ENGINEMAN  1  &  C the reed in the neutral position, the fluid pressure is relieved to chamber C through the nozzles and a pressure drop allows the centering springs to return the spool valve to a central position; in this position, the valve blocks the pressure and return line, creating a hydraulic lock in chambers E and D.  By  energizing  the  left-hand  solenoid,  the magnetic reed will move to the left and the entire process  will  be  reversed. The position of the spool valve can be adjusted by using the centering screw. Fixed orifices are used so that the pressure drop in the hydraulic servovalve will not create a pressure drop in the opposite nozzle which is closed. Note that the ser- vovalve is basically a sliding spool valve. This type of valve has many other applications in hydraulic systems. For example, servovalves are used in the guidance systems of missiles and in the control systems of aircraft. DISTILLING PLANTS This  section  will  deal  with  the  operation, troubleshooting,  and  repairing  of  the  submerged tube and the flash type distilling plants that are used by the Navy. For additional and more de- tailed  information  than  is  provided  by  this  train- ing  manual,  consult  the  manufacturer’s  technical manual for the type of distilling plant installed on your ship. Distilling plants in naval ships are of three general types: (1) vapor compression, (2) low- pressure  steam,   and   (3)   heat   recovery.   The major differences between the three types are the kinds of energy used to operate the units and the pressure  under  which  distillation  takes  place. Vapor compression units use electrical energy (for heaters and compressors). Low-pressure steam distilling units use low-pressure steam from either the auxiliary exhaust steam systems or the aux- iliary steam system. Heat recovery distilling units use diesel engine jacket water instead of steam as the heat source. Vapor compression units boil the feedwater at a pressure slightly above atmospheric pressure.  Low-pressure  steam  and  heat  recovery units  depend  on  a  relatively  high  vacuum  for operation. Vapor compression type distilling units are used in submarines and small diesel-driven sur- face craft where the daily requirements do not exceed 4000 gallons per day (gpd). Since the vapor compression  type  found  on  surface  crafts  is being replaced with the heat recovery distilling units,  vapor  compression  distilling  units  will  not be covered in this manual. Chapter 531 (9580-II) of  Naval  Ships’  Technical  Manual contains  infor- mation on these plants. The low-pressure steam distilling unit is used in all steam-driven surface ships and nuclear sub- marines.  Enginemen  usually  share  responsibility with  Machinist’s  Mates  for  the  maintenance  and operation  of  the  low-pressure  steam  distilling plants. There are two reasons why low-pressure steam distilling units are considered “low pressure”: (1) they  use  low-pressure  steam  as  the  source  of energy, and (2) their operating shell pressure is less than atmospheric pressure. The  three  major  types  of  low-pressure  steam distilling units are submerged tube, flash type, and vertical  basket. In  this  section  of  the  chapter  we  will  be discussing only two of these distilling units—the submerged tube and the flash type. SUBMERGED TUBE PLANTS Low pressure, submerged tube plants differ from ship to ship, but the operating conditions and the maintenance procedures are basically the same. In almost all instances, the personnel who stand watches on distilling plants are also respon- sible for the maintenance of the plants. This gives them   ample   opportunity   to   detect   abnormal operating  conditions  before  such  conditions  reach advanced  stages.  When  operating  troubles  do occur, it is the responsibility of the EN1 or ENC on duty to locate the trouble and to make the necessary adjustments or repairs. Steady  operating  conditions  are  essential  for satisfactory  results.  Except  under  emergency  con- ditions, no plant should be forced beyond its rated capacity,  because  higher  steam  pressures  will  be required and the resulting higher temperatures will cause more rapid scaling of the evaporator tubes. During operation, the various elements of any plant are interdependent due to the heat and fluid balances throughout the plant. Adjustment of any one control can produce widespread effects on these balances. For example, an increase in the 7-22