Several  designs  of  capacity  control  systems  are  in use. One of the most common is shown in figure 10-6. The capacity control system consists of a power element and its link for each controlled cylinder, a step control hydraulic relay, and a capacity control valve. The  system’s  components  are  all  integrally  attached to the compressor. The suction or crankcase pressure of the refrigeration plant is sensed by the capacity control valve to control the system. In other words, a change in the refrigeration load on the plant will cause a change in suction  pressure.  This  change  in  suction  pressure  will then   cause   the   capacity   control   system   to   react according to whether the suction pressure increased or decreased.  The  working  fluid  of  the  system  is compressor oil pump pressure. Compressor oil pump pressure is metered into the system through an orifice. Once the oil passes the orifice, it becomes the system control oil and does work. Locate the following components on figure 10-6, and refer to them as you read the next two paragraphs. (A) (B) (C) (D) (E) (M) (G) (H) (O) (J) (K) Compressor  oil  pump  pressure  tap-off Control  oil  strainer Hydraulic  relay Hydraulic relay piston Unloader  power  element Unloader  power  element  piston Lifting  fork Unloader  sleeve Suction  valve Capacity  control  valve Crankcase (suction) pressure sensing point The  following  functions  take  place  when  the compressor  is  started  with  a  warm  load  on  the refrigeration  system. Compressor  oil  (A)  is  pumped  through  the  control oil strainer (B) into the hydraulic relay (C). There the oil flow to the unloader power elements is controlled in steps by the movement of the hydraulic relay piston (D). As soon as pump oil pressure reaches a power element (E), the piston (F) rises, the lifting fork (G) pivots, and the  unloader  sleeve  (H)  lowers,  permitting  the  suction valve (1) to seat. The system is governed by suction pressure, which actuates the capacity control valve (J). This valve controls the movement of the hydraulic relay piston by metering the oil bleed from the control oil side of the hydraulic relay back to the crankcase. Suction  pressure  increases  or  decreases  according to  increases  or  decreases  in  the  refrigeration  load requirements  of  the  plant.  After  the  temperature pulldown period with a subsequent decrease in suction pressure,  the  capacity  control  valve  moves  to  increase the control oil bleed to the crankcase from the hydraulic relay. There is a resulting decrease in control oil pressure within the hydraulic relay. This decrease allows the piston  to  be  moved  by  spring  action.  This  action successively  closes  oil  ports  and  prevents  compressor oil pump pressure from reaching the unloader power elements.  As  oil  pressure  leaves  a  power  element,  the suction valve rises and that cylinder unloads. With an increase in suction pressure, this process is reversed, and the controlled cylinders will load in succession. The loading process is detailed in steps 1 through 7 in figure 10-6. Condenser The  compressor  discharges  the  high-pressure, high-temperature  refrigerant  vapor  to  the  condenser, where it flows around the tubes through which seawater is being pumped. As the vapor gives up its superheat (sensible heat) to the seawater, the temperature of the vapor  drops  to  the  condensing  point.  The  refrigerant, now  in  liquid  form,  is  subcooled  slightly  below  its condensing point. This is done at the existing pressure to ensure that it will not flash into vapor. A  water-cooled  condenser  for  an  R-  12  refrigeration system is shown in figure 10-7. Circulating water is obtained through a branch connection from the fire main or by means of an individual pump taking suction from the  sea.  The  purge  connection  (fig.  10-7)  is  on  the refrigerant side. It is used to remove air and other noncondensable gases that are lighter than the R-12 vapor. Most  condensers  used  for  naval  refrigeration  plants are of the water-cooled type. However, some small units have air-cooled condensers. These consist of tubing with external fins to increase the heat transfer surface. Most  air-cooled  condensers  have  fans  to  ensure  positive circulation  of  air  around  the  condenser  tubes. Receiver The receiver (fig. 10-8) acts as a temporary storage space and surge tank for the liquid refrigerant. The receiver also serves as a vapor seal to keep vapor out of the liquid line to the expansion valve. Receivers are constructed for either horizontal or vertical installation. 10-8