Boilers come in all different types, sizes, and shapes and are classified by configuration of the furnace, or firesides, and the watertubes, or water holding volume. The two main boiler types consist of firetube and watertube boilers, and deciding which of the two will best fit your process needs is not as easy as it may seem. It is like comparing apples to oranges; it’s not as simple as just comparing prices for similar nameplate steam capacity, and there are some significant differences to consider.
With firetube boilers, flue gases are directed through vertical or horizontal steel tubes that are surrounded by the water for heating, and typically go through two, three, or four passes, or changes in direction. In watertube boilers, the arrangement is reversed. Instead of the water being outside the tubes, it circulates inside the tubes and is heated externally by the combustion gases. Fuel is burned inside the furnace, which heats the water in the steam-generating tubes. The water then rises to the steam drum where saturated steam is drawn from the top of the drum. The watertube boiler has some relative advantages over the firetube boiler design, which are outlined below.
Watertube boilers are safer by design and generally considered to last much longer than the firetube boiler. They are available in much larger sizes and recover faster than firetube boilers, can handle pressures up to 5000 psig, and have the ability to reach very high temperatures with the use of superheaters. Because of their inherent design, firetube boilers are not suitable for pressures above 350 psig. Also, the steam output is less than the nameplate at actual operating conditions. Watertube boilers are rated at actual operating conditions. However, from a manufacturing standpoint, firetube boilers are typically lower in cost than a comparable sized watertube boiler, the firesides are easier to clean, and the straight firetubes are easily replaceable.
Let’s not forget that there is a tremendous difference in water volume between a firetube boiler and a watertube boiler. From a cold start, the firetube boiler, due to its larger water volume, takes longer to bring up to operating temperature or pressure vs. a similar sized watertube boiler. With less water volume, a watertube boiler can follow load swings more precisely and generally has better turndown. However, one advantage that the firetube boiler does offer is that once it is up to operating pressure, it can handle a sudden upward load surge better because of the large steam disengaging area. The drawback to this feature is once you drop pressure on the firetube, it takes longer to catch back up.
The rule of thumb is that if you have a very constant and steady load that doesn’t vary much, than the firetube boiler may work well. If you have a swing or varying load, then the watertube boiler is generally a better choice.
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