An industrial-sized boiler is a complex and intricate mechanism that requires periodic adjustment to operate most efficiently. The manufacturer’s guidelines are the best sourcebook for maintenance scheduling, although there are some general tips that apply to any furnace or boiler.
The burners are the most critical part of a boiler. The fuel is vaporized there, in the case of oil burners, or expanded into a vapor in the case of gas burners. A fixed quantity of fuel is combined with the corresponding amount of oxygen needed for a complete combustion of the fuel. The fuel-to-air ratio can be affected by such factors as insufficient oxygen, excess fuel, or poor mixing of the two. A burner with the correctly adjusted fuel-to-air ratio creates the most heat from combustion and also the least number of toxic products that are discharged to the atmosphere via the flue.
The burners need to be cleaned occasionally. A dirty element can deliver the incorrect amount of fuel. It can also cause the fuel to be atomized or mixed incompletely with the combustion air. Another less obvious, but equally detrimental, situation can affect burner efficiency: the supply of outside air that’s available to the boiler.
Many building codes stipulate louvered openings in outside walls of mechanical rooms with a specific minimum free area. This permits sufficient combustion air to be available to the unit and provided in such a way that it’s not drawn from occupied spaces. An improper installation can draw conditioned air from the building, and even reduce the oxygen content of the building air itself. In addition, the building codes require that boilers are adequately vented so the fumes and other products of combustion do not infiltrate back into the conditioned area.
Be alert for a boiler room with louvered outside-air openings that have been blocked. Storage containers may have reduced the free area of the louvers, to the extent that the boiler may be starved for fresh air, reducing its efficiency. Or the louvers may have been eliminated entirely with construction subsequent to the installation of the boiler room. It’s imperative that sufficient outside air be made available to the boiler, via either new louvers or ductwork installed in accordance with the applicable code authority.
Fuel-oil boilers have a filter or strainer to keep impurities out of the supply lines. Even a small particle lodged in the burner can affect the operation of the boiler, so it’s important to clean or replace the filtering elements according to the manufacturer’s directions. During times of heavy use, this is especially important, and it may be advisable to check the strainer more often then, for dirt and debris. Excess particulates may also occur after the boiler has been fired up following a few months of idle time, due to settling of suspended solids in the storage tank or rust flaking from the tank or piping. Keeping clean filters is a vital precaution for older systems.
Fuel-oil-fired boilers often have heaters to warm the fuel before it enters the burner. This makes the oil less viscous and easier to atomize in the burner, for complete mixing with the combustion air. These fuel-oil heaters should be inspected frequently to verify proper operation. The heating elements should be cleaned of caked solids that can reduce the heat transferred to the oil and cause more energy to be spent warming the fluid.
Boilers are generally located in a mechanical room that is at ambient temperature, which can be quite cold in the wintertime. The operating temperature within the boiler itself is several hundred degrees Fahrenheit. As a result of this great disparity in temperatures, there is the potential for much heat to be lost from the boiler if the insulation is damaged or lacking. A periodic inspection of the insulating surfaces of the boiler is called for, to include the refractory and firebrick of large units. Another boiler part that deteriorates with use is the bundle of heat- exchange tubes. Water circulates through the inside of the tubes, and the hot gases of combustion flow across the outside of the tube bundle, heating the fluid within. Impurities in the combustion fuel, improperly adjusted burners, and unfiltered combustion air can cause carbon deposition on the outside of the boiler tubes.
This acts to insulate the tube walls and reduces the heat transferred to the circulating fluid. The result is a loss in boiler efficiency such that the water outlet temperature drops—and the flue gas temperature increases—as more heat is lost up the stack. The heat-exchange coils should be pulled from the boiler and the buildup cleaned off when the solid deposition worsens.
A similar process takes place on the inside of the boiler tubes. As water flows through the tubes, cool spots occur in the fluid circuit due to uneven heating by the products of combustion. In addition, there are times when the fluid continues to circulate through the boiler while the fire is not at full strength, and the temperature of the water drops. In both situations, the solids dissolved in the water—due to natural hardness or to chemicals added to the water for corrosion and bacteria-growth inhibition—have a lower solubility. If the temperature drops enough, the dissolved solids precipitate out of solution. Often these solids adhere to the pipe and restrict the flow passages. This coating acts to insulate the water from the heat source, further diminishing the heat-transfer rate through the tube walls.
This phenomenon is evidenced by an increased pressure drop across the hot-water coils on the water side. The pump has to work harder to push water through these restricted-flow passages, and the volume of water it is able to pump diminishes. This means that not only is more energy wasted, as the boiler is unable to transfer all the heat of combustion to the water, but also additional energy is used by the pump to maintain the flow rate through the building loop.
Properly designed systems reduce the solids content of the water by a combination of water softeners, filters, and chemical treatment. These devices significantly reduce the scaling problem in the piping but do not eliminate it altogether. Thus, it’s important to be aware of the problem and to inspect the tubing at least once a year, or more often if there is a problem in maintaining water solids and chemical balance or if the flow rate decreases substantially.
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Excerpt from Energy Conservation Projects.