Draft control

The steam generator is a balanced draft, controlled circulation, multichamber unit which incorporates NO control and final burnout of the fuel-rich MHD combustion gases. The MHD generator exhaust is cooled in a primary radiant chamber from about 2310 to 1860 K in two seconds, and secondary air for afterburning and final oxidation of the gas is introduced in the secondary chamber where seed also condenses. Subsequent to afterburning and after the gas has been cooled down sufftciendy to soHdify condensed seed in the gas, the gas passes through the remaining convective sections of the heat recovery system. 

Mercury from cinnabar ore 225 tons ore/day (95% recovery) (2) 18,0 ft. diam, 8 hearth furnaces Furnaces fired on hearths 3 to 7, inclusive retention time of 1,0 hr, furnaces are oil-fired with low-pressure atomizing air burners all air, both primary and secondary, introduced through the burners draft control by Monel cold-gas fans downstream from mercury condensers. 

Clelland, P. J., "Recent Furnace Draft Control Considerations," Proc. IEEE Conf. on Dec, and Cont., 1977, 1101. 

Flue gas is supplied to the scrubbing system with a forced draft fan. Typically, the fan is designed for 20 inches of water pressure drop and is equipped with the typical draft control mechanism. A secondary damper is connected to the vacuum side of the fan. The secondary damper admits makeup air and thereby automatically maintains optimum gas flow in the scrubber during varying loads in the boiler operation. 

Draft Control Avoid positive pressure to prevent safety hazards and provide sufficient primary air for burners. 

I feel the need to provide additional comments on excess air as many plants have an O2 reduction program. O2 reduction (or minimum excess air) must be built upon the basis of proper draft control. Minimum excess air for the fired heater can be obtained when it is reduced to the point where combustibles begin to appear in the stack. For modern fired heaters, this occurs at 8% excess air equivalent to 1.8% of oxygen level in the flue gas. However, practical constraints prevent achieving this minimum excess air in operation, and these constraints include variations in fuel quality, feed rates, and other process variables. Thus, operation without flame impingement sets the limit for practical minimum excess air. The optimal flue... 

Draft should be monitored and maintained as required for the specific fired heater design. Fired heaters without draft control should be periodically checked. 

High/low fiimace pressure Loss of draft control High pressure Loss of containment, fire and gas escapes causing damage to property, seal, and personnel, explosion Low pressure Furnace implosion, damage to seal, excess dtaft, combustion... 

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

A notable example of controlled water reuse was utilization of secondary sewage effluent from the Back River Wastewater Treatment Plant in Baltimore by the Sparrows Point Works of Bethlehem Steel (6). The Sparrows Point plant was suppHed primarily by weUs located near the brackish waters of Baltimore harbor. Increased draft on the weUs had led to saltwater intmsion. Water with chloride concentration as high as 10 mg/L is unsuitable for many steelmaking operations. Rollers, for example, are pitted by such waters. However, treated effluent from the Back River Plant can be used for some operations, such as coke quenching, and >4 x 10 m /d (10 gal/d) are piped 13 km to Sparrows Point. This arrangement has proved economical to both parties for >40 yr. 

Vapors (from drying) are removed at the feed end of the dtyer to the atmosphere through a natural-draft stack and settling chamber or wet scrubber. When employed in simple drying operations with 3.5 X 10 to 10 X 10 Pa steam, draft is controlled by a damper to admit only sufficient outside air to sweep moisture from the cylinder, discharging the air at 340 to 365 K and 80 to 90 percent saturation. In this way, shell gas velocities and dusting are minimized. When used for solvent recovery or other processes requiring a sealed system, sweep gas is recirculated throu a scrubber-gas cooler and blower. 

Bucket elevators, skip hoists, and cranes are used for top feeding of the furnace. Retention and downward flow are controlled by timing of the bottom discharge. Gases are propelled by a blower or by induced draft from a stack or discharge fan. In normal operation, the downward flow of sohds and upward flow of gas are constant with time, maintaining ideal steady-state conditions. 

Fuel-Staged Burners Use of fuel-staged burners is the preferred combustion approach for NO control because gaseous fuels typically contain little or no fixed nitrogen. Figure 27-36 illustrates a fuel-staged natural draft refineiy process heater burner. The fuel is spht into primaiy (30 to 40 percent) and secondary (60 to 70 percent) streams. Furnace gas may be internally recirciJated by the primaiy... 

Part 5 (Draft) Programmable logic controllers for safety applications (August 1998) VDIA DE 3542, Safety terms for automation systems. 

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