There are an amazing variety of boiler designs. Without an effective boiler, a steam engine is just a work of art.

The simplest boilers are often called pot boilers - they are easiest to make, but they also take the longest to get a good head of steam and they use a lot of fuel for the steam they produce. Other types of boilers include the fire tube boiler and the water tube boiler. Flash boilers are a type of water tube boiler.

Fire tube boilers have tubes, called flues, that allow hot gases from the fire to travel close to the water and help heat it. This type was very popular with steam locomotives. Even today, given the amount of steam produced for the amount of fuel used, it is a very attractive boiler design.

I've been on a bit of a quest to build a boiler for miniature steam engines I like to tinker with. Here is a concept for a small fire tube boiler I've put together with my CAD software.

The tank in this drawing is semi-transparent so you can see the fire tubes inside. This is a vertical boiler. Fire tubes in a vertical boiler that go straight through the boiler tend to be rather inefficient - this is because the hot gases from the fuel can go through the exhaust without transferring much of their heat to the water. Many fire tube boilers are horizontal.

In this design concept, there are 16 1/8" flues fitted into a 2" diameter base. These then go into a heating plate just above the water line. The heating plate is made of two parts press fit together - the bottom has 16 holes for the flues to connect with, the top connects to the exhaust. Between the top and bottom of the heat plate, there is a 1/16" gap for the hot gases to move around in.

Theory is the hot gases would give up a lot of their heat to the heat plate, which in this case would help heat the steam since it is above the water line.

Here is an exploded parts diagram:

And here is a cut-away drawing showing the inside of the heat plate:

I'm a bit torn between making one from stainless steel and making one from copper or bronze. I don't seem to have sufficient scraps right now to build it from anything - except maybe aluminum. While aluminum can be used in a boiler, especially a miniature low pressure one such as this, it is more difficult to join, as I don't have a TIG welder. I've thought about building it with aluminum and threading everything - even the flues. That would make it easy to dissemble and clean things too.