Economizer

Stirling engine

Robert Stirling's innovative contribution to the design of Stirling's engine enough to make it commercially successful in particular applications, and has since been a component of every air engine that is called a Stirling engine.

Boilers

In boilers, economizers are heat exchange devices that heat fluids, usually water, up to but not normally beyond the enthalpy in fluid streams that are hot, but not hot enough to be used in a boiler, thereby recovering more useful enthalpy and improving the boiler's efficiency. They are a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to preheat the cold water used to fill it (the feed water).

History

The first successful design of economizer was used to increase the steam-raising soot.

Economizers were eventually fitted to virtually all stationary steam engines in the decades following Green's invention. Some preserved stationary steam engine sites still have their Green's economizers although usually they are not used. One such preserved site is the Claymills Pumping Engines Trust in Staffordshire, England, which is in the process of restoring one set of economizers and the associated steam engine which drove them.

Powerplants

Modern-day boilers, such as those in coal-fired feedwater heaters and heat the condensate from turbines before it is pumped to the boilers.

Economizers are commonly used as part of a HRSG in a boiler and then a superheater. The economizer also prevents flooding of the boiler with liquid water that is too cold to be boiled given the flow rates and design of the boiler.

A common application of economizers in steam powerplants is to capture the waste heat from flue gas) and transfer it to the boiler feedwater. This raises the temperature of the boiler feedwater thus lowering the needed energy input, in turn reducing the firing rates to accomplish the rated boiler output. Economizers lower stack temperatures which may cause condensation of acidic combustion gases and serious equipment corrosion damage if care is not taken in their design and material selection.

HVAC

Air-side economizers can save energy in buildings by using cool outside air as a means of cooling the indoor space. When the enthalpy of the outside air is less than the enthalpy of the recirculated air, conditioning the outside air is more energy efficient than conditioning recirculated air. When the outside air is sufficiently cool, no additional conditioning of it is needed; this portion of the air-side economizer control scheme is called free cooling.

Air-side economizers can reduce HVAC energy costs in cold and temperate climates while also potentially improving indoor air quality, but are most often not appropriate in hot and humid climates. For information on how economizers and other controls can affect energy efficiency and indoor air quality in buildings, see the U.S. Environmental Protection Agency report, "Energy Cost and IAQ Performance of Ventilation Systems and Controls" [1]

When the outside air's dry- and wet-bulb temperatures are low enough, water-side economizers use water cooled by a wet thermodynamic cycle. Also, instead of passing the cooling tower water through a strainer and then to the cooling coils, which causes their fouling, more often a plate-and-frame heat exchanger is inserted between the cooling tower and chilled water loops.

Good controls, and valves or dampers, as well as maintenance, are needed to ensure proper operation of the air- and water-side economizers.

Examples of economizers in chillers would be Flasc Economizers, Flash Economizers, and Flash Flasc Economizers.

See also

• Countercurrent exchange
• Regenerator
• Regenerative heat exchanger

• Feedwater heater
• Thermal efficiency

References

• Richard L. Hills (1989). Power from steam: A history of the stationary steam engine. Cambridge University Press. ISBN 0-521-45834-X.