Introduction
This article introduces boilers. Common boiler designs are discussed, along with how they work and their applications.
Purpose of a Boiler
Boilers are used to supply hot water, or steam. A boiler supplying hot water is technically not a boiler, as the water is below its boiling point. Despite this, hot water heaters are colloquially referred to as boilers.
Irrespective of if a boiler is providing hot water or steam, the purpose of a boiler is to supply heat that can be used to complete a desired purpose. Providing the heat required to heat a building or room (a.k.a. space heating) is a common boiler task. A typical system setup for space heating using hot water is shown below.
Typical Hot Water System
A typical system setup for space heating using steam is shown below.
Typical Steam System
Fired and Un-Fired
Open boiler vessels are not used due to their inefficiency, closed vessels are used instead. Closed vessels with an internal furnace are the most common industrial boiler design. A closed vessel under pressure is referred to as an un-fired pressure vessel.
Compressed Air System Un-Fired Pressure Vessel
A closed vessel that uses a means of combustion to generate pressure within the vessel, is referred to as a fired pressure vessel. Combustion usually occurs within the pressure vessel i.e. an internal fired pressure vessel, or outside of the pressure vessel i.e. an external fired pressure vessel.
Fire Tube Boilers (Internal Fired Pressure Vessel)
Fire Tube and Water Tube
There are two main types of boiler used in the industrial engineering world, these are the fire tube boiler and water tube boiler. As the name implies, the difference between the two boilers is where the gases of combustion and water are relative to the boiler tubes
Water tube boilers have water in the tubes and gases of combustion outside of the tubes.
Fire tube boilers have gases of combustion within the tubes and water outside of the tubes.
Water tube boiler furnaces are surrounded by tubes (pipes) full of water, which allows a large amount of heat from combustion to be transferred to the tubes very quickly.
Fire tube boiler furnaces are usually inside the pressure vessel, but may also be external to the pressure vessel. Exhaust gases of combustion travel through tubes surrounded by water.
Direction of Heat Transfer
Almost all large industrial plants will have a boiler on site. Most industrial plant boilers are of the fire tube design whilst water tube boilers are more favoured in the power generation industry. Large and very large steam capacity boilers are always water tube boilers.
Water Tube Boiler Tubes
Water and Fire Tube Boilers Compared
A detailed overview of water tube and fire tube boilers is not possible within the confines of this course, but the below table highlights some of the main differences between the two designs.
|
Water Tube |
Fire Tube |
|
Water in the tubes. |
Combustion gases in the tubes. |
|
Combustion gases surrounding the tubes. |
Water surrounding the tubes. |
|
Maximum allowable working pressures (MAWP) in excess of 2,900 psi (200 bar). |
Maximum allowable working pressure (MAWP) up to 362 psi (25 bar). |
|
Very high steam generation rate. |
Comparatively low steam generation rate. |
|
More efficient than a fire tube boiler (typically greater than 90%). |
Less efficient than a water tube boiler (typically 80-85%). |
|
Explosion risk is higher due to higher MAWP. |
Explosion risk lower due to lower MAWP. |
|
Suitable for the power generation industry. |
Not suitable for the power generation industry. |
Water Tube and Fire Tube Comparison
For a detailed look at fire tube and water tube boilers, please see the associated saVRee courses.
Electric Boilers
There are two common types of electric boiler, these are the electrode boiler and resistance boiler. Both boiler designs are relatively simple as they do not require a combustion system.
Electrode boilers generate electrical arcs which heat the water until steam is formed. The number of electrodes per boiler, MAWP and their steam generation rates, varies depending upon the boiler design.
Electrode Boiler
Resistance boilers pass electrical current through submerged resistive elements in order to generate heat. Heat from the elements is transferred to the surrounding water, which gradually turns to steam. Resistance boilers are often used for hot water heating applications.
Resistance Boilers
Electric boilers are emission free (no gases of combustion) and comparatively small compared to combustion type boilers. The largest disadvantage associated with electric boilers is the cost compared to combustion type boilers.
Boiler Resistor Elements
Package and Field Erected Boilers
Package boilers are sold as turn-key boilers, they are delivered to the site completely assembled and only need to be connected to the various systems they require to operate (feedwater, steam, fuel, exhaust gas etc.).
A large advantage associated with package boilers is that they can be tested completely by the original equipment manufacturer (OEM) at the factory before shipping, and can be replaced by the same (or similar) model quite quickly.
Field erected boilers are too large to ship as a single product and must be assembled on-site. Water tube boilers used in the power generation industry are examples of field erected boilers.
Additional Resources
https://en.wikipedia.org/wiki/Boiler
https://www.heat-timer.com/domestic-hot-water-systems
https://mirmarine.net/stati-na-anglijskom/marine-boiler/692-types-and-working-of-marine-boiler