What is a Smoke Tube Boiler? Definition, Types, FAQ’s [PDF]

Smoke Tube Boiler Definition

A smoke tube boiler is a kind of boiler where the smoky gases go through some tubes while being surrounded by water. This makes the water turn into steam as it absorbs the heat from the smoky gases. These boilers are quite straightforward and are commonly used in small businesses and industries.

what is smoke tube boiler definition

What is an Internal Furnace Smoke Tube Boiler?

An internal furnace smoke tube boiler is a kind of boiler where there’s a furnace inside the boiler shell. The furnace is surrounded by water, and the heat from burning stuff in the furnace goes into the water through the furnace walls. After that, the smoky gases pass through some tubes that are also surrounded by water. This way of moving heat around is called convection. Internal furnace smoke tube steam boilers are popular for several good reasons. They are very efficient. You can use different types of fuels in them.

what is internal smoke tube boiler

Main parts of a Smoke Tube Boiler

Following are the main parts of a smoke tube boiler:

Feed water valve

The Feed Water Valve controls the flow of water. It has the role of allowing water to enter and fill the boiler while also preventing water from escaping when the boiler is sealed or when it contains steam.

Water Level Indicator

Boilers, often referred to as steam generators, operate continuously. As water turns into steam, the water level decreases. The purpose of the Water Level Indicator is to display and alert us to the amount of water in the boiler. It functions to avoid overfilling and adds water when there is a shortage to maintain a safe water level.

Steam Check Valve

The Steam Check Valve is important because it prevents steam from flowing backward after it has been generated in a steam container.

Pressure Gauge

The Pressure Gauge monitors the water and the steam pressure in a boiler. By reading the pressure gauge, the boiler operator can ensure that the closed container is maintained within the appropriate pressure range.


Since boilers operate continuously at high temperatures and require maintenance, it’s essential to have access inside them. Manholes are entry points into the boiler. It allows personnel to enter for cleaning the interior and maintenance.


The Chimney is an external pathway that facilitates the release of burnt gases or flue gases from the boiler or container.

Blow Off Valve

The Blow Off Valve is used to eliminate impurities and to empty water from the boiler. Typically, Blow Off Valves are located at the lower bottom sides of a boiler for this purpose.

Main Types Of  Smoke Tube Boiler

Smoke Tube Boilers are divided into 3 main types:

Dry Back Boilers

In dry back boilers, the back part of the furnace, including the smoke box and side plates, is exposed to the surrounding environment. This exposure causes a slight increase in radiation losses to the surroundings.

what is dry back boiler

Semi-Wet Back Boilers

In semi-wet back boilers, the side shell of the wrapper drum that’s linked to the furnace is surrounded by water, while the rear end of the wrapper drum is open to the external environment. These boilers do experience radiation losses, but they are lower compared to dry back boilers.

what is wet back boiler

Fully Wet Back Boilers

Fully wet back boilers are the most energy-efficient type of boilers. In these boilers, the entire wrapper drum that connects to the furnace, where the flue gases change direction, is enveloped by water. It minimizes or eliminates radiation losses to the surroundings.

what is fully wet back boiler

Advantages of Smoke Tube Boilers

Following are the main advantages of Smoke Tube Boilers:

  • Operating and maintaining them is easy.
  • They have a compact design.
  • Installation is simple.
  • They are fuel-efficient
  • It maximize fuel usage efficiency.

Disadvantages of Smoke Tube Boilers

  • Operates at low pressures.
  • Steam is less dry compared to Water Tube Boilers.
  • Slower steam production due to more Fire Tubes.
  • Lower efficiency
  • Limited steam production rate
  • Requires more ground space due to horizontal tube design.

Video on Smoke Tube Boiler

Difference between Smoke Tube Boiler and Water Tube Boiler

Smoke Tube BoilerWater Tube Boiler
Low evaporation rateHigh evaporation rate
Less boiler pressureHigh boiler pressure
Water surrounds the tube containing gasSmoke surrounds the tube containing water
Used where steam demand is lowUsed where steam demand is high
No need to consider water quality water quality needs to be considered
Less compactMore compact


How much does the smoke tube boilers cost?

The cost of a smoke tube boiler can vary based on factors like size, type, and manufacturer. Small ones may cost a few thousand dollars. Larger models can range from 10 lakhs to 80 lakhs.

What are the maintenance requirements for smoke tube boilers?

1. Regular cleaning to eliminate sediment and debris.
2. Inspecting for leaks and corrosion.
3. Replacing worn or damaged components as needed.

What are the alternatives of smoke tube boilers?

Several alternative boiler types can be used in place of smoke tube boilers. These alternatives include water tube boilers, fire tube boilers, and heat recovery steam generators (HRSGs).

Are smoke tube boiler and fire tube boiler the same?

Yes. Fire tube boilers are also known as smoke tube boilers.


That was all about smoke tube boilers. If you have any questions feel free to ask in comments below.

Print Friendly, PDF & Email