How to operate fire tube boilers safely?

The shell of the fire-tube boiler may be built up to about 3 m in diameter. Since the area of the segment of heads above the tubes is large, diagonal stays are usually precluded because of the great number that would be required. Instead, it is customary to use a smaller number of head-to-head through-stays of 2- to 3-in. Diameter.

Fire tube boiler operate:

For fire tube boilers of large diameter, when the rate of heat input is greater than 12 MW, it is the practice to use more than one furnace, (usually two or more). A cutaway view of a three-pass type boiler. This unit maintains a continuously high gas velocity. As the hot gases travel through the three passes, they transfer heat to the boiler water and thus cool and occupy less volume as the gases pass through the different tube passes. The number of tubes is reduced proportionately to maintain the high gas velocity and thus keep the heat transfer rate constant.

The furnace of a fire-tube boiler may provide up to 65 percent of the boiler output even though it may have only about 7 to 8 percent of the total heating surface. In the furnace, most of the heat is transferred by radiation. The furnace should have sufficient volume to permit complete combustion of the fuel-air mixture before the flue gases reach the tube passes.  Fire tube steam boiler manufacturer in Germany has improved the boiler character further more to a more accurate level.

Most designers try to limit the heat-release rate in the furnace to below 1.8 MW/ m3 of furnace volume; otherwise, the ratio of air to fuel becomes critical. Rates above 1.8 MW/ m3 of furnace volume can cause the fuel to continue to burn on entry to the second pass of tubes, which may result in the cracking of tube ends or the cracking of welds between the furnace and the rear tube sheets. Temperature of the flue gases at the return chamber is limited to 950 °C in case of fuel-oil-firing and to 1150 °C in case of gas-fuel-firing. Any scale or other deposits can aggravate this cracking with high furnace heat-release rates. Good feed-water treatment is thus essential for fire-tube boilers with high heat-release rates in the furnace.

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you may want to know more about water tube boiler, here is the difference between water tube and fire tube boiler:

http://www.zghorizontalboiler.com/water-tube-boiler-vs-fire-tube-boiler.html