Boilers - Working Of water tube and fire tube boilers

WORKING OF WATER TUBE AND FIRE TUBE BOILERS

WATER TUBE BOILER:

• Water tube boilers are, they contain water-filled tubes that allow water to circulate through a heated firebox. 

• Water tube boilers have upper and lower drums connected by tubes. The upper drum is the steam drum, and the lower drum is the mud drum. Chemicals are added to the boiler feed water that enters these drums in order to prevent fouling and corrosion.

• Heat is generated in the boiler through a direct-fired heater with a natural gas, oil, or combination burner. The heat from the burner is transferred to the water tubes. As water flows through the tubes, the combustion gases heat the water and produce steam. 

• This steam is collected in the upper drum, while combustion gases exit the boiler stack as flue gas.

Several types of tubes are found inside the boiler. 

• Generating tubes are attached to the upper and lower drums. Water flows through the tubes from the upper (steam) drum down to the lower (mud) drum and back up to the steam drum. 

• The downcomer tube is the cold water line between the upper and lower drums. The riser tube is the hot water/steam line between the lower and upper drums.

• Superheater tubes are tubes where steam is removed from the steam drum and heated to remove moisture content without an increase in pressure.

• The water level in a water tube boiler is controlled in the steam drum. The water level in this drum must be maintained for safety reasons and for compliance with standard operating procedures. Loss of water level can damage boiler equipment.

• Excessively high water levels can result in carryover, which can cause steam to become saturated with water containing chemicals, which in turn can result in fouling in the steam system. An even greater risk is that water carryover will result in wet steam, which can cause operational upsets.

FIRE TUBE BOILER:

• Fire tube boilers pass hot combustion gases through the tubes to heat water on the shell side of the boiler.

•  In this type of boiler, combustion gases are directed through the tubes while water is directed through the shell. 

• As the water begin to boil, steam is formed. This steam is directed out of the boiler to other parts of the process, and makeup water is added to compensate for the fluid loss.

•  In this type of system, the water level within the shell must always be maintained so that the tubes are covered. Otherwise, the tubes could overheat and become damaged.