Heaters, fired

Indirect-fired heaters of the box type, also called fired heaters, process heaters, and furnaces, are commonly used to heat and/or vaporize non-reacting process streams at elevated temperatures beyond where steam is normally employed. The fuel for combustion is either gas or 

Fired heaters for specific purposes are discussed below in Section 16.6. 

After being heated to 1,000°F and before entering the reactor, the combined feed in Example 16.9 is heated further to 1,200 F in a fired heater. Determine the f.o.b. purchase cost of a fired heater at a CE index of 394. 

5 Purchase Costs of the Most Widely Used Process Equipment 527 

The calculated absorbed heat duty, Q, is 18,390,000 Btu/hr. Assume a design pressure for the tubes of 700 psig. Because of the significant hydrogen concentration in the combined feed, stainless steel tubes are required. From Eq. (16.49), the base cost is 

Normally, a PRV is located on the downstream equipment of the fired heater, e.g., the distillation column at the discharge of a fired heater. This is acceptable provided  

1 Factors controlling the integrity and life of fired heaters 

The life of radiant tubes in fired heater service is governed by the combination of several possible time-dependent degradation mechanisms and by the tube 

Tube materials in CRU fired heaters are usually of 2V4Cr 1 Mo specification, although ferritic tubes with higher chromium content, SCrlMo and 9CrlMo are encountered, the last mainly in CCR service. Tube dimension design and corrosion allowance generally follow API RP-530 (various dates). 

2 Assessment procedures for fired heaters prior to shutdown 

Assessment procedures capable of giving an estimate of condition, probability of failure and serviceable lifetime while the plant is operating are based on calculation (Fig. 2.1). An inverse design (e.g. API RP-530) deterministic approach can be adopted, but this invariably produces conservative results. Thus, in line with the current trend for risk-based information, the more accurate approach is to deploy probabilistic techniques and a more rigorous 

Many processes are heat driven, take place at elevated temperatures, or require product drying. As a result, process heaters and dryers are common equipment in processing facilities. Many of these are fired units fueled by a variety of gas or liquid fuels frequently by natural gas. They may be used to heat a process stream directly, to heat an intermediate heat transfer fluid, or to 

For process liquid and gas stream heating, most designs heat the process stream as it flows through tubes that pass through fireboxes, convection sections, or combustion gas stacks, although a few fire-tube heaters exist. 

Fires involving liquid process streams are the most common heater loss. Most involve a ruptured process stream tube leading to a firebox fire or a pool fire under or near the heater. The two most common causes are failure of the tubes due to overheating and rupture of the tubes as the result of a fire box explosion. 

When overheated, hydrocarbons tend to breakdown, leaving carbon residues (coke). This coke builds up on the inside of the heater tubes, slowing the transfer of heat from the tube walls to the product by restricting the flow of product and acting as an insulator. As the control system attempts to maintain the process outlet temperature at the setpoint, the fuel valves will open and the tubes subjected to an increased heat load. With the diminished ability of this heat to be transferred to the process fluid, the temperature of the tubing will increase. 

Solid materials are often dried or heated using combustion gas exhaust from a fired heater as the material is conveyed through a hot combustion gas zone. Solids handling dryers may take a number of forms, e.g., a rotary kiln. Losses involving dryers usually involve internal fires or explosions. 

In some situations, process heat needs to be supplied  

After the flue gas leaves the combustion chamber, most furnace designs extract further heat from the flue gas in horizontal banks of tubes in a convection section, before the flue gas is vented to the atmosphere. The temperature of the flue gases at the exit of the radiant section is usually in the range 700 to 900°C. The first few rows of tubes at the exit of the radiant section are plain tubes, known as shock tubes or shield tubes. These tubes need to be robust enough to be able to withstand high temperatures and receive significant radiant heat from the radiant section. Heat transfer to the shock tubes is both by radiation and by convection. After the shock tubes, the hot flue gases flow across banks of tubes that usually have extended surfaces to increase the rate of heat transfer to the flue gas. The heat transferred in the radiant section will usually be between 50 and 70% of the total heat transferred. 

Heat input to the fired heater is from three sources  

The heat output from the fired heater is to four sinks  

Radiant Section Tubes Horizontally or Vertically Mounted 

With such data, an estimate can be made of a possible evaporator configuration for a required duty, that is, the diameter, length, and number of tubes can be specified. Then heat transfer correlations can be applied for this geometry and the surface recalculated. Comparison of the estimated and calculated surfaces will establish if another geometry must be estimated and checked. This procedure is described in Example 8.12. 

High process temperatures are obtained by direct transfer of heat from the products of combustion of fuels. Maximum flame temperatures of hydrocarbons burned with stoichiometric air are 

In fired heaters and furnaces, heat is released by combustion of fuels into an open space and transferred to fluids inside tubes which are ranged along the walls and roof of the combustion chamber. 

The heat is transferred by direct radiation and convection and also by reflection from refractory walls lining the chamber. 

Horizontal tube supports are made of refractory steel to withstand the high temperatures. Hangers for vertical tubes make for a less expensive construction per unit of tube surface. Furnaces are lined with shaped light weight refractory brick 5-8 in. thick. A 1 in. layer of insulating brick is placed between the lining and the metal shell. 

409) and in Marks Mechanical Engineers Handbook, (1978, p. 4.57). With excess air to ensure complete combustion the temperatures are lower, but still adequate for the attainment of process temperatures above 2000°F when necessary. Lower temperatures are obtained with heat transfer media such as those of Table 8.2 which are in turn serviced in direct-fired heaters. 

Because of the elevation of boiling point by dissolved solids, the difference in temperatures of saturated vapor and boiling solution may be 3-10°F which reduces the driving force available for heat transfer. In backward feed [Fig. 8.17(b)] the more concentrated solution is heated with steam at higher pressure which makes for lesser heating surface requirements. Forward feed under the influence of pressure differences in the several vessels requires more surface but avoids the complications of operating pumps under severe conditions. 

Several comprehensive examples of heat balances and surface requirements of multiple effect evaporation are worked out by Kern (1950). 

High process temperatures are obtained by direct transfer of heat from the products of combustion of fuels. Maximum flame temperatures of hydrocarbons burned with stoichiometric air are about 3500°F. Specific data are cited by Hougen, Watson, and Ragatz (Chemical Process Principles, Vol. I, Wiley, New York, 1954, p. 409) and in Marks Standard Handbook for Mechanical Engin- 

The latter two units raquira no refractory consequently they can be rapidly started up and shut down. Rafractories must ba haatad or cooled slowly to avoid damage to the refractories. 

Electrical energy as a source of haat for avaporation systems can affectively compete with conventional fuels only whan tha cost for alactricity is low or soma unusual process condition or configuration exists. Principal advantages of alactrical heating methods are  

End—fired horizontot tube cobin type furnace. 

Side—fired horizontai tube cabin type furnoce. 

Alt—rodiont vertical cylindrical furnoce with a continuous heticot coil. 

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Fire control Fire damp Fired heaters Fire extinguishants Fire extinguisher Fire extinguishers... 

Fresh reducing gas is generated by reforming natural gas with steam. The natural gas is heated in a recuperator, desulfurized to less than 1 ppm sulfur, mixed with superheated steam, further preheated to 620°C in another recuperator, then reformed in alloy tubes filled with nickel-based catalyst at a temperature of 830°C. The reformed gas is quenched to remove water vapor, mixed with clean recycled top gas from the shaft furnace, reheated to 925°C in an indirect fired heater, and injected into the shaft furnace. For high (above 92%) metallization a CO2 removal unit is added in the top gas recycle line in order to upgrade the quaUty of the recycled top gas and reducing gas. 

Reducing gas is generated from natural gas in a conventional steam reformer. The natural gas is preheated, desulfurized, mixed with steam, further heated, and reformed in catalyst-filled reformer tubes at 760°C. The reformed gas is cooled to 350°C in a waste heat boiler, passed through a shift converter to increase the content, mixed with clean recycled top gas, heated to 830°C in an indirect-fired heater, then injected into reactor 4. 

The way the equipment is located on the background is based on the process flow sequence. Again, certain equipment such as fired heaters can be situated first to put them at a safe distance from other equipment. Other large equipment may have to be located where the soH-beariag load is best. 

Reboilers need to be located next to the tower they serve, except for the pump-through types, which can be located elsewhere. Fired heater reboilers are always located away from the associated tower and use a pump to circulate the bottoms. Ketde-type reboders are preferred from an operational and hydraulic standpoint because they can be designed without the worry of having to ensure sufficient head for circulation required by thermosyphon reboders. However, ketde reboders require a larger-diameter shed that is more cosdy, and the reboder must be supported at a sufficient elevation to get the product to the bottoms pump with adequate NPSH. 

Fired Heaters. The fired heater is first a reactor and second a heat exchanger. Often, in reafity, it is a network of heat exchangers. 

Fired Hester a.s a. Reactor. When viewed as a reactor, the fired heater adds a unique set of energy considerations, such as. Can the heater be designed to operate with less air by O2 and CO analy2ers How does air preheating affect fuel use and efficiency How can a lower cost fuel (coal) be used Can the high energy potential of the fuel be used upstream in a gas turbine ... 

Table 2. Lost-Work Analysis for a Fired Heater...

Fired Heater as a Heat-Exchangee System. Improved efficiency in fired heaters has tended to focus on heat lost with the stack gases. When stack temperatures exceed 150°C, such attention is proper, but other losses can be much bigger when viewed from a lost-work perspective. For example, a reformer lost-work analysis by Monsanto gave the breakdown shown in Table 2. 

Simple heat losses through the furnace walls are also significant. This follows from the high temperatures and large size of fired heaters, but these losses are not inevitable. In an optimized system, losses through insulation (1) are roughly proportional to... 

Like the fired heater, the dryer is physically large, and proper insulation of the dryer and its aUied ductwork is critical. It is not uncommon to find 10% of the energy input lost through the walls in old systems. 

Equipment Tests. Procedures for rigorous, detailed efficiency determination are available (ASME Test Codes) but are rarely used. For the objective of defining conservation potentials, relatively simple measurements are adequate. For fired heaters, stack temperature and excess O2 ia stack should be measured for turbiaes, pressures (ia and out) and temperatures (ia and out) are needed. 

In extremely cold environments, engines can quickly become difficult, sometimes nearly impossible, to start. If ordinary gasoline- or diesel-oil-fired heaters are used, the coolant circulation pump, air fan, etc, must be powered from the vehicle s batteries, thus curtailing the time the system can be used, especially at very low temperatures when it is needed the most. By adding PbTe thermoelectrics to such heater systems, about 2% of their thermal output can be turned into electricity to mn the heater s electronics, fuel pump, combustion fan, and coolant circulation pump, with stiH sufficient power left over to keep the vehicle s battery fliUy charged. The market for such units is in the hundreds of thousands if manufacturing costs can be reduced. 

Disposal of the spent caustic solution can be a troublesome environmental problem. Depending on the plant location, acid gases are either sent to a fired heater or treated in a Claus unit for conversion of hydrogen sulfide to elemental sulfur. 

When manipulating a stream whose flow is independently determined, such as flow of a product or a heat-transfer fluid from a fired heater, a three-way valve is used to divert the required flow to the heat exchanger. This does not alter the linearity of the process or its sensitivity to supply variations and even adds the possibility of independent flow variations. The three-way valve shomd have equal-percentage characteristics, and heat-flow control may be even more beneficial. 

A. Pikiilik and H. E. Diaz [Chem. Eng., 84, 106-122 (Oct. 10, 1977)] presented a graphical method for estimating the fabricated cost of distillation cohimns and pressnre vessels, storage tanks, fired heaters, pnmps and drivers, compressors and drivers, and vacnnm eqnipment. 

Air recirculation. Prevailing winds and the locations and elevations of buildings, equipment, fired heaters, etc., require consideration. All air-cooled heat exchangers in a bank are of one type, i.e., all forced-draft or all induced-draft. Banks of air-cooled exchangers must be placed far enough apart to minimize air recirculation. 

FIG. 12-104 Open spray-drying system with direct-fired heater. (NIRO, Inc. )... 

FIG. 23-1 Heat transfer to stirred tank reactors, a) Jacket, (h) Internal coils, (c) Internal tubes, (d) External heat exchanger, (e) External reflux condenser. if) Fired heater. (Walas, Reaction Kinetics for Chemical Engineers, McGraw-Hill, 1959). 

Mild thermal cracking is conducted in the tubes of a fired heater, sometimes fol-... 

FIG. Noncatalytic gas phase reactions, a) Steam cracking of light hydrocarbons in a tnhnlar fired heater, (h ) Pehhle heater for the fixation of nitrogen from... 

NO -laden fumes are preheated by effluent from the catalyst vessel in the feed/effluent heat exchanger and then heated by a gas- or oil-fired heater to over 600° F. A controlled quantity of ammonia is injected into the gas stream before it is passed through a metal oxide, zeolite, or promoted zeolite catalyst bed. The NO is reduced to nitrogen and water in the presence or ammonia in accordance with the following exothermic reactions ... 

Brick buildings severely damaged, 75% external wall collapse. Fired heaters badly damaged. Storage tanks leak from base. Threshold for eardrum damage to people. Domino or knock-on radius. Pipe bridges may move. 

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 (dis-... 

Fired heaters differ from other indirect-fired processing equipment in that the process stream is heated by passage through a coil or tubebank enclosed in a furnace. Fired heaters are classified by function and by coil design. 

Function Berman (Chem. Eng. 85(14) 98-104, June 19, 1978) classifies fired heaters into the following six functional categories, providing descriptions that are abstracted here. 

Fractionator-feed preheaters partially vaporize charge stock from an upstream unfired preheater en route to a fractionating column. A typical refinery application a crude feed to an atmospheric column enters the fired heater as a liquid at 505 K (450°F) and leaves at 644 K (700°F), having become 60 percent vaporized. 

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