NUMBER OF FURNACES CONTROL  OF  SUPERHEAT All boilers commonly used in the propulsion A   boiler   that   provides   some   means   of plants of naval ships may be classified as either controlling  the  degree  of  superheat  independently SINGLE-FURNACE   BOILERS   or   DOUBLE- of  the  rate  of  steam  generation  is  said  to  have FURNACE  BOILERS.  The  D-type  boiler  is  a CONTROLLED   SUPERHEAT.   A   boiler   in single-furnace  boiler;  the  M-type  boiler  is  a which  such  separate  control  is  not  possible  is  said double-furnace   (divided-furnace)   boiler. to  have  UNCONTROLLED  SUPERHEAT. Normally,  the  term  superheat  control  boiler BURNER  LOCATION is  used  to  identify  a  double-furnace  boiler.  The term  uncontrolled   superheat   boiler   is  used  to Naval boilers are also classified on the basis identify  a  single-furnace  boiler. of where their burners are located. Most burners in  naval  propulsion  plants  are  located  at  the  front of  the  boiler.  These  are  called  FRONT-FIRED OPERATING  PRESSURE BOILERS. Other ships, such as the AO-177 and LKA-113  class  ships,  have  their  burners  on  the top  of  the  boilers.  These  are  called  TOP-FIRED BOILERS. FURNACE PRESSURE Another   convenient   boiler   classification   is based on the air pressure used in the furnace. Most boilers in use in naval propulsion plants operate with a slight air pressure (seldom over 5 psig) in the  boiler  furnace.  This  slight  pressure  is  not enough  to  justify  calling  these  boilers  pressurized- furnace boilers. However, some boilers installed on   naval   ships   are   truly   pressurized-furnace boilers.  They  are  called  PRESSURE-FIRED  or SUPERCHARGED   BOILERS.   These   furnaces are  maintained  under  a  positive  air  pressure  of about  65  psia  (about  50  psig)  when  operated  at full  power.  The  air  pressure  in  these  boiler furnaces  is  maintained  by  special  air  compressors called superchargers. TYPE OF SUPERHEATERS On almost all boilers used in the propulsion plants of naval ships, the superheater tubes are protected from radiant heat by water screen tubes. The water screen tubes absorb the intense radiant heat of the furnace, and the superheater tubes are heated  by  convection  currents  rather  than  by direct  radiation.  These  superheaters  are  called CONVECTION-TYPE    SUPERHEATERS. In a few older ships, the superheater tubes are not screened by water screen tubes but are exposed directly  to  the  radiant  heat  of  the  furnace. Superheaters  of  this  design  are  called  RADIANT- TYPE   SUPERHEATERS. For some purposes, it is convenient to classify boilers   according   to   operating   pressure.   Most classification of this type are approximate rather than  exact.  Header-type  boilers  and  some  older drum-type   boilers   are   often   called   400-PSI BOILERS even though their operating pressures range  from  about  435  psi  to  700  psi. The term high-pressure boiler is at present used rather loosely to identify any boiler that operates at  a  substantially  higher  pressure  than  the  so- called  600-PSI  BOILERS.  In  general,  we  will  con- sider any boiler that operates at 751 psi or above as a high-pressure boiler. Many boilers in naval ships  operate  at  about  1200  psi.  These  boilers  are referred  to  as  1200-PSI  BOILERS. As you can see, classifying boilers by operating pressure  is  not  very  precise  since  actual  operating pressure may vary widely within any one group. Also,   any   classification   based   on   operating pressure  may  easily  become  obsolete.  What  is  called a  high-pressure  boiler  today  may  well  be  called a  low-pressure  boiler  tomorrow. BOILER  COMPONENTS Boilers  used  onboard  naval  ships  have essentially the same components: steam and water drums,  generating  and  circulating  tubes,  super- heaters,  economizers,  and  accessories  and  fittings for  controlling  steam  pressure  and  temperature and other aspects of boiler control and operation. Figure 4-1 shows a cutaway view of a D-type boiler. You should refer to this figure as a guide to the arrangement of the boiler components. As we  discuss  the  boiler  and  its  components,  imagine that you are assembling a similar boiler. As you 4-4