Turndown ratio

This ratio, often simply termed turndown or t/d, is the quotient of (high-fire rate)/(low-fire rate). Typical values for industrial heating operations are in the range of 3 1 to 6 1. If higher ratios are needed, the cost of the control valve and burner will increase. Because of the square root law relating pressure drop to flow, a 10 1 flow turndown ratio requires a 100 1 pressure turndown ratio a 40 1 turndown ratio requires a 1600 1 pressure turndown ratio. (See table 6.4.) 

Turndown may be limited by (a) burner stability range, flammability limits, mixing quality, (b) valve leak or process low-flow limit, either of which raises the denominator in the t/d equation, (c) flow controller range limit, (d) low-pressure air atomizer for liquid fuel, (e) flame detector range, and (f) transmitter turndown (4 to 20 ma 5 1 t/d). 

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

Atomizers The common need to disperse a liquid into a gas has spawned a large variety of mechanical devices. The different designs emphasize different advantages such as freedom from plugging, pattern of spray, small droplet size, uniformity of spray, high turndown ratio, and low power consumption. 

Critical Burner Dimensions - The position of the flameholder s and burner lines relative to the bottom or the stack is critical for efficient operation. For example, the multijet flare has a turndown ratio of 10 1 when the flameholder centerline is 125 mm below the bottom of the stack but only 2 1 when it is 150 mm above the bottom of the stack. 

The air paths within these baffles are arranged to provide the correct amount of combustion air. Turndown ratios of up to 35 to 1 are obtainable, providing the correct gas flow for air temperature control. 

At low vapor rates, valve trays will weep. Bubble cap trays cannot weep (unless they are damaged). For this reason, it is generally assumed that bubble cap trays have nearly an infinite turndown ratio. This is true in absorption processes (e.g., glycol dehydration), in which it is more important to contact the vapor with liquid than the liquid with vapor. However, this is not true of distillation processes (e.g., stabilization), in which it is more important to contact the liquid with the vapor. 

Figure 8-71 A. Nutter MVO high performance fixed valve tray with 4 1 turndown ratio. Used in new installations and to replace sieve trays. Used by permission, Nutter Engineering, Harsco Corp., Bui. CN-4.

Turndown ratio is the ratio of the maximum allowable vapor rale at or near flooding conditions (rates) to the minimum vapor rate when weeping or liquid leakage becomes significant it may be termed the minimum allowable vapor velocity [193, 199, 200]. 

Turndown ratio A term used by designers to denote ratio of minimum-allowable to operating throughput. 

Wide turndown ratio limited only by distributors... 

The turndown ratio is an indication of the ability of the burner to maintain a stable flame at lower firing rates, and is a ratio of the maximum and minimum firing rates. Turndown can be low for average burners of both natural-and forced-draft burners, 3 1 being a typical figure with 5 1a maximum although up to 40 1 is possible with special burners. 

With twin-flue design boilers, it is usual to have completely separate gas passes through the boiler with twin wetback chambers. It is then possible to operate the boiler on one flue only, which effectively doubles its turndown ratio. For example, a boiler rated at 20,000 kg/h F and A100°C may reasonably be expected to operate down to 2500 kg/h F and A100°C on oil or gas providing suitable combustion equipment and control is incorporated. If prolonged periods of single-flue operation become necessary. 

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. 

These partially overcome the weakness of the simplex pressure jet regarding turndown ratio by spilling back the unconsumed fuel at part load. In this way, the swirl velocity in the exit chamber is maintained constant but the diameter of the exit hole remains the same. A further... 

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. 

Advantages of this type include an ability to burn all fuels including those containing solid particles, good turndown ratio (4 to 10 1 typically) and an insensitivity to oil conditions such as pressure and temperature. It is widely used in shell boilers, and the only real limitation is that the cup surface has to be cleaned daily. The most common atomizer layout is shown in Figure 24.7. Variants include direct driven cup and separate mounting of the primary air fan. 

Characteristics of various atomizers are given in Table 24.1. Primary considerations are selecting the best principle for the type of fuel, the size of the burner/boiler and the type of application. Other important characteristics are ability to operate with the minimum of excess air, turndown ratio and questions of durability and maintenance. 

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

Gas burners Type Size Dual-fuel Fuel type Gas pressure Turndown ratio Flame Characteris tics Main applications... 

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

Modem burner design typically provides for high turndown ratios (8-10 1) to reduce purge cycle losses under low and variable load conditions. 

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. 

More modem industrial boiler designs also provide facilities for easy fuel changeover with simultaneous firing, low NOx burners, high turndown ratios, and low excess air capabilities (down to only 10% excess air). 

Bubblecap trays are used only when a liquid ievei must be maintained at iow turndown ratio they can be designed for lower pressure drop than either sieve or valve trays. 

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. 

Main Advantages Minimizes weeping good turndown ratio lower pressure drop can incorporate caps as stiffening members can support internals. 

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