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Berman Part 1 PDF

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337 views6 pages

Berman Part 1 PDF

Uploaded by

dinakaranpatel
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Finding the basic design for yourapplication = C1 Energy conservation is not new. The economic sue- cess of any competitive process requires the efficient use of energy. Of the energy consumed in typical chemical processing or petroleum refining plants, approximately. 175% is burned in the form of hydrocarbon fuel in fired heaters and steam boilers. Conservation, therefore, af- fords a strong and timely incentive for scrutinizing the design criteria and construction features commonly used in fired heaters. What is a fired heater? A fired heater, for our purposes, will include a num- bber of devices in which heat liberated by the combus- tion of fuel within an internally insulated enclosure is transferred to fluid contained in tubular coils. Typi- cally, the tubular heating elements are installed along the walls and roof of the combustion chamber, where heat transfer occurs primarily by radiation, and if eco- nomically justifiable, in a separate tube bank, where heat tansler is accomplished mainly by convection. Industry identifies these heaters with such names as process heater, furnace, process furnace and direct-fired heater, all of which are interchangeable. ‘The fundamental function of a fired heater is to supply a specified quantity of heat at clevated tempera: ture levels to the fluid being heated. It must be able to do so without localized overheating of the fluid or of the structural components, Fired-heater size is defined in terms of its design heat-absorption capability, or duty. Duties range from about a half-million Btu/h for small, specialty units to about one billion Beu/h for superproject facilities such as the mammoth steam hydrocarbon-reformer heaters, By and large, the vast majority of fired-heater installa tions fall within the 10- to 350-million-Btu/h range. Process industry requirements for fired heaters are divided, in the main, into a half-dozen general service categories. These categories can be designated and de- scribed as follows: Column reboiles. This is normally considered one of the mildest and least critical of fired-heater applications. ‘The charge stock taken from a distillation column is a recirculating liquid that is partially vaporized in the fired heater. The mixed vapor-liquid stream renters CREAT NOREEN itex Petroleum Carp, the column, where the vapor condenses and releases the heat of vaporization. Reboiler applications are charac- terized by relatively small differentials between the inlet and outlet fluid temperatures across the fired heater, and by substantial vaporization (typically, 50% or more of the charge stock is vaporized). Depending on the particular application, reboiler heater outlet tempera- tures generally fall in the range of 400°F to 550°F. Froctionating-colunn feed preheaters, Fired heaters in this service tend to be the workhorses of many process oper- ations. The charge stock (usually all liquid, although tome feeds may contain a nominal amount of vapor at the inlet) ise tothe fred heater following upateam preheating in unfired equipment. In the fred heater, the fluid temperature is usally rated high enough to achieve peril vaporization ofthe charge och Atypical example of this service i the feed heater for an atinonpherie dsilaton column in th erudeol unit of a petroleum refinery. Here, crude oll entering the fred heater as 0 50°F liquid might ext near 700°F with about 6O% ofthe charge stock vaporized Reacorfed prebeatrs Fired heaters inthis application raise the chargestock temperature to a level necessary for controlling a chemical reaction taking place in an adjoining reator ves. The nate ofthe charge stock and the heater operating temperatures and preswures an vary considerably, depending on the proses. The following example illustrate the diversity of the appl cations performed by reacor-feed preheater: MF Singlephase/single-component heating such as steam superheating in the reaction sections of styrene manufacturing proses, In this service, the fad tem. perature acrom the fred heater inreases from an inet temperature of about 700°F to an exit temperature of approximately 1500°F. Single-phase/molticomponent heating, such asthe heating of mixtures of vaporized hydrocarbons and recycle hydrogen gas prior to catalyti reforming in a fern thieves he charge tock enter the red cater at about 00°F and exits at approximately 1,000°F. “ Tn reformers, the fd pressure may range fom about 250'to 600 psig. Severe restrictions on uid presure drop are normally associated with this service a 99 a. Vertical cylindrical, all radiant Nene eet Mixed-phase/multicomponent heating, such as the heating of mixtures of liquid hydrocarbons and recycle hydrogen gas for reaction in a refinery hydro- cracker. Fluid temperatures typically run fom 700°F at the inlet to 850°F at the outlet. Operating pressures may reach 3000 psig, depending on the process, Heat supplied to heat-tansfer media, Many plants furnish, heat to individual users via an intermediate heat-trans- fer medium. A fired heater is generally employed to elevate the temperature of the recirculating medium, which is typically a heating oil, Dowtherm, Therminol, molten salt, etc. Fluids flowing through the fired heater in these systems almost always remain in the liquid phase from inlet to outlet, Heat supplied to viscous fluids. Oftentimes heavy oil must bbe pumped from one location to another for processing. At low temperatures, where the oil may have so high a viscosity as to render pumping infeasible, a fired heater is employed to warm the oil to a temperature that will facilitate pumping, Fired reactors. In this category ate heaters in which a chemical reaction occurs within the tube coil. As a class, these units represent the fired-heater industry's most sophisticated technology. The following two applica tions typify the majority of installations: Stcam hydrocarbon-reformer heaters, in which the tubes of the combustion chamber function individually. 100 b. Verticalcylindrical, helical coil CERI ERENTTRING J Convection call, ‘convection section ce ee | as vertical reaction vessels filled with nickel-bearing catalyst. In reformers that yield hydrogen, fluid outlet temperatures range from 1,450 to 1,650°F. Pyrolysis heaters, used to produce olefins from ¢gascous feedstocks such as ethane and propane and from liquid feedstocks such as naphtha and gasoil, In crack | ing heaters, where chemical reactions occur in the coi the tubes and burers are arranged so as to assure pinpoint firing control. Fluid outlet temperatures in heaters designed for liquid feedstocks are in the 1,500°F to 1,650°F range. Many variations in design and layout ‘There are many variations in the layout, design, and. detailed construction of fired heaters. A consequence of this flexibility is that virtually every fired heater is ‘custom-engineered for its particular application. ‘The simplest type of fired heater follows the so-called “all radiant” design, wherein the entire tube coil is arranged along the walls of the combustion chamber or radiant section, This design is characterized by low thermal efficiency and normally represents the lowest capital investment for a specified duty. The terminol- ogy “all radiant” is somewhat of a misnomer, Convec- tion currents do exist, due to the flow of flue gases through the combustion chamber, and these currents ©. Vertical-cylindrical with crossflow Ver! integre hc pl convection ‘cil Extended Radiant coi} 4. Vertical-cylindrical with integral convection section account for a portion of the total heat absorbed in the radiant section. In addition to the radiant section, most modern fired heaters include a separate convection section. The re~ sidual heat of the flue gases leaving the radiant is recovered in this section, primarily by convection. Using this heat for preheating the charge stock, or for other supplementary heating services, increases. the thermal efficiency of the fired heater. “The first few rows of tubes in the convection section are subject to radiant heat transfer, in addition to con- vective transfer from the hot flue gases as they flow ‘across the tubes. Because these tube rows are usually being subjected to the highest heat-transfer rates in the fired heater, they are aptly termed “shield” or “shock” tubes. Horizontal vs. vertical ‘The principal classification of fired heaters, however, relates to the orientation of the heating coil in the radiant section; i., whether the tubes are vertical or horizontal. Vertical arrangements are shown in Fig. 1; horizontal arrangements in Fig. 2. Salient features of ‘each configuration are noted here: Vertical-clindrcal, all radiant. Here the tube coil is placed vertically along the walls of the combustion @, Atbor or wicket type RETIN FONE 4. Vertical-tube, single-row, double-fired chamber. Firing is also vertical, from the floor of the heater. ‘Heaters of this type represent 2 low-cost, low~ cfficieney design, which requires a minimum of plot ‘area. Typical duties are 0.5 to 20 million Bru/h, Vertical-clindrical, helical coil. In these units, the col is arranged helically along the walls of the combustion chamber, and firing is vertical from the floor. Although these heaters are grouped with others having vertical tube designs, their in-tube characteristics resemble those of horizontal-tube fired heaters. ‘This design also represents low cost, low efficiency, and requires a minimum of plot area. ‘The tube coil is inherently drainable. One limitation on these units is that generally only one flow path is followed by the process fluid, Heating duties run from 0.5 to 20 million Bru/h. Vertcal-glindrical, with crossflow convection. These heat- cers, also fired vertically from the floor, feature both radiant and convection sections. The radiant-section tube coil is disposed in a vertical arrangement along the walls of the combustion chamber, ‘The convection sec- tion tube coil is arranged as a horizontal bank of tubes positioned above the combustion chamber. "This configuration provides an economical, high- cfficiency design that requires a minimum of plot area. ‘The majority of new, vertical-tube fired-heater installa- 101 Radiant Burners ft ESA a. Cabin tions fall into this category. Typical duty range is 10 to 200 million Bru/h, Vertcal-clindrical, with integral convection. Although this design is rarely chosen for new installations, the vast number of existing units of this type warrants its men- tion in any review of fired heaters ‘As with the previous types, this design is likewise vertically fired from the floor, with its tube coil installed in a vertical arrangement along the walls. The distin- guishing feature of this type is the use of added surface area on the upper reaches of each tube to promote convection heating. This surface area extends into the annular space formed between the convection coil and a central baffle sleeve. Medium efficiency can be achieved with a minimum of plot area. Typical duty for this design is 10 to 100 million Beu/h.

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