Economizers |


Cain economizers installed on boilersIntroduction

Flue gases from large boilers are typically 450 – 650°F.  Stack Economizers recover some of this heat for pre-heating water. The water is most often used for boiler make-up water or some other need that coincides with boiler operation.  Stack Economizers should be considered as an efficiency measure when large amounts of make-up water are used (ie:  not all condensate is returned to the boiler or large amounts of live steam are used in the process so there is no condensate to return) or there is a simultaneous need for large quantities of hot water for some other use.

The savings potential is based on the existing stack temperature, the volume of make-up water needed, and the hours of operation.  Economizers are available in a wide range of sizes, from small coil-like units to very large waste heat recovery boilers.

See also Flue Gas Condensers that are used when water and flue gases come in direct contact with each other.

How They Work

Boiler stack economizers are simply heat exchangers with hot flue gas on one side and water on the other. Or, in direct contact condensing units, the make-up water is in direct contact with the flue gases.

Economizers must be sized for the volume of flue gas, its temperature, the maximum pressure drop allowed through the stack, what kind of fuel is used in the boiler, and how much energy needs to be recovered. Economizers designed for natural gas only, would likely plug-up if installed on a coal boiler and would face increased risk of corrosion if installed on an oil-fired boiler. Some units are designed to keep the flue gases above condensation temperature, and others are made of materials that resist the corrosive effect of condensed flue gases.

Economizer Schematic


The savings potential is a function of how much heat can be recovered, which is a function of how much cold water needs to be heated. A generally accepted “rule of thumb” is that about 5% of boiler input capacity can be recovered with a properly sized economizer. A higher percentage can be recovered with a Flue Gas Condenser, assuming there is enough cold water to condense all of the flue gas that is available. Therefore, for ‘ball parking’ purposes, start by comparing boiler input capacity with the need to heat water.

For example: consider a 500 hp boiler with a gas input of 20 million BTUs per Hour.

20,000,000 BTUs x 5% = 1,000,000 BTUs (100% Load Factor)

1,000,000 BTUs / (1,200 BTUs per Gallon of 200F water) = 833 Gallons per Hour

(1,000,000 BTUs / 80% efficiency) = ~1.2 MCF x $7.00 per MCF Natural Gas = $8.40 per Hour Value

Savings is reduced by 50% for a 50% Load Factor, etc.

If there is a need for that much hot water, the savings potential of $8.40 per hour would be multiplied by the number of boiler run hours, or the number of hours that the hot water can be used. In each application, be sure to consider the boiler Load Factor, the efficiency that the hot water is otherwise produced at, the cost of natural gas, and the installation cost of the equipment.

If the economizer would be used to heat boiler make-up water, it is necessary to determine the volume and temperature at the inlet of the economizer. The lower the amount of condensate return, the higher the volume of make-up water and the higher savings potential.

An economizer that recovers 5% of boiler input should easily have a 2 year payback in a year-round application.

More Information

Flue Gas Condensers

Exhaust Draft Control

Boiler Efficiency




Cain EconomizerCain Industries
P.O. Box 189
W194 N11826 McCormick
Germantown, WI 53022
Telephone: 262-251-0051

Web site:

Cain Industries produces a wide variety of exhaust heat transfer products for boilers, incineration and cogeneration systems.


Combustion Capacity: 5,000 – 250,000 pph (300,000,000 Btu/hr maximum input approx.)
Entering Gas Temps.: 350 – 1,250°F
Heat Sink Types: Boiler feedwater, makeup water, hot water return hot water storage tank, condensate tank, process water, potable water.


Combustion Capacity: 50 – 10,000 scfm
Entering Gas Temps.: 350 – 1,400°F
Heat Sink Types: Boiler feedwater, makeup water, process water, potable water, thermal fluids, run-around systems.


Combustion Capacity: 40 to 800 Bhp
Entering Gas Temps.: 350 – 700°F
Heat Sink Types: Boiler feedwater, makeup water, hot water return, hot water storage tank, condensate tank, process water, potable water


Combustion Capacity: 200,000 – 6,400,000 Btu/hr input
Entering Gas Temps.: 350 – 700°F
Heat Sink Types: Boiler feedwater, makeup water, hot water return, hot water storage tank, condensate tank, process water, potable water


Combustion Capacity: 100 – 30,000 scfm Entering Gas Temps.: 350 – 1,400°F Heat Sink Types: Boiler feedwater, makeup water, process water, hot water return, potable water, thermal fluids, run-around systems

Cannon Boiler Works, Inc.
510 Constitution Blvd.
New Kensington, Pa 15068

Web site:

Cannon Boiler Economizers and Feedwater preheatersCannon economizers are custom designed for optimum heat recovery and service life for your unique application.  Cannon will design your economizer for gas, oil, coal, or special waste gas streams.  These units are available in open lattice or bulkhead tubesheet designs, and Cannon manufactures the nickel-brazed helical finned tubes used in all of our economizers.

Combustion & Energy Systems, Ltd
25 Royal Crest Court,Suite 110
Markham, Ontario
Canada L3R 9X4
Telephone: 905-415-9400
Fax: 905-415-9482

Web site:

Condex Diagram

P.O. Box 35258
Tulsa, OK 74153
Telephone: 918-665-1930

Web site

E-Tech Economizer

E-Tech is your complete waste heat recovery source.  We provide economizers, waste heat boilers, air heaters, blowdown recovery systems and cogeneration recovery units to the industry.   Unlike other manufacturers, E-Tech offers complete packaged systems including ductwork, transitions, stacks, support steel, dampers and boiler trim.

Heat Sponge
Boilerroom Equipment Inc.
Telephone: 866-666-8977 FREE

Web site:

The three words that describe Heatsponge are:

  • Simple: Our unit was designed to be easy to procure, install, and operate
  • Effective: Our unit is designed to provide for high performance heat recovery
  • Economical: The Heatsponge is competitively priced and easy to service

Heat SpongeAll Heatsponge rectangular economizers provide for easy ongoing maintenance featuring removable tubes the connect to our headers with a compression fitting. Repairs are fast, easy, and require no welding. We stock tube elements allowing for rapid dispatch to minimize downtime. Since every tube makes its own circuit you can even operate your unit with one tube out of service to maintain your energy savings while waiting for a convenient time to shut down to perform the repair. Cleaning the tube bundle is fast and easy. When the boiler is offline simply unbolt the rear access door and use a water spray to hose out the bundle. Water catch pans in the bottom of the unit will collect and drain the water from the unit and prevent it from entering the boiler

Kentube Boiler EconomizerKentube Engineered Products
555 West 41st Street
Tulsa, Oklahoma 74107-7012
Telephone: 918-446-4561

Web site:

Since 1962, Kentube Engineered Products has delivered high-quality, innovative heat transfer products, service and satisfaction. We offer a full range of engineering and customization capabilities, to meet the most demanding requirements.

From small, cylindrical economizers for boiler applications to large, modular, field-assemblies, Kentube has thousands of customized units in service, saving hundreds of millions of dollars in fuel each year in power plants, hospitals, universities, paper mills, canneries, and a variety of Fortune 500 companies worldwide.

Kentube also manufactures all types of economizer retrofits, special heat recovery units, tubular air heaters, stool coolers-and specializes in sulfuric acid applications, HRSG fabrication, and our own line of finned tubing.

The cylindrical design covers a range of sizes for both firetube and watertube boilers. For firetube selections the cylindrical economizer suits boiler horsepower sizes of 400 HP and larger. For watertube boilers, 100,000 PPH is the largest size handled. The cylindrical economizers are cost effective designs that fit into the stack, thus requiring no expensive transitions. Sootblowers, when required, are an integral part of the design.

The rectangular designs are used for the larger industrial watertube boilers but are adaptable for sizes from as low as 15,000 PPH to 500,000 PPH and greater. Rectangular units offer the most flexible design with configurations for vertical or horizontal gas flow, finned or bare tube designs, and many other options.

Sidel USA UnitSidel USA Systems
PO Box 1868
Atascadero, CA 93423
Telephone: 805-462-1250 or
800-668-5003 FREE

Web site:

Sidel Systems has been in operation in Canada since 1978, in the United States the company was incorporated in 1992. Sidel Systems specializes in the design and installation of hot water heating systems for commercial greenhouses.

The Sidel SRU series waste heat recovery units are built in North America in accordance with the ASME codes. An SRU flue gas waste heat recovery system can be installed with any natural gas or LPG fired power burner boiler or heating unit. When the heating unit is required to operate on oil the flue gases will have to be bypassed from the waste heat recovery unit. Flue gas recovery systems have been used around the world for over 20 years. In most applications savings of 12-15% are realized.

Standard sizes are for equipment with inputs from 2.1 to 42 million BTUper hour, 50 to 1,000 Boiler Horse Power.

Source: Text Bob Fegan 3/2003; manufacturers descriptions and graphics from their web sites referenced and linked above 3/2003; rev 1/2004; schematic of economizer from SpiraxSarco learning web site 3/2005; rev 3-2005; rev 9/2007;

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