Economizer
Economizers, or in British English economisers, are mechanical devices intended to reduce energy consumption, or to perform another useful function like preheating a fluid.  The term economizer is used for other purposes as well.  Boiler, powerplant, and heating, ventilating, and air-conditioning (HVAC) uses are discussed in this article. In simple terms, an economizer is a heat exchanger.

# Stirling engine
Robert Stirling's innovative contribution to the design of hot air engines of 1816 was what he called the 'Economiser'. Now known as the regenerator, it stored heat from the hot portion of the engine as the air passed to the cold side, and released heat to the cooled air as it returned to the hot side. This innovation improved the efficiency 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 boiling point of that fluid.  Economizers are so named because they can make use of 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 efficiency of the boilers of stationary steam engines.  It was patented by Edward Green in 1845, and since then has been known as Green's economizer.  It consisted of an array of vertical cast iron tubes connected to a tank of water above and below, between which the boiler's exhaust gases passed.  This is the reverse arrangement to that of fire tubes in a boiler itself; there the hot gases pass through tubes immersed in water, whereas in an economizer the water passes through tubes surrounded by hot gases.  The most successful feature of Green's design of economizer was its mechanical scraping apparatus, which was needed to keep the tubes free of deposits of 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 power stations, are still fitted with economizers which are descendants of Green's original design.  In this context they are often referred to as 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 combined cycle power plant.  In an HRSG, water passes through an economizer, then 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 boiler stack gases (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" 
When the outside air's dry- and wet-bulb temperatures are low enough, water-side economizers use water cooled by a wet cooling tower to cool buildings without operating a chiller.  They are historically know as the strainer cycle, but the water-side economizer is not a true 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.