All About Steam Boilers

This short article is about steam boilers. A typical steam boiler is formed as a closed vessel with in which water or some other liquid is heated to elevated temperatures. The water used in a boiler is not necessarily heated to its boiling point although most often it is. Once in operation the heated water or liquid exits the boiler system where it is used in various industrial processes or heating applications, including water heating, central or space heating, steam-based power generation, cooking, and disinfection and many more applications.

Types of Steam Boiler

There are several types of steam boiler some of which are described below:

Water-Tube Boilers

Water tube boilers incorporate tubes filled with water situated inside a furnace in a number of configurations. Typically the water tubes connect large drums, the lower ones containing water and the upper ones steam and water.

With mono-tube steam boilers, water is circulated by a pump through a series of coils. This type of steam boiler usually provides high rates of steam, but less storage capacity than mono-tube boilers. Water tube boilers can also be designed to use several different sources of heat and are often used in high-pressure steam applications as the high-pressure steam generated can be contained within small diameter pipes which are able to withstand the pressure.

Fire-Tube Boilers

Fire tube boilers use water which partially fills a boiler barrel with a small void left above the water surface that accommodates the steam. This is the type of tube boiler is commonly found in most steam trains.

The heat source powering the boiler is situated inside a firebox or furnace and it is important that this firebox kept immersed in water to maintain the temperature of the heating surface below boiling point. The firebox can be located at one end of a fire-tube which extends the path of the hot boiler gases, so augmenting the heating surface which can be further increased by making the boiler gases reverse direction through a second parallel tube or a bundle of multiple tubes (2-pass or return flue boiler).  As an alternative the boiler gases may be run along the sides and then beneath the boiler through flues (3-pass boiler).

With a steam locomotive-type boiler, a boiler barrel extends from the firebox and the hot gases pass through a bundle of fire tubes inside the barrel which greatly increases the heat transfer surface efficiencies.

Fire-tube boilers typically deliver a lower rate of steam production, but a high steam storage capacity. Fire-tube boilers tend to use solid fuels including coal, but can be easily converted to use liquid or gas type fuels.

Pot Boilers or Haycock Boilers

These steam boilers are based on a basic “steam-kettle” type arrangement where a heat source, usually a fire heats a partially filled water container from underneath.
Haycock boilers date from the 18th century and typically produced and stored large volumes of very low-pressure steam, often hardly above that of the atmosphere. The heat source would usually be provided by burning wood or more commonly coal. The heat transfer efficiency of this type of boiler system was typically very low.

Sectional Boilers

Sectional steam boilers are typically fabricated from cast iron sections where the boiler water is stored. During installation the individual sections of the boiler are usually assembled on site.

Flash Boilers

A flash boiler is a special type of water-tube boiler in which tubes are fitted close together and water is pumped through them. The tubes inside flash boilers are maintained at very high temperatures, so hot that the water feed is rapidly converted into steam and then superheated.

Flued Boilers

This type of flued boiler usually incorporated with one or two large flues and was an early form of fire-tube boiler.

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