Considerations When Selecting a Condensing Economizer Page: 1 of 2
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- Determine your boiler capacity,
combustion efficiency, stack gas
temperature, annual hours of
operation, and annual fuel
- Identify in-plant uses for low-
temperature heated water (plant
space heating, boiler makeup
water heating, preheating, or
- 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.
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 Web site
bestpractices to access these and
many other industrial efficiency
resources and information on
Considerations When Selecting a Condensing Economizer
Boilers equipped with condensing economizers can have an overall efficiency that
exceeds 90%. A condensing economizer can increase overall heat recovery and steam
system efficiency by up to 10% by reducing the flue gas temperature below its dew
point, resulting in improved effectiveness of waste heat recovery.
This tip sheet is a companion to one entitled Consider Installing a Condensing
Economizer; and discusses two types of condensing economizer: indirect and direct
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 200F while achieving exit gas
temperatures as low as 75F. 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.
water or hot
process water load D
, 45-85'F heat exchange
Figure 1. Indirect contact condensing economizer
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.
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Considerations When Selecting a Condensing Economizer, report, August 1, 2007; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc887515/m1/1/: accessed January 21, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.