Negative pressure furnace

Large WT boilers generally have a balanced draft system (i.e., a combination of both FD and ID fans) and usually operate at negative furnace pressure (-0.1 to -0.5 inch of water) to provide for better control of both the furnace flue-gas pressure and the fuel-air combustion process. 

A system of balancing induced draft and forced draft to a large WT boiler controlled primarily by dampers. In fact, large WT boilers tend to be specifically designed to operate at a slightly negative furnace pressure. 

Note. All losses are much greater with negative furnace pressure. (1) = least loss (4) = worst loss. 

Furnace pressure or draft is normally controlled by a damper in the stack, thus choking off the outfiow of gases and pressurizing the furnace. (See sec. 6.6.3.) If negative furnace pressure is needed, use a speed control on an induced draft fan, a pressure (volume) control on an eductor jet, or a barometric damper. (See sec. 6.7.1 on Turndown Devices.)... 

Negative furnace pressure increases fuel consumption. A recent complaint about a car furnace that could not reach capacity was found to be a problem with a i in. gap all around the large car that admitted so much cold tramp air... 

Negative furnace pressure diminished product quality by admitting cold drafts that cause temperature nonuniformity, and may change the metallurgically required atmosphere in the furnace. Poor product quality raises fuel, labor, and material costs because the job has to be done all over again. It may cost loss of business and customers. 

The only gain from a negative furnace pressure is lowered fan or blower costs (operating and capital). 

Bottom flueing is preferred, but in-the-wall vertical flues have been found too costly, and they pull a harmful negative pressure at the hearth level. With top firing, the best arrangement is hearth-level flues with automatic furnace pressure (damper) control. If fired with top and bottom burners, use of a roof flue with automatic furnace pressure control is suggested. The flue location should be determined to enhance the design circulation pattern. (See chap. 7.)... 

Bottom firing (i.e., burners below the loads) delivers heat to the usually cooler hearth, making up for hearth losses that otherwise would be taken from the loads or from the gas blanket. (See fig. 7.3.) Bottom firing is sometimes used with roof vents, but roof flues can be undesirable because at low-firing rates, the gases may short-circuit direct to the roof flues (giving poor temperature uniformity and poor fuel economy). Roof vents also can cause negative or low furnace pressure therefore, oversize vents should be avoided, and furnace pressure should be controlled with a stack closure. Tall furnaces are especially susceptible to this problem. 

Vibrations in fired equipment and afterburn. Balanced-draft or induced-draft furnaces and boilers are intended to be operated with a small negative pressure (ca. -0.1 in H20), just below the first row of convective tubes, i.e., just below the shock tubes. If we operate such a piece of equipment with a severe shortage of air in the firebox and massive air... 

In general, with decreasing hydrocarbon partial pressure, unsaturated components such as acetylene, ethylene, propylene, and butadiene increase whereas BTX, pyrolysis fuel oil, and saturated components such as methane, ethane, and propane decrease. Low hydrocarbon partial pressure can be attained either by high steam dilution or by low absolute pressure in the cracking coil, which is determined by furnace outlet pressure and pressure drop in the cracking coil. For each specific case there is an optimum steam dilution. Reduction of steam dilution influences yields, utilities, running times and, in the case of a new ethylene plant, of course, investment costs—but in different ways, either positive or negative. Thus, an optimization has to be carried out to identify the most economic steam dilution. 

AM, but the furnace went to a negative pressure and fluctuated once again. Another burner lockout occurred this time because the clean liquor pump was not running. At 12 28 AM. the DFS duct ACAMS alarmed and the site was masked again and the furnace was bottled up at 12 32 AM. The alarm cleared and the site was unmasked at 1 07 AM. DAAMS tubes from the perimeter were collected around 6 55 AM and subsequent analysis showed no detectable agent. The analysis of the stack DAAMS tubes indicated a stack release of 18-36 mg. 

Most furnace camera systems are air-cooled. Two-stage coolant systems, using high-volume air and compressed air from separate sources, are very effective. High-volume, low-pressure air travels through a protective wall box, the primary coolant shroud that encloses the lens tube. This air is supplied either by an FD fan or is pulled in by a negative-pressure furnace. The lens tube itself is cooled by compressed air. 

Retraction systems automatically withdraw the furnace camera several feet back from the firebox if the systems sense loss of either water, air coolant, or high lens tube temperature. This gives the lens additional protection from the heated air that would blow against the lens if fans stopped and negative pressure changed to positive pressure. The retraction system is often air-operated, using an air-reserve tank as a purely pneumatic system component. This makes it totally independent of electricity. On loss of air pressure, water coolant, or high lens tube temperature, a solenoid valve opens and activates a rodless cylinder to withdraw the lens from harms way. The system will not return the camera to its inserted position until the problem is corrected. 

With multiple flues, if anything (scale, refractory crumbs, misplaced loads) partially blocks one or more of the hearth-level flues, that flue s low flow will cause it to cool and other hotter flues will carry more flue gas load, causing them to get hotter. This results in irregular heating of the loads in the furnace, and may eventually cause runaway overheating of the hotter flues. This same sort of unbalance of flue loads can be caused by different firing rates in adjacent zones or by burner locations that create localized positive or negative pressure on one flue entrance more than on another. 

External means of mechanical circulation are induced draft fans and forced draft fans. Neither can do as thorough a job of in-furnace circulation as well-planned and strategically placed burner jets, but these draft fans or blowers do assist in overall transport or movement of gases out of and into a furnace. Induced draft fans have their inlet connected to the furnace, and fherefore create a suction or negative pressure forced draft fans and blowers have their outlet connected to the furnace, and therefore create a positive pressure. Large power boilers often have both induced and forced... 

The superior environmental performance derives essentially fiom the totally enclosed furnace operating under negative pressure and fiom the sealing of all ports via specially designed devices. When processing secondary lead materials, the post combustion of process gases at temperatures in excess of 1300°C results in levels of dioxins and fiirans that are well below the statutory levels. These unique features are particularly well suited to the requirements of the lead smelting industry. 

The Ausmelt reactor is totally enclosed with up to four small openings in the furnace roof to accommodate the furnace feed, lance, standby burner and sample tool entry. The furnace is operated under negative pressure, typically -20 to -40 Pa, to minimise fugitive emissions caused by pressure fluctuations. These must be further reduced to very low levels by the use of additional sealing devices on the feed and lance ports. Sealing systems used in current operations include a rotatory feed valve and spinning disc feeder on the feed ports, and water and compressible bellow seals on the lance ports. 

Figure 18.8 shows the flow chart of the hollow fiber membrane system for oxygen production with the vacuum operating mode. The membrane module is placed under a vertically positioned tubular furnace with the sealing points kept out of the bottom inlet of the furnace tube. An oil-free vacuum pump is connected to the lumen of the fibers to yield a negative pressure and to collect the oxygen product. 

The fluid-bed dryer operates under negative pressure in which drying gases are drawn from the heat source through a fluidizing chamber. Dryer and furnace temperature controllers are employed in the control system to readjust the heat input to match the evaporative load changes. 

On one 50,000-B/D crude unit, steps were taken to eliminate air leakage through the furnace skin fuel use subsequently dropped by 6%. To correct excessive draft, one should slowly pinch back on the stack damper until the draft gauge indicates a small negative pressure and open the secondary air registers until the excess air target is reached. 

There are two modifications of the sodium swelling test ISO/WD 15379-1.2) [23,24] with and without pressure. In both variants, negative potential is applied to the test sample of the cathode carbon block that is dipped in cryolite and aluminium in the vessel while the vessel is in the furnace. 

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