concerns the different designs and construction features of fire-tube boilers. The basis for identifying the two types is as follows: WATER-TUBE BOILERS are those in which the products  ofcombustion  surround  the  tubes  through which the water flows. FIRE-TUBE  BOILERS  are  those  in  which  the products of combustion pass through the tubes and the water surrounds them. WATER-TUBE  BOILERS Water-tube boilers may be classified in a number of ways.  For  our  purpose,  they  are  classified  as  either straight tube or bent tube.   These  classes  are  discussed separately in succeeding sections. To avoid confusion, make  sure  you  study  carefully  each  illustration  referred to throughout the discussion. Straight Tube The STRAIGHT-TUBE class of water-tube boilers includes three types: 1. Sectional-header cross drum 2.  Box-header  cross  drum 3.  Box-header  longitudinal  drum In   the   SECTIONAL-HEADER   CROSS   DRUM boiler with vertical headers, the headers are steel boxes into which the tubes are rolled. Feedwater enters and passes down through the downcomers (pipes) into the rear sectional headers from which the tubes are supplied. The water is heated and some of it changes into steam as it flows  through  the  tubes  to  the  front  headers.  The steam-water  mixture  returns  to  the  steam  drum  through the circulating tubes and is discharged in front of the steam-drum baffle that helps to separate the water and steam. Steam is removed from the top of the drum through the dry pipe. This pipe extends along the length of the drum and has holes or slots in the top half for steam to enter. Headers, the distinguishing feature of this boiler. are usually made of forged steel and are connected to the drums with tubes. Headers may be vertical or at right angles to the tubes. The tubes are rolled and flared into the header. A handhold is located opposite the ends of each  tube  to  facilitate  inspection  and  cleaning.  Its purpose is to collect sediment that is removed by blowing down the boiler. 1-3 Baffles are usually arranged so gases are directed across  the  tubes  three  times  before  being  discharged  from the boiler below the drum. BOX-HEADER CROSS DRUM boilers are shallow boxes made of two plates—a tube-sheet plate that is bent to form the sides of the box, and a plate containing the handholds that is riveted to the tube-sheet plate. Some are designed so that the front plate can be removed for access to tubes. Tubes enter at right angles to the box header and are  expanded  and  flared  in  the  same  manner  as  the sectional-header  boiler.  The  boiler  is  usually  built  with the drum in front. It is supported by lugs fastened to the box headers. This boiler has either cross or longitudinal baffling  arranged  to  divide  the  boiler  into  three  passes. Water  enters  the  bottom  of  the  drum,  flows  through connecting tubes to the box header, through the tubes to the rear box header, and back to the drum. BOX-HEADER  LONGITUDINAL  DRUM boilers have either a horizontal or inclined drum. Box headers  are  fastened  directly  to  the  drum  when  the drum  is  inclined.  When  the  drum  is  horizontal,  the front box header is connected to it at an angle greater than 90 degrees. The rear box header is connected to the drum by tubes. Longitudinal or cross baffles can be used with either type. Bent Tube Bent  tube  boilers  usually  have  three  drums.  The drums are usually of the same diameter and positioned at different  levels  with  each  other.  The  uppermost  or highest positioned drum is referred to as the STEAM DRUM,  while  the  middle  drum  is  referred  to  as  the WATER DRUM, and the lowest, the MUD DRUM. Tube banks connect the drums. The tubes are bent at the ends to enter the drums radially. Water enters the top rear drum, passes through the tubes to the bottom drum, and then moves up through the tubes to the top front drum. A mixture of steam and water is discharged into this drum. The steam returns to the top rear drum through the upper row of tubes, while the water travels through the tubes in the lower rear drum by tubes extending across the drum and enters a small collecting header above the front drum. Many types of baffle arrangements are used with bent-tube boilers. Usually, they are installed so that the inclined tubes between the lower drum and the top front drum absorb 70 to 80 percent of the heat. The water-tube boilers discussed above offer a number of worthwhile advantages.  For  one  thing,  they  afford  flexibility  in starting up. They also have a high productive capacity