Principle of Fire Tube Boilers Part 1

Stromquist and Company does not sell fire tube boilers but we do sell a vast amount of controls for these boilers from manufactures such as Conbraco, Honeywell, Asco, McDonnell & Miller, Maxitrol, and numerous other control manufacturers. The understanding of these boilers is a major concern in our continuing efforts to keep our readers informed.


In a fire tube steam boiler, heat and gases of combustion pass through the tube surrounded by water. Fire tube boilers steam boilers maybe either high or low pressure boilers. The three types of fire tube steam boilers are horizontal return tubular boiler, scotch marine boiler, and vertical fire tube boiler.

All fire tube boilers have the same basic operating principles. The heat produced by the gases of combustion pass through the tubes while the water surrounds the tubes. However, fire tube boilers have different designs like 2 pass, 3 pass, and 4 pass based on application and installation considerations.

Fire tube boiler tubes are always measured by their outside diameter (O.D.). Fire tube boilers are usually designed for pressures up to a maximum of 250 psi and approximately 750 horsepower.


When water is heated, it increases in volume and becomes lighter. This warmer water, now lighter, rises and the cooler water drops to take its place. The steam bubbles that eventually form break the surface of the water and enter the steam space.









The addition of tubes inside the drum containing the water increases the heating surface. The heating surface is that part of the boiler with water on one side and the heat and gases of combustion on the other. By increasing the heating surface, more heat is taken from the gases of combustion. This results in a more rapid water circulation and faster formation of steam bubbles.

When larger quantities of steam are released, the thermal efficiency of the boiler increases. Thermal efficiency is the ratio of the heat supplied from the fuel to the heat absorbed by the water. Modern fire tube boilers with improved design and heat transfer rates have achieved thermal efficiency rates as high as 80% to 85%.

Placing an internal furnace within the boiler shell greatly increases the heating surface allowing for maximum absorption of heat thus reducing the time to create steam.










Because of the large volume of water fire tube boiler contain, disastrous explosions may occur. Explosions may occur because of a sudden drop in pressure without a corresponding drop in temperature. Knowledge of basic principles of boiler operation can prevent serious accidents.

Water will boil and turn into steam when it reaches 212 degrees F at atmospheric pressure.

The higher the steam pressure, the higher the boiling point of the water in the boiler.

As steam pressure in the boiler increases, there is a corresponding increase in temperature.


When a steam boiler is operating at 100 psi gauge pressure the temperature of the water and steam will be about 337 degrees F. If there is a sudden drop in pressure from 100 psi to 0 psi without a corresponding drop in temperature, the water at 337 degrees F flashes into steam. When water flashes into steam its volume increases tremendously. This can result in a disastrous explosion.

It is imperative that maximum care is exercised in the operation and maintenance of the fire tube boiler. This includes annual boiler inspections of the waterside and fireside of the boiler. Controls such as the low water cut off, relief valves, flame safeguards must all be in correct working order.

To see videos of Fire tube boilers in action click here.

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13 Responses

  1. Easy way to identify most firetube boiler set ups is a 2 and 4 pass FT the stack is in front of the boiler a 3 pass FT boiler has the stack in the rear of the boiler.

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