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Considerations When Selecting a Condensing Economizer

December 17, 2007
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Boilers equipped with condensing economizers can have an overall efficiency that exceeds 90 percent. A condensing economizer can increase overall heat recovery and steam system efficiency by up to 10 percent by reducing the flue gas temperature below its dew point, resulting in improved effectiveness of waste heat recovery.

This article is a companion to one titled “Consider Installing a Condensing Economizer” (Dec. 3, 2007 issue), and it discusses two types of condensing economizer: indirect and direct contact.

Figure 1. Indirect contact condensing economizer. (Click on the image for an enlarged view.)

An indirect contact condensing economizer (see Figure 1) removes heat from hot flue gases by passing them through one or more shell-and-tube or tubular heat exchangers. This economizer can heat fluids to a temperature of 200°F while achieving exit gas temperatures as low as 75°F. The indirect contact economizer is able to preheat water to a higher outlet or process supply temperature than the direct contact economizer. The condensing economizer must be designed to withstand corrosion from condensed water vapor produced by the combustion of hydrocarbon fuels such as natural gas or light oils. The condensed water vapor is acidic and must be neutralized if it is to be discharged into the sewer system or used as process water.

Another heat recovery option is to use a direct contact condensing economizer (see Figure 2), which consists of a vapor-conditioning chamber followed by a countercurrent spray chamber. In the spray chamber, small droplets of cool liquid come into direct contact with the hot flue gas, providing a non-fouling heat transfer surface. The liquid droplets cool the stack gas, condense and disentrain the water vapor. The spray chamber may be equipped with packing to improve contact between the water spray and hot gas. A mist eliminator is required to prevent carryover of small droplets. The direct contact design offers high heat transfer coupled with water recovery capability since heated water can be collected for boiler feedwater, space heating, or plant process needs. Recovered water will be acidic and may require treatment prior to use, such as membrane technology, external heat exchangers, or pH control.

Figure 2. Direct contact condensing economizer with packed bed and external heat exchanger. (Click on the image for an enlarged view.)

The site must have substantial heating requirements for low-temperature process or cold make-up water if a direct contact condensing economizer is to be a viable heat recovery alternative. Because direct contact condensing economizers operate close to atmospheric pressure, altitude and flue gas temperature limit makeup water temperature to 110°F to 140°F.

When considering whether to install a condensing economizer, evaluate changes in system operating parameters. These economizers preheat boiler makeup water and reduce deaerator steam requirements, thereby providing more steam for plant processes. The energy savings potential is decreased if the majority of the deaerator steam is supplied from blowdown heat recovery. The condensing economizer could also limit or decrease backpressure steam turbine energy production if the turbine discharge is used to balance a low-pressure header. The reduction in stack gas exit temperature reduces plume buoyancy and must be considered when modeling pollutant dispersion. Performance characteristics of both indirect and direct contact economizers are summarized in the table below.

Condensing economizers require site-specific engineering and design, and a thorough understanding of the effect their operation will have on the existing steam system and water chemistry.

Suggested Actions

• Determine your boiler capacity, combustion efficiency, stack gas temperature, annual hours of operation, and annual fuel consumption.

• Identify in-plant uses for low-temperature heated water (plant space heating, boiler makeup water heating, preheating, or process requirements).

• Verify the thermal requirements that can be met through installing a condensing economizer, and potential annual fuel energy and cost savings.

• Determine the cost-effectiveness of a condensing economizer, ensuring that system changes are evaluated and modifications are included in the design (e.g., mist eliminator, heat exchangers). Simple paybacks for condensing economizer projects are often less than 2 years.

Resources

U.S. Department of Energy - DOE’s software, the Steam System Assessment Tool and Steam System Scoping Tool, can help you evaluate and identify steam system improvements. In addition, refer to Improving Steam System Performance: A Sourcebook for Industry for more information on steam system efficiency opportunities.

Visit the BestPractices Website at www.eere.energy.gov/industry/bestpractices to access these and other efficiency resources.

Reprinted from Steam Tip Sheet #26B, “Considerations When Selecting a Condensing Economizer,” from the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy. For more information, visit www.eere.energy.gov.

Publication date: 12/17/2007

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