James R. Stringer
Boilers are used to generate heat, power, or both at the same time. This post will be focusing on heating boilers that use natural gas or electricity as a fuel source.
Low pressure steam boilers have an operating limit of 14.7 PSI (103 kPa) while hot water heating boilers have a limit of 121C or 160 PSI (1100 kPa).
Hot water heaters for domestic water act the same as heating boilers but they are not classified as a boiler as long as they have less than 450 litres of water capacity, the water temperature remains below 99C, and the energy input is less than 58.6 kW (200 000 BTU).
Heating boilers attain max efficiency when they are operated continuously at full load. A rule of thumb is that for 90% of the heating season, the heating plant needs 60% or less of heating capacity. A single large capacity boiler operates at full capacity 10% of the time.
Firetube Boilers
Firetube boilers have straight tubes where the hot combustion gases travel through from the burner flame, generally making multiple passes before exiting through the stack (chimney) into the atmosphere. Firetube boilers are simple boilers that are of rugged construction and have a lower cost to produce. Firetube boilers have a large capacity of water in them and thus they are slow to bring to operating temperature but react and meet load changes quickly; they are ideal for on-off operation. Because of the large capacity of water, a pressure failure can cause large amounts of water damage.
Packaged boilers (both firetube and watertube) are made complete from the manufacturer with all fittings and tested before shipping. Once the boiler arrives to its destination, it can be installed and prepared for operation right away.
Watertube Boilers
As the name suggests, in watertube boilers – water flows through the tubes and is heated by flames and combustion gases. They have much less water capacity than firetube boilers, considered to be safer, and responds rapidly to load changes. The serpentine tubes are flexible and eliminate the stresses from expansion/contraction. The tubes are also easier to replace as they do not require welding or expanding.
Watertube boilers tend to have a higher cost to produce and more expensive, while also requiring closer supervision of water treatment. These boilers are used not only in heating plants but also power plants, gas plants, and refineries.
Cast Iron Sectional Boilers
Cast iron sectional boilers arrive on site in sections so they can fit through any narrow doorways to be installed. Sections are connected by push nipples that allow water or steam to flow through to each section. The sections are held together with tie rods and screw jacks.
Cast iron sectional boilers have a high resistance to corrosion, they are relatively easy to install with many choices of assembly, and sections can be added to increase the capacity for the facility.
Operating several smaller boilers can be much more efficient for a heating plant and also provide redundancy in face of a boiler failure. Burners and controls are more cost effective for smaller boilers, the water content is much less, they are lighter weight, and less floor space is required in the mechanical room.
Condensing Boilers
High efficiency condensing boilers work by using the hot water return to the boiler to condense the flue gas; this greatly increases the efficiency of the boiler as more flue gas condensed, the higher the efficiency. In simple terms, the exhaust leaving the boiler is a much lower temperature and thus more heat energy is transferred to the water to be heated instead of as a waste pollutant.
Non-condensing boilers operate around 180-200F while condensing boilers have a minimum of 140F and can operate at a lower temperature if the heating capacity for the need of the facility is met. For this reason, these types of boilers are great options for facilities with heat pumps, in-floor radiant heating, snow melt systems, domestic water heating, and pool heating systems.
Condensing boilers are much more expensive and require stainless steel or aluminum heat exchangers as the condensed flue gases are very acidic that will corrode away untreated metal.
Electric Boilers
Electric boilers are used for heating facilities, providing humidification, sterilizing equipment – to name a few examples. The advantages of electric boilers are numerous: they are compact as they require no chimney to vent exhaust as they do not burn hydrocarbons (which means they need no storage space for fuel), they are silent during their operation, safer as low water conditions mean that no electricity is conducted, and they are 98% efficient. Though they generate zero pollution – the power plant that generates the electricity for boiler operation may cause pollution. Scale is also limited in electric boilers and kept as sludge since the metal of the boiler cannot get hotter than the water inside the boiler.
Electrode type boilers create an electrical current through the water and heats it. Pure water does not conduct electricity so an electrolyte (ex. salt) is added for this to occur. The more of the electrode that is covered by water, the more current is produced, and the more the water is heated and/or steam is produced – and vice versa.
Immersion type boilers use heating elements that is submerged into the water. The water does not conduct electricity; electricity causes the heating elements to get very hot due to their resistance and heats the water they are submerged in. Temperature or steam control is through shutting off or turning on elements as needed. The heating elements are susceptible to scale from the high quantities of heat they produce.
Disadvantages for the electric boiler is the high price of electricity. They are also limited to pressures up to 304 PSI or 2100 kPa due to the high temperatures on the heating elements or electrodes.
BONUS PICTURES!
In the unfortunate event that a boiler (or several) has become inoperable, mobile boilers can be brought on site and integrated into the heating system until the heating plant’s boilers are back in operation. This hot water heating plant suffered from plugged tubes due to inadequate chemical treatment throughout the years and finally failed, at the beginning of winter of course!
References:
Power Engineering Fifth Class Edition 3.5; Chapters 1 – 4
CleaverBrooks; Total System Efficiency: An Introduction to Condensing Boilers by David Grassl