Fired heaters furnaces and boilers

When high temperatures and high flow rates are required, fired-heaters are used. Fired heaters are directly heated by the products of combustion of a fuel. The capacity of fired heaters ranges from 3 to 100 MW. 

Process feed-stream heaters such as the feed heaters for refinery crude columns (pipe stills) in which up to 60 per cent of the feed may be vaporised. 

Reboilers for columns, using relatively small size direct-fired units. 

Direct-fired reactors for example, the pyrolysis of dichloroethane to form vinyl chloride. 

Reformers for hydrogen production, giving outlet temperatures of 800-900°C. 

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In such cases, radiant heat transfer is used from the combustion of fuel in a fired heater ox furnace. Sometimes the function is to purely provide heat sometimes the fired heater is also a reactor and provides heat of reaction. The special case of steam generation in a fired heater (a steam boiler) will be dealt with in Chapter 23. Fired heater designs vary according to the function, heating duty, type of fuel and the method of introducing combustion air. However, process furnaces have a number of features in common. A simple design is illustrated in Figure 15.19. The chamber where combustion takes place, the radiant section... 

Both types of boiler systems may incorporate finned copper heating coils, which are located above the furnace and gas-pass tubes (smoke tubes or fire tubes) and provide for indirect heating of domestic HW. Where coils are fitted and the boilers are only fired during winter months, domestic HW heating usually is provided via gas heaters for the summer. 

J. Use of fired heaters the presence of boilers or furnaces, heated by the combustion of fuels, increases the probability of ignition should a leak of flammable material occur from a process unit. The risk involved will depend on the siting of the fired equipment and the flash point of the process material. The factor to apply is determined with reference to Figure 6 in the Dow Guide. 

Indirect-Fired Equipment (Fired Heaters) Indirect-fired combustion equipment (fired heaters) transfers heat across either a metallic or refractory wall separating the flame and products of combustion from the process stream. Examples are heat exchangers (discussed in Sec. 11), steam boilers, fired heaters, muffle furnaces, and melting pots. Steam boilers have been treated earlier in this section, and a subsequent subsection on industrial furnaces will include muffle furnaces. 

If you try to operate a furnace, fired heater, or boiler with too little combustion air to starve the burners of oxygen to smother or bog down the firebox, then you will likely cause afterburn or secondary combustion in the stack, you will not be able to operate on automatic temperature control, and may even destroy the equipment altogether. 

Fire 1.4. Steam boiler and furnace arrangements. [Steam, Babcock and Wilcox, Barberton, OH, 1972, pp. 3.14, 12.2 (Fig. 2), and 25.7 (Fig. 5)]. (a) Natural circulation of water in a two-drum boiler. Upper drum is for steam disengagement the lower one for accumulation and eventual blowdown of sediment, (b) A two-drum boiler. Preheat tubes along the Roor and walls are cormected to heaters that feed into the upper drum, (c) Cross section of a Stirling-type steam boiler with provisions for superheating, air preheating, and flue gas economizing for maximum production of 550,000 Ib/hr of steam at 1575 psia and 900°F. 

A fired heater or furnace is used to heat large quantities of hydrocarbons for industrial use in a distillation system or reactor. Fired heaters are characterized by three basic designs cabin, cylindrical, and box. The basic components of a furnace include shell, refractory lining, burners, radiant tubes, convective tubes, damper, stack, and firebox. Air and fuel are proportionally balanced as temperatures in the furnace are held constant. Figure 7-12 shows the two standard symbols used for a fired heater or furnace and a boiler. 

Fired henteri and furnaces Cfldi CpFdftfFf/Y Fj is the superheat correction factor for Steam boilers (F,- = 1 for citiier heaters and furnaces) and is given by Fr = 1 + O.OD184iT-0.000003350 where AT b the amount of superheat in °C. 

Minimizing steam consumption by using fired heater reboilers will minimize steam plant and boiler feedwater preparation facilities. An alternate approach is to use a hot oil loop where one furnace fires enough heat for all reboilers on the service. 

Boiler and fired heater tubes develop scale on the fired side of the tube. Past procedures required furnace shutdown for cleaning on a regular basis. On-line cleaning with combustible abrasives allows treatment without shutdown. With on-line cleaning, one refinery experienced a CO2 emissions reduction of 1,800 toimes per year, 60,000 fuel per year savings, 300,000 per year yield improvement, 800,000 per year throughput increase, and an overall 1.5% improvement in efficiency for the unit. 

Orsat analysis A measurement of the oxygen, carbon dioxide, and carbon monoxide in a mixture of gases, usually from the exhaust of combustion processes such as boilers, furnaces, fired heaters, and combustion engines. Named after its inventor H. Orsat in 1873, it involves absorption of the gases onto materials contained in pipette tubes. The method has been largely replaced by other techniques. 

In fossil fuel-fired boilers there are two regions defined by the mode of heat transfer. Fuel is burned in the furnace or radiant section of the boiler. The walls of this section of the boiler are constmcted of vertical, or near vertical, tubes in which water is boiled. Heat is transferred radiatively from the fire to the waterwaH of the boiler. When the hot gas leaves the radiant section of the boiler, it goes to the convective section. In the convective section, heat is transferred to tubes in the gas path. Superheating and reheating are in the convective section of the boiler. The economizer, which can be considered as a gas-heated feedwater heater, is the last element in the convective zone of the boiler. 

Early SM boilers were manufactured with between two and four corrugated furnace tubes in wet-back and dry-back versions and generally incorporated heat recovery equipment such as economizers and air heaters. Some designs also provided for superheaters and for coal, oil, or gas fuel firing. Many of the best features are incorporated in the SM boilers commonly available today. 

For corrosion and safety reasons, the condensate recovered from these sources is best not returned to the deaerator for use as boiler feedwater. However, depending on the contaminant, the condensate may be reused for a number of services. Our favorite reuse of such contaminated condensate is as a replacement for velocity steam in the heater-tube passes of a fired furnace. 

Combustion calculations show that an oil-fired watertube boiler requires 200,000 lb/h (25.2 kg/s) for air of combustion at maximum load. Select forced- and induced-draft fans for this boiler if the average temperature of the inlet air is 75°F (297 K) and the average temperature of the combustion gas leaving the air heater is 350°F (450 K) with an ambient barometric pressure of 29.9 inHg. Pressure losses on the air-inlet side are, in inFLO air heater, 1.5 air supply ducts, 0.75 boiler windbox, 1.75 burners, 1.25. Draft losses in the boiler and related equipment are, in inH20 furnace pressure, 0.20 boiler, 3.0 superheater, 1.0 economizer, 1.50 air heater, 2.00 uptake ducts and dampers, 1.25. Determine the fan discharge pressure and horsepower input. The boiler burns 18,000 lb/h (2.27 kg/s) of oil at full load. 

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