Firetube Boilers

what is a firetube boiler

Inside a firetube boiler.

What is a firetube boiler? What are the three main types of firetube boilers? Don’t know yet? No worries, we are going to cover it. You will most likely run into those two questions again if you plan on taking the boiler licensing exam, at least one of them, and it is usually the one about the three types of firetube boilers.

A couple weeks ago, a post went up that went over boiler classifications. High and low pressure steam boilers, the basic pressure requirements, and the type of fuel used all contribute to the boiler classification.

Another way to separate boilers is by the type of tubing inside or internal to the boiler. There are firetube and watertube boilers. Again, this post is going to focus only on firetube boilers. We will list quickly define the three types of firetube boilers, then get into the operation of one, such as, how they work or how they differ from watertube boilers. Or you could just wait another month until that post goes up and compare the two lol.

What Is A Firetube Boiler?

Trying to separate these two classifications of boilers, firetube and watertube is easy by definition or operation. The problem is, sometimes you can’t tell from looking at the boiler just from the outside. Anyways, here is the simple definition of a firetube boiler:

Heat and the gases of combustion pass through tubes that are immersed in water in a firetube boiler.

The tubes are submerged. Surrounded. Covered by the water. A watertube boiler is the opposite, we are just going to focus on the firetube class in this post though.

There are 3 common types of firetube boilers. They are:

  • Horizontal Return Tubular Boilers
  • Vertical Firetube
  • Scotch Marine

At a later date we will go into each type of firetube boiler a little, in a separate post. For now, and for a boiler licensing exam, remember these 3 common types and be able to barely describe how each one works, or the differences.

Firetube boilers can be either high pressure or low pressure, keep that in mind. Firetube boilers though are designed to handle pressures up to 250 psi and around 750 boiler horsepower. All firetube boilers have different designs though, based upon the needs of the user or demands of a system.

The tubes inside of a firetube boiler, where the gases and heat flow through, are measured by their outside diameter.

How Firetube Boilers Work

how does a firetube boiler work

Inside a firetube boiler – from

Here are the nuts and bolts of what happens inside a firetube boiler. We have talked about energy conversions before and this is exactly why. Now you know what goes on with energy transfers and such, let’s focus on what actually happens internally.

As water is heated, the water molecules become more active and bounce around. The water becomes lighter and it increases in volume.

The light, warm water naturally wants to rise so it makes its way towards the surface. As it does, new, cooler water “falls” into the previous place. Steam bubbles eventually form, making the warm and cooler water circulate even better, promoting more heat transfer. Some steam bubbles will be able to break the surface tension of the water, especially as the water heats up even more, and then they enter like a steam space.

Now, some brainy person decided to add these long tubes inside the boiler. What does this do? It gives the gases of combustion and the heat more opportunity to give up their heat as the water gurgles around the outside of the tubes. The tubes increase the size of the heating surface.

What is a Heating Surface?

In a boiler a heating surface is:

  • That part of a boiler with water on one side, and heat and gases of combustion on the other.

When the heating surface is made as large, while staying as safe, as possible, we can capture more energy, heat, from the gases rushing past. This transfer of energy promotes even more water circulation. More water circulation means more steam bubbles, etc. It’s awesome.

As more and more steam collects in the steam chamber of the firetube boiler, the efficiency of the boiler increases. More specifically, the thermal efficiency.

Thermal efficiency in the case of a boiler is:

  • the ratio of heat absorbed by the water inside the boiler to the amount of heat that was available in the fuel for combustion.

Firetube boilers can hot thermal efficiency rates as high as 80-85%. Meaning that only 15-20% is lost between the fuel burning and giving up it’s heat to the atmosphere and transferring it to the water.

The furnace of a firetube boiler is physically where the combustion happens. An internal furnace also greatly increases the heating surface. This helps by allowing the maximum absorption of heat, creating steam more quickly.

Firetube Boilers – Advantages

Here are some features of firetube boilers that people, like construction managers and building engineering staff, look at when choosing between the two styles of boilers. They weigh the pros and cons and factor in what they need the boiler system to do and the add in system demands.

Some advantages of firetube boilers are:

  • Low Initial Expense
  • Holds More Water
  • Less of a Footprint
  • Easier Controlled Assembly
  • Needs Less Brickwork

A firetube boiler can hold a larger amount, or volume, of water compared to a same size watertube boiler. What does this mean? It means that we can hold more energy. If we can use more water but stay at the same size, efficiency increases.

And easier controlled assembly only means that firetube boilers can be put together in a factory so quality control is better.

Firetube Boilers – Disadvantages

And of course there are disadvantages to operating a firetube boiler. The book I’m using to reference all this, High Pressure Boilers, lists one major one. As you may notice, the disadvantage also is an advantage from above.

  • Holds More Water

Since it does contain more water, explosions associated with firetube boilers are often worse that others. More energy just waiting to be released. This can happen when there is a rapid pressure drop without a corresponding drop in temperature.


In this post we discussed firetube boilers. We started out with what they are, and then listed 3 common types of firetube boilers: the horizontal return tubular boiler, the vertical firetube, and the Scotch Marine. After that we moved into some basic principles of operation and then finally we wrapped it up by listing some pros and cons of firetube boilers.

If any of you jokers have a question or comment about what you just read on firetube boilers, or anything else on the site, let me know. You can use the comment section or send me an email through the site. Peace, have a good one.

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