Radiant Section Rating

Choose a center-to-center spacing for the radiant tubes which is compatible with the selected tube size. Wide tube-spacing permits high radiant absorption rates with relatively low firebox temperatures and gives good circumferential heat 

Using the approximate radiant tube surface determined previously, choose firebox dimensions to accommodate the required total tube length. The exact proportions depend on judgment and past experience. Long furnaces minimize the number of return bends required, thus decreasing total cost. Shorter and wider fireboxes, on the other hand, usually give more uniform heat distribution and lessen the probability of flame impingement on the tube surface. 

The procedures described here will be covered step-by-step in Example 1. 

If preheated air or fuel is used, calculate the heat content of each stream above 60° F and its ratio to 

If the assumed value of tg was correct, the computed point will be on the absorption curve for the average tube wall temperature. Usually, of course, there will be a discrepancy. In that case, select another value of tg on the other side of the absorption curve and repeat the above procedure. 

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The design conditions for the convection section are set by the overall design conditions and by the radiant section rating. The flue gas rate, the stack heat content, the stack temperature and the intube... 

In the radiant section of a boiler the fourth power of the wall temperature is typically less than 2 per cent of the fourth power of the mean flame and gas temperature. The effects of waterside conditions and wall thickness on the heat transfer rate are therefore negligible. 

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. 

One of the most effective reformer modifications is to use heat from the convection section to preheat radiant section feed. This will reduce radiant section duty and firing rate. The effectiveness of this option is limited only by the risk of coking in the preheat coil, the metallurgy of the preheat coil and the metallurgy of the radiant inlet system. This option has been used to increase capacity by 10% without increasing the arch temperature in the radiant section86. 

Operation The continuous operation time is approximately two years without decoking. The high conversion rate is 55%, due to the vapor EDC-feed. No iron enters the radiant section. 

Heat transfer to the tubes on the furnace walls is predominantly by radiation. In modern designs this radiant section is surmounted by a smaller section in which the combustion gases flow over banks of tubes and transfer heat by convection. Extended surface tubes, with fins or pins, are used in the convection section to improve the heat transfer from the combustion gases. Plain tubes known as shock tubes are used in the bottom rows of the convection section to act as a heat shield from the hot gases in the radiant section. Heat transfer in the shield section will be by both radiation and convection. The tube sizes used will normally be between 75 and 150 mm diameter. The tube size and number of passes used depend on the application and the process-fluid flow rate. Typical tube velocities will be from 1 to 2 m/s for heaters, with lower rates used for reactors. Carbon steel is used for low temperature duties stainless steel and special alloy steels, for elevated temperatures. For high temperatures, a material that resists creep must be used. 

Thermocouple installation is important to ensure the proper temperature will be measured. Figure 5.3 shows a thermocouple located at the top of the radiant section in a process heater used to measure the temperature of fhe heater. As will be discussed later, this type of fhermocouple measurement needs to be corrected to get the actual temperature. The location of this thermocouple is important because if it is located too close to the wall, then the temperature will be lower due to the lower temperature tubes that are cooled by process fluid. That lower temperature would not be representative of the average heater temperature. Figure 5.4 shows a photo of the thermocouples used to measure the water outlet temperatures from calorimeters in a flame impingement heating study [17]. The thermocouples were positioned so that water would have to flow over the junctions, regardless of the flow rate. If the thermocouples were positioned, for example, perpendicular to a vertically downward flow of water, there is a good chance the junction would not... 

For each square foot of radiant section plus the convection section, the heat absorbed per hour is Qr + Qc. The surface thus found is the equivalent plane surface having the same radiant absorption as the most exposed element of the tubes. This can be converted into other forms by applying the factors from Table 1-1. After finding the overall efficiency, the fuel rate and the weight of the combustion gases can be computed. The convection bank can then be designed by conventional methods. 

Then, from enthalpy data on the process fluid, calculate the crossover temperature from convection to radiant section. If the computed temperature is significantly different from that assumed in starting the rating, it may be necessary to go back and repeat the calculation with a new value of average tube wall temperature. The tube wall... 

The tube size, the number of tubes per row, the area per row and the minimum cross-sectional area for flue gas flow are considered in this rating procedure. Usually, in fired heaters the tube size in the radiant section is the same as in the convection section, and the number of tubes per row is such that the flue gas rate is about or below 15 feet per second. The area per row is the exposed area per tube times the number of tubes. The minimum cross-section area for flue gas flow is the total cross-sectional area of the convection section less the projected area of one row of tubes. 

Example 2. Using the same basis as the rating example in the radiant section, it is easy to compare plain and finned tubes in the convection section. 

Process gas fiom the reaction shaft enters the uptake shaft at 21,000 Nm /h, 1375°C, and 14-18% SO2. The uptake shaft is constructed entirely with water-cooled copper elements. It is joined to the reaction shaft by four copper jackets called the bullnose as shown in Figure 2. The KIVCET waste heat boiler is divided into three sections radiant, downcomer, and convection. The radiant section consists of 717-m of membrane wall and reduces the process gas temperature at the top of the uptake shaft to 800-830 C. The process gas flows through the down comer section with a surface area of486 m and is cooled to a temperature of600-630 C followed by the convection boiler with a surface area of 1012 m. The outlet gas temperature from the waste boiler is 325-350 C. Typically, the waste heat boiler produces 23-25 t/h of steam at charge rates of 56 t/h. The on-line cleaning of the waste heat boiler consists of spring hammers for the down comer and radiant sections and pneumatic rappers for the convection section. 

One manufacturer uses single-drum, watertube type waste heat boilers on incineration systems. Watertube bodets are also used by other manufacturers in installations where high steam pressures and flow rates are required. Another manufacturer offers heat recovery systems with water wall or radiant sections in the primary chamber. These water wall sections, which are usually installed in series with a convective type waste heat boiler, can increase overall heat recovery efficiencies by as much as 10 to 15%. 

The law states that the rate of heat transfer (2), in the radiant section of a fired heater, varies highly nonlinearly with the temperature (7). 

Pyrolysis Exhibit 7-4 illustrates a pyrolysis furnace whose produa tubes are placed in the center of the radiant section because of a relatively short residence time, high heat transfer rate, and need for even temperature distribution in the tubes. An integral waste heat recovery system that employs the use of a steam drum and a transfer line exchanger (TlX) is also shown. Steam decoking is required to clean the internal walls of the process tubes. 

The absorption cross section (rate of photon absorption for unit radiant energy flux) is cr(co) = W (o)lc (c being the light speed), while the molar extinction coefficient is simply NAoico) (Na being Avogadro s number). 

My utility plant operators have observed that when they preheat their boiler combustion air from 20°C to 220°C, steam production drops by around 3 or 4 percent. They are forced to reduce the firing rate, because the radiant section of the boiler starts to overheat. With the radiant section just as hot with or without the air preheat, I would think that steam production would remain constant, so why would it drop off "... 

Fuel oils that contain both vanadium and sodium are a special problem. I ve discussed this problem in a later example dealing with corrosion. The difficulty is that a mixture of 99 percent vanadium and 1 percent sodium forms a eutectic mixture, which melts at about 1300°F. The viscous liquid that results is corrosive. I ve seen rapid rates of localized tube thinning on vertical, radiant section of tubes. 

If the deposits don t melt, they will accumulate on the surface of the radiant section tubes. I ve seen an 0.125 inch of vanadium evenly coating a 300 mm Btu/hr heater. A heater flux rate of 14,000 Btu/hr/fF had been sustained for five years with no tube damage. The vanadium acted as a protective coating for the tubes, except where it melted and caused a tube leak. 

In the isoflow stills (g) and (h), finned tubes are used in tke convection section. This greatly reduces the amount of tubing required and resulU in rates of heat transfer in the convection section as large as or sometimes larger than in the radiant section. In still h) the upper ends of the tubes are finned and thus there is no distinct convection section. The Selas still (k) employs ceramic burner cups, spaced at about 30 in., which cover both walls. 

Preheating the air that is used in combustion has an effect that is the reverse of flue-gas recirculation. It tends to increase the absorption rate in the radiant section and to increase the flame temperature. To be most effective, the air should be heated by only the gases that are passing to the stack. 

A study of this equation for the effect of the type of fuel is useful. Tabl 18-3 is based on the fuel analyses and data given in Chap. 14. The radiant-absorption factors and rates of absorption per square foot of projected area are computed for 30 p>er cent excess air when half of the heat liberation is absorbed in the radiant section. 

Example 18-1.. Rate of Absorption in the Radiant Section. A pipestill uses 7,110 lb per hr of a cracked gas (net heating value 20,560 Btu per lb or 1900 Btu per cu ft). The radiant section contains 1,500 sq ft of projected area, and the tubra (5 in. outdde diameter) are spaced at a center-to>center distance of 10 in. Th e is only one row of radiant tubes, and they are 40 ft long. The ratio of air to fuel is 21.0 (30 per cent excess air). What percentage of the heat liberation is absorbed in tbe radiant section and how many Btu are absorbed per hour through each square foot of projected area ... 

The rate of radiant absorption varies at different parts of the radiant section. In ordinary stills the exact distribution of radiation is not important but for (1) heating sensitive stocks such as treated lubricating oils, (2) heating to very high temperatures as in cracking stills, and (3) heating two different stocks in separate coils in the same radiant section, the rate of heat absorption in the different parts of the furnace box becomes very important. Data on several stills have been reported that indicate radian tion rates that varied by 400 per cent in different parts of the radiant section. Combustion space is not of direct importance in pipestills. 

The rate of heat absorption in the convection section can be approached with more assurance than for the radiant section. The only troublesome points are (1) the combination of radiation and convection heating that occurs through the first rows of the convection section and (2) the radiar-tion from the hot gas and the walls of the convection section. The first difficulty is often handled by counting these tubes twice i.e., the first two rows are counted as radiant tubes, and they are also counted as convection tubes. The second difficulty is discussed later. 

According to Fig. 23-6, a stack temperature of 600 F wiU be economical. This corresponds to a stack loss of about 15 to 16 per cent (Fig. 14-3). Wall losses for a steaight-up-type still (Fig. 18-ld) will be only about 4 per cent (2 per cent in the radiant and 2 per cent in the convection section). In the topping of crude oil, a radiant-absorption rate or q of 35,000 Btu p sq ft of projected area (Table 18-5) is safe, and a velocity of 3 ft per sec will not give a prohibitive pressure drop. According to Tig 18-4, the percentage of the heat liberation that is absorbed in the radiant section is about 39.4, but since the gas fuel used here is different from the one of Fig. 18-4, an R of 41 will be used. Note that this applies only to radiant sections in which two rows of tubes are used, and for center-to-center spacings that are twice the tube diameter. The heat balance is... 

Considering the distribution of tubes between the radiant and convection sections, it would probably cost little more if more tubes were put in the radiant section, and this would be highly advisable because it would reduce the radiant-absorption rate to such low values that no danger of overheating would exist. In addition, the radiant section is so small that only one row of radiant tubes (rather than two) would allow a better arrangement of surface. 

The heating operation is not a difficult one, and hence the radiant section can be designed for a radiant-absorption rate of about 35,000 Btu per sq ft of projected area per hr (Table 18-5). 

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