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54-403: HRSGs consist of four major components: the economizer , evaporator , superheater and water preheater . The different components are put together to meet the operating requirements of the unit. See the attached illustration of a Modular HRSG General Arrangement. Modular HRSGs can be categorized by a number of ways such as direction of exhaust gases flow or number of pressure levels. Based on

108-443: 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 . 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 . Subcooling may also be produced by superheating

162-454: A fully assembled unit from the factory. They can be used in waste heat or turbine (usually under 20 MW) applications. The packaged HRSG can have a water-cooled furnace, which allows for higher supplemental firing and better overall efficiency. Some HRSGs include supplemental, or duct firing. These additional burners provide additional energy to the HRSG, which produces more steam and hence increases

216-427: A high degree of flexibility as the sections are allowed to grow or contract based on the heat load being received from the gas turbine. The absence of drums allows for quick changes in steam production and fewer variables to control, and is ideal for cycling and base load operation. With proper material selection, an OTSG can be run dry, meaning the hot exhaust gases can pass over the tubes with no water flowing inside

270-586: A reheat/IP (intermediate pressure) section, and an HP (high pressure) section. Each section has a steam drum and an evaporator section where water is converted to steam. This steam then passes through superheaters to raise the temperature beyond the saturation point. The steam and water pressure parts of an HRSG are subjected to a wide range of degradation mechanisms, for example creep , thermal fatigue , creep-fatigue, mechanical fatigue, Flow Accelerated Corrosion (FAC), corrosion and corrosion fatigue, amongst others. Packaged HRSGs are designed to be shipped as

324-456: A solid (see pitch drop experiment ) as well. In particle physics , the concept is extended to include fluidic matters other than liquids or gases. A fluid in medicine or biology refers to any liquid constituent of the body ( body fluid ), whereas "liquid" is not used in this sense. Sometimes liquids given for fluid replacement , either by drinking or by injection, are also called fluids (e.g. "drink plenty of fluids"). In hydraulics , fluid

378-477: A solid, shear stress is a function of strain , but in a fluid, shear stress is a function of strain rate . A consequence of this behavior is Pascal's law which describes the role of pressure in characterizing a fluid's state. The behavior of fluids can be described by the Navier–Stokes equations —a set of partial differential equations which are based on: The study of fluids is fluid mechanics , which

432-407: 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

486-403: 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

540-418: 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

594-972: Is a term which refers to liquids with certain properties, and is broader than (hydraulic) oils. Fluids display properties such as: These properties are typically a function of their inability to support a shear stress in static equilibrium . By contrast, solids respond to shear either with a spring-like restoring force —meaning that deformations are reversible—or they require a certain initial stress before they deform (see plasticity ). Solids respond with restoring forces to both shear stresses and to normal stresses , both compressive and tensile . By contrast, ideal fluids only respond with restoring forces to normal stresses, called pressure : fluids can be subjected both to compressive stress—corresponding to positive pressure—and to tensile stress, corresponding to negative pressure . Solids and liquids both have tensile strengths, which when exceeded in solids creates irreversible deformation and fracture, and in liquids cause

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648-427: 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,

702-417: 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

756-428: Is not measured on an absolute temperature scale, only on a relative scale as a temperature difference. Fluid In physics , a fluid is a liquid , gas , or other material that may continuously move and deform ( flow ) under an applied shear stress , or external force. They have zero shear modulus , or, in simpler terms, are substances which cannot resist any shear force applied to them. Although

810-461: Is not taken in their design and material selection. 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

864-459: 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

918-539: 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. A standard two staged cycle of this kind has an expansion valve that expands and modulates

972-544: Is subdivided into fluid dynamics and fluid statics depending on whether the fluid is in motion. Depending on the relationship between shear stress and the rate of strain and its derivatives , fluids can be characterized as one of the following: Newtonian fluids follow Newton's law of viscosity and may be called viscous fluids . Fluids may be classified by their compressibility: Newtonian and incompressible fluids do not actually exist, but are assumed to be for theoretical settlement. Virtual fluids that completely ignore

1026-439: 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. Recently, machines exclusively designed for this purpose have been developed. In Chile, the manufacturer EcoPac Systems developed

1080-403: 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 . 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

1134-402: 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

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1188-490: 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

1242-503: 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. In boilers , economizers are heat exchange devices that heat fluids, usually water, up to but not normally beyond

1296-405: 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

1350-549: The HRSG will have to be split and the SCR placed in between the two sections. Some low-temperature NOx catalysts have recently come to market that allow for the SCR to be placed between the evaporator and economizer sections (350–500 °F [177–260 °C]). A specialized type of HRSG without boiler drums is the once-through steam generator. In this design, the inlet feedwater follows a continuous path without segmented sections for economizers, evaporators, and superheaters. This provides

1404-409: The HRSG. Some may contain a selective catalytic reduction system to reduce nitrogen oxides (a large contributor to the formation of smog and acid rain) or a catalyst to remove carbon monoxide . The inclusion of an SCR dramatically affects the layout of the HRSG. NOx catalyst performs best in temperatures between 650 and 750 °F (343–399 °C). This usually means that the evaporator section of

1458-462: 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

1512-400: The appropriate controls, economizers can be used in climates which experience various weather systems. 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

1566-483: 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

1620-489: 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. On average over the years, boiler combustion efficiency has risen from 80% to more than 95%. The efficiency of heat produced

1674-420: 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

Heat recovery steam generator - Misplaced Pages Continue

1728-412: 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

1782-486: 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 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. This means there

1836-683: 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

1890-635: 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. 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

1944-552: 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

1998-416: 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 are chilled to 38 °C and there is 15% excess air, then the efficiency will be 94%. The condensing economizer can thus recover the sensible and latent heat in the steam condensate contained in the flue gases for

2052-618: The flow of exhaust gases, HRSGs are categorized into vertical and horizontal types. In horizontal type HRSGs, exhaust gas flows horizontally over vertical tubes whereas in vertical type HRSGs, exhaust gas flow vertically over horizontal tubes. Based on pressure levels, HRSGs can be categorized into single pressure and multi pressure. Single pressure HRSGs have only one steam drum and steam is generated at single pressure level whereas multi pressure HRSGs employ two (double pressure) or three (triple pressure) steam drums. As such triple pressure HRSGs consist of three sections: an LP (low pressure) section,

2106-408: 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

2160-408: 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

2214-401: The gas leaving the evaporator and heading to the gas compressor . 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

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2268-421: 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. The need to use two compressors in a booster set-up tends to increase

2322-623: The notion of increasing efficiencies. 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). 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. A system

2376-627: The onset of cavitation . Both solids and liquids have free surfaces, which cost some amount of free energy to form. In the case of solids, the amount of free energy to form a given unit of surface area is called surface energy , whereas for liquids the same quantity is called surface tension . In response to surface tension, the ability of liquids to flow results in behaviour differing from that of solids, though at equilibrium both tend to minimise their surface energy : liquids tend to form rounded droplets , whereas pure solids tend to form crystals . Gases , lacking free surfaces, freely diffuse . In

2430-409: The output of the steam turbine . Generally, duct firing provides electrical output at lower capital cost. It is therefore often utilized for peaking operations. HRSGs can also have diverter valves to regulate the inlet flow into the HRSG. This allows the gas turbine to continue to operate when there is no steam demand or if the HRSG needs to be taken offline. Emissions controls may also be located in

2484-478: 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

2538-493: 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. Another use of the term occurs in industrial refrigeration, specifically vapor-compression refrigeration . Normally,

2592-530: The process. The economizer is made of an aluminium and stainless steel alloy. 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. 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

2646-403: 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

2700-421: 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. 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

2754-427: The term fluid generally includes both the liquid and gas phases, its definition varies among branches of science . Definitions of solid vary as well, and depending on field, some substances can have both fluid and solid properties. Non-Newtonian fluids like Silly Putty appear to behave similar to a solid when a sudden force is applied. Substances with a very high viscosity such as pitch appear to behave like

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2808-410: The tubes. This eliminates the need for a bypass stack and exhaust gas diverter system which is required to operate a combustion turbine with a drum-type HRSG out of service. Economizer 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

2862-517: 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

2916-479: 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. A common form of refrigeration economizer

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