A firetube boiler is one of many different types of industrial boilers available, and is popular due to its relatively simple design and ease of operation, as well as its compact footprint. The term “firetube” is a bit of misnomer, since there is no actual fire inside the tubes; however, the name does rightfully imply that the inside of the tubes is hot, and the heat is used to boil water surrounding the outside of the tubes. Firetube boilers are found in a great number of industrial and commercial facilities, supplying low pressure (up to 450 psig), saturated steam to processes, buildings, and machinery.
In its simplest form, a firetube boiler has a fuel source, one or more tubes, a boiler shell, water inlet, steam outlet, and a chimney. Fuel is heated in a chamber with excess air. The hot gasses leave the chamber and pass through the inside of the tube(s), then exit through a chimney or stack. On the outside of the tubes, water contained in a cylindrical boiler shell is heated to its boiling point. The steam given off during boiling rises to the top of the boiler shell into a dome, where it is extracted and distributed to the facility.
Early firetube boilers were used to drive steam engines (railroad and marine), drive industrial equipment, power line shafts in manufacturing facilities, and provide process heat. They had a firebox that contained fuel (usually a solid fuel such as wood or coal), a once-through bank of horizontal tubes, and a stack through which to exhaust flue gasses. Water was pumped into the bottom of the boiler shell and steam was extracted from the top.
A modern firetube boiler takes advantage of today’s clean-burning fuels and is designed to maximize the use of the available heat. The fuel source is usually a self-contained, high-efficiency gas or oil burner, and the gasses make two, three, or four passes through the boiler shell before being exhausted to take advantage of all the heat contained in the system. In some cases, the flue gasses are used to preheat the water entering the boiler, thus increasing thermal efficiency. External Superheaters are also occasionally included to dry out and heat up the steam leaving the boiler.
From the outside, a firetube boiler is a skid-mounted horizontal cylinder. The cylindrical shell contains the process water at operating pressure, and inside the cylinder is an array of horizontal tubes, secured on each end by a tubesheet. At one end sits a burner unit, and an exhaust stack is located on the same or opposite end based on the number of passes the flue gas makes through the boiler shell. Internally, the design gets more and more complex as the number of gas passes increases. The flue gasses have to be turned around at each end and redirected back through the boiler shell for each pass, so chambering becomes an integral part of the head design. There may also be multiple water inlets and multiple steam outlets, depending on the size of the unit.
Firetube boilers are compact in design, relatively easy to install, operate, and maintain, and less expensive than many other types of industrial boilers. They can provide saturated steam at pressures up to 450 psig, with capacities up to 2500 HP, and their high-efficiency burner design means they can comply with federal and state emissions limits. They are an ideal choice for any small or medium-size facility that needs a constant supply of low-pressure steam economically.