Economizer
Encyclopedia
Economizers or economisers (UK/international), are mechanical devices intended to reduce energy consumption, or to perform another useful function such as 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
.
's innovative contribution to the design of hot air engine
s 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.
s, 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
)
of the boilers of stationary steam engine
s. 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 usually but not always seen in the fire tubes of a boiler; there the hot gases usually pass through tubes immersed in water, whereas in an economizer the water passes through tubes surrounded by hot gases. While both are heat exchange devices, in a boiler the burning gases heat the water to produce steam to drive an engine, whether piston or turbine, whereas in an economizer, some of the heat energy that would otherwise all be lost to the atmosphere is instead used to heat the water and/or air that will go into the boiler, thus saving fuel. 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.
-fired power station
s, are still fitted with economizers which are descendants of Green's original design. In this context they are often referred to as feedwater heater
s and heat the condensate from turbines
before it is pumped to the boilers.
Economizers are commonly used as part of a heat recovery steam generator 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, lowering the needed energy input, in turn reducing the firing rates needed for 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.
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 both sufficiently cool and sufficiently dry (depending on the climate) the amount of enthalpy in the air is acceptable and 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. With the appropriate controls, economizers can be used in climates which experience various weather systems. 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 Study." http://www.epa.gov/iaq/largebldgs/energy_cost_and_iaq/index.html
When the outside air's dry- and wet-bulb temperatures are low enough, water-side economizers use water cooled by a wet cooling tower
or a dry cooler (also called fluid cooler) to cool buildings without operating a chiller
. They are historically known 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.
.
For example, for a walk-in freezer that is kept at -20°F, the main refrigeration components would include: an evaporator coil (a dense arrangement of pipes containing refrigerant and thin metal fins used to remove heat from inside the freezer), fans to blow air over the coil and around the box, an air-cooled condensing unit sited outdoors, and valves and piping. The condensing unit would include a compressor and a coil and fans to exchange heat with the ambient air.
There are two types of economizers, flash and sub-cooling.
A subcooling
economizer is simply a type of heat exchanger
that uses the cold gas leaving the evaporator coil to cool the high-pressure liquid that is headed into the start of the evaporator coil via a thermal expansion valve. The gas is used to chill a chamber that has a series of pipes for the liquid running through it. The warmer gas then proceeds on to the compressor to be returned to a liquid form. The sub-cooling term refers to cooling the liquid below its boiling point. 10 degrees of sub-cooling means it is 10 degrees colder than boiling at a given pressure. An example of the effect of the sub-cooler: 105°F liquid refrigerant leaving the compressor could be reduced to 50°F liquid.
The flash economizer is more complex. http://www.refrigeration-engineer.com/forums/showthread.php?t=8244&highlight=economizer&page=2
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
. The term economizer is used for other purposes as well. Boiler
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
, powerplant, and heating, ventilating, and air-conditioning (HVAC
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
) uses are discussed in this article. In simple terms, an economizer is a heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
.
Stirling engine
Robert StirlingRobert Stirling
The Reverend Dr Robert Stirling was a Scottish clergyman, and inventor of the stirling engine.- Biography :Stirling was born at Cloag Farm near Methven, Perthshire, the third of eight children...
's innovative contribution to the design of hot air engine
Hot air engine
A hot air engine is any heat engine which uses the expansion and contraction of air under the influence of a temperature change to convert thermal energy into mechanical work...
s 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
Stirling engine
A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work....
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 boilerBoiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
s, economizers are heat exchange devices that heat fluids, usually water, up to but not normally beyond the boiling
Boiling
Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid...
point of that fluid. Economizers are so named because they can make use of the enthalpy
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
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
Boiler feedwater
Boiler feedwater is water used to supply a boiler to generate steam or hot water. At thermal power stations the feedwater is usually stored, pre-heated and conditioned in a feedwater tank and forwarded into the boiler by a boiler feedwater pump....
)
History
The first successful design of economizer was used to increase the steam-raising efficiencyThermal efficiency
In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...
of the boilers of stationary steam engine
Stationary steam engine
Stationary steam engines are fixed steam engines used for pumping or driving mills and factories, and for power generation. They are distinct from locomotive engines used on railways, traction engines for heavy steam haulage on roads, steam cars , agricultural engines used for ploughing or...
s. It was patented by Edward Green
Edward Green
Edward Green may refer to:* Edward Green , Edward "Eddie" L. Green, former head football coach at NC State and UNC Chapel Hill* Edward Howland Robinson Green , known as Colonel Green, American philatelist and numismatist...
in 1845, and since then has been known as Green's economizer. It consisted of an array of vertical cast iron
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
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 usually but not always seen in the fire tubes of a boiler; there the hot gases usually pass through tubes immersed in water, whereas in an economizer the water passes through tubes surrounded by hot gases. While both are heat exchange devices, in a boiler the burning gases heat the water to produce steam to drive an engine, whether piston or turbine, whereas in an economizer, some of the heat energy that would otherwise all be lost to the atmosphere is instead used to heat the water and/or air that will go into the boiler, thus saving fuel. 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
Soot
Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...
.
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
Claymills pumping station
Claymills Pumping Station is a restored Victorian sewage pumping station on the north side of Burton upon Trent, Staffordshire, England. It was designed by James Mansergh and used to pump sewage to the sewage farm at Etwall....
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 coalCoal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
-fired power station
Power station
A power station is an industrial facility for the generation of electric energy....
s, are still fitted with economizers which are descendants of Green's original design. In this context they are often referred to as feedwater heater
Feedwater heater
A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system...
s and heat the condensate from turbines
Steam turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....
before it is pumped to the boilers.
Economizers are commonly used as part of a heat recovery steam generator in a combined cycle
Combined cycle
In electric power generation a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators...
power plant. In an HRSG, water passes through an economizer, then a boiler
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
and then a superheater
Superheater
A superheater is a device used to convert saturated steam or wet steam into dry steam used for power generation or processes. There are three types of superheaters namely: radiant, convection, and separately fired...
. 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
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
stack gases (flue gas
Flue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...
) and transfer it to the boiler feedwater. This raises the temperature of the boiler feedwater, lowering the needed energy input, in turn reducing the firing rates needed for 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 HVAC (heating, ventilating, and air conditioning) can save energy in buildings by using cool outside air as a means of cooling the indoor space. When the enthalpyEnthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
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 both sufficiently cool and sufficiently dry (depending on the climate) the amount of enthalpy in the air is acceptable and 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
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
energy costs in cold and temperate climates while also potentially improving indoor air quality
Indoor air quality
Indoor air quality is a term referring to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants....
, but are most often not appropriate in hot and humid climates. With the appropriate controls, economizers can be used in climates which experience various weather systems. 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 Study." http://www.epa.gov/iaq/largebldgs/energy_cost_and_iaq/index.html
When the outside air's dry- and wet-bulb temperatures are low enough, water-side economizers use water cooled by a wet cooling tower
Cooling tower
Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or in the case of closed circuit dry cooling towers rely...
or a dry cooler (also called fluid cooler) to cool buildings without operating a chiller
Chiller
A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required.-Use in air conditioning:...
. They are historically known 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.
Refrigeration
Another use of the term occurs in industrial refrigeration, specifically vapor-compression refrigerationVapor-compression refrigeration
Vapor-compression refrigeration is one of the many refrigeration cycles available for use. It has been and is the most widely used method for air-conditioning of large public buildings, offices, private residences, hotels, hospitals, theaters, restaurants and automobiles...
.
For example, for a walk-in freezer that is kept at -20°F, the main refrigeration components would include: an evaporator coil (a dense arrangement of pipes containing refrigerant and thin metal fins used to remove heat from inside the freezer), fans to blow air over the coil and around the box, an air-cooled condensing unit sited outdoors, and valves and piping. The condensing unit would include a compressor and a coil and fans to exchange heat with the ambient air.
There are two types of economizers, flash and sub-cooling.
A subcooling
Subcooling
In refrigeration, subcooling is the process by which a saturated liquid refrigerant is cooled below the saturation temperature, forcing it to change its phase completely. The resulting fluid is called a subcooled liquid and is the convenient state in which refrigerants may undergo the remaining...
economizer is simply a type of heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
that uses the cold gas leaving the evaporator coil to cool the high-pressure liquid that is headed into the start of the evaporator coil via a thermal expansion valve. The gas is used to chill a chamber that has a series of pipes for the liquid running through it. The warmer gas then proceeds on to the compressor to be returned to a liquid form. The sub-cooling term refers to cooling the liquid below its boiling point. 10 degrees of sub-cooling means it is 10 degrees colder than boiling at a given pressure. An example of the effect of the sub-cooler: 105°F liquid refrigerant leaving the compressor could be reduced to 50°F liquid.
The flash economizer is more complex. http://www.refrigeration-engineer.com/forums/showthread.php?t=8244&highlight=economizer&page=2
See also
- Countercurrent exchangeCountercurrent exchangeCountercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or...
- Regenerative heat exchangerRegenerative heat exchangerA regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where the flow through the heat exchanger is cyclical and periodically changes direction. It is similar to a countercurrent heat exchanger. However, a regenerator mixes the two fluid flows while a...
- Feedwater heaterFeedwater heaterA feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system...
- Thermal efficiencyThermal efficiencyIn thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...