Fuel costs are a major portion of the operating costs of fuel-fired equipment, such as boilers. With continuing increases in fuel costs and an emphasis on economy of operation, there is an ever-increasing focus on exploring all available options to increase system fuel efficiency.

Boiler economizers offer an efficient and low-cost way to save energy dollars during a time of high fuel prices. Plus, economizers are environmentally friendly and offer benefits beyond energy savings.

Using an Economizer
Economizers preheat boiler feedwater by using energy still present in exhaust-stack gases. Therefore, fuel that would have been needed by a boiler to transfer this energy to feedwater is saved. Economizers can improve fuel efficiency by 3 to 10 percent, depending on the stack-gas temperature from which energy is being recovered. The temperature to which flue gas can be cooled is dictated by the fuel. A good rule of thumb for natural-gas-fired boilers is that every 40°F drop in an economizer's gas temperature corresponds to a 1-percent efficiency gain.

With natural-gas energy prices averaging $7 per thousand standard cubic feet (MSCF) and doubling recently, the installed payback of a typical economizer is measured in months, rather than years. With these high fuel costs, what used to be a large-boiler domain now incorporates a wider range of boiler sizes. Commercially available packaged economizers run from 30 bhp to 400,000 lb per hour and larger.

Decreasing the amount of fuel consumed decreases emissions proportionately. Additionally, economizers help boilers respond to load changes faster because the feedwater entering the boilers is hotter.

Choosing an Economizer Type
With improvements in heat-transfer surfaces, economizers have become compact and more efficient. Both internally and externally enhanced tubes are used in economizers to improve the transfer of heat from flue gas to boiler feedwater. Enhancements always should be on the side that has the lower and, therefore, controlling heat-transfer coefficient. In boiler-feedwater economizers, the water side has a very high heat-transfer coefficient. Therefore, the enhancement is on the gas side in the form of finned tubes for economizers with gas on the shell side or internally ribbed tubes for firetube economizers.

There are applications in which heat from flue gas is not being transferred to water, but to a heat-transfer fluid on the tube side. In such cases, the tube-side heat-transfer coefficient also is low. Such units can benefit from double-sided augmentation. However, these have to be evaluated based on tube-side-fluid physical properties.

Economizers are available in both firetube (Photo A) and watertube designs. Watertube designs are available in either a cylindrical-coiled (Photo B) or rectangular (Photo C) design.

For boiler sizes from 30 to 400 bhp, a firetube economizer is ideal because most boilers in this range have on/off feedwater pumps. A firetube economizer has a large water reservoir. Because of the water mass' thermal inertia, the water is extremely resistant to steaming. There is no need for expensive feedwater-proportioning systems.

For boilers 400 bhp (13,800 pph) to 100,000 pph in size, a cylindrical design fits the bill. In most instances, these economizers fit directly into the stack, eliminating the need for costly transition pieces. Because there are fewer welds, cylindrical economizers have the lowest frequency of repair. These economizers are compact and can be used for a wide variety of fuels because of the availability of online soot blowing.

When a boiler's size is greater than 100,000 pph, a rectangular design is necessary. These economizers offer the most customization in terms of meeting required heat duty, gas- and water-side pressure drops, and constrained dimensions.

Things to Consider When Selecting an Economizer
Economizers that are designed properly and selected for the application have useful lives exceeding 20 years. Materials of construction should be dictated by design-temperature requirements. In other words, do not choose what is not needed. Pay particular attention to corrosion concerns. An economizer is a pressure vessel, just like a boiler. As such, it will be stamped under American Society of Mechanical Engineers (ASME) Boiler and Pressure Code, Section I: Power Boilers, or, if materials of construction require it, ASME Boiler and Pressure Code, Section VIII: Pressure Vessels.

Carbon steel is the most common material of construction for shell-side design temperatures up to 750°F. As design temperatures increase, Cor-ten, A387 Grade 11 steel, and higher stainless alloys are required.

Tube-side material selection also depends on tube-side design temperature. Tube-side wall temperature is not controlled by gas temperature, but, rather, by water temperature. Welded tubes are suitable for most applications. For higher design pressures, seamless tubes can be used.

The heart of a heat exchanger is the finned tubing. Fin-to-tube welds need a high tensile strength. Tension-wrapped fins or those with dissimilar metals should be avoided because they may have different coefficients of expansion that may result in the fins becoming detached from the tubing. In fact, high-frequency resistance-welded fins are the only kind accepted by most boiler manufacturers. Serrated fins typically are used for cleaner-burning applications (Photo D). Solid fins are used for "dirtier" applications (Photo E).

When the fuel being fired may be dirty (e.g., coal or heavy oil) cleanability is an issue. Therefore, an economizer must have provisions for soot blowing. An inline-tube layout is preferred (Figure 1). Finned tubes still can be used for fouling applications, as long as the proper fin pitch and fin-tip clearances are maintained.

For applications with particulate-laden flue gas, gas velocities should be maintained so erosion is not a concern. If need be, tube shields can be provided for select rows.

Corrosion Considerations
Corrosion is an area that is not highlighted enough or for which confusing or misleading information often is available. Users, as well as designers, need to be aware of three concerns:

• Oxygen-pitting corrosion. Feedwater contains dissolved oxygen that will come out of solution as it is heated because oxygen solubility decreases as temperature increases. This oxygen will corrode carbon-steel construction in no time. It is important to have adequately deaerated water available for economizers with carbon-steel construction. Atmospheric preheating alone is not always sufficient because the oxygen-solubility curve shows an asymtotic value beyond180°F. To remove oxygen sufficiently, mechanical deaeration in combination with chemical deaeration typically is needed. If deaerated feedwater is not available, economizers still can be used. However, it is important to use stainless-steel tubes to combat oxygen-pitting corrosion.
• Water dew-point concerns. Tube-wall temperature in an economizer is controlled predominantly by the water temperature because the tube-side heat-transfer coefficient is orders of magnitude higher than the gas-side coefficient. Moisture present in the flue gas will condense out at the water dew-point temperature (typically, 135°F) if the water temperature is lower. This can cause problems with an economizer. Acid dew-point concerns. If sulfur is present in fuel, sulfur dioxide and some sulfur trioxide will dissolve in the moisture present in the flue gas to form corrosive acids. These acids will condense out of the flue gas at much higher temperatures (approximately 200 to 250°F). This is the acid dew-point temperature. It depends on the amount of sulfur present in the fuel (Figure 2). The only economical way to prevent this is to keep the water-inlet temperature high enough so that the wall temperature stays above the acid dew-point temperature.Water or flue-gas bypassing is not the solution to acid dew-point corrosion.

Saving With an Economizer
This is a representative example of how fuel savings with an economizer are calculated:

A natural-gas-fired, 300-hp boiler operates at 80-percent efficiency with 150-psig operating pressure. Flue-gas temperature from the boiler is 460°F. Heat input into the boiler is 12.55 Mmbtu per hour. Flue-gas mass flow is 10,975 lb per hour.