LOCATION OF FIRE AND  WATER  SPACES One  of  the  basic  classifications  of  boilers  is according to the relative location of the fire and water  spaces.  By  this  method  of  classification, boilers are divided into two classes, FIRE-TUBE BOILERS and WATER-TUBE BOILERS. In the fire-tube  boilers,  the  gases  of  combustion  flow through the tubes and thereby heat the water that surrounds  the  tubes.  In  water-tube  boilers,  the water flows through the tubes and is heated by the gases of combustion that fill the furnace and heat  the  outside  metal  surfaces  of  the  tubes. All propulsion boilers used in naval ships are of the water-tube type. Auxiliary boilers may be either  fire-tube  or  water-tube  boilers. TYPE  OF  CIRCULATION Water-tube   boilers   are   further   classified according  to  the  method  of  water  circulation. Water-tube boilers may be classified as NATURAL CIRCULATION  BOILERS  or  FORCED  CIR- CULATION   BOILERS. In natural circulation boilers, the circulation of  water  depends  on  the  difference  between  the density of an ascending mixture of hot water and steam  and  a  descending  body  of  relatively  cool and  steam-free  water.  The  difference  in  density occurs because the water expands as it is heated, and  thus,  becomes  less  dense.  Another  way  to describe  natural  circulation  is  to  say  that  it  is caused by convection currents which result from the uneven heating of the water contained in the boiler. Natural   circulation   may   be   either   free   or accelerated.   In   a   boiler   with   free   natural circulation,  the  generating  tubes  are  installed almost  horizontally,  with  only  a  slight  incline toward  the  vertical.  When  the  generating  tubes are   installed   at   a   much   greater   angle   of inclination,   the   rate   of   water   circulation   is definitely  increased.  Therefore,  boilers  in  which the tubes slope quite steeply from steam drum to water drum are said to have natural circulation of  the  accelerated  type. Most  naval  boilers  are  designed  for  accelerated natural  circulation.  In  such  boilers,  large  tubes (3   inches   or   more   in   diameter)   are   installed between  the  steam  drum  and  the  water  drum. These   large   tubes,   or   DOWNCOMERS,   are located  outside  the  furnace  and  away  from  the heat of combustion. They serve as pathways for the downward flow of relatively cool water. When enough downcomers are installed, all small tubes can be generating tubes, carrying steam and water upward,  and  all  downward  flow  can  be  carried by  downcomers. The  size  and  number  of downcomers  installed  varies  from  one  type  of boiler  to  another,  but  downcomers  are  installed in all naval boilers. Forced circulation boilers are, as their name implies, quite different in design from the boilers that  use  natural  circulation.  Forced  circulation boilers  depend  upon  pumps,  rather  than  upon natural differences in density, for the circulation of   water   within   the   boiler.   Because   forced circulation   boilers   are   not   limited   by   the requirements that hot water and steam must be allowed  to  flow  upward  while  the  cooler  water flows downward, a great variety of arrangements may  be  found  in  forced  circulation  boilers. ARRANGEMENT  OF  STEAM AND  WATER  SPACES Natural   circulation   water-tube   boilers   are classified  as  DRUM-TYPE  BOILERS  or  HEADER- TYPE BOILERS, depending on the arrangement of the steam and water spaces. Drum-type boilers have one or more water drums (and usually one or  more  water  headers  as  well).  Header-type boilers  have  no  water  drum;  instead,  the  tubes enter many headers which serve the same purpose as water drums. What is a header, and what is the difference between a header and a drum? The term  header is commonly used in engineering to describe any tube,  chamber,  drum,  or  similar  piece  to  which tubes or pipes are connected in such a way as to permit the flow of fluid from one tube (or group of tubes) to another. Essentially, a header is a type of manifold or collection point. As far as boilers are  concerned,  the  only  distinction  between  a drum and a header is size. Drums maybe entered by  a  person  while  headers  cannot.  Both  serve basically  the  same  purpose. Drum-type   boilers   are   further   classified according  to  the  overall  shape  formed  by  the steam and water spaces—that is, by the tubes. For example,  double-furnace  boilers  are  often  called M-type  boilers  because  the  arrangement  of  the tubes  is  roughly  M-shaped.  Single-furnace  boilers are often called D-type boilers because the tubes form  a  shape  that  looks  like  the  letter  D. 4-3