Fired heaters tube design

Direct-fired furnace. Fired heaters are designed to increase the process temperature of oil and gas streams. This increase of temperature in most every case does not change molecular structure. Thus, temperatures up to 500°F maximum with 400°F design are very common. Designs are usually cylindrical, with vertical radiant tube banks fired by oil/gas combination burners. 

Fired heater reboilers should be designed to keep the tube wall temperature below 300°F which limits the heat flux to 6,500 to 8,500 Btu/(hr)(ft2) (Manning and Thompson, 1991 Ballard, 1966). Bacon (1987) recommends that the fired heater tube skin temperature be limited to a maximum of 350°F and heat flux should be limited to 6,000 to 8,000 Btu/(hr)(ft2). These constraints may require inlet ferrules to restrict heat flux at the fired inlet. 

The simplest unit employing vacuum fractionation is that designed by Canadian Badger for Dominion Tar and Chemical Company (now Rttgers VFT Inc.) at Hamilton, Ontario (13). In this plant, the tar is dehydrated in the usual manner by heat exchange and injection into a dehydrator. The dry tar is then heated under pressure in an oil-fired hehcal-tube heater and injected directly into the vacuum fractionating column from which a benzole fraction, overhead fraction, various oil fractions as side streams, and a pitch base product are taken. Some alterations were made to the plant in 1991, which allows some pitch properties to be controlled because pitch is the only product the distillate oils are used as fuel. 

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."... 

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). 

The,se heaters represent a low-cost, low-ot ficiency design that requires a minimum of plot area. Typical duties run from 0.5 to 200 MMBtu/hr. Six iy[)cs of veitical-tube-fired heaters are shown in Figure 3-20. 

The radiant section tube coils of horizontal fired heaters are arranged horizontally so as to line the sidewalls and the roof of the combustion chamber. In addition, tliere is a convection section of tube coils, winch are positioned as a horizontal bank of tubes above the combustion cham her. Nonnally the tubes are fired vertically from the floor, but they can also be fired horizontally by side wall mounted burners located below the tube coil. Tins economical, high dficiency design currently represents the majority of new horizontal-tube-t1icd heater installations. Duties run from 5 to 250 MMBtu/hr. Six types o) horizontal-tube-fired heaters arc-shown in Figure 3-21. 

Figure 3-21. Six bask designs used in horizontal-lube fired heaters. Radian section coil is horLmtai. (a) Cabin, (b) Two-cell box. (c) Cabin with dividing bridgewall, (d) End-fired box. e) End-fired box, with side-mounted convection section, (f) Horizontal-tube, single-row, double-h red. [From Chem. Eng., 102-103 (June 19, 1978).]...

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. 

Process furnace or direct-fired heater Design type, absorbed heat duty, pressure, tube material, capacity... 

The selected system comprises three direct-fired hot oil heaters, surge vessels ond circulation pumps. The hot oil is circulated in a closed loop through the tube bundles in the glycol reboilers. The system allows accurate control of the glycol tempercture ond by designing the fired heaters os a redundant utility system cvailoble to the three process troins the reliability of the system is thereby improved. 

For fired heaters subject to creep problems, make sure that the tube metal temperature was considered in materials selection, hi the absence of better information, assume the fireside temperature is 100°F (38°C) higher than the process temperature. (If tube-side fouling is anticipated [e.g., coke formation], assume the tube metal temperature is 150°F [85°C] higher than the process temperature.) If necessary, make a note on the template to ensure that creep is accommodated during design of heater tubes, in accordance with API 530 [23]. 

The basic rating method presented here is applicable to extended surface equipment in general, but it is designed especially for the finned tube convection section of fired heaters. The rating problem is divided into three parts ... 

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