by Joe Farsetta, Certified Master Inspector® and Certified Commercial Inspector
Steam boilers can be simple or complex, depending on their application, the fuel available, and specific requirements and demands, including steam pressure and volume. Steam systems in commercial and industrial settings will typically require the services of a licensed Professional Engineer for the design and installation phases. The appropriateness and safety of the design, installation and operation are critical. The overall operating conditions of an active steam plant may be within the inspector’s responsibility, depending on the specific inspection job and the mutually agreed-upon scope of work.
Fire Tube Boiler
One of the most basic types of boilers, the fire tube boiler is also one of the oldest designs.
The burner shoots the flame into a tube that is immersed in water contained in the main vessel of the boiler itself. There may be multiple fire tubes. The heat of the flame transfers into the water, heating it to its boiling point. The resultant steam is captured in the space above the water and escapes through a steam outlet to whatever purpose it is intended for, whether heating a radiator or driving a locomotive. But because both steam and water are contained within this shared vessel, steam pressure is somewhat limited. In the event that the main vessel should succumb to increased pressure beyond its capacity, there could be catastrophic consequences.
Within the realm of fire tube boilers are two main types. These types are dependent on the physical location of the furnace (flame production). These are described as external and internal furnaces.
Each of the two main types includes subsets. For example, there are three types of external furnaces:
- horizontal-return tubular fire tube boiler;
- short firebox fire tube boiler; and
- compact fire tube boiler.
For internal furnaces, there are two subsets:
- horizontal tubular; and
- vertical tubular fire tube boiler.
Let’s examine the workings of a horizontal return tubular fire tube boiler. In this configuration, a large steam drum is installed horizontally within the housing and supporting structures. Multiple fire tubes emanate from the furnace and are also aligned in a horizontal fashion within the drum. This makes sense, as the drum is horizontal. As the drum is filled with water, the tubes become submerged.
Fuel is combusted within the furnace, heating gases that pass through the tubes, transferring heat to the water, which, in turn, heats the water to its boiling point. Spent gases pass through the fire tubes and into a smoke box that is connected to an exhaust stack. However, as steam is produced in the drum, so is pressure. A pressurized system results in water boiling at a higher temperature. Hence, the more that steam pressure builds inside the drum, the boiling temperature of the water increases, resulting in a reduction in steam production overall. In essence, the boiler is self-controlled via the steam pressure it produces through its operation.
Types of Internal Furnace Fire Tube Boilers
Internal fire tube boilers are characterized as having their firebox within the unit. They include the following.
Cornish Fire Tube Boiler
This style of boiler includes a plain cylindrical shell, and a single heat pipe through its center. It has a single exhaust flue connected to its single heat pipe.
Lancashire Fire Tube Boiler
The construction of the Lancashire boiler is similar to the Cornish, except it has two internal fire tubes and two exhaust flues.
Locomotive Fire Tube Boiler
The locomotive boiler is an obsolete design. Originally found in steam locomotives, wood or coal was burned in the combustion chamber (furnace). A unit with a horizontal tube design, it was a rugged piece of equipment, with a high steam capacity.
Vertical Fire Tube Boiler
A vertical tube boiler is a simple boiler with a vertical cylinder design. It includes cross tubes, and the furnace is located at the bottom of the boiler. Combustion cases escape through the top of the unit via the flue.
Cochran Fire Tube Boiler
The Cochran boiler is also a vertical-type boiler of multi-tubular design. It includes multiple horizontal fire tubes.
Immersion Fire Tube Boiler
This style of boiler incorporates a horizontal single-pass design. Flames are pushed into corresponding small-diameter tubes. The multiple small-diameter tubes are individually immersed in water. There is one tube for each flame nozzle.
Water Tube Boilers
Water tube boilers offer advances in steam-producing technology. It is different than fire tube technology in that the flame from the furnace is shot into a large containment area, where it literally bounces off the rear wall and distributes heat more evenly and efficiently, thus reducing fuel costs. Fire tubes send flames into tubes immersed in water, as opposed to a chamber. As there is less water to produce a steam explosion in the event of a failure, operations are much safer, and the unit is easier to inspect and maintain.
Within the chamber is a series of tubes containing water. The tubes run the length of the boiler. Tubes are sealed and can withstand pressure individually, not affecting the adjacent tubes. Heat is transferred through the metallic tubes to the water flowing through them.
This is a notable difference from fire tube boilers, in which the water that’s being used to create steam will surround the heat source. With water tube boilers, the heat surrounds the water tubes. This results in less stress on the boiler as a unit, as compared to fire tube designs.
The water that has been heated inside the tubes rises to the top of the boiler and into the steam drum. The process typically takes only a few minutes. Steam is produced at a high rate. The effects of heating and cooling create a condition known as thermal siphoning, which allows for the circulation of water in the boiler. Efficiency of the operation of the boiler allows it to occupy a smaller footprint than fire tube units. Water tube boilers are mainly used for generating steam at higher pressures and at higher volumes.
Components of a Water Tube Boiler
Water tube boilers typically include the following components:
- the boiler shell, which is the external cylindrical component of the pressure vessel;
- the mud drum, at the space located at the bottom of the water container. This is where the impurities resulting from the water turning to gas eventually gather. It is also where bottom blowdown occurs;
- the strainer, which is a type of filter that strains out any solid elements;
- a sight glass to observe water levels; and
- a burner, which is the source of the fire that heats the water
Difference Between Fire Tube Boilers and Water Tube Boilers
There are many differences between these types of boilers. At the base level, one has fire immersed in water, and the other has water immersed in fire. That is the single most striking difference between the two designs. There are differences in performance as well.
Working pressure, steam production cycles, materials utilized in production, physical sizing, efficiencies, load handling, and the cost of maintenance and operations are all factors in the differences between fire tube boilers and water tube boilers.
- The water tube design allows for a higher pressure range.
- Load fluctuations in the fire tube are not easily compensated for, whereas water tube fluctuations are easily accommodated.
- Water tube boilers can take up less physical space for large-capacity units.
- Water tube boilers can be 90% efficient, as compared to a 75% limit for fire tube designs.
- The design of water tube boilers is generally more complex than for fire tube boilers, and maintenance is typically greater.
- Expertise is required for efficient water tube boiler operations, as compared to fire tube units, which require little or no expertise.
Advantages and Disadvantages
The fire tube boiler does have certain advantages, including its compact design. It is also cheaper to purchase than a water tube boiler. However, it also comes with distinct disadvantages, including a limitation on the steam pressure it is capable of producing. Water and steam are kept in the same vessel; so, because the fire tubes are trying to heat a vessel full of water, it takes longer to bring the water to temperature. The biggest disadvantage, however, is the possibility of drum (vessel) failure and a huge explosion.
Fire tube boilers include a pressure gauge and water-level indicator. It may or may not be equipped with a Hartford loop, depending on how the condensate is returned to the unit. Condensate pumps eliminate the need for the loop.
A majority of smaller steam boilers are pre-packaged and are fire tube units. They are factory constructed and physically small. They can be manufactured, shipped, unpacked, and installed with relative ease. A majority sold domestically are fired via natural gas.
On the other hand, water tube boilers are installed in large-capacity operations. Most are assembled and constructed in the field, as things like ducts, steel supports, and vibration dampers must also be constructed in the field. Exhaust stacks, as opposed to simple flues, are also required most of the time. Due to the sheer size and weight of the components, it is virtually impossible to ship a system as an assembled and complete unit. The main fuel for these units is also natural gas, though crushed coal is also sometimes utilized in industrial settings.
Electric Steam Boilers
Electric steam boilers rely primarily on electric resistance heating elements to heat the water and produce steam. They are often seen in smaller operations, which may include laundries, food processing, and hospitals, though a hospital will likely have a steam plant in place.
Although more costly to operate than fuel-fired counterparts, this type of boiler is popular because of its simplicity. Thermal efficiency is directly related to the cost of electricity. Although technically efficient and “green” due to its zero emissions from the burning of fuel, electric steam boilers can be expensive to operate. depending on the local electricity rates. In addition, the generation of electricity often translates to the burning of fuels at the power-plant level, so the generation of electricity to “fuel” these boilers produces exhaust gases.
Packaged electric steam boilers are available to produce low- and high-pressure steam at rates up to at least 165 BHP. They are often found in application-specific deployments, including food and beverage manufacturing or packaging. Electric boilers are also often seen in sterilization processes.
An inspector may see a large commercial fuel-fed steam boiler and a smaller electric boiler at the same location. Depending on steam requirements, which will typically be high in winter months, many facilities augment their steam plants with smaller electric units. When the demand for steam is lower, some plants will shut down their larger boiler and switch to their smaller electric unit. This may be to due to high fuel costs or the need for maintenance or repair. Regardless, don’t be surprised if two types of systems are present.
Although the main plant boiler usually supplies steam and hot water for comfort heating and humidification, there may be cases where it is cost-effective to install an electric boiler for localized heating in a plant expansion. Similarly, electric boilers are ideal for new process facilities where large, fossil-fueled boilers are impractical or not required.
Steam generation principles are essentially the same, with electric resistance heating elements that heat the water to produce steam. These units have many of the same operational components as conventional boilers, minus the burner, chimney, and exhaust stacks. Electrode systems are also electric, but they rely on water as the conductor. In these systems, the water becomes charged, so there are many safety concerns associated with the operation of an electrode steal boiler.
A common application for small commercial steam boilers is in light commercial settings, such as health spas, where there are often steam generators for use in steam rooms. A steam boiler is different than a steam generator, which is typically small-scale and is designed to handle a single limited application.
Maintenance on electric boilers is minimal beyond routine inspections of water levels and monthly inspections of wiring. As with all boilers, they do require scale control measures and periodic blowdowns to maintain their efficiency. Heating element replacement, when required, is easily accomplished through the boiler door.
More commercial inspector resources:
Commercial Steam Boilers: A Primer
Steam Boiler Mechanics and Their Application in Commercial and Industrial Settings