- Operate boilers with the right level of air to improve combustion efficiency.
- Maintain steam traps and steam pipe insulation for significant energy savings.
- A feedwater economizer reduces fuel requirements by up to 10 percent.
Nearly half of the boilers currently installed in commercial and industrial facilities are at least 40 years old, and these units use a significant amount of energy. While many of these older systems are inefficient compared to newer models, replacement is not always an option. However, you can take steps to optimize the efficiency of your system and save significantly on operating costs.
Optimize air/fuel ratio
Combustion efficiency is a measure of how effectively the heat content of the fuel is transferred into usable heat and depends on the stack temperatures and flue gas oxygen (or carbon dioxide) concentrations. Operating your boiler with the right amount of air will reduce heat loss through the stack and improve combustion efficiency.
In practice, excess air must be supplied to burn fuel completely. Inadequate excess air results in combustibles while too much air results in heat loss because of increased flue gas flow, lowering the overall fuel-to-steam efficiency. In a well-designed, natural gas-fired system, an excess air level of 10 percent is attainable. In general, boiler efficiency can be increased by 1 percent for each 15 percent reduction in excess air.
The correct amount of excess air is determined by analyzing flue-gas oxygen concentrations. Testing equipment ranges in price and capabilities. Online oxygen analyzers, which are used when fuel composition or steam flows are highly variable, provide immediate feedback to burner controls.
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 steam traps may have failed. 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.
Steam traps should be tested at specific intervals based on pressure levels:
- Weekly: 150 pounds per square inch (psig) or above
- Monthly or quarterly: 30 to 150 psig
- Annually: under 30 psig
Wireless monitoring systems are now available that can reduce steam leakage by up to 95 percent.
Uninsulated steam distribution and condensate return lines are a constant source of wasted energy. Insulation can reduce energy losses by 90 percent and helps to ensure proper steam pressure for plant equipment. Any surface of more than 120°F should be insulated, including boiler surfaces and steam and condensate return piping and fittings. You should also:
- Repair or replace damaged or wet insulation immediately to avoid compromising the insulating valve.
- Eliminate sources of moisture prior to insulation replacement.
- 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.
Recover waste heat
A feedwater economizer can reduce fuel requirements by 5 percent to 10 percent 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. Boiler efficiency can be increased by 1 percent for every 40°F reduction in flue gas temperature.
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.
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 exists within the boiler, a feedwater economizer is appropriate for removing combustion heat. Boilers that exceed 100 horsepower, operate at pressures exceeding 75 psig or above and are significantly loaded all year long, are excellent candidates for economizer retrofits.