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Turndown ratio refers to the width of the operational range of a device, and is defined as the ratio of the maximum capacity to minimum capacity. For example, a device with a maximum output of 10 units and a minimum output of 2 units has a turndown ratio of 5. The term is commonly used with measurement devices and combustion plant like boilers and gasifiers.
In flow measurement, the turndown ratio indicates the range of flow that a flow meter is able to measure with acceptable accuracy. It is also known as rangeability. It is important when choosing a flow meter technology for a specific application. If a gas flow to be measured is expected to vary between 100,000 m3 per day and 1,000,000 m3 per day, the specific application has a turndown ratio of at least 10:1. Therefore, the meter requires a turndown ratio of at least 10:1. For example: if the meter had an advertised maximum flow of 2,000,000 m3 per day then the required turndown ratio would be 20:1.[ citation needed ]
The turndown ratio of each type of meter is limited by theoretical considerations and by practical considerations. For example, orifice meters create a pressure drop in the measured fluid proportional to the square of the velocity. Therefore, the range of differential pressure can become too large and compromise accuracy. It can also create process problems such as hydrate formation, and in the case of measuring the discharge of a compressor, there is a limit to how much pressure loss is acceptable.
The examples are here for gas flow, but the same meter types can be used on liquids as well, with similar turndown ratios. Note that meter manufacturers state their products' turndown ratios—a specific product may have a turndown ratio that varies from the list below.[ citation needed ]
A thermal mass flow meter has a turndown ratio of 1000:1.
An orifice plate meter has a practical turndown ratio of 3:1.
A turbine meter has a turndown ratio of 10:1.
Rotary positive displacement meters have a turndown ratio of between 10:1 and 80:1, depending on the manufacturer and the application. Diaphragm meters are considered to have a turndown ratio of 80:1.
Multipath ultrasonic meters often have a stated turndown ratio of 50:1.
Boiler turndown ratio is the ratio of maximum heat output to the minimum level of heat output at which the boiler will operate efficiently or controllably. Many boilers are designed to operate at a variety of output levels. As the desired temperature/pressure point is approached, the heat source is progressively turned down. If pressure/temperature falls, the heat source is progressively turned up. If a boiler application requires it to operate at a low proportion of its maximum output, a high turndown ratio is required. Conversely, in applications where the operational conditions are not expected to vary significantly (for example, a large power plant), a low turndown ratio will be sufficient.
If the heating plant is only working at a small fraction of its maximum and the turndown ratio is too low, at some point the burner will still need to be shut off when the desired pressure/temperature is achieved. This in turn leads to a rapid reduction in temperature/pressure, requiring the boiler to restart. Cycling frequency can be as high as 12 times per hour. [1] This is undesirable, as flue gases are purged during both the shut-down and start-up phases, leading to energy losses and therefore inefficiency. Additionally, typical startup times for boilers are in the order of one to two minutes, leading to an inability to respond to sudden load demands. [1]
Electricity
As there are no combustion losses associated with electricity, nor delays in system startup, is it unusual to have any means of modulating down the energy supply (i.e., turndown ratio is 1).[ citation needed ]
Gas
Gas boilers can be designed for turndown ratios of 10–12 with little to no loss in combustion efficiency, while some gas burners may achieve a ratio of 35. [2] [ unreliable source? ] However, typical turndown ratio is 5. [3]
In the search for increased efficiency, even very small gas boilers have modulating burners these days. In practice only boilers with fan assisted fuel/air circulation will have the modulating feature. The fan also mixes gas and air more thoroughly, so achieving more efficient combustion. If the boiler is of the high efficiency condensing type, high turndown ratios are feasible and the higher the turndown ratio, the more efficient it will be.
Every time a gas/oil boiler stops, it has to be "purged" with cold air to remove any combustible gases that may have accumulated in the boiler before restarting. (This to prevent possible explosion.) This cold air takes heat from the boiler every time this happens. Higher turndown ratios mean fewer stops and starts and hence fewer losses.
Oil
Oil burning boilers can achieve turndown ratios as high as 20, [2] but typically only 2 to 4 with conventional burner designs. [3]
Small domestic "vaporising" (i.e. burning kerosene or 28 second oil) burners do not modulate at all and are relatively inefficient. Boilers using the pressure jet type of burner, i.e. with a fan, (usually with 35 second oil) can achieve a turndown ratio of 2, while the rotary cup type burner can achieve 4. [3] Condensing oil boilers are fairly unusual; the condensate from the combustion of oil is far more aggressive than gas, mainly due to sulphur content. These days oil companies are reducing sulphur content of oil on environmental grounds, so this may change. However, due to problem of mixing the oil and air, turndown ratios of greater than four are uncommon.
Coal
These days mechanised coal boilers only occur in large industrial plant due to the convenience and easy availability of gas. Theoretically coal burning plant can have quite a high turndown ratio, and in the days of hand firing coal boilers this was common.
On systems where coal is burned on a grate, turndown ratio must be greater than 1 due to the fact that a sudden reduction/cessation of the load can leave many tons of burning fuel on the grate.[ citation needed ]
A boiler is a closed vessel in which fluid is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.
A furnace, referred to as a heater or boiler in British English, is an appliance used to generate heat for all or part of a building. Furnaces are mostly used as a major component of a central heating system. Furnaces are permanently installed to provide heat to an interior space through intermediary fluid movement, which may be air, steam, or hot water. Heating appliances that use steam or hot water as the fluid are normally referred to as a residential steam boilers or residential hot water boilers. The most common fuel source for modern furnaces in North America and much of Europe is natural gas; other common fuel sources include LPG, fuel oil, wood and in rare cases coal. In some areas electrical resistance heating is used, especially where the cost of electricity is low or the primary purpose is for air conditioning. Modern high-efficiency furnaces can be up to 98% efficient and operate without a chimney, with a typical gas furnace being about 80% efficient. Waste gas and heat are mechanically ventilated through either metal flue pipes or polyvinyl chloride (PVC) pipes that can be vented through the side or roof of the structure. Fuel efficiency in a gas furnace is measured in AFUE.
Fluidized bed combustion (FBC) is a combustion technology used to burn solid fuels.
An afterburner is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and combat. The afterburning process injects additional fuel into a combustor ("burner") in the jet pipe behind the turbine, "reheating" the exhaust gas. Afterburning significantly increases thrust as an alternative to using a bigger engine with its attendant weight penalty, but at the cost of increased fuel consumption which limits its use to short periods. This aircraft application of "reheat" contrasts with the meaning and implementation of "reheat" applicable to gas turbines driving electrical generators and which reduces fuel consumption.
A stove or range is a device that generates heat inside or on top of the device, for -local heating or cooking. Stoves can be powered with many fuels, such as natural gas, electricity, gasoline, wood, and coal.
A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turbine (CCGT) plant, which is a kind of gas-fired power plant. The same principle is also used for marine propulsion, where it is called a combined gas and steam (COGAS) plant. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs.
A fire-tube boiler is a type of boiler invented in 1828 by Mark Seguin, in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.
A thermal power station, also known as a thermal power plant, is a type of power station in which the heat energy generated from various fuel sources is converted to electrical energy. The heat from the source is converted into mechanical energy using a thermodynamic power cycle. The most common cycle involves a working fluid heated and boiled under high pressure in a pressure vessel to produce high-pressure steam. This high pressure-steam is then directed to a turbine, where it rotates the turbine's blades. The rotating turbine is mechanically connected to an electric generator which converts rotary motion into electricity. Fuels such as natural gas or oil can also be burnt directly in gas turbines, skipping the steam generation step. These plants can be of the open cycle or the more efficient combined cycle type.
In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc.
Condensing boilers are water heaters typically used for heating systems that are fueled by gas or oil. When operated in the correct circumstances, a heating system can achieve high efficiency by condensing water vapour found in the exhaust gases in a heat exchanger to preheat the circulating water. This recovers the latent heat of vaporisation, which would otherwise have been wasted. The condensate is sent to a drain. In many countries, the use of condensing boilers is compulsory or encouraged with financial incentives.
A pulverized coal-fired boiler is an industrial or utility boiler that generates thermal energy by burning pulverized coal that is blown into the firebox.
Economizers, 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.
An oil burner is a heating device which burns #1, #2 and #6 heating oils, diesel fuel or other similar fuels. In the United States, ultra low sulfur #2 diesel is the common fuel used. It is dyed red to show that it is road-tax exempt. In most markets of the United States, heating oil is the same specification of fuel as on-road un-dyed diesel.
Engine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work. There are two classifications of thermal engines-
A wood-burning stove is a heating or cooking appliance capable of burning wood fuel, often called solid fuel, and wood-derived biomass fuel, such as sawdust bricks. Generally the appliance consists of a solid metal closed firebox, often lined by fire brick, and one or more air controls. The first wood-burning stove was patented in Strasbourg in 1557. This was two centuries before the Industrial Revolution, so iron was still prohibitively expensive. The first wood-burning stoves were high-end consumer items and only gradually became used widely.
Tankless water heaters — also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters — are water heaters that instantly heat water as it flows through the device, and do not retain any water internally except for what is in the heat exchanger coil unless the unit is equipped with an internal buffer tank. Copper heat exchangers are preferred in these units because of their high thermal conductivity and ease of fabrication. However, copper heat exchangers are more susceptible to scale buildup than stainless steel heat exchangers.
The circulating fluidized bed (CFB) is a type of fluidized bed combustion that utilizes a recirculating loop for even greater efficiency of combustion. while achieving lower emission of pollutants. Reports suggest that up to 95% of pollutants can be absorbed before being emitted into the atmosphere. The technology is limited in scale however, due to its extensive use of limestone, and the fact that it produces waste byproducts.
Pellet heating is a heating system in which wood pellets are combusted. Other pelletized fuels such as straw pellets are used occasionally. Today's central heating system which run on wood pellets as a renewable energy source are comparable in operation and maintenance of oil and gas heating systems.
A pellet boiler is a heating system that burns wood pellets. Pellet boilers are used in central heating systems for heat requirements from 3.9 kW (kilowatt) to 1 MW (megawatt) or more. Pellet central heating systems are used in single family homes, and in larger residential, commercial, or institutional applications. Pellet boiler systems run most efficiently at full load and can usually be regulated down to 30% of full load. Since the warm up phase of pellet boilers usually takes longer than for oil or gas firing systems, short burning phases have negative effects on the fuel efficiency. In order to improve energy efficiency and reduce harmful emissions, pellet boilers are usually combined with buffer systems, such as insulated water tanks.
Computational fluid dynamics (CFD) are used to understand complex thermal flow regimes in power plants. The thermal power plant may be divided into different subsectors and the CFD analysis applied to critical equipment/components - mainly different types of heat exchangers - which are of crucial significance for efficient and trouble free long-term operation of the plant.