Boilers

Boilers heat the water for recirculating hot water systems used for building heating. They can provide enough heat for entire buildings. A boiler (Fig. 24-3) is a closed arrangement of vessels and tubes in which water is

Water supply to boiler

Water supply to boiler

Water level gauge

Low water cutoff

Figure 24-3 Steam boiler

Water level gauge

Low water cutoff

Figure 24-3 Steam boiler heated or steam is generated. The type of boiler used depends on the size of the heating load, the heating fuels available, the efficiency needed, and whether the boilers are single or modular. Fuels for boilers include wood, coal, solid waste, fuel oil, gas, or electricity, and some boilers use more than one fuel. Fossil-fuel burning boilers need flues to exhaust gases, fresh air for combustion, and pollution-control equipment. A horizontal pipe carries exhaust gas from the boiler, and is connected to a vertical flue section called the stack. Boilers also need ventilation air, with an inlet and outlet on opposite sides of the room.

In older coal-fueled boilers, the coal was shoveled by hand into the boiler's firebox. Newer boilers use a mechanized stoker and automated ash-removal system. The ash is removed to a landfill, or can be used to improve the qualities of concrete. Coal boilers require antipollution equipment to control fly ash, which consists of various sizes of particles, and flue gas containing sulfur and nitrogen. Flue gases contribute to acid rain, which damages plant life and animals and erodes ancient monuments and ruins as well as modern statues and buildings. Acid rain pollutes water systems and affects tree growth.

Electric boilers eliminate the need for combustion air, flues, and air pollution in the building. However, these improvements are offset by the use of high-grade electrical energy for the low-grade task of heating. As discussed earlier, electricity is inefficient to generate, and produces pollution at the electrical generating plant. It is also expensive.

Boilers sometimes use recovered industrial waste heat to generate steam, often in combination with oil or gas. Hot water converters use a steam or hot water heat source such as geothermal, district heating, or a central steam boiler to heat hot water for building use. Hot water converters are essentially heat exchangers that transmit heat from a steam or hot water source to the water to be heated.

Boilers have safety release valves that open when vapor pressure is above a set level, allowing vapor to escape until the pressure is reduced to a safe or acceptable level. An air cushion tank, also known as a compression tank or expansion tank, is a closed tank containing air that is usually located above the boiler. Heated water expands and compresses air in the tank, which keeps the water from boiling and avoids frequent opening of the pressure relief valve.

Boiler systems require a fuel, a heat source, and a pump or fan to move the water. A distribution system, heat exchanger or terminal within the space to be heated, and a control system complete the equipment. Any heat escaping through the boiler's walls also helps heat the building. If a boiler is too small for the building, the building temperatures will be too low. Boilers that are too large waste money and space.

Cast-iron boilers are used for low-pressure steam and low-temperature hot water systems, and are generally less efficient than other types. Small steel boilers, called portable boilers, are assembled from welded steel units. They are prefabricated on a steel foundation and transported as a single package from the factory. Large boilers are installed in refractory brick settings built at the site.

Gas-fired cast-iron hot water boilers use hot gases rising through cast-iron sections to heat water inside. Additional heat is collected from a heat extractor in the flue. Oil-fired, cast-iron hot water boilers regulate the amount of air at a burner unit. The flame enters a refractory (resistant to heat) chamber, and continues around the outside of water-filled cast-iron sections.

In oil-fired steel boilers, the hot flame from burning oil produces combustion within a refractory chamber and fire tubes. The refractory chamber heats water outside the chamber. A domestic hot water coil can be connected, but this requires a larger boiler for support. An aquastat (a thermostat for water) turns on to keep the water hot. Newer, small-dimension compact boilers have high thermal efficiencies. They are available with venting options suitable for small equipment rooms.

Boilers are most efficient when in operation continuously. Modular boilers provide an efficient alternative, as sections can be used as needed, resulting in ease of maintenance and smaller size.

Gas-fired pulse boilers are even smaller and more energy efficient. They produce 60 to 70 small explosions per second, which cause the flue gases to pulse, and create a highly efficient heat transfer. Gas-fired pulse boilers operate at lower temperatures at efficiencies as high as 90 percent. They exhaust moist air, not hot smoke. To receive ENERGY STAR certification, a boiler must have an annual fuel usage efficiency of 85 percent or higher.

Steam boilers for space heating and domestic hot water are usually low-pressure. They are also used for electrical power generation for hospitals, kitchens, and industrial processes. A pressure gauge and an automatic safety control shut down boiler operation if needed, and a relief valve will blow off dangerous excess pressure. A glass water-level gauge shows the water level in the boiler and a shutoff valve in the water supply line permits water to be added manually as needed. Some boilers have an automatic water feed instead of a manual one. A low-water cutoff automatically shuts down the boiler if the water level drops too far.

With a pump added, low-pressure water systems can serve larger areas, including high-rise buildings. Medium-high-pressure steam boilers are found in high-rise buildings. High-pressure steam or water can carry more heat, allowing for smaller distribution piping for heat delivery throughout the building. High-pressure plants are more complex, and require added safety precautions and the presence of a qualified operating engineer when in use. Hot water is quieter and easier to distribute than steam.

/ Control

Compression tank x

Pump

Figure 24-4 Hot water (hydronic) heating system.

Pump

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