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Figure  14-9.Auger  boring  head  nomenclature.
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Equipment Operator Basic - Beginning construction equipment operators manual
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the NCF, are governed by a pressure control system, adjusted to compress air to a maximum pressure of 100 pounds per square inch (psi). Compressor units are available in reciprocating, rotary, or screw design. Compressors are classified as either  single-stage  or multistage.  A  single-stage  compressor  has  one compressing  element  that  compresses  the  air  from  the initial intake pressure to the final discharge pressure in one step.  The  multistage  compressor  has  more  than  one compressing element. The first stage compresses the air to  an  intermediate  pressure,  then  through  one  or  more additional  stages  to  final  discharge  pressure.  The multistage  system  is  more  efficient  than  the  single-stage system, because the air cooling that occurs between stages reduces buildup of pressure due to a temperature rise. All  military  construction  compressors  are  governed by  a  pressure  control  system.  In  a  reciprocating compressor, this control system causes the engine to idle  and  the  suction  valve  to  remain  open  when  the pressure reaches a set maximum. When the air pressure drops below a set minimum, the pressure control system causes the engine to increase speed and the suction valve to  close,  starting  the  compression  cycle  again.  The rotary compressor output is governed by varying the engine  speed.  The  engine  operates  automatically  at  the speed  needed  to  compress  enough  air  to  supply  the demand at a fairly constant pressure. When the engine has  slowed  to  idle  because  of  low  demand,  a  valve throttles  the  amount  of  free  air  that  may  enter  the compressor. The  screw   compressor   output   is   controlled automatically and provides a smooth, uninterrupted capacity from full load to no load in response to the demand  for  air.  This  capacity  is  achieved  by  a  floating speed  engine  control  and  a  variable  inlet  compressor. COMPRESSOR  CAPACITY The capacity of an air compressor is determined by the amount of free air (at sea level) that it can compress to a specified pressure, usually 100 psi per minute, under the conditions of 68°F and a relative humidity of 38 percent. This capacity is expressed in cubic feet per minute  (cfm)  and  is  usually  included  in  the nomenclature  of  the  compressor. The number of pneumatic tools that can be operated at one time from an air compressor depends on the air requirements of each tool; for example, a 55-pound class rock drill requires 95 cfm of air at 80 psi. A 210-cfm compressor can supply air to operate two of the drills, because  their  combined  requirements  is  190  cfm. However, if a third such drill is added to the compressor, the combined demand is 285 cfm, and this condition overloads  the  compressor  and  the  tools  and  results  in serious   wear. NOTE: When the pressure and volume of air to a pneumatic tool drops 10 percent below the designed minimum, the tool efficiency is reduced 41 percent. Compressor  Location Install the compressor unit so it is as close to level as possible. Compressor design permits a 15-degree lengthwise   and   a   15-degree   sidewise   limit   on out-of-level operation. The engine, not the compressor, is the limiting factor. When the unit is to be operated out of level, you should be sure to do the following: 1. Keep the engine crankcase oil level on the full mark with the unit level. 2. Ensure the compressor oil gauge shows full with the  unit  level. Compressed Air System A compressed air system consists of one or more compressors, each with the necessary power source, regulation, intake air filter, aftercooler, air receiver, connecting piping, and a distribution system to carry the air  to  points  of  use. The  object  of  installing  a compressed air system is to provide enough air at the work  area  at  pressures  adequate  for  efficient  operation of pneumatic tools. Many construction jobs require more cfm than one compressor can produce. Also, terrain conditions often create problems of distance from the compressor to the operating tool. Since the air line hose causes a loss of pressure (friction loss) at distances farther than 200 feet, a system has been devised for efficient transmission of compressed air over longer distances. This system is air manifolding (fig. 14- 10). Figure  14-10.—Methods  of  manifolding  compressors. 14-8







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