- Operating boilers with the right level of air will reduce heat loss and improve combustion efficiency.
- Maintaining steam traps and steam pipe insulation can result in significant energy savings.
- A feedwater economizer can recover waste heat and reduce fuel requirements by up to 10 percent.
Boilers use a significant amount of energy. According to the American Boiler Manufacturers Association, 47 percent of boilers in commercial and industrial facilities are at least 40 years old. While many of these older systems operate far below the efficiency standards of newer models, replacement is not always an option. However, you can take steps to optimize the efficiency of your current boiler systems and save significantly on operating costs.
Optimize air to fuel ratio
Combustion efficiency is a measure of how effectively the heat content of a fuel is transferred into usable heat. The stack temperatures and flue gas oxygen (or carbon dioxide) concentrations are good indicators of combustion efficiency. Operating your boiler with the right amount of air will reduce heat loss through the stack and improve combustion efficiency.
In practice, combustion conditions are never ideal, and excess air must be supplied to burn fuel completely. Inadequate excess air results in combustibles (fuel, soot, smoke and carbon monoxide) while too much air results in heat loss because of increased flue gas flow; lowering the overall boiler fuel-to-steam efficiency. In a well-designed, natural gas-fired system, an excess air level of 10 percent is attainable. A rule of thumb is that boiler efficiency can be increased by 1 percent for each 15 percent reduction in excess air or 40°F reduction in stack gas temperature.
The correct amount of excess air can be determined by analyzing flue-gas oxygen concentrations. Low-cost test kits measure the percentage of oxygen in the flue gas. Higher-priced analysis equipment displays oxygen, temperature and boiler efficiency, but it is only recommended for boiler systems with annual fuel costs exceeding $50,000. Online oxygen analyzers should be considered when fuel composition or steam flows are highly variable. They provide immediate feedback to burner controls to reduce excess air and optimize fuel-to-air ratios.
Inspect and repair steam traps
Steam trap maintenance is often overlooked, but it can result in significant energy savings. In steam systems that have not been maintained for three to five years, up to 30 percent of the installed steam traps may have failed, thus allowing live steam to escape into the condensate return system. In systems with regularly scheduled maintenance programs, leaking traps should account for less than 5 percent of the trap population. If your steam system includes more than 500 traps, a steam trap survey will most likely reveal significant steam losses.
Establish a program for regular, systematic inspection, testing and repair of steam traps that includes the documentation of energy and dollar savings. Steam traps should be tested at specific intervals based on pressure levels: weekly for 150 pounds per square inch (psig) or above; monthly or quarterly for 30 to 150 psig; and annually for systems under 30 psig.
Insulate steam distribution and condensate return lines
Uninsulated steam distribution and condensate return lines are a constant source of wasted energy. Insulation can often reduce energy losses by 90 percent and help to ensure proper steam pressure for plant equipment. Any surface over 120°F should be insulated, including boiler surfaces, steam and condensate return piping and fittings.
Insulation frequently becomes damaged or is removed and never replaced during steam system repair. Damaged or wet insulation should be repaired or replaced immediately to avoid compromising the insulating valve. Eliminate sources of moisture prior to insulation replacement. Causes of wet insulation include leaking valves, pipe leaks, tube leaks or leaks from adjacent equipment. Also, use removable insulating jackets for valves, flanges, steam traps and other fittings. After steam lines are insulated, changes in heat flows can influence other parts of the steam system.
Use feedwater economizers to recover waste heat
A feedwater economizer reduces fuel requirements by transferring heat from the flue gas to incoming feedwater. Boiler flue gases are often rejected to the stack at temperatures 100°F to 150°F higher than the temperature of the generated steam. Generally, boiler efficiency can be increased by 1 percent for every 40°F reduction in flue gas temperature. By recovering waste heat, an economizer can often reduce fuel requirements by 5 percent to 10 percent and pay for itself in less than two years.
A feedwater economizer is not effective in every application, so a careful analysis is required. First, determine the stack temperature after the boiler has been tuned to manufacturer's specifications. The boiler should be operating near optimum excess air levels, with all heat-transfer surfaces clean. Next, establish the minimum temperature to which stack gases can be cooled, subject to criteria, such as dew point, cold-end corrosion and economic heat-transfer surface. The lowest temperature to which flue gases can be cooled depends on the type of fuel used, such as 250°F for natural gas.
When insufficient heat-transfer surface exists within the boiler, a feedwater economizer is appropriate for removing combustion heat. Boilers that exceed 100 horsepower (hp), operate at pressures exceeding 75 psig or above, and are significantly loaded all year long, are excellent candidates for economizer retrofits.