Hubbry Logo
EconomizerEconomizerMain
Open search
Economizer
Community hub
Economizer
logo
7 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Economizer
Economizer
Economizer
View on Wikipedia
from Wikipedia

Economizers (US and Oxford spelling), or economisers (UK), are mechanical devices intended to reduce energy consumption, or to perform useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, heating, refrigeration, ventilating, and air conditioning (HVAC) may all use economizers. In simple terms, an economizer is a heat exchanger.

Stirling engine

[edit]

Robert Stirling's innovative contribution to the design of hot air engines of 1816 was what he called the 'Economiser'. Later 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 the Stirling 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

[edit]

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).

Steam boilers use large amounts of energy raising feed water to the boiling temperature, converting the water to steam and sometimes superheating that steam above saturation temperature. Heat transfer efficiency is improved when the highest temperatures near the combustion sources are used for boiling and superheating, while using the residual heat of the cooled combustion gases exhausting from the boiler through an economizer to raise the temperature of feed water entering the steam drum.

An indirect contact or direct contact condensing economizer will recover the residual heat from the combustion products. A series of dampers, an efficient control system, as well as a ventilator, allow all or part of the combustion products to pass through the economizer, depending on the demand for make-up water and/or process water. The temperature of the gases can be lowered from the boiling temperature of the fluid to little more than the incoming feed water temperature while preheating that feed water to the boiling temperature. High pressure boilers typically have larger economizer surfaces than low pressure boilers. Economizer tubes often have projections like fins to increase the heat transfer surface on the combustion gas side.[1] On average over the years,[clarification needed] boiler combustion efficiency has risen from 80% to more than 95%. The efficiency of heat produced is directly linked to boiler efficiency. The percentage of excess air and the temperature of the combustion products are two key variables in evaluating this efficiency.

The combustion of natural gas needs a certain quantity of air in order to be complete, so the burners need a flow of excess air in order to operate. Combustion produces water steam, and the quantity depends on the amount of natural gas burned. Also, the evaluation of the dew point depends on the excess air. Natural gas has different combustion efficiency curves linked to the temperature of the gases and the excess air. For example, if the gases[clarification needed] are chilled to 38 °C and there is 15% excess air, then the efficiency will be 94%.[citation needed] The condensing economizer can thus recover the sensible and latent heat in the steam condensate contained in the flue gases for the process. The economizer is made of an aluminium and stainless steel alloy.[citation needed] The gases pass through the cylinder, and the water passes through the finned tubes. It condenses about 11% of the water contained in the gases.[citation needed]

History

[edit]
One of two original 1940s 'Green's Economisers' inside the Killafaddy Board Mills boiler house on the outskirts of Launceston

The first successful economizer design 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 economiser. 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 economisers 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 economisers and the associated steam engine which drove them. Another such example is the British Engineerium in Brighton & Hove, where the economiser associated with the boilers for Number 2 Engine is in use, complete with its associated small stationary engine. A third site is Coldharbour Mill Working Wool Museum, where the Green's economiser is in working order, complete with the drive shafts from the Pollit and Wigzell steam engine.

Power plants

[edit]
Main article: Feedwater heater

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 heat recovery steam generator (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 power plants 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.

HVAC

[edit]

A building's HVAC (heating, ventilating, and air conditioning) system can make use of an air-side economizer to save energy in buildings by using cool outside air as a means of cooling the indoor space. When the temperature of the outside air is less than the temperature of the recirculated air, conditioning with the outside air is more energy efficient than conditioning with 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.[2]

When the outside air's dry- and wet-bulb temperatures are low enough, a water-side economizer can use water cooled by a wet cooling tower or a dry cooler (also called a 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 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

[edit]

Cooler economizer

[edit]

A common form of refrigeration economizer is a "walk-in cooler economizer" or "outside air refrigeration system". In such a system outside air that is cooler than the air inside a refrigerated space is brought into that space and the same amount of warmer inside air is ducted outside. The resulting cooling supplements or replaces the operation of a compressor-based refrigeration system. If the air inside a cooled space is only about 5 °F warmer than the outside air that replaces it (that is, the ∆T>5 °F) this cooling effect is accomplished more efficiently than the same amount of cooling resulting from a compressor based system. If the outside air is not cold enough to overcome the refrigeration load of the space the compressor system will need to also operate, or the temperature inside the space will rise.

Vapor-compression refrigeration

[edit]

Another use of the term occurs in industrial refrigeration, specifically vapor-compression refrigeration. Normally, the economizer concept is applied when a particular design or feature on the refrigeration cycle, allows a reduction either in the amount of energy used from the power grid, in the size of the components (basically the gas compressor's nominal capacity) used to produce refrigeration, or both. For example, for a walk-in freezer that is kept at −20 °F (−29 °C), 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.

An economizer display takes advantage of the fact that refrigeration systems have increasing efficiencies at increasing pressures and temperatures. The power the gas compressor needs is strongly correlated to both the ratio and the difference, between the discharge and the suction pressures (as well as to other features like the refrigerant's heat capacity and the type of compressor). Low temperature systems such as freezers move less fluid in same volumes. That means the compressor's pumping is less efficient on low temperature systems. This phenomenon is notorious when taking in account that the evaporation temperature for a walk-in freezer at −20 °F (−29 °C) may be around −35 °F (−37 °C). Systems with economizers aim to produce part of the refrigeration work on high pressures, condition in which gas compressors are normally more efficient. Depending on the application, this technology either allows smaller compression capacities to be able to supply enough pressure and flow for a system that normally would require bigger compressors, increases the capacity of a system that without economizer would produce less refrigeration, or allows the system to produce the same amount of refrigeration using less power.

The economizer concept is linked to subcooling as the condensed liquid line temperature is usually higher than that on the evaporator, making it a good place to apply the notion of increasing efficiencies.[3] Recalling the walk-in freezer example, the normal temperature of the liquid line in that system is around 60 °F (16 °C) or even higher (it varies depending on the condensing temperature). That condition is by far less hostile to produce refrigeration, than the evaporator at −35 °F (−37 °C).

Economizer setups in refrigeration

[edit]

Several configurations of the refrigeration cycle incorporate an economizer, and benefit from this idea. The design of these systems requires expertise and extra components. Pressure drop, electronic valve control, and oil drag, must all be considered.

Two Staged System.
Two staged systems may need to double the pressure handlers installed in the cycle. The diagram displays two different thermal expansion valves (TXV) and two separate stages of gas compression.

Two staged systems and boosters

[edit]

A system is said to be a two staged set-up if two gas compressors work together in serial to produce the compression. A normal booster installation is a two staged system that receives fluid to cool down the discharge of the first compressor, before it is input to the second compressor. The fluid that arrives at the interstage of both compressors comes from the liquid line and is normally controlled by expansion, pressure and solenoid valves.

Subcooled Booster Setup.
A subcooled booster has a subcooling heat exchanger (SHX) that provides subcooling for the condensed liquid line.

A standard two staged cycle of this kind has an expansion valve that expands and modulates the amount of refrigerant incoming at the interstage. As the fluid arriving at the interstage expands, it will tend to evaporate, producing a temperature drop and cooling the second compressor's suction when mixed with the fluid discharged by the first compressor. This kind of set-up may have a heat exchanger between the expansion and the interstage, which may be a second evaporator to produce an additional refrigeration effect, though not as cool as the main evaporator (for example to produce air conditioning or for keeping fresh products). A two staged system is said to be set-up as a booster with subcooling, if the refrigerant arriving at the interstage passes through a subcooling heat exchanger that subcools the main liquid line arriving at the main evaporator of the same system.[4]

Economizer With Flash Tank.
Some screw compressor manufacturers offer them with economizer. This systems can use flash-gas for the economizer input.

Economizer gas compressors

[edit]

The need to use two compressors in a booster set-up tends to increase the cost of a refrigeration system. A two staged system also needs synchronization, pressure control and lubrication. To reduce these costs, specialized equipment has been developed.

Economizer With Subcooling Exchanger.
A subcooled economizer reduces the amount of gas compressors in the system.

Economizer screw compressors are built by several manufacturers like Refcomp, Mycom, Bitzer and York. These machines merge both compressors of a two staged system into one screw compressor with two inputs: the main suction and an interstage side entrance for higher pressure gas.[citation needed] This means there is no need to install two compressors and still benefit from the booster concept.

There are two types of economizer setups for these compressors, flash and subcooling. The latter works like a two staged booster with subcooling. The flash economizer is different because it doesn't use a heat exchanger to produce the subcooling. Instead, it has a flash chamber or tank, in which flash gas is produced to lower the temperature of the liquid before the expansion. The flash gas that is produced in this tank leaves the liquid line and goes to the economizer entrance of the screw compressor.[5]

System With Cycle Optimization.
Refrigeration cycle optimizers such as EcoPac's E-Series, keep the original design of the refrigeration cycle without modification.

Subcooling and refrigeration cycle optimizers

[edit]

The above systems produce an economizer effect by using compressors, meters, valves and heat exchangers within the refrigeration cycle. In some refrigeration systems the economizer can be an independent refrigeration mechanism. Such is the case of subcooling the liquid line by any other means that draws the heat out of the main system. For example, a heat exchanger that preheats cold water needed for another process or human use, may take heat from the liquid line, effectively subcooling it and increasing the system's capacity.[6]

Recently, machines exclusively designed for this purpose have been developed. In Chile, the manufacturer EcoPac Systems developed a cycle optimizer able to stabilize the temperature of the liquid line and allow either an increase in the refrigeration capacity of the system, or a reduction of the power consumption.[7] Such systems have the advantage of not interfering with the original design of the refrigeration system and are a way to expand a single staged system that does not possess an economizer compressor.[8]

Internal heat exchangers

[edit]

Subcooling may also be produced by superheating the gas leaving the evaporator and heading to the gas compressor.[9] These systems withdraw heat from the liquid line and heat up the gas compressor's suction line. This is a very common solution to insure that gas reaches the compressor and liquid reaches the valve. It also allows maximum heat exchanger use as minimizes the portion of the heat exchangers used to change the temperature of the fluid, and maximizes the volume in which the refrigerant changes its phase (phenomena involving much more heat flow, the base principle of vapor-compression refrigeration).

An internal heat exchanger is simply a heat exchanger that uses the cold gas leaving the evaporator coil to cool the high-pressure liquid that is headed into the beginning of the evaporator coil via an expansion device. The gas is used to chill a chamber that normally has a series of pipes for the liquid running through it. The superheated gas then proceeds on to the compressor. The subcooling term refers to cooling the liquid below its boiling point. 10 °F (5.6 °C) of subcooling means it is 10 °F colder than boiling at a given pressure. As it represents a difference of temperatures, the subcooling value is not measured on an absolute temperature scale, only on a relative scale as a temperature difference.

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Economizer

View on Grokipedia
from Grokipedia
An economizer is a mechanical device used in engineering systems to enhance energy efficiency by recovering waste heat from exhaust gases or fluids and transferring it to preheat incoming fluids, thereby reducing overall fuel consumption and operational costs. In boiler and thermal power plant applications, the economizer functions as a heat exchanger typically installed in the flue gas stack, where it captures residual heat from combustion gases—often at temperatures between 300°F and 500°F (149–260°C)—to raise the temperature of boiler feedwater, typically entering at 180–230°F (82–110°C), to near-saturation levels before it enters the boiler drum. This preheating process lowers the energy required to generate steam, improving boiler efficiency by 3–6% in non-condensing types and 10–15% or more in condensing variants that cool flue gases below their dew point to recover latent heat. Common types include non-condensing economizers, common in coal-fired plants to avoid acid corrosion by limiting gas cooling to about 250°F (120°C), and condensing economizers suited for cleaner fuels like natural gas that achieve lower temperatures around 80°F (25°C) for greater heat recovery. Key components often feature gilled tubes, coiled tubes, or finned tubes made of corrosion-resistant materials to facilitate efficient heat transfer while handling high-pressure fluids. Beyond power generation, economizers play a vital role in (HVAC) systems for commercial buildings, where they serve as dampers and control mechanisms that introduce cool outdoor air for space cooling instead of relying on energy-intensive compressors when external conditions—such as and —are favorable. This "free cooling" mode can reduce HVAC use by drawing in ambient air through vents and sensors that monitor levels, potentially cutting cooling costs by up to 30% in suitable climates. Economizers in HVAC are often roof-mounted and integrated with rooftop units, using integrated or differential controls to optimize ventilation and maintain . Economizers are also used in systems and other to optimize use. The invention of the economizer traces back to 1845, when British engineer Edward Green patented the first practical design to boost the efficiency of stationary steam engine boilers by reusing exhaust heat, marking a significant advancement in industrial thermodynamics during the early stages of the Industrial Revolution. Today, these devices contribute to lower emissions, extended equipment lifespan, and compliance with energy standards across industries, with payback periods often within a few years due to fuel savings of up to 20%.

Fundamentals

Definition and Purpose

An economizer is a heat recovery device designed to capture from gases, exhaust streams, or process fluids and transfer it to preheat incoming fluids, primarily such as , thereby reducing the overall input required for heating processes. This function is typically achieved through a configuration that extracts that would otherwise be lost to the atmosphere. Preheating of air is typically handled by separate devices known as air preheaters. The primary purpose of an economizer is to enhance in systems like boilers and power plants by recovering from exhaust streams, which can improve overall system efficiency by 5-10% depending on the design and operating conditions. For instance, non-condensing economizers typically achieve 5-7% gains by maintaining temperatures above the , while condensing types can reach 10% or more by also recovering , though they require corrosion-resistant materials to handle acidic condensate. Beyond efficiency, economizers minimize consumption, leading to significant cost savings—for example, recovering about 5% of a boiler's input capacity can reduce costs by approximately $8.40 per hour for a 500 horsepower unit operating at full load—and lower emissions of pollutants like CO2 and by reducing use proportionally. These benefits also aid compliance with energy efficiency standards, such as those in the EU Energy Efficiency Directive, which promotes heat recovery to meet broader environmental and regulatory goals. Economizers are classified into types such as finned-tube designs that directly transfer heat to feedwater. This allows for tailored applications prioritizing liquid preheating to optimize combustion.

Operating Principles

Economizers operate on the thermodynamic principle of recovering waste heat from exhaust gases, aligning with the second law of thermodynamics, which governs the spontaneous transfer of heat from higher to lower temperature regions to increase overall system entropy while enabling useful work extraction. This heat recovery primarily occurs through convective heat transfer from the hot exhaust fluid to the cooler working fluid across a separating surface, with conduction playing a secondary role within the exchanger walls; arrangements typically employ counterflow configurations for maximum efficiency, though parallel flow may be used in simpler designs to minimize pressure losses. The fundamental heat transfer rate in an economizer is quantified by the equation Q=m˙⋅Cp⋅ΔTQ = \dot{m} \cdot C_p \cdot \Delta T where QQ is the heat transfer rate, m˙\dot{m} is the of the fluid, CpC_p is the , and ΔT\Delta T is the difference across the exchanger. Efficiency is then calculated as η=Tout−TinTexhaust−Tin×100%\eta = \frac{T_{\text{out}} - T_{\text{in}}}{T_{\text{exhaust}} - T_{\text{in}}} \times 100\% where ToutT_{\text{out}} and TinT_{\text{in}} are the outlet and inlet temperatures of the preheated fluid, and TexhaustT_{\text{exhaust}} is the inlet exhaust temperature; this metric reflects the fraction of available exhaust heat captured. Design considerations emphasize material selection, such as alloys to resist from acidic condensates in gases, alongside managing drops to avoid excessive penalties from fans or pumps—typically limited to 0.5-2 kPa on the gas side. prevention involves fin spacing and coatings to mitigate ash or scale buildup, while pinch point analysis ensures optimal sizing by identifying the minimum temperature difference (usually 5-15°C) that drives without oversizing the unit. Performance is evaluated through metrics like approach temperature, the difference between the preheated fluid outlet and exhaust gas outlet, commonly maintained at 10-20°C to balance recovery and cost; reducing stack temperatures by 20-40°C can improve overall system efficiency by 1-2% by minimizing heat loss to the atmosphere. In boiler applications, this preheats feedwater to enhance combustion efficiency.

History

Early Inventions

The invention of the economizer marked a significant advancement in steam technology during the mid-19th century, driven by the escalating demands of the for more in factories, mills, and locomotives. The first successful design was patented by British engineer Edward Green in 1845, specifically tailored for Cornish boilers commonly used in stationary steam engines. This device consisted of an array of vertical tubes connected to upper and lower water tanks, through which flowed while hot es passed externally, recovering to preheat the water and thereby reducing consumption. Green's innovation addressed the inefficiency of early steam systems, where much of the heat from combustion was lost through exhaust, by integrating a simple yet effective tubular into the flue gas path. Early economizers like Green's faced notable challenges, particularly soot accumulation on the tube surfaces, which insulated the metal and impeded , and due to the acidic nature of flue gases interacting with cast-iron materials. To mitigate soot buildup, Green's design incorporated a mechanical scraping apparatus operated by levers and chains, allowing periodic without shutting down the , a feature that distinguished it from prior unsuccessful attempts and ensured practical viability. Corrosion issues in these initial cast-iron models were exacerbated by high-temperature exposure and impurities in coal-derived gases, often requiring frequent maintenance or material reinforcements to extend service life. These early devices delivered initial efficiency gains of approximately 10-15% in fuel savings for and boilers by elevating feedwater temperatures from ambient levels to around 100-150°C, depending on conditions, thereby boosting overall thermal performance without major alterations to existing setups. Adoption began rapidly in the following the 1845 patent, with Green's company installing thousands of units by the 1870s in industrial centers like and , spreading to the amid the post-Civil War manufacturing boom as American engineers adapted similar tubular designs for and mills. By the late , economizers had become standard in large-scale steam operations, laying the groundwork for further refinements in efficiency.

Key Developments

In the early , economizer technology advanced with the adoption of steel tube designs, which provided greater structural integrity and corrosion resistance compared to earlier variants. In 1910, companies like manufactured their first economizers during a period of rapid production expansion, integrating steel tubes to enhance heat recovery in industrial applications. These developments marked a shift toward more robust systems capable of handling higher pressures in generation. Further milestones in the mid-20th century included the introduction of finned surfaces in the , which significantly improved convective by increasing the surface area exposed to gases. Steel H-finned tubes, in particular, became widely adopted for their optimal balance of heating surface and self-cleaning properties in fouling-prone environments. Post-World War II, economizers were integrated into supercritical boilers as part of advanced once-through designs that eliminated traditional steam drums and enabled operations above the critical point of water (221 bar and 374°C) to boost overall cycle efficiency. Modern economizer materials have evolved to include specialized alloys such as SA-210, a grade standardized by ASME for seamless tubes in boilers and superheaters. This offers excellent high-temperature resistance, withstanding up to 538°C (1000°F) while maintaining tensile strength of at least 415 MPa, making it ideal for economizers in high-pressure environments. Since the , condensing economizers have emerged for gas-fired systems, cooling gases below the (around 57°C for ) to recover from , achieving additional efficiency gains of 5-10% over non-condensing types. Initial installations in the faced challenges with sulfur-induced but proved viable for low-sulfur fuels like . Regulatory frameworks have driven widespread economizer adoption. The U.S. Clean Air Act of 1970 mandated stricter emission controls, incentivizing efficiency improvements in boilers to reduce fuel use and pollutant output, thereby promoting economizer retrofits as a cost-effective compliance strategy. Similarly, the EU Energy Efficiency Directive, originally 2012/27/EU and revised in 2018 and 2023 as Directive (EU) 2023/1791, sets binding targets including an additional 11.7% reduction in final energy consumption by 2030 compared to projections (exceeding the prior 32.5% ambition), encouraging the integration of heat recovery technologies like economizers in industrial and building systems to meet national efficiency obligations. Post-2010, smart economizers incorporating sensors for real-time monitoring have enabled dynamic optimization, using algorithms like reinforcement learning to adjust airflow and temperature based on occupancy and weather data, reducing energy waste by up to 20% in HVAC applications. Efficiency trends reflect these innovations: early 20th-century economizers provided about 10% gains by preheating feedwater, while by the , advanced designs in combined-cycle contribute to overall efficiencies exceeding 60%, with economizers recovering up to 25% or more of in gas turbine exhaust. For instance, GE's advanced combined-cycle systems, featuring optimized heat recovery steam generators with integrated economizers, achieve net plant efficiencies of 64% in applications through enhanced materials and cycle configurations.

Developments in HVAC Economizers

While boiler economizers originated in the 19th century, HVAC economizers evolved separately in the early 20th century as part of ventilation systems in commercial buildings. Early designs used simple outdoor air dampers to provide "" during favorable weather, gaining prominence during the 1970s oil crisis with the integration of controls to optimize energy use and . By the 1980s, roof-mounted units with differential sensors became standard, reducing cooling energy by up to 20% in moderate climates without duplicating mechanical refrigeration.

Boiler and Power Plant Applications

Boiler Economizers

Boiler economizers are heat recovery devices installed on to preheat feedwater using residual from flue gases, thereby improving overall . Typically positioned between the and the or stack, they capture exhaust gases at temperatures ranging from 200°C to 300°C, transferring to the incoming feedwater through tube arrangements that can be configured horizontally or vertically depending on space constraints and flow dynamics. This placement minimizes loss to the atmosphere while ensuring the preheated water enters the at a higher , reducing the energy required for generation. Economizers come in two primary types: non-condensing and condensing, each suited to different sources and operational needs. Non-condensing economizers, commonly used with fuels like or , operate above the to avoid from acidic condensate, featuring robust designs such as fire-tube arrangements in packaged boilers where hot gases pass through tubes surrounded by feedwater. In contrast, condensing economizers, optimized for gaseous fuels like , allow temperatures to drop below the , recovering and achieving up to 90% overall heat recovery efficiency by condensing in the exhaust. Fire-tube economizers, for instance, are prevalent in smaller industrial packaged boilers due to their simplicity and ease of integration. In terms of performance, boiler economizers can reduce fuel consumption by 5% to 15% in industrial applications, depending on the initial stack temperature and load conditions. These gains stem from the sensible heat transfer in the flue gases, which would otherwise be wasted, enhancing boiler efficiency without requiring major system overhauls. Maintenance of boiler economizers is crucial to sustain performance and prevent downtime, focusing on regular cleaning to remove ash buildup from solid fuel combustion and implementing water treatment protocols to mitigate corrosion, particularly in condensing types exposed to acidic condensates. Integration with deaerators helps remove dissolved oxygen from feedwater, further protecting against internal tube corrosion and extending equipment life. Soot blowers or automated cleaning systems are often employed in high-ash environments to maintain heat transfer surfaces, ensuring optimal operation over extended periods.

Power Plant Integration

In power plants operating on the , economizers play a critical role by preheating using residual heat from es, thereby improving overall in coal-fired, nuclear, and gas-fired facilities. Positioned in the path after the , economizers ensure that the feedwater enters the at a higher temperature, reducing the energy required for evaporation and superheating while minimizing fuel consumption. This integration is essential for high-pressure , where the preheated feedwater supports generation for drive, enhancing the cycle's performance across diverse fuel types. In combined cycle power plants, economizers within heat recovery steam generators (HRSGs) recover heat from gas turbine exhaust gases, typically at 500-600°C, to generate steam for the bottoming , significantly boosting plant efficiency to over 60% on a lower heating value basis. These HRSG economizers utilize multi-pass tube bundles to maximize from the hot exhaust, enabling steam production that complements the topping and achieves higher overall conversion compared to simple cycle plants. Economizer designs in large-scale , such as 1000 MW coal-fired units, often feature finned or bare tube bundles arranged in multiple passes to handle high-temperature gases effectively, reducing stack temperatures from approximately 150°C to 120°C and capturing additional recoverable heat. However, in subject to load cycling, economizers face challenges from thermal stresses induced by rapid temperature fluctuations, which can lead to tube fatigue and require robust materials and design modifications for longevity. Despite these issues, integration yields 2-5% gains in overall plant efficiency through enhanced heat recovery and supports reduced emissions by lowering temperatures and optimizing conditions.

HVAC Applications

Heating Systems

In building heating systems, economizers enhance efficiency by recovering from exhaust air or other sources to preheat supply air or water, thereby reducing the demand on mechanical heating equipment such as boilers or furnaces. These devices are particularly valuable in (VAV) systems, where they integrate with dampers and controls to modulate and minimize loss during the heating season. Unlike their cooling counterparts, heating-focused economizers prioritize heat retention and recovery to offset the required for warming incoming outdoor air, which can otherwise represent a significant load in cold climates. Air-side economizers in heating applications employ heat recovery mechanisms, such as wheels or plate exchangers, to transfer from outgoing exhaust air to incoming outdoor air streams. This process preconditions the supply air, lowering the heating coil load and enabling systems to operate with less or consumption. In VAV configurations, these economizers use modulating dampers to maintain optimal outdoor air intake while avoiding excessive cold air infiltration that could increase heating demands; for instance, they ensure that warmer return air mixes appropriately with outdoor air when conditions permit, common in commercial buildings like offices and schools. Controls often incorporate sensors to monitor conditions and prevent over-ventilation, aligning with broader principles where transfer dominates in dry winter environments. Water-side economizers facilitate heat recovery through closed-loop systems, such as run-around coils, which circulate a glycol-water between exhaust and supply air handlers to capture and transfer across remote zones. In these setups, warm exhaust air from one area heats the fluid in an exhaust coil, which then preheats the supply air or hot water loop in another via a supply coil, effectively recovering that would otherwise be vented. This approach is ideal for buildings with separated air streams, like multi-zone offices, where direct air-to-air transfer is impractical, and it integrates seamlessly with systems to boost overall heating efficiency without cross-contamination risks. As per ASHRAE Standard 90.1-2022, energy recovery systems (including air- or water-side heat recovery economizers) are required for HVAC systems with high outdoor air fractions (e.g., outdoor air exceeding 70% of supply air or specific airflow thresholds like 5,000 cfm), with a minimum enthalpy recovery ratio of 50% or sensible recovery effectiveness of 60% in applicable cases. For cooling economizers, controls must disable excessive outdoor air intake during cold weather to avoid increasing building heating energy use and ensure compliance. Enthalpy-based sensors measure total heat content (temperature and humidity) of outdoor and return air, enabling precise modulation— for example, activating recovery only when outdoor enthalpy exceeds a setpoint to optimize preheat without introducing moisture issues. These requirements ensure economizers contribute to energy codes by promoting recovery effectiveness of at least 50-60% in applicable systems, such as those with high ventilation rates. The implementation of heating economizers yields substantial benefits, including savings of 20-30% in mild climates where transitional weather allows frequent recovery operation, primarily by cutting heating loads through preheated supply. In office buildings, integrating water-side economizers with boilers has demonstrated reductions in heating use by up to 25%, as seen in case studies of large commercial facilities where run-around coils recovered exhaust heat to support hot water loops, lowering operational costs and emissions without major retrofits.

Ventilation and Air Conditioning

In heating, ventilating, and air conditioning (HVAC) systems, cooling economizers utilize outdoor air to provide "free cooling" when ambient conditions are favorable, thereby reducing reliance on mechanical refrigeration equipment such as chillers and compressors. These devices are typically integrated into air handling units or packaged rooftop systems, where motorized dampers modulate the intake of outdoor air to mix with return air, cooling the supply air stream without activating the compressor. This approach is mandated by energy efficiency standards like ASHRAE Standard 90.1-2022 for most commercial cooling systems exceeding 33,000 Btu/h (9.7 kW) capacity in applicable climate zones, excluding extremely humid areas such as zones 1A and 1B. Control strategies for cooling economizers primarily involve differential dry-bulb temperature or enthalpy sensors to determine optimal outdoor air usage. Differential dry-bulb control compares outdoor air temperature to return air temperature, enabling economizer mode when outdoor air is at least 2–3°F (1.1–1.7°C) cooler, often with a high-limit setpoint of 55–75°F (13–24°C) depending on climate. Enthalpy-based controls, which measure total heat content (sensible plus latent), offer superior performance in humid regions by preventing the intake of moist air that could overload dehumidification; differential enthalpy activates when outdoor enthalpy falls below return air enthalpy, typically below 28 Btu/lb (65 kJ/kg). These controls ensure seamless integration with chillers, modulating damper positions from minimum outdoor air to 100% outdoor air as needed, while interlocks prevent simultaneous compressor and economizer operation to avoid inefficiencies. Economizers also support ventilation requirements by facilitating the delivery of minimum outdoor air fractions as prescribed by Standard 62.1, which specifies ventilation rates to maintain based on occupancy and space type. For instance, in variable occupancy spaces like offices or retail areas, economizers integrate with demand-controlled ventilation (DCV) systems using CO₂ sensors to dynamically adjust outdoor air , ensuring compliance while minimizing energy use—CO₂ levels above 800–1,000 ppm trigger increased ventilation without excess. This synergy recovers energy during cooling-dominant periods, as the economizer's outdoor air stream satisfies both thermal loads and air quality needs, often incorporating ventilators for management in transitional modes. Common in commercial buildings, cooling economizers are frequently housed in roof-mounted packaged units, which serve single zones or multiple spaces efficiently due to their accessibility and . During shoulder seasons—spring and fall—when outdoor temperatures range from 50–70°F (10–21°C) with moderate , these systems achieve significant gains, reducing cooling by 30–50% compared to full mechanical operation by leveraging natural . Despite these benefits, cooling economizers face limitations in high- environments, where introducing outdoor air can elevate indoor relative above 60%, potentially fostering mold growth and compromising air quality. To mitigate this, modern designs incorporate mixed-air plenums—insulated chambers that blend outdoor and return air streams for uniform temperature and distribution—along with high-limit cutoffs and dehumidification interlocks. Proper , including damper sealing and , is essential to prevent issues like over-ventilation or contaminant ingress.

Refrigeration Applications

Vapor-Compression Cycles

In vapor-compression refrigeration cycles, the economizer plays a crucial role by being placed immediately after the condenser, where it subcools the condensed and separates the flash gas that forms during partial expansion. This separation prevents the vapor from entering the , where it would otherwise occupy space and diminish the absorption capacity, while the subcooled proceeds to the expansion device with higher for improved . As a result, the capacity increases by 15-20%, allowing the system to handle greater cooling loads without proportionally larger equipment. The typical configuration employs a flash tank economizer, an intermediate pressure vessel that receives the throttled refrigerant from the condenser outlet. Here, the pressure reduction causes a fraction of the liquid (often 10-15% by mass) to evaporate into flash vapor, which is then vented and injected into the compressor at an interstage port for recompression at a lower work input compared to high-pressure compression. The remaining subcooled liquid is throttled further to the evaporator pressure. On a pressure-enthalpy (p-h) diagram, the economizer cycle shifts the liquid line to the left, achieving greater subcooling and illustrating enthalpy gains through a wider area under the evaporation process curve, which quantifies the enhanced thermodynamic efficiency. The refrigeration effect per unit mass is expressed as the enthalpy difference across the : qL=h1−h4q_L = h_1 - h_4 where h1h_1 is the at the outlet and h4h_4 is the reduced at the inlet post-economizer , leading to a larger qLq_L than in non-economized cycles. This modification typically elevates the (COP) by 5-10%, for instance, improving from a baseline of 3.0 to around 3.3 in moderate operating conditions, by minimizing irreversible losses from flash gas in the . Economizers find widespread application in commercial refrigerators and cold storage facilities, where space and energy efficiency are paramount. For example, in R-134a-based systems, the flash tank configuration reduces overall work by recompressing the separated vapor at intermediate pressure, yielding measurable energy savings while maintaining reliable low-temperature performance in environments like supermarket display cases or cooling.

Multi-Stage and Optimized Setups

In multi-stage systems, economizers are integrated between compression stages to facilitate intercooling, where a portion of the high-pressure liquid is expanded and evaporated to cool the intermediate-pressure gas from the low-stage , thereby reducing the work required for subsequent compression. This setup enhances overall by lowering the discharge temperature and improving the (COP), with studies showing up to a 10% COP improvement in two-stage configurations compared to single-stage systems. Booster compressors in these systems utilize flash gas generated during the throttling , directing it back to the low-stage to reduce superheat and increase mass flow through the , which optimizes without excessive compressor loading. In industrial applications, such as anhydrous ammonia systems, this flash gas integration allows booster compressors to operate at intermediate s (e.g., 40–80 psia), contributing to savings of approximately 1.3% per °F reduction in condensing through floating head pressure control. Subcooling optimizers, including suction-line heat exchangers, further enhance performance by transferring heat from the liquid refrigerant line to the line, providing additional cooling of the liquid before expansion and preventing flash gas formation. In direct expansion (DX) refrigeration systems, the incorporation of such economizers can improve by enhancing and evaporator effectiveness. Internal heat exchangers operate on a counterflow between the exiting the condenser and the gas from the , maximizing for and . The effectiveness of these exchangers is quantified by the equation ϵ=Tcond−TsubcoolTcond−Tsuction\epsilon = \frac{T_{\text{cond}} - T_{\text{subcool}}}{T_{\text{cond}} - T_{\text{suction}}} where TcondT_{\text{cond}} is the condenser saturation , TsubcoolT_{\text{subcool}} is the subcooled , and TsuctionT_{\text{suction}} is the gas entering the exchanger; higher ϵ\epsilon values indicate better performance in boosting cycle efficiency. Economizer configurations in cascade systems enable significant COP enhancements—up to 80% over traditional baselines—for ultra-low temperature applications such as -40°C freezers, while minimizing energy use in varying ambient conditions. Recent advancements as of include integration with low-GWP refrigerants like CO₂ in transcritical cycles using vapor injection and expander-boosted to further improve efficiency and comply with environmental regulations.

Specialized Applications

Stirling Engines

In Stirling cycle engines, the regenerator functions as an internal economizer by storing heat extracted from the during the cooling phase and releasing it back during the heating phase, thereby minimizing thermal losses between the hot and cold sides of the engine. This heat recovery mechanism significantly enhances overall efficiency, with studies indicating that engines equipped with regenerators require up to five times less external heat input to achieve comparable performance levels compared to those without. The regenerator is particularly effective in alpha-type configurations, where separate compression and expansion are connected via the regenerator matrix, and in beta-type configurations, where it is integrated coaxially within a single housing both the displacer and power , allowing for compact in both setups. External economizers in engines take the form of additional heat exchangers, such as preheaters, that utilize residual from the combustion exhaust or heat source to further warm the —often , chosen for its superior thermal conductivity and low —before it enters the primary heater. For instance, in solar-powered engines employing dish concentrators, these preheaters contribute to achieving thermal efficiencies approaching 30%, as demonstrated in systems where concentrated drives the external input. The core principle of the regenerator involves cyclic facilitated by the displacer , which shuttles the through the porous matrix during isochoric processes: absorbing heat as the fluid moves from the hot side to the cold side and releasing it on the return path to preheat the fluid approaching the hot side. The effectiveness of this regenerator, denoted as ηreg\eta_{\text{reg}}, quantifies the heat recovery and is given by ηreg=Thot,out−Tcold,inThot,in−Tcold,in,\eta_{\text{reg}} = \frac{T_{\text{hot,out}} - T_{\text{cold,in}}}{T_{\text{hot,in}} - T_{\text{cold,in}}}, where Thot,inT_{\text{hot,in}} and Thot,outT_{\text{hot,out}} are the inlet and outlet temperatures on the hot side, and Tcold,inT_{\text{cold,in}} is the inlet temperature on the cold side; high values of ηreg\eta_{\text{reg}} (approaching 0.95 or more) are essential for near-ideal cycle efficiency. Stirling engines incorporating advanced regenerators find applications in micro-combined heat and power (micro-CHP) systems for residential use, where they convert or heat into and usable hot water with overall efficiencies exceeding 90% when accounting for both outputs. In space power generation, has developed free-piston convertors for radioisotope systems, leveraging regenerators to achieve reliable, long-duration operation in deep-space missions with minimal mass. However, challenges persist, particularly pressure drops in porous regenerator matrices, which can reduce net power output by increasing pumping losses; numerical analyses show these drops are pronounced during oscillatory flows, necessitating optimized geometries like sintered metals to balance and fluid friction.

Industrial and Emerging Uses

In , economizers play a crucial role in recovery, particularly in high-temperature sectors like production and manufacturing. In kilns, economizers capture heat from clinker cooling air (typically 350–900°F) and exhaust gases (300–500°F) to preheat combustion air or generate , enhancing overall plant efficiency without requiring advanced R&D. Similarly, in mills, finned-tube economizers recover from exhaust gases in blast furnaces (1,950–2,050°F) and furnaces (1,000–1,800°F), with mini-mills achieving up to 35% recovery of heat input through integration with organic Rankine cycles. For processes involving ovens, such as in food manufacturing or coatings applications, economizers utilize clean exhaust gases (300–600°F) to preheat process air or , recovering sensible and to support energy-efficient operations. Emerging applications of economizers extend to cooling and . In s, water-side economizers use condenser water loops to directly cool chilled water, bypassing mechanical chillers during favorable outdoor conditions and reducing cooling energy demands by up to 50% in temperate climates. For low-grade heat sources, economizers integrate with organic Rankine cycles (ORCs) in geothermal and plants, where they preheat working fluids from exhaust streams, contributing to cycle efficiencies of 10–25% depending on heat source temperature. Innovations in economizer design address challenging environments, such as high . Post-2015 developments in membrane-based ventilators enable selective transfer of heat and moisture across semi-permeable membranes, achieving annual HVAC energy savings of 18–49% in humid climates like through zone-level integration in systems. In smart factories, AI-optimized controls enhance economizer performance by analyzing real-time sensor data from IoT devices to dynamically match loads, monitor losses, and implement recovery strategies, thereby improving overall energy efficiency in processes. Case studies illustrate practical benefits in specialized sectors. In hybrid vehicles, heat recovery systems function as automotive economizers by capturing to accelerate warm-up and support cabin heating, improving fuel economy by reducing cold-start inefficiencies and lowering emissions. In petrochemical plants, boiler economizers recover heat to preheat feedwater, yielding 5–10% fuel savings with a typical of two years, as demonstrated in refining operations where energy costs constitute a major expense.

References

  1. https://www.electrical4u.com/economiser-in-thermal-power-plant-economiser/
  2. https://thermodyneboilers.com/economizers/
  3. https://www.powerhouse.com/en/boiler-education/maintenance-care/proper-boiler-feed-water-temperature-management
  4. https://svach.lbl.gov/what-is-an-economizer/
  5. https://www.epa.gov/indoor-air-quality-iaq/indoor-air-quality-and-energy-efficiency
  6. https://www.gracesguide.co.uk/E._Green_and_Son
  7. https://odr.chalmers.se/bitstreams/eda69ff4-e6d7-4c22-9c3e-6dbffa3ec154/download
  8. http://cleanboiler.org/learn-about/boiler-efficiency-improvement/efficiency-index/economizers/
  9. https://www.energy.gov/sites/prod/files/2014/05/f15/steamhandbook.pdf
  10. https://www.osti.gov/servlets/purl/244535
  11. https://energy.ec.europa.eu/topics/energy-efficiency/energy-efficiency-targets-directive-and-rules/energy-efficiency-directive_en
  12. https://processbarron.com/increasing-boiler-efficiency-economizerpreheater-combos/
  13. https://www.researchgate.net/publication/223189731_Second_law_analysis_of_a_waste_heat_recovery_steam_generator
  14. https://asmedigitalcollection.asme.org/heattransfer/article/134/11/111801/455268/Geometric-and-Thermodynamic-Optimization-of-a-Heat
  15. https://www.ijert.org/research/thermal-design-of-economizer-for-waste-heat-recovery-boiler-whrb-IJERTV7IS100037.pdf
  16. https://www.cainind.com/downloads/Cain-Boiler-Economizer.pdf
  17. https://www.researchgate.net/publication/245392682_Thermoeconomic_optimization_of_the_pinch_point_and_gas-side_velocity_in_heat_recovery_steam_generators
  18. https://apps.dtic.mil/sti/tr/pdf/ADA187419.pdf
  19. https://www.magothermal.com/common-maintenance-issues-for-economizer/
  20. https://insulation.org/io/articles/the-history-of-the-steam-generating-boiler-and-industry/
  21. https://www.babcock.com/home/about/corporate/history
  22. https://www.eriger.co.za/products-services/economiser-tubes
  23. https://www.sciencedirect.com/topics/engineering/supercritical-boiler
  24. https://www.solitaire-overseas.com/asme-sa210-a1-carbon-steel-tubes-supplier-exporter.html
  25. https://www.vicsteelpipe.com/info/what-is-the-temperature-range-of-sa210-boiler-97950096.html
  26. https://www.slideshare.net/slideshow/condensing-economizers/66271803
  27. https://www.epa.gov/clean-air-act-overview/clean-air-act-and-economy
  28. https://dl.acm.org/doi/fullHtml/10.1145/3600100.3625685
  29. https://visualizingenergy.org/power-plant-efficiency-since-1900/
  30. http://sites.science.oregonstate.edu/~hetheriw/energy/topics/doc/elec/natgas/cc/advanced.technology.combined.cycle.GER3936a.pdf
  31. https://www.ashrae.org/technical-resources/ashrae-handbook/hvac-systems-and-equipment
  32. https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf
  33. https://www.epa.gov/sites/default/files/2015-08/documents/coalfired.pdf
  34. https://www.powermag.com/how-overcycling-induces-economizer-tube-failures/
  35. https://www.ijerd.com/paper/vol4-issue8/O040894100.pdf
  36. https://www.pnnl.gov/projects/om-best-practices/air-side-economizers
  37. https://www.trane.com/content/dam/Trane/Commercial/global/products-systems/education-training/engineers-newsletters/airside-design/enews3_3.pdf
  38. https://www.energystar.gov/products/data_center_equipment/16-more-ways-cut-energy-waste-data-center/use-air-side-economizer
  39. https://hccoils.com/coils/run-around-coils/
  40. https://www.ashrae.org/file%2520library/technical%2520resources/standards%2520and%2520guidelines/standards%2520addenda/90_1_2019_y_20211209.pdf
  41. https://www.energycodes.gov/sites/default/files/2023-11/ASHRAE90-1_2022_20231116_webinar.pdf
  42. https://hvacseoagency.com/blog/hvac-economizers-energy-savings
  43. https://www.tandfonline.com/doi/abs/10.1080/00038628.1995.9696772
  44. https://www.trane.com/content/dam/Trane/Commercial/global/products-systems/education-training/engineers-newsletters/airside-design/ADM-APN054-EN_05202015.pdf
  45. https://www.trane.com/content/dam/Trane/Commercial/global/products-systems/education-training/engineers-newsletters/airside-design/admapn020en-0406.pdf
  46. https://www.mdpi.com/2071-1050/15/14/10777
  47. https://www.pnnl.gov/main/publications/external/technical_reports/pnnl-20955.pdf
  48. https://patents.google.com/patent/US5692389A/en
  49. https://www.conceptsnrec.com/blog/using-an-economizer-in-centrifugal-compressor-chiller-cycles
  50. https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1794&context=iracc
  51. https://core.ac.uk/download/pdf/4954823.pdf
  52. https://patents.google.com/patent/US20100058781A1/en
  53. https://irc.wisc.edu/export.php?ID=64
  54. https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=3368&context=iracc
  55. https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2761&context=iracc
  56. https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=3423&context=iracc
  57. https://art.torvergata.it/retrieve/7deff5ac-5440-4fe9-82d5-d4123f1a4fc3/IJR%20180%20%282025%29%20503-517.pdf
  58. https://www.sciencedirect.com/science/article/pii/S2214157X25003843
  59. https://uphf.hal.science/hal-03450107v1/file/Gheith2018.pdf
  60. https://ieeexplore.ieee.org/document/10086916/
  61. https://www.sciencedirect.com/science/article/abs/pii/S2451904918304566
  62. https://www.sciencedirect.com/science/article/abs/pii/S0196890424007477
  63. https://ntrs.nasa.gov/api/citations/19880003045/downloads/19880003045.pdf
  64. https://info.ornl.gov/sites/publications/files/Pub52987.pdf
  65. https://www.energystar.gov/products/data_center_equipment/16-more-ways-cut-energy-waste-data-center/consider-water-side-economizers
  66. https://www.turboden.com/solutions/1053/waste-heat-recovery
  67. https://www.sciencedirect.com/science/article/abs/pii/S0378778815303236
  68. https://edgemethods.com/temp-unleashing-ai-and-iot-for-enhanced-energy-efficiency-in-smart-factories/
  69. https://www.sae.org/papers/development-effective-exhaust-gas-heat-recovery-system-a-hybrid-electric-vehicle-2011-01-1171
  70. https://www.osti.gov/servlets/purl/937505
Add your contribution
Related Hubs
User Avatar
No comments yet.