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Economizers Improve Boiler Efficiency

Key Points
  • Economizers recover waste heat, improving boiler efficiency by up to 10 percent.
  • Efficiency depends on the stack temperature and the amount of hot or makeup water required.
  • An economizer allows a boiler to respond more quickly to rapid load demands.

boilerBoilers can waste a lot of heat during the combustion process, especially if that heat isn't being transferred into steam or hot water. Recovering this wasted energy with economizers helps to improve combustion heat transfer and increase boiler efficiency.

Economizer technology

Economizers are heat exchangers. Most consist of bundles of straight finned tubes installed in a rectangular enclosure. There are two types of economizers—condensing and non-condensing.

Non-condensing (feedwater) economizers are the most common and least expensive type. They capture heat from the flue gas and transfer it to the feedwater. Since the feedwater is at a higher temperature, the boiler doesn't have to add as much heat to create steam, thus using less fuel and saving money. Non-condensing units can improve boiler efficiency by as much as 5 percent.

Condensing economizers cool the downstream flue gas below its dew point, which is about 135°F for natural gas combustion. The economizer reclaims both sensible heat from the flue gas and latent heat by condensing flue gas water vapor, improving boiler efficiency by up to 10 percent. Corrosion-resistant heat exchangers are required to accommodate the corrosive fluids generated during condensation. Because these economizers extract additional heat, they lower the outlet exhaust temperature even more. In certain applications, the water can be used elsewhere in the facility. Note that a condensing economizer should not be used with boilers firing #2 oil because sulfur from the #2 oil would combine with condensate and form sulfuric acid. 

Improving boiler efficiency

Economizers are most effective when large amounts of makeup water are used. Efficiency and potential savings depend on the boiler’s stack temperature and hours of operation. The following table shows how combustion efficiency is improved and stack temperature reduced, especially when both types of economizers are used.

Comparison of Combustion Efficiency*
System Configuration Combustion Efficiency Stack Gas Temperature
No Economizers 78 to 83 percent 350°F to 550°F
Feedwater Economizer
84 to 86 percent 250°F to 300°F
Both Feedwater and Condensing Economizers
92 to 95 percent 75°F to 150°F
*at 4 percent excess oxygen. Source: U.S. Department of Energy
 

Although economizers are usually more cost-effective for larger boilers (>150 BHP) producing high-pressure steam, smaller economizers are available for boilers down to 100 BHP. These smaller economizers consist of one or more coiled tubes inside a cylindrical shell, installed in-line in the flue. The cost can exceed $1,000 per million Btu/h of boiler capacity.

For larger boilers, economizers cost from $200 to $600 per million Btu/h. Equipment prices range from a few thousand for the smaller units to over $100,000 for the larger units. Installation costs vary widely but will be higher if piping and equipment must be moved to make room for the economizer. In some cases, the installation cost can equal the equipment cost. Lifecycle cost analysis typically shows an attractive financial return on investment.

In considering an economizer upgrade, work with the boiler equipment manufacturer to follow these guidelines:

  • Determine the stack temperature after tuning the boiler to the manufacturer's specifications.
  • Make sure the boiler operates at close to optimum excess air levels, and all heat transfer surfaces are clean.
  • Determine the minimum temperature to which stack gases can be cooled based on dew point, cold-end corrosion and the type of heat transfer surface.
  • Consider fuel costs; equipment and installation costs; and the configuration of piping and boiler equipment.

We can help

National Grid has tools and energy solutions that can help grow your business and foster strong client relationships. For more information, call us at 844-280-4325 or email retrofit@nationalgrid.com.

 

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