Turndown Ranges

Choose a type of boiler with a fairly constant and high efficiency over its full turndown range. Obtain its efficiency curve. 

Limited turndown with conventional controls, and are somewhat nonlinear over the turndown range. This nonlinearity is particularly problematic during volumetric discharge as frequent calibrations may be required to guarantee accurate feed rates. 

TABLE 6.4 Some typical turndown ranges (for listed pressures only). 

A wide range of loadabihty (turndown ratio) is required. Valve trays are particiilarly desirable in this case. 

Generally, this style of unit will remove particles of 12 to 15 microns efficiently. The typical droplet separator is shown for an air-water system in Figure 4-17A. This will vary for other systems with other physical properties. The variations in capacity (turndown) handled by these units is in the range of 3 to 6 times the low to maximum flow, based on k values [33]. 

Trays are usually designed with F-factor from 0.25 to 2.0 for a turndown of 8 1. Pressure drop per theoretical stage falls between 3 and 8 mm Hg. Note that bubble cap trays are on the high side and sieve trays are on the lower end of the range. Varying tray spacing and system efficiency, the HETP for trays are usually between 24 in. and 48 in. [133]. The C-factor is the familiar Souders and Brown capacity equation. 

Power savings of 15% or more may result from the use of such vanes,depending on the design and final operating point on the compressor s performance curves. The inlet guide vanes increase the turndown and, thus, increase the operating range. 

Oil-, gas- and dual-fired boilers are available with a range of combustion appliances. The smaller units have pressure jet-type burners with a turndown of about 2 1 while larger boilers may have rotary cup, medium pressure air (MPA) or steam-atomizing burners producing a turndown ratio of between 3 1 and 5 1, depending upon size and fuel. The majority has rotary cup-type burners, while steam- or air-atomizing burners are used where it is essential that the burner firing is not interrupted even for the shortest period. 

Steam is the preferred atomizing medium, since it is more economic than compressed air. Steam consumption is typically less than 0.5 per cent of the fuel burnt on a mass basis, although this rises in direct proportion to turndown ratio. On very large burners, the steam flow is modulated in proportion to fuel burnt. Turndown ratios range from about 5 1 for small shell boilers to 12 1 in watertube applications, making this one of the most versatile burners. The steam condition is important in that it must be dry saturated or slightly superheated at the nozzle to avoid condensate formation. On small or non-continuously running plant where no steam is available for start-up a compressed air supply must be provided until steam becomes available from the boiler. 

Oil burners Type Size range Dual-fuel capability Fuel type Atomizing Atomizing pressure viscosity (bar) (cS) Turndown ratio Flame characteristics Main applications... 

Coal burners Type Size range (MW) Fuels Grate thermal loading (mW/m ) Bed thickness Turndown ratio Ashing system Main applications... 

Boilers are designed to operate within a range of firing levels the greater this range (turndown ratio), the more flexible they are in operation. However, boiler efficiency changes with variation in steam output, and most boilers produce their maximum efficiency at about 80 to 85% of MCR. 

Inflexibility. A given design is not easily adapted to a range of conditions. Performance is strongly dependent upon flow rate and feed composition, and the turndown ratio (range of operation) is small. 

Dual- Orifice 20-200 A variety of aircraft and industrial gas turbines Good atomization, Turndown ratio 50 1, Relatively constant spray angle Poor atomization in transition range, Complexity in design, Susceptibility of small passages to blockage... 

One of the problems in combustors that utilize premixed flames is the attainment of stable performance over an extended range of operation (turndown ratio). The condition, at which the combustion wave is driven back causing the flame to be extinguished when the flow velocity exceeds the burning velocity everywhere in the flow field, is of particular interest to this study. The physical mechanisms responsible for the blow-out limits and flame stabilization of jet flames is still a topic of extensive research [1, 2]. The flame stabilization technique discussed in this paper is aimed to control the velocity gradient in the region close to... 

In summary, from all currently available test data, the four-in-one 33-tnm (1.4-in.] hydrocyclones exhibit high efficiencies over a range of 318 to 1113 m3/d water (2,000 to 7,000 BWPD] per unit, giving a turndown ratio of 3.5 1. The upper limit is considered to be a pressure-drop restriction on available reject rate, not droplet shearing caused by the high flow rate. 

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