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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 a heating unit used to heat up an entire building. Furnaces are mostly used as a major component of a central heating system. The name derives from Latin word fornax, which means oven. 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 boiler or residential hot water boiler. 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 and in rare cases coal or wood. 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 PVC pipes that can be vented through the side or roof of the house. Fuel efficiency in a gas furnace is measured in AFUE. Furnaces primarily run on natural gas or electricity. Furnaces that are used to boil water are called boilers.
Fluidized bed combustion (FBC) is a combustion technology used to burn solid fuels.
Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated to steam have many uses.
A stove is a device in which fuel is burned to heat either the space in which the stove is situated, or items placed on the heated stove or inside it in an oven.
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. 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 in which hot gases pass from a fire through one or (many) 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 is a power station in which heat energy is converted to electric power. In most, a steam-driven turbine converts heat to mechanical power as an intermediate to electrical power. Water is heated, turns into steam and drives a steam turbine which drives an electrical generator. After it passes through the turbine the steam is condensed in a condenser and recycled to where it was heated. This is known as a Rankine cycle. The greatest variation in the design of thermal power stations is due to the different heat sources: fossil fuel, nuclear energy, solar energy, biofuels, and waste incineration are all used. Certain thermal power stations are also designed to produce heat for industrial purposes, for district heating, or desalination of water, in addition to generating electrical power.
In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a steam turbine or a steam engine, a boiler, furnace, or a refrigerator for example. For a heat engine, thermal efficiency is the fraction of the energy added by heat that is converted to net work output. In the case of a refrigeration or heat pump cycle, thermal efficiency is the ratio of net heat output for heating, or removal for cooling, to energy input.
Condensing boilers are water heaters fueled by gas or oil. They achieve high efficiency by condensing water vapour in the exhaust gases and so recovering its latent heat of vaporisation, which would otherwise have been wasted. This condensed vapour leaves the system in liquid form, via 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) uses are discussed in this article. In simple terms, an economizer is a heat exchanger.
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 appliance capable of burning wood 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, two centuries before the Industrial Revolution, which would make iron an inexpensive and common material, so such stoves were high end consumer items and only gradually spread in use.
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An inverter compressor is a gas compressor that is operated with an inverter.
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.