Fired heaters heat transfer

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. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

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. 

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). 

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-... 

In the vei tical-tube single-row double-fired heater, a single row of vertical tubes is arrayed along the center plane of the radiant section that is fired from both sides. Usually this type of heater has an overhead horizontal convec tion bank. Although it is the most expensive of the fired heater designs, it provides the most uniform heat transfer to the tubes. Duties are 21 to 132 GJ/h (20 to 125 10 Btu/h) per cell (twin-cell designs are not unusual). 

A horizontally fired burner is located at one end of the heater. The flame extends along the central longitudinal axis of the heater. In this way the wickets are exposed to the open flame and can be subjected to a maximum rate of radiant heat transfer. The tubes should be sufficiently far away from the flame to prevent hot spots or flame pinching. 

In an indirect bath heat exchanger, the heating medium provides Iil u to an intermediary fluid, which then transfers the heat to the fluid h)cuig heated. An example of this is the common line heater used on many gas well streams to keep the temperature above the hydrate formal ion lem perature. A fire tube heats a water bath, which provides heat to tlie v.all siieam flowing through a coil immersed in the bath. Details pertaining to dcsi jit of indirect bath heaters are presented in Chapter 5. 

For safely reasons, heaters are most often used to heat a heal medium system (water, steam, or heat transfer fluid) rather than to heat the gas or oil stream directly. If the fluid to be heated contains hydrocarbons, (lie heatci can he located safely away from other equipment, If it catches on fire, damage can be limited. 

Gas-fired water heaters use the same general method of construction, except that the elements are replaced with a burner beneath the tank. The combustion products from the burner are vented through a flue made out of the same thickness steel as the tank, that goes up through the center of the tank. To increase heat transfer from the hot flue gases to the inner wall of the flue, a baffle is inserted down the flue. This baffle is a twisted strip of sheet metal with folds and tabs on it. The folds and tabs are designed to... 

Gas-Fired water heaters are also made more efficient by a variety of designs that increase the recov-ei y efficiency. These can be better flue baffles multiple, smaller-diameter flues submerged combustion chambers and improved combustion chamber geometry. All of these methods increase the heat transfer from the flame and flue gases to the water in the tank. Because natural draft systems rely on the buoyancy of combustion products, there is a limit to the recovery efficiency. If too much heat is removed from the flue gases, the water heater won t vent properly. Another problem, if the flue gases are too cool, is that the water vapor in the combustion products will condense in the venting system. This will lead to corrosion in the chimney and possible safety problems. 

Radiation is not generally considered in conventional heat transfer equipment except for direct gas/oil-fired heaters and cracking units. These later types are not a part of this chapter, because they are specialty items of their own as far as design considerations are concerned. 

In modem, packaged horizontal FT boilers, the furnace is the most important heat-transfer component, typically providing 50 to 60% of the total heat transfer from only 30% or so of the total available heating surfaces. This level of heat transfer, coupled with the additional heat extraction obtained by the various multiple-pass designs (four passes is a practical maximum) provide efficiencies of 80 to 83% GCV. As a result, there generally is little additional benefit to be obtained from the use of economizers or air heaters, especially when using oil-fired boilers, which can operate at up to a 3% or so higher efficiency level compared to gas-fired units. 

Fired heaters radiant rate, 12,000 Btu/(hr)(sqft) convection rate, 4000 cold oil tube velocity, 6 ft/sec approx equal transfers of heat in the two sections thermal efficiency 70-75% flue gas temperature 250-350°F above feed inlet stack gas temperature 650-950°F. 

The tube inside heat transfer coefficients and pressure drop can be calculated using the conventional methods for flow inside tubes see Section 12.8, and Volume 1, Chapter 9. If the unit is being used as a vaporiser the existence of two-phase flow in some of the tubes must be taken into account. Bergman (1978b) gives a quick method for estimating two-phase pressure drop in the tubes of fired heaters. 

WlMPRESS, N. (1978) Chem. Eng., NY 85 (May 22nd) 95. Generalized method predicts fired-heater performance. WOLVERINE (1984) Wolverine Tube Heat Transfer Data Book—Low Fin Tubes (Wolverine Division of UOP Inc.). 

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... 

Fired heaters are extensively used in the oil and gas industry to process the raw materials into usable products in a variety of processes. Fuel gas is normally used to fire the units which heat process fluids. Control of the burner system is critical in order to avoid firebox explosions and uncontrolled heater fires due to malfunctions and deterioration of the heat transfer tubes. Microprocessor computers are used to manage and control the burner system. 

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... 

The two governing modes of heat transfer in the construction of a fired heater are radiation and convection. 

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. 

Heat transfer in the radiant zone of a fired heater occurs largely by radiation from the flue gas (90% or so) but also significantly by convection. The combined effect is represented by... 

In the convection zone of the heater, some heat also is transferred by direct radiation and reflection. The several contributions to overall heat transfer specifically in the convection zone of fired heaters were correlated by Monrad [Ind. Eng. Chem. 24,505 (1932)]. The combined effects are approximated by item 10 of Table 8.16, which is adequate for estimating purposes. The relation depends on the temperature of the gas film which is taken to be the sum of the average process temperature and one-half of the log mean temperature difference between process and flue gas over the entire tube bank. The temperature of the gas entering the convection zone... 

Figure 17.33. Heat transfer to stirred-tank reactors (a) jacket (b) internal coils (c) internal tubes (d) external heat exchanger (e) external reflux condenser (f) fired heater (Walas, 1959).

A major fire erupted in a nonflammable solvents manufacturing unit in a U.S. Gulf Coast chemical complex. A furnace tube in a natural-gas-fired heater ruptured due to overheating. At least 1,800 gallons (6,800 liters) of a combustible heat transfer fluid spilled and burned intensely. Within about 25 minutes, the intense hot fire damaged four levels of structure and associated process equipment. The plant on-site emergency squad quickly and properly responded. However, the price tag for short-lived incident was over 1.5 million in direct property damage and over 4 million in business interruption (U.S. 1979). Fortunately, there were no injuries. [2]... 

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