Intercoolers When air is compressed, heat is generated. This heat causes the air to expand, thus requiring an increase in power for further compression. If this heat is successfully removed  between  stages  of  compression,  the  total  power required for additional compression may be reduced by as  much  as  15  percent.  In  multistage  reciprocating compressors,  this  heat  is  removed  by  means  of intercoolers  that  are  heat  exchangers  placed  between each  compression  stage.  Rotary  air  compressors  are cooled by oil and do not use intercoolers. Aftercoolers It is obvious that the presence of water or moisture in an air line is not desirable. The water is carried along through  the  line  into  the  tool  where  the  water  washes away  the  lubricating  oil,  causing  the  tool  to  run sluggishly  and  increases  maintenance.  The  effect  is particularly pronounced in the case of high-speed tools where  the  wearing  surfaces  are  limited  in  size  and excessive wear reduces efficiency by creating internal air leakage. Further  problems  may  result  from  the  decrease  in temperature caused by the sudden expansion of air at the tool.  This  low  temperature  creates  condensation  that freezes   around   the   valves,   ports,   and   outlets.   This condition  obviously  impairs  the  operational  efficiency  of the tool and cannot be allowed. The  most  satisfactory  means  of  minimizing  these conditions is the removal of the moisture from the air immediately after compression and before the air enters the  distribution  system.  This  may  be  accomplished  in reciprocating   compressors   through   the   use   of   an aftercooler  that  is  an  air  radiator  that  transfers  heat  from the  compressed  air  to  the  atmosphere.  The  aftercooler reduces  the  temperature  of  the  compressed  air  to  the condensation   point   where   most   of   the   moisture   is removed.  Cooling  the  air  not  only  eliminates  the difficulties which moisture causes at points where air is used  but  also  ensures  better  distribution. Receiver  Tank The receiver tank is of welded steel construction and is installed on the discharge side of the compressor. It acts as a surge tank as well as a condensation chamber for the removal  of  oil  and  water  vapors.  It  stores  enough  air during operation to actuate the pressure control system and is fitted with at least one service valve, a drain or blow-by valve, and a safety valve. Pressure-Control  System All  portable  air  compressors  are  governed  by  a pressure-control   system.   The   control   system   is designed  to  balance  the  compressor's  air  delivery  and engine speed with varied demands for compressed air. In a reciprocating compressor the pressure-control system causes the engine to idle and the suction valves to   remain   open   when   the   pressure   reaches   a   set maximum,   thus   making   the   compressor   unit inoperative.  When  the  air  pressure  drops  below  a  set minimum,  the  pressure-control  unit  causes  the  engine to   increase   speed   and   the   suction   valves   to   close, thereby resuming the com pression cycle. The  rotary  compressor  output  is  governed  by varying  the  engine  speed.  The  engine  will  operate  at the speed required to compress enough air to supply the demand at a fairly constant pressure. When the engine has slowed to idling speed as a result of low demand, a valve controls the amount of free air that may enter the compressor. A screw compressor output is governed by automatic control   that   provides   smooth,   stepless   capacity regulation from full load to no load in response to the demand  for  air.  From  a  full  load  down  to  no  load  is accomplished  by  a  floating-speed  engine  control  in combination  with  the  variable-inlet  compressor. AIR  COMPRESSOR  MAINTENANCE A  number  of  built-in  features  that  make  portable compressors  easy  to  maintain  include: an automatic blowdown valve for releasing air pressure when the engine is stopped, a valve for draining moisture that accumulates in the receiver tank, a drain cock at the bottom of the piping at the bottom of the oil storage tank, an air filter service indicator to show when the filter needs servicing, and a  demister,  or  special  filter,  that  separates lubricating oil from compressed air. Remember a good maintenance program is the key to a long machine life. So it is up to both the operator and  the  mechanic  to  ensure  that  the  maintenance  is performed on time every time. 3-33