CHAPTER 5 REFRIGERATION  AND  AIR  CONDITIONING As  an  EN3,  you  have  learned  the  principles  of refrigeration and air conditioning; the components and accessories that make up the system; and how to start, operate,  and  secure  refrigeration  and  air-conditioning plants.   As   an   EN2,   you   will   perform   routine maintenance   jobs,   such   as   cleaning,   lubricating, troubleshooting,  servicing  the  system,  using  correct procedures  for  leak  detecting,  and  charging  the refrigeration   and   air-conditioning   plants.   As   you advance in rate, you will be expected to have a greater knowledge of the construction and operating principles of  refrigeration  and  air-conditioning  plants.  You  will  be required  to  perform  more  complicated  maintenance jobs, to make repairs as required, and to determine the causes  of  inefficient  plant  operation  and  accomplish  the necessary corrective procedures. This chapter provides some   general   information   on   the   construction   and maintenance   of   refrigeration   and   air-conditioning equipment  and  the  detection  and  correction  of  operating difficulties. Refer  to  the  manufacturer’s  technical  manual  for details  of  the  plant  on  your  ship.  If  you  have  any questions about the basic theory of refrigeration and air conditioning, refer to EN3, chapters 16 and 17. R-12 REFRIGERATION SYSTEM We will present the R-12 system as though it had only   one   evaporator,   one   compressor,   and   one condenser.  A  refrigeration  system  may  (and  usually does)  include  more  than  one  evaporator,  and  it  may include an additional compressor and condenser units. COMPRESSORS Many different types and sizes of compressors are used  in  refrigeration  and  air-conditioning  systems.  They vary  from  the  small  hermetic  units  used  in  drinking fountains and refrigerators to the large centrifugal units used  for  air  conditioning. One of the most common compressors on modem ships is a high-speed unit with a variable capacity. This compressor  is  a  multicylinder,  reciprocating  design  with an  automatic  device  built  into  the  compressor  to  control its output. This automatic capacity control provides for continuous  compressor  operation  under  normal  load conditions.  The  capacity  of  the  compressor  is  controlled by unloading and loading the cylinders. This is a very desirable design feature of the unit. If the compressor had  to  be  started  under  a  load,  or  with  all  cylinders working,  a  much  greater  amount  of  torque  would  be required,  and  it  would  be  necessary  to  have  a  much larger  drive  motor.  Also,  if  the  compressor  ran  at constant  capacity  or  output,  it  would  reach  the low-temperature  or  low-pressure  limits  or  be  constantly starting and stopping, thereby putting excessive work on the unit. Unloading  of  the  cylinders  in  the  compressor  is accomplished by lifting the suction valves off their seats and holding them open. This method of capacity control unloads  the  cylinders  completely  but  still  allows  the compressor to work at as little as 25 percent of its rated capacity  . Unloader Mechanism When   the   compressor   is   not   in   operation,   the unloader  mechanism  is  in  the  unloaded  position  as shown in figure 5-1. The mechanism is operated by oil pressure  from  the  capacity  control  valve.  The  oil pressure pushes the unloader spring against the unloader piston. This action moves the unloader rod to the left, thereby  rotating  the  cam  rings.  As  the  cam  rings  are rotated, the lifting pins are forced upward, raising the suction valve off its seat. The suction valve is held in this  position  until  the  compressor  is  started  and  oil pressure of approximately 30 psi is reached. At that time, the oil pressure from the capacity control valve pushes the unloader piston back to the right against the unloader spring.  The  motion  transmitted  through  the  pushrod rotates  the  cam  ring.  This  lowers  the  lifting  pins  and allows the suction valve to close or operate normally and the  cylinder  to  become  loaded  (fig.  5-2).  On  most compressors the unloader is connected to the cylinders in pairs. Capacity  Control  Valve The capacity control valve (fig. 5-3) is located in the compressor crankcase cover. The valve is actuated by oil pressure from the main oil pump. It admits or relieves oil to or from the individual unloader power elements, 5-1