Turndown

Sieve trays have a poor turndown (about 2 1). The minimum operating throughput in sieve trays is almost always restricted by excessive weeping, Turndown of sieve trays can be improved by 

Valve tray turndown is normally about 4 to 5 1. The minimum operating rate in valve trays is usually restricted by excessive weeping, but it may also be restricted by the onset of vapor channeling (Sec. 6.2,13). 

A third design (98) has a caged vertically moving plate with an or- 

Incorporate pulse firing, which takes advantage of all the energy of high fire velocity (momentum) in limited time firings instead of throttling burners to low 

There are other ways to increase mixing energies and mass flows. For example, 5 to 10% of the maximum airflow can be in a jet down the center of the fuel tube of the burner. This will allow the use of the pressure upstream of the air control valve to provide 10 of water column to accelerate the air to mix with the fuel 66.2 (10/0.356)°- =350 fps. 

The use of excess air to achieve temperature uniformity costs more fuel, but so does holding the furnace in a soak mode for a long time to achieve uniformity. An alternative to high excess air is to use pulse firing so that the desired high mass flow is either high or off. 

Which mode of heat transfer travels only in straight lines Which can go around corners  

Radiation travels straight, like light therefore has a shadow problem. Convection can go anywhere that a moving gas stream can. 

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Design continuous processes for flexible operation, e.g., high turndown rate rather than shutdown. 

A perforated plate can be flat, concave, convex, or double-dished. The main advantages of the perforated plate are that it is simple, inexpensive, easy to modify, and easy to clean. The disadvantages of a perforated plate are the possibiUty of soflds leaking, ie, weeping through it into the plenum lower turndown capabiUty than other distributors the requirement of a peripheral seal and a relatively high pressure drop requited for good distribution. 

Several cap-type distributors are shown in Figure 12. These minimize weeping and have good turndown, but are difficult to clean and modify, and are more expensive than perforated plates. A peripheral seal is also requited as for a perforated plate. 

Figure 13 shows two pipe distributors, one in a branched and one in a ring configuration. These distributors minimize weeping, have good turndown, may requite the lowest pressure drop, and avoid the need for a plenum chamber. They are also well suited to multiple-level fluid injection. The disadvantages of these distributors are that there are defluidized soHds beneath the distributor and the mechanical design is more complex. 

Spinning-cup atomisers are used ia some plants to provide finer atomisation, allowiag smaller burner chambers and easier turndown, but with the burden of added rotating equipment. Rotary kiln burners were once popular to bum lower quaHty sulfur, but few are stiU ia operation. Spray burners can be operated intermittently and used at higher rates than rotary burners. 

A compressor is typically a specially designed device, and comes with far less surplus capacity than other process components. As a result compressors merit great care in specification of flow, inlet pressure, and discharge pressure. Similarly, the control system and equipment need to be carefully matched to provide turndown with maximum efficiency. 

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. 

Pressure. Flow 0t(AP/pd - Only source of energy is from fluid being atomized. Simplicity and low cost. Limited tolerance for solids uncertain spray with high-viscosity liquids susceptible to erosion. Need for special designs (e.g., hypass) to achieve turndown. 

Oil Burners The structure of an oil flame is shown in Fig. 27-28, and Fig. 27-29 illustrates a conventional circiilar oil burner for use in boilers. A combination of stabilization techniques is used, typically including swirl. It is important to match the droplet trajectories to the combustion aerodynamics of a given burner to ensure stable ignition and good turndown performance. 

Speetrum analysis (shaft, pinions), using eddy eurrent probes at normal load and turndown... 

For positive displacement pumps, a bypass-type control valve should be furnished to set the primary lube system pressure. The valve should be able to maintain system pressure during pump startup and pump transfers, which includes relieving the capacity of one pump, while both are running. The valve should provide stable, constant pressure during these transients. Flow turndown of 8 to 1 is not unusual. Multiple valves in parallel should be used if a single valve is not suitable. The valve should be sized to operate between 10 and 90% of the flow coefficient (Cv). Additional pressure control valves should be furnished as required to pro ide any of the intermediate pressure levels. 

If turndown is a consideration, again the reciprocating compressor, with its many unloading options, does meet the challenge. This flexibility is difficult to achieve reliably in the dynamic machine and may require the compressor to operate at an unfavorable point on its characteristic curve. 

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. 

On oxygen steel conversion furnaces, primary fume control is usually achieved by a separate close-capture hood positioned over the vessel mouth. The enclosure is then used for secondary fume control during charging, turndown, tapping, and slagging. 

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