As you know, hot air is capable of holding great amounts of water. The water is removed as the compressed air is routed through the coolers. The coolers remove the heat from the airstream and cause some of the water vapor to condense into liquid   (condensate).   The   condensate   must   be periodically  drained  from  the  compressor. Although the coolers will remove some of the water from  the  air,  simple  cooling  between  the  stages  of compression (intercooling) and cooling of the airstream after it leaves the compressor (aftercooling) will not make the air dry. pneumatic control required  air  from When  clean  dry  air  suitable  for and other shipboard systems are the compressor is routed through air-drying  units.  Many  air-drying  units  are  capable  of removing enough water vapor from the airstream to cause the dew point to be as low as -60°F. Some of the more common devices used to remove water vapor from the airstream, such as dehydrators, are discussed later in this  chapter. CLASSIFICATION OF AIR COMPRESSORS An air compressor may be classified according to pressure (low, medium, or high), type of compressing element, and whether the discharged air is oil free. Because  of  our  increasing  need  for  oil-free  air aboard ship, the oil-free air compressor is gradually replacing  most  of  the  standard  low-pressure  and high-pressure air compressors. For this reason, most of this discussion is focused on the features of oil-free air compressors. The  Naval  Ships’  Technical  Manual  (NSTM), chapter 551, lists compressors in three classifications: 1. Low-pressure air compressors (LPACs), which have a discharge pressure of 150 psi or less 2.  Medium-pressure  compressors,  which  have  a discharge pressure of 151 psi to 1,000 psi 3.  High-pressure  air  compressors  (HPACs),  which have a discharge pressure above 1,000 psi Low-Pressure  or  Ship’s  Service  Air Compressors The two types of LPACs that are used on naval ships are the screw type and the reciprocating type. SCREW  TYPE.–  The  helical-screw  type  of compressor is a relatively new design of oil-free air compressor.  This  low-pressure  air  compressor  is  a single-stage,    positive-displacement,    axial-flow, helical-screw type of compressor. It is often referred to as a screw-type compressor. Figure 10-23 shows the general  arrangement  of  the  LPAC  unit. In the screw-type LPAC, compression is caused by the meshing of two helical rotors (a male and a female rotor, as shown in fig. 10-24) located on parallel shafts and enclosed in a casing. Air inlet and outlet ports are located on opposite sides of the casing. Atmospheric air is   drawn   into   the   compressor   through   the filter-silencer.   The   air   passes   through   the   air cylinder-operated unloader (butterfly) valve and into the 10-25